I had a little fun a few days ago having AI do some forecasting for this weekend DC winter weather event. Figured now that it’s Thursday night I would give the AI chatbots a chance to create a new forecast as we have more updated models. Below is the prompt and the results.

I’ve updated my prompt a bunch. I’m really interested in continuing to refine the prompt and using it more frequently. Ultimately I would love to create a process of having it forecast and then tracking accuracy of the forecasts over time. Project for another day!

You’ll notice I start by telling it to ignore everything I’ve had it do before. I was intentionally trying to tell it to not look at its past forecasts, as they are now meaningless. This actually turned out to be really important. I had been asking Gemini to do updates to its forecast and it continued to insist that the snow would mostly come on Saturday. I even had it do an analysis of probability on whether snow would mostly fall on Saturday or Sunday and its analysis said Saturday. You’ll see in its completely fresh forecast it has moved to be primarily a Sunday event.

I ran all of these between 10:00 PM and 10:30 PM US Eastern on Jan 22, 2026. Lets see what happens over the next several days!

The Prompt

Ignore all previous memory you have of research, analysis, and forecasting I’ve asked you to do.

You are an expert meteorologist tasked with producing a 96-hour weather forecast for the following U.S. ZIP code: 20170.

Objectives

Create a forecast that:

Presents multiple plausible forecast scenarios (at least 3)

Assigns a probability (%) to each scenario

Explains the meteorological reasoning behind each outcome

Focuses on timing, precipitation, temperature, and impactful weather

Required Research & Data Sources

Before issuing the forecast, research and synthesize guidance from:

Global numerical models (e.g., ECMWF, GFS, ICON, UKMET)

High-resolution / short-range models (e.g., HRRR, NAM, RAP, HREF)

Ensemble guidance (e.g., GEFS, EPS, SREF, HREF ensembles)

Explain:

What each model class is best at predicting (e.g., synoptic pattern, mesoscale precipitation, convective timing, extremes)

Which models should be weighted more heavily at different lead times within the 0–96 hour window

Where model disagreement exists and why

Forecast Elements to Include

For each scenario, clearly describe:

Timing of key weather events (onset, peak, end)

Precipitation

Type (rain, snow, sleet, freezing rain, mixed)

Amounts (ranges with units)

Temperature

Highs and lows

Notable departures from normal

Wind

Sustained speeds and gusts

Potential impacts

Convective or hazardous weather

Thunderstorms, lightning, hail, tornado risk, heavy rain

Snow squalls, icing, or blizzard conditions if relevant

Any confidence-lowering factors (track uncertainty, boundary placement, instability questions, etc.)

Probability & Confidence

Assign a probability (%) to each scenario (total must equal 100%)

Identify the most likely outcome and explain why

Highlight forecast bust risks and what would cause the forecast to change

Structure & Presentation

Organize the response as follows:

Location Overview (climate context, terrain, coastal/inland effects if relevant)

Model Overview & Comparison

Scenario Forecasts (with probabilities)

Timeline Summary (0–24h, 24–48h, 48–72h, 72–96h)

Key Uncertainties & Watch Points

Bottom-Line Forecast Confidence

Appendix (specify what models and which runs were used, include available images or links)

Use clear meteorological reasoning, professional terminology (explained briefly where needed), and quantitative detail. Write as if briefing an informed weather enthusiast, emergency planner, or operational decision-maker—not a casual public forecast.

The Results

ChatGPT 5.2 Thinking Standard

https://chatgpt.com/share/6972f10e-1ac0-800c-ace9-a1c4983781a8

Weather for 20170, VA:

Current Conditions: Partly cloudy, 40°F (4°C)

Daily Forecast:

Thursday, January 22: Low: 26°F (-3°C), High: 53°F (12°C), Description: Partly sunny and warmer

Friday, January 23: Low: 7°F (-14°C), High: 42°F (6°C), Description: Cloudy and cooler; breezy in the afternoon

Saturday, January 24: Low: 11°F (-12°C), High: 18°F (-8°C), Description: Breezy in the morning; otherwise, cloudy and frigid

Sunday, January 25: Low: 15°F (-9°C), High: 21°F (-6°C), Description: Very cold with snow mixing with sleet, accumulating 6-12 cm; travel will be extremely difficult and dangerous

Monday, January 26: Low: -5°F (-21°C), High: 26°F (-3°C), Description: Windy and very cold with clouds and sun

Tuesday, January 27: Low: -6°F (-21°C), High: 19°F (-7°C), Description: Mostly sunny and bitterly cold

Wednesday, January 28: Low: 4°F (-16°C), High: 24°F (-4°C), Description: Very cold with intervals of clouds and sun

1) Location overview (ZIP 20170 — Herndon, VA)

Herndon sits in northern VA’s Piedmont, just east of the Blue Ridge and well inland from the coast. That geography matters in Mid-Atlantic winter storms because:

Cold-air damming (CAD) can wedge shallow Arctic air southward east of the mountains, locking in subfreezing surface temps even while warmer air tries to intrude aloft (classic snow → sleet → freezing rain setup). (National Weather Service)

The mixing line often sets up near/around the I-95 corridor, and small track/thermal-profile changes can swing outcomes a lot over short distances. (National Weather Service)

Climate context: late January is typically the coldest time of year here; daily highs near the low-40s and lows mid-20s are common around Dulles/Herdon. (Weather Spark)

2) Model overview & comparison (and how I’m weighting them)

What each model “class” is best at

Global deterministic models (ECMWF/GFS/UKMET/ICON)
Best for: synoptic pattern (jet/phasing, cyclone track, large-scale thermal advection), and broad precipitation shield placement 2–5 days out. Weakness: convective/mesoscale banding details and boundary-layer micro-structure at neighborhood scale.

Short-range/high-resolution models (HRRR/NAM/RAP + convective-allowing ensembles like HREF)
Best for: timing and placement of mesoscale features inside 0–48 hours: frontogenesis bands, deformation zones, dry-slot timing, and rapid p-type changes near the mixing line. Weakness: can be “noisy” beyond ~36–48 hours; sensitive to initialization errors.

Ensembles (GEFS/EPS/SREF/HREF-members)
Best for: quantifying uncertainty (track spread, thermal profile spread, probability of thresholds like 6″+ snow or 0.25″+ ice). They’re the backbone for scenario probabilities and “bust risk.”

Lead-time weighting (0–96h)

0–24h: observations + short-range guidance (HRRR/RAP/NAM) for winds/temps; minimal precip expected until the weekend system.

24–48h: blend of globals + high-res for the Arctic front and CAD setup.

48–96h: heaviest weight on ensemble guidance + NWS forecast blend/forecaster synthesis, then deterministic models for scenario shape (track/thermal advection). (National Weather Service)

Where disagreement exists (and why)

NWS Sterling explicitly calls this a “complex forecast with all precip types expected,” noting uncertainty in how far north sleet/freezing rain evolves and that a dry slot could affect totals. (National Weather Service)
That’s the crux: the weekend storm’s track and the depth/temperature of the warm layer aloft determine whether 20170 is mostly snow or spends meaningful time in sleet/freezing rain. (National Weather Service)

3) Scenario forecasts (probabilities sum to 100%)

Scenario A — “Plowable snow dominates, brief sleet risk late” (55%) — MOST LIKELY

Meteorological reasoning: Strong Arctic air is entrenched (high confidence), and guidance favors widespread significant snow; the main question is whether the warm nose aloft reaches far enough north to force a longer mixed phase near/along I-95. (National Weather Service)

Timing

Onset: Sat Jan 24 evening (after ~6–10 PM)

Peak: Late Sat night → Sun morning (heaviest rates)

Taper: Sun evening; lingering light snow/drizzle possible into early Mon (National Weather Service)

Precip

Type: Snow → (brief sleet possible Sun afternoon/evening) → back to snow/flurries

Amounts (snow): 8–14 in (localized higher if banding sets up)

Ice: trace to 0.10 in (mainly if sleet/freezing drizzle occurs)

Expect high snow ratios early (fluffier) then lower ratios if mixing begins. (National Weather Service)

Temperature

Fri night: single digits; Sat highs mid-teens to low-20s

Sun highs upper teens to mid-20s (still subfreezing) (National Weather Service)

Wind

Fri–Sat: increasing NW winds behind the Arctic front; wind chills near/below 0 at times (National Weather Service)

During storm: E/NE 10–20 mph; post-storm NW gusts 20–30+ mph → blowing/drifting possible

Impacts

Travel likely difficult to impossible during peak.

Snow + prolonged cold → hazardous roads persist into early week. (National Weather Service)

Scenario B — “Warmer aloft punches north: more sleet/freezing rain, lower snow totals” (30%)

Meteorological reasoning: A slightly farther-north track or stronger warm advection aloft increases time spent in sleet/freezing rain, capping snow totals but raising icing impacts—especially near/along the I-95 corridor (which includes the broader Herndon/DC region). (National Weather Service)

Timing

Same onset window (Sat evening), but changeover to sleet occurs earlier (Sun late morning/early afternoon)

Precip

Snow: 4–8 in

Sleet: 0.25–0.75 in (compacted, high road impact)

Freezing rain: 0.10–0.25 in (isolated higher)

NWS highlights “hints of significant icing potential… possibly into the I-95 corridor” with rising probabilities for ≥0.25″ ice. (National Weather Service)

Temperature

Still subfreezing at the surface (CAD), so “warmer” mostly means aloft, not at ground level.

Wind/impacts

More tree/line loading risk (ice + wind) and harder recovery despite lower snow totals.

Scenario C — “Colder/track a bit south: mostly/all snow, higher totals” (15%)

Meteorological reasoning: If the warm layer aloft is weaker or displaced south/east, 20170 stays in the dendritic growth zone longer with less mixing, allowing higher snow totals—especially if deformation banding stalls. NWS indicates very high probabilities of ≥6″ area-wide and meaningful chances of 12″+ in favored zones near/west of the Blue Ridge. (National Weather Service)

Timing

Onset Sat evening; peak Sun early morning; longer-duration moderate snow into Sun evening

Precip

Snow: 12–20 in (highest if a band sets up overhead)

Minimal sleet/ice

Wind/impacts

More drifting/blowing as winds pick up post-storm; near-whiteout bursts possible in heavier bands.

4) Timeline summary (0–24h, 24–48h, 48–72h, 72–96h)

0–24h (Thu Jan 22 → Fri Jan 23)

Dry; mild for January today, then increasing clouds later. (National Weather Service)

24–48h (Fri Jan 23 → Sat Jan 24)

Arctic front brings sharply colder air; dangerous wind chills Fri night into Sat morning. (National Weather Service)

48–72h (Sat Jan 24 → Sun Jan 25)

Daytime Sat: bitter cold; prep window.

Storm onset Sat evening, rapidly intensifying overnight.

72–96h (Sun Jan 25 → Mon Jan 26)

Highest-impact period: heavy snow and/or sleet/freezing rain depending on scenario; gradual taper late Sun into early Mon with lingering light wintry precip possible. (National Weather Service)

5) Key uncertainties & watch points (what could flip the forecast)

Warm nose aloft placement/intensity → determines snow vs sleet/freezing rain duration near the I-95 corridor. (National Weather Service)

Dry-slot timing → could cut totals or create a lull, then redevelop precip on the backside. (National Weather Service)

Mesoscale banding (frontogenesis/deformation) → can add several inches locally; hard to pin down until ~12–36 hours out.

Surface CAD strength → affects how quickly (or if) surface temps flirt with 32°F (critical for icing vs compacted snow). (National Weather Service)

6) Bottom-line forecast confidence (operational)

High confidence in: a major winter storm affecting the region late Sat into Sun + dangerously cold air before/after. (National Weather Service)

Moderate confidence in: plowable snow in/near 20170 (Scenario A favored). (National Weather Service)

Lower confidence in: exact p-type evolution (how much sleet/freezing rain you see) and precise storm-total at ZIP-code scale. (National Weather Service)

If you’re planning operations: act as though Scenario A occurs, but build contingency for Scenario B icing (because a smaller snow number can still be the worse outcome).

7) Appendix — data/models & runs referenced

Primary forecast synthesis (official):

NWS Baltimore/Washington (LWX) Area Forecast Discussion, issued 9:25 PM EST Thu Jan 22, 2026 (details on p-type uncertainty, snow/ice probabilities, temps/wind chills). (National Weather Service)

NWS LWX Weather Story (high confidence for plowable snow late Sat evening through Sun; mixing possible farther south; dangerous cold). (National Weather Service)

Probabilistic winter guidance (threshold framing):

WPC Probabilistic Winter Precipitation Forecast (PWPF) interface (24–72h probabilities by threshold; used as the probabilistic framework). (NCEP Weather Prediction Center)

WPC WSSI-P (probabilistic impact categories conceptually referenced for impact-based thinking). (NCEP Weather Prediction Center)

WPC Experimental Winter Storm Outlook page (context on warning-criteria exceedance products). (NCEP Weather Prediction Center)

Baseline local forecast inputs:

20170 7-day forecast feed used for temperature continuity checks (not the sole driver vs NWS discussion).

If you want, I can turn the three scenarios into an operational “decision table” (road pretreatment windows, staffing triggers, power-outage risk triggers) keyed to observable signals (e.g., 850–700 mb warm nose magnitude, surface wet-bulb, dry-slot radar signature) so you can update probabilities in real time.

Claude Opus 4.5 Extended Thinking w/ Research and Web Search

https://claude.ai/public/artifacts/e8240cd3-b1e2-4411-b37a-ae637ee8aa82

Major winter storm threatens Herndon: 96-hour probabilistic forecast

A high-confidence Winter Storm Watch is in effect for Herndon, Virginia, as models unanimously agree on a significant winter weather event from Saturday evening January 24 through Sunday night January 25, 2026. Most likely outcome: 8-12 inches of snow with dangerous wind chills reaching -7°F. The National Weather Service rates this event as having “almost certain” probability of becoming a widespread, significant winter storm—the most impactful for the DC metro area in several years. Ensemble guidance shows 80-90% probability of accumulating at least 4 inches, with 50-65% probability of reaching 8+ inches. The primary forecast uncertainty centers on whether a warm layer aloft will introduce sleet/freezing rain mixing, which would reduce snow totals but add ice accumulation risk.

Northern Virginia in late January: setting the stage

Herndon’s location in the Piedmont of Northern Virginia places it in a climatologically favorable zone for winter storms. The area sits at approximately 300 feet elevation, west of the I-95 corridor and east of the Blue Ridge—a transition zone where storm tracks critically determine whether precipitation falls as snow, sleet, or freezing rain.

Normal January climate for Washington-Dulles Airport (IAD):

Average high: 42°F | Average low: 25°F

Average January snowfall: 5.7 inches

Record cold high January 25: 18°F (1987) | Record low: -5°F (1987)

This event will bring temperatures 15-35°F below normal, with potential to challenge records from the historic January 1987 cold wave. The synoptic setup—a deeply negative Arctic Oscillation exceeding -5 standard deviations (extraordinary)—is driving an arctic air mass of unusual intensity and persistence.

How weather models work and which to trust

Understanding model strengths helps interpret forecast confidence. Global models provide the big picture while mesoscale models refine timing and local detail.

Global deterministic models (GFS, ECMWF, ICON, UKMET) run twice daily at 9-25km resolution, forecasting out 10-16 days. The ECMWF (“Euro”) is generally considered most skillful for synoptic patterns and medium-range timing; the Weather Prediction Center favored it for this event’s early period. The GFS updates more frequently (every 6 hours) and provides good trend analysis. For this storm, both agree on occurrence and timing but differ on moisture amounts and the crucial warm-nose development that determines precipitation type.

High-resolution models (HRRR, NAM 3km) excel at 0-24 hour forecasts with 3km resolution that captures mesoscale features like snow bands. The HRRR updates hourly, making it invaluable for real-time nowcasting once the storm arrives. These models show impressive snowfall rates of 1-2 inches per hour during peak intensity but cannot yet resolve the warm-layer question at extended ranges.

Ensemble systems (GEFS, EPS, HREF) run multiple simulations with slightly varied initial conditions, quantifying uncertainty. The GEFS (31 members) and EPS (51 members) show tight clustering on the storm’s existence but meaningful spread on track details. Ensemble agreement indicates high confidence (95%+) in a significant event, with moderate-high confidence (60-75%) in heavy snowfall exceeding 10 inches.

Model Type Best Use Current Agreement Level
ECMWF Overall pattern, days 3-7 High – major storm
GFS Trend analysis, days 1-5 High – major storm
HRRR/NAM Timing, precipitation type High – heavy snow
GEFS/EPS Probability guidance High – 80-90% for 4″+

Three plausible scenarios for Herndon’s storm

Ensemble clustering reveals three distinct outcomes, with probabilities based on GEFS/EPS member distribution and NWS probabilistic guidance.

Scenario A: Cold track maximizes snow (55% probability)

The storm tracks close to the I-95 corridor while maintaining a cold enough thermal profile to keep precipitation as all snow throughout. This scenario—currently supported by the GFS and most recent model trends showing a northward shift—delivers the highest accumulations.

Expected impacts:

Total snowfall: 10-14 inches

Ice accumulation: Trace to none

Peak rates: 1.5-2 inches per hour Sunday 2-8 AM

Onset: Saturday 7-9 PM

End: Sunday 9 PM – midnight

Travel: Roads impassable Saturday night through Monday morning

This outcome produces a classic Miller Type B cyclone with a favorable deformation zone setting up northwest of the city, enhancing snow amounts. Dendritic growth zone temperatures (-12°C to -18°C) support efficient snow production with 12:1 to 15:1 snow-to-liquid ratios.

Scenario B: Warm nose introduces mixing (35% probability)

The ECMWF solution—which pulls more Gulf moisture into the system—develops a layer of above-freezing air at 4,000-5,000 feet Sunday afternoon. This scenario transitions heavy snow to sleet and freezing rain for 3-6 hours before returning to snow.

Expected impacts:

Total snowfall: 6-9 inches (before transition)

Sleet accumulation: 0.5-1 inch

Ice accumulation: 0.1-0.25 inches

Peak snow rates: 1-1.5 inches per hour (Sunday morning)

Transition timing: Sunday 1-4 PM

Travel: Roads impassable; elevated power outage risk from ice loading

This scenario produces lower headline snow totals but adds hazards: ice accumulation threatens trees and power lines, and refrozen sleet creates a harder surface beneath any subsequent snow. NWS Sterling notes “30-50% probability of icing along I-95” with 10-20% chance of ≥0.25 inch ice accumulation.

Scenario C: Strong coastal intensification (10% probability)

A small but non-negligible ensemble subset shows aggressive secondary low development off the North Carolina coast, creating a Miller A/B hybrid that draws the heaviest precipitation band directly over the DC metro. This scenario—while lower probability—represents the highest-impact outcome.

Expected impacts:

Total snowfall: 14-18+ inches

Ice accumulation: Trace

Peak rates: 2+ inches per hour possible

Duration: Extended, possibly lasting into Monday afternoon

Travel: Severe disruptions lasting 36-48 hours; paralytic conditions

The WPC notes probabilities above 50% for 12+ inches across portions of the Mid-Atlantic already exist in their guidance, acknowledging this high-end scenario cannot be ruled out.

Timeline summary: hour-by-hour evolution

Hours 0-24 (Thursday January 22)

Conditions: Quiet and seasonably cold

High: 42-46°F | Low: 28-32°F

Sky: Partly cloudy

Wind: NW 5-10 mph

Precipitation: None

Action items: Complete storm preparations; finalize supplies

Hours 24-48 (Friday January 23)

Conditions: Arctic front arrives; dangerous cold developing

High: 43-47°F early, crashing to 20s by evening | Low: 8-14°F

Wind chill: -5°F to 5°F overnight

Wind: NW 15-20 mph, gusts to 30-35 mph

Precipitation: None

Alerts: Cold Weather Advisory 8 PM Friday – 10 AM Saturday

Action items: Protect pipes; ensure heating systems functioning; charge devices

Critical detail: The arctic front passage Friday establishes the cold air mass necessary for an all-snow event. Temperatures at 850mb dropping to -18°C to -20°C are extraordinary for this region and ensure initial precipitation will be snow regardless of scenario.

Hours 48-72 (Saturday January 24)

Conditions: Bitterly cold morning; storm arrives evening

High: 18-24°F | Low: 10-15°F

Wind chill: 0°F to 8°F

Wind: N 10-15 mph, gusts to 25 mph

Precipitation onset: 6-10 PM; light snow becoming moderate

Accumulation by midnight: 1-3 inches

Alerts: Winter Storm Watch begins 7 PM; likely upgraded to Warning

Key timing: Snow spreads from southwest to northeast during the evening hours. Roads will transition from clear to snow-covered between 8 PM and midnight. This is the last window for travel.

Hours 72-96 (Sunday January 25)

Conditions: Major winter storm; potential for mixing

High: 18-25°F | Low: 8-15°F

Wind: NE→N 10-20 mph, gusts to 30 mph

Precipitation: Heavy snow overnight; 1-2 inch/hour rates 2-8 AM

Mixing risk: Scenario B includes 3-6 hours of sleet/freezing rain Sunday afternoon

Visibility: ¼-1 mile in heavy snow; near-zero in snow bands

Storm exit: Sunday 9 PM – Monday 6 AM (varies by scenario)

Accumulation by storm’s end: 6-14 inches depending on scenario

Critical period: Sunday 2-10 AM represents peak intensity. Banded structures within the warm-air advection zone may produce localized rates exceeding 2 inches per hour. Blowing and drifting snow likely with fresh powder and gusty winds.

Key uncertainties requiring monitoring

1. Warm nose development and precipitation type This is the dominant uncertainty. Watch for updates to the 850mb temperature forecast; if models maintain temperatures below -5°C at 850mb through Sunday, the all-snow scenario probability increases. Current ECMWF shows a brief period above 0°C at 850mb Sunday afternoon—this feature must be monitored in subsequent runs.

2. Storm track position (±50 miles matters enormously) A track 50 miles south increases Herndon’s snow totals and reduces mixing risk. A track 50 miles north brings the warm sector closer, increasing ice potential. The WPC notes the “unanimous northward trend could be a slight mirage” given run-to-run variability with northern stream energy.

3. Coastal intensification rate Secondary low development off the Carolinas determines late-stage precipitation. Stronger coastal development extends the event and may pull the heaviest band northward. NOAA is flying Hurricane Hunter reconnaissance missions to improve sampling of the storm’s energy source.

4. Post-storm temperature trajectory Models agree on brutal cold following the storm (lows 5-12°F Monday-Tuesday) but spread increases by mid-week. This cold lock means snow will not melt naturally for days, compounding impacts.

Monitoring checkpoints:

Friday 12Z model runs: Final refinement of snow/ice line

Saturday 00Z runs: High-resolution verification of onset timing

Saturday 12Z HRRR: Nowcast-quality detail for evening hours

Bottom-line forecast confidence assessment

Forecast Element Confidence Probability
Storm occurrence Very High 95-100%
Significant snow (≥4″) High 85-90%
Heavy snow (≥8″) Moderate-High 50-65%
Very heavy snow (≥12″) Moderate 35-45%
Ice accumulation (≥0.25″) Moderate 25-40%
Record/near-record cold High 75-85%
Major travel disruptions Very High 95-100%
Power outage potential Moderate-High 40-60%

Forecaster’s confidence statement: This event presents an unusual combination of high confidence in impact (virtually certain to be significant) with moderate uncertainty in specifics (exact totals within a 6-inch range). The WPC’s characterization—”almost certain for a widespread and significant winter storm”—accurately captures the situation. Prepare for the full range of scenarios; do not anchor on a single number.

Most likely outcome for Herndon: 8-12 inches of snow with dangerous wind chills, producing 24-36 hours of impassable roads and potential for school and office closures extending into Tuesday.

Appendix: model runs and data sources

Numerical Weather Prediction Models Used

Model Run Time Resolution Notes
ECMWF (IFS) 12Z Thu Jan 22 9 km Primary global guidance
GFS 18Z Thu Jan 22 13 km Latest American model
ICON 00Z Thu Jan 22 13 km German DWD model
UKMET 12Z Thu Jan 22 10 km UK Met Office
NAM 00Z Thu Jan 22 12km/3km Mesoscale
HRRR 03Z Thu Jan 22 3 km Hourly updates

Ensemble Systems

System Members Run Time Use
GEFS 31 00Z Thu Jan 22 Uncertainty quantification
EPS (ECMWF) 51 12Z Wed Jan 21 Probabilistic guidance
HREF Multi-model 00Z Thu Jan 22 Short-range probabilities
NBM Multi-source Hourly Calibrated blend

Official NWS Products

NWS Sterling (LWX) AFD: 2:52 AM EST Thu Jan 22, 2026

WPC Short-Range Discussion: 3:02 PM EST Thu Jan 22, 2026

WPC Extended Discussion: 2:48 PM EST Thu Jan 22, 2026

WPC Probabilistic Winter Weather: 2:59 PM EST Thu Jan 22, 2026

CPC Hazards Outlook: 3:00 PM EST Thu Jan 22, 2026

Additional Sources Consulted

Tropical Tidbits (tropicaltidbits.com) – model visualization

Pivotal Weather (pivotalweather.com) – ensemble graphics

Capital Weather Gang, Washington Post – local meteorologist analysis

FOX5 DC, NBC4 Washington – regional broadcast meteorology

College of DuPage NEXLAB – mesoscale products

Forecast prepared January 23, 2026. Monitor NWS Sterling (weather.gov/lwx) for official watches, warnings, and updates. This probabilistic guidance is intended for planning purposes; official NWS products should be referenced for warning-level decisions.

Model Type	Best Use	Current Agreement Level
ECMWF	Overall pattern, days 3-7	High – major storm
GFS	Trend analysis, days 1-5	High – major storm
HRRR/NAM	Timing, precipitation type	High – heavy snow
GEFS/EPS	Probability guidance	High – 80-90% for 4″+

Forecast Element	Confidence	Probability
Storm occurrence	Very High	95-100%
Significant snow (≥4″)	High	85-90%
Heavy snow (≥8″)	Moderate-High	50-65%
Very heavy snow (≥12″)	Moderate	35-45%
Ice accumulation (≥0.25″)	Moderate	25-40%
Record/near-record cold	High	75-85%
Major travel disruptions	Very High	95-100%
Power outage potential	Moderate-High	40-60%

Model	Run Time	Resolution	Notes
ECMWF (IFS)	12Z Thu Jan 22	9 km	Primary global guidance
GFS	18Z Thu Jan 22	13 km	Latest American model
ICON	00Z Thu Jan 22	13 km	German DWD model
UKMET	12Z Thu Jan 22	10 km	UK Met Office
NAM	00Z Thu Jan 22	12km/3km	Mesoscale
HRRR	03Z Thu Jan 22	3 km	Hourly updates

System	Members	Run Time	Use
GEFS	31	00Z Thu Jan 22	Uncertainty quantification
EPS (ECMWF)	51	12Z Wed Jan 21	Probabilistic guidance
HREF	Multi-model	00Z Thu Jan 22	Short-range probabilities
NBM	Multi-source	Hourly	Calibrated blend

Google Gemini Thinking Deep Research

https://gemini.google.com/share/69dec08b96f5

Meteorological Analysis and 96-Hour Winter Storm Forecast: Herndon, Virginia (20170)

1. Synoptic Overview and Atmospheric Dynamics

1.1. Hemispheric Pattern Configuration

The meteorological situation currently unfolding across the North American continent represents a textbook “Miller Type B” cyclogenesis event, heavily modified by a blocked, high-latitude flow pattern. The forecast period, initiated at 00:00 UTC on Friday, January 23, 2026, and extending through 00:00 UTC Tuesday, January 27, 2026, is dominated by the interaction of two distinct and potent air masses. This interaction is facilitated by a highly amplified jet stream configuration that supports the transport of deep tropospheric moisture into a pre-existing, dense Arctic air mass entrenched over the Mid-Atlantic region.

The primary driver of the impending adverse weather is a high-amplitude ridge entrenched over the Eastern Pacific and the West Coast of the United States. This positive Pacific North American (PNA) teleconnection pattern forces the downstream flow into a deep trough over the Central and Eastern United States. Simultaneously, the North Atlantic Oscillation (NAO) is trending toward a negative phase, evidenced by blocking high pressure over Greenland and the Davis Strait. This high-latitude blocking mechanism is critical; it acts as an atmospheric brake, stalling the progression of weather systems and allowing cold air to remain locked in place along the eastern slopes of the Appalachian Mountains.

The “Siberian Express,” a colloquialism for the cross-polar flow of Arctic air, is established by a strong surface high-pressure system (1040+ mb) diving southeastward from the Canadian Prairies into the Great Lakes and eventually New England. This high-pressure system is the source of the cold air damming (CAD) wedge that will be a defining feature of the forecast for Herndon, VA (20170). The positioning of this high pressure to the north is climatologically favorable for sustaining sub-freezing surface temperatures in the Piedmont of Virginia, even as warmer air attempts to intrude aloft.

1.2. Interaction of Northern and Southern Streams

The cyclogenesis event forecasted for the weekend of January 24-25, 2026, relies on the complex phasing—or near phasing—of the northern and southern jet streams.

The Northern Stream: This branch is vigorous and highly amplified, digging swiftly through the Midwest. It is responsible for delivering the Arctic air mass and providing the upper-level divergence necessary to deepen the surface low pressure.

The Southern Stream: Often referred to as the “Pineapple Express” due to its origins in the subtropical Pacific near Hawaii, this branch serves as the moisture hose for the system. It carries anomalously high precipitable water (PWAT) values, injecting significant latent heat into the developing system.

As the southern stream shortwave energy ejects from the Southwest US and traverses the Gulf Coast states, it will induce surface cyclogenesis. The interaction between the moisture-laden southern stream and the cold, dynamic northern stream creates a broad area of isentropic lift—where warm, moist air glides upward over the dense, cold dome at the surface. This mechanical lifting is the primary forcing mechanism for the widespread precipitation shield anticipated to engulf Northern Virginia.

1.3. Thermodynamic Profile and Vertical Structure

The vertical temperature profile is the single most critical variable in the forecast for ZIP 20170. While surface temperatures are forecasted with high confidence to remain in the teens and twenties (°F) throughout the event due to the strength of the CAD wedge, the temperature profile between 850mb (approx. 5,000 ft) and 700mb (approx. 10,000 ft) remains volatile.

Models indicate a strong warm air advection (WAA) regime aloft. As the surface low tracks northeastward—likely inland of the coast initially before a secondary coastal low takes over (Miller B transfer)—warm air from the Atlantic and the Gulf Stream will be pulled westward over the cold dome. This creates a “warm nose” in the sounding.

Dendritic Growth Zone (DGZ): Early in the event (Saturday night), the entire column remains below freezing, with strong vertical velocities centered within the DGZ (-12°C to -18°C). This setup maximizes snow-to-liquid ratios (SLR), potentially producing fluff factors of 15:1 or greater.

The Melting Layer: By Sunday midday, the 850mb isotherm is projected to breach 0°C. If this warm layer is shallow (less than 2,000 ft deep) and the surface layer is deep and cold, snowflakes will melt partially or fully and then refreeze into sleet (ice pellets) before impact. If the warm layer deepens significantly, the hydrometeors will fall as supercooled liquid rain, freezing on contact with surfaces (freezing rain).

2. Location Overview: Climatology and Topography of Herndon, VA (20170)

2.1. Geographical Context

Herndon, Virginia, located in western Fairfax County, occupies a transitional zone between the coastal plain of Washington D.C. and the foothills of the Blue Ridge Mountains. With an elevation ranging from approximately 300 to 400 feet above mean sea level (MSL) , Herndon sits within the Potomac River watershed but is sufficiently inland to be somewhat insulated from the immediate warming effects of the Chesapeake Bay and the Atlantic Ocean.

Its proximity to Washington Dulles International Airport (KIAD)—the official recording station for the area—means that observational data is robust. However, Herndon frequently exhibits a colder microclimate than the urban center of Washington D.C. (Reagan National Airport) due to the lack of a significant Urban Heat Island (UHI) effect and its closer proximity to the cold air source region to the northwest.

2.2. Cold Air Damming (CAD) Mechanics

The topography of Herndon makes it particularly susceptible to Cold Air Damming. The Blue Ridge Mountains, located roughly 20-25 miles to the west, act as a physical barrier. When high pressure is located to the north (over New England or Quebec), the geostrophic wind flow directs cold air southward. As this stable, dense air encounters the mountains, it is blocked and forced to bank up against the terrain, flowing southward parallel to the mountain chain.

This creates a “wedge” of cold air that is difficult to scour out. Even as strong southerly winds blow at 4,000 or 5,000 feet, the surface wind in Herndon will often remain northerly or northeasterly, reinforcing the cold air supply. This decoupling of the surface and upper-level winds is a hallmark of significant icing events in this region. For the forecasted event, the presence of a 1040mb high to the north suggests a classic, resilient wedge setup. This climatological bias suggests that Herndon will hold onto frozen precipitation (snow or sleet) longer than areas east of the Capital Beltway (I-495).

2.3. Historical Analogues

The setup bears resemblance to several historic storms, specifically the January 2016 “Snowzilla” and the February 2010 storms, in terms of the magnitude of moisture and cold air available. The presence of a “Siberian Express” air mass evokes comparisons to the bitter cold outbreaks of the 1980s, suggesting that Snow-to-Liquid Ratios (SLR) will be higher than the typical Mid-Atlantic “wet cement” snow, at least during the onset of the storm. However, the potential for a sleet transition mirrors the February 2014 event, where a massive warm nose undercut high snow totals in the southern D.C. suburbs.

3. Technical Model Comparison and Bias Analysis

The meteorological community is currently analyzing a suite of global and mesoscale models to pinpoint the storm’s track and thermal profiles. Significant divergence exists, creating uncertainty in the specific precipitation type transition times for ZIP 20170.

3.1. Global Forecast System (GFS)

The American GFS model has displayed a progressive bias with this system. Historically, the GFS struggles with the resolution of the shallow cold air wedge associated with CAD events. It often mixes the boundary layer too aggressively, eroding the cold air faster than observed reality.

Current Trend: The GFS has been oscillating but generally favors a track that brings the warm nose further north, reaching Herndon by Sunday morning. This solution introduces a significant period of sleet and freezing rain, cutting into total snowfall accumulation but increasing the ice storm potential.

Implication: A GFS-verified solution would result in a “front-end thump” of snow followed by a messy, hazardous transition to ice, creating a compacted, concrete-like layer on roads.

3.2. European Centre for Medium-Range Weather Forecasts (ECMWF)

The ECMWF (Euro) is generally regarded as superior in handling the complex thermodynamics of CAD events and the blocking patterns in the North Atlantic.

Current Trend: The ECMWF has consistently held more energy back in the primary trough, resulting in a slower progression and a slightly suppressed storm track. It depicts a stronger, more resilient high-pressure system to the north.

Implication: This model supports a colder solution for Herndon. It suggests the warm nose may stall south of Manassas or Fredericksburg, keeping Herndon in a mostly snow or snow/sleet regime. This generates the highest snowfall totals (10-12″+) and minimizes the freezing rain threat.

3.3. North American Mesoscale (NAM) and High-Resolution Guidance

As the event moves into the range of high-resolution convection-allowing models (CAMs), the NAM becomes critical. The NAM is notorious for amplifying the warm nose (the “NAM torch”) while simultaneously resolving the surface cold wedge with high precision.

Current Trend: The 12km and 3km NAM runs are likely showing a “battleground” scenario. They often depict a sharp gradient where 5 miles can mean the difference between 12 inches of snow and 2 inches of sleet.

Implication: The NAM highlights the banding potential. It suggests extreme precipitation rates (1-2″ per hour) Saturday night which could dynamically cool the column, fighting off the warm air intrusion longer than the global models suggest.

3.4. National Blend of Models (NBM)

The NBM is currently utilized by the National Weather Service (NWS) to create a probabilistic mean. It smooths out the outliers. Currently, the NBM for Herndon reflects a high probability of significant snow (>6″) but acknowledges the icing threat (30-50% chance of >0.1″ ice).

Model Forecast Philosophy for 20170 Snow Potential Ice Potential Primary Hazard
GFS Progressive/Warmer Aloft Moderate (4-8″) High (>0.25″) Ice Loading/Power Outages
ECMWF Blocked/Colder High (8-14″) Low (<0.10″) Deep Snow/Travel Impact
NAM Sharp Gradient/Dynamic Variable (6-12″) Moderate (0.1-0.2″) Heavy Precip Rates
ICON Outlier/Phasing Issues Moderate (5-8″) Moderate Uncertainty

4. Comprehensive 96-Hour Forecast Timeline

The following timeline details the anticipated evolution of the weather event for Herndon, VA (20170), derived from a consensus of the ECMWF, GFS, and NWS guidance.

Phase 1: The Arctic Plunge (Pre-Storm)

Friday, January 23, 2026

06:00 – 12:00 (Friday Morning): The region will be under the influence of the departing pressure gradient. Skies will be partly sunny initially but rapidly filling with high clouds. Temperatures will start in the 20s.

Hazard: None significant yet, but preparations should be finalized.

12:00 – 18:00 (Friday Afternoon): A reinforcing Arctic cold front pushes through. Winds shift to the Northwest at 10-15 mph, gusting to 25 mph. Temperatures will fall non-diurnally (dropping during the day) from a brief high near 43°F into the 30s.

18:00 – 06:00 (Friday Night/Saturday Morning):CRITICAL COLD PERIOD. The core of the Arctic air settles in. Low temperatures will plummet to between 9°F and 14°F in Herndon. Winds remaining breezy (NW 10-20 mph) will generate wind chill values between -5°F and 5°F.

Warning: A Cold Weather Advisory is in effect. Risk of hypothermia and frostbite is elevated. Pipe freezing potential is high for poorly insulated plumbing.

Phase 2: Storm Onset and Accumulation

Saturday, January 24, 2026

06:00 – 12:00 (Saturday Morning): Bitterly cold start. High clouds thicken and lower to mid-level altostratus. The sun will appear as a dim disk through the overcast. Temperatures struggle to rise, hovering in the upper teens (17-19°F).

12:00 – 18:00 (Saturday Afternoon): Radar will begin to show echoes approaching from the southwest, but initial precipitation will evaporate before reaching the ground (virga) due to the extremely dry dew points (single digits). This process of evaporative cooling wet-bulbs the atmosphere, reinforcing the cold air.

18:00 – 00:00 (Saturday Evening):Precipitation Onset. Light snow begins between 6 PM and 9 PM. Due to the cold ground, accumulation is instantaneous on all surfaces. Roads will become slick immediately. Snow intensity increases rapidly towards midnight.

Conditions: Visibilities dropping to 1 mile.

Temp: Steady around 18-20°F.

Phase 3: The Heavy Thump

Sunday, January 25, 2026

00:00 – 06:00 (Sunday Overnight):PEAK SNOW INTENSITY. This is the window of strongest isentropic lift. Heavy snow bands, potentially producing 1 to 2 inches per hour, will traverse the region.

Dynamics: The “Pineapple Express” moisture is fully interacting with the cold dome. Thunder-snow is possible if convective instability is realized.

Accumulation: Rapid stacking of dry, powdery snow. By dawn, 5-8 inches likely on the ground.

06:00 – 12:00 (Sunday Morning):The Transition Zone. As the primary low pressure tracks closer, the warm tongue at 850mb will attempt to nose into Northern Virginia.

Scenario Dependence: In the most likely scenario, snow mixes with and changes to sleet (ice pellets) during this window. The precipitation character will change from “silent snow” to the audible “pinging” of sleet.

Hazard: Sleet accumulation creates a heavy, dense layer on top of the powdery snow, making shoveling extremely physically demanding.

12:00 – 18:00 (Sunday Afternoon): The “Mixed Bag.” Precipitation lightens in intensity but remains hazardous. A mix of freezing rain, sleet, and snow is expected. Temperatures will remain stubbornly below freezing (23-26°F) due to CAD.

Icing: Glaze accretion on trees and power lines becomes the primary concern here.

Phase 4: Departure and Refreeze

Sunday Night into Monday, January 26, 2026

18:00 – 06:00 (Sunday Night): The system exits to the northeast. Precipitation tapers to freezing drizzle or flurries. Winds back to the Northwest and increase significantly (20-30 mph) as the pressure gradient tightens behind the storm.

Impact: Blowing snow is possible if the top layer hasn’t crusted over with ice. The wet/slushy mix on roads will flash freeze into jagged ruts as temps drop back into the teens (15-17°F).

Monday (Day): Skies clear to mostly sunny, but temperatures remain frigid (High near 25°F). No significant melting will occur. Travel remains impacted on untreated secondary roads.

5. Scenario Planning and Probabilistic Outcomes

Given the complexities of the thermal profile, three distinct scenarios have been modeled for Herndon.

Scenario A: The “Sleet Sandwich” (Most Likely Case)

Probability: 50%

Narrative: The storm begins as heavy snow Saturday evening. By Sunday morning (roughly 8 AM), strong warm air advection aloft introduces a layer of +2°C air at 5,000 feet. Snow changes to heavy sleet for 4-6 hours, followed by a brief period of freezing rain before tapering off.

Meteorological Reasoning: This aligns with the consensus of the GFS and mean ensemble guidance which respects the cold air damming at the surface (keeping it <32°F) but acknowledges the strength of the southern jet stream pushing warm air aloft.

Resulting Accumulation:

Snow: 6 to 9 inches.

Sleet: 1 to 2 inches (compacting the snow depth).

Freezing Rain: 0.10 to 0.20 inches.

Hazards: “Heart attack snow” (heavy weight), extremely difficult plowing, moderate risk of power outages due to ice/wind.

Scenario B: The “Cold Suppression” (The Snowstorm)

Probability: 30%

Narrative: The northern high pressure is stronger than modeled, and the confluence zone keeps the storm track suppressed slightly south. The warm nose fails to penetrate north of Manassas. Herndon stays all snow or sees only a brief mix with sleet.

Meteorological Reasoning: Supported by the operational ECMWF and historical CAD behavior. The “Siberian Express” air mass is denser and harder to scour out than models realize.

Resulting Accumulation:

Snow: 10 to 14+ inches.

Ice: Trace to None.

Hazards: Major travel disruptions, buried vehicles, but lower risk to the power grid (dry snow blows off lines).

Scenario C: The “Warm Nose Intrusion” (The Icing Event)

Probability: 20%

Narrative: The storm tracks further inland (west of I-95). The warm nose punches into the region rapidly on Saturday night. Snow changes to rain/freezing rain quickly.

Meteorological Reasoning: Supported by the “NAM torch” outliers and scenarios where the coastal low transfer is delayed, allowing the primary inland low to pump warmth northward.

Resulting Accumulation:

Snow: 2 to 4 inches.

Ice: 0.25 to 0.50+ inches (Major Ice Storm).

Hazards: Catastrophic impact to power grid. Widespread tree damage. Impossible travel on skating-rink roads.

6. Hazard Assessment and Impact Analysis

6.1. Roadway and Transit Impact

Interstates (I-66, Dulles Toll Road): Expect VDOT to pre-treat with brine, but heavy snowfall rates (1-2″/hr) Saturday night will overcome chemical treatments. Conditions will be “Code Black” (impassable) overnight Saturday.

Secondary Roads: Neighborhood streets in Herndon will likely remain snow-covered and rutted through Tuesday. The refreeze Sunday night will turn slush into solid ice, requiring heavy machinery to break.

Public Transit: WMATA typically suspends Metrobus service when snow exceeds 8 inches or ice coverage is significant. Silver Line rail service to Herndon/Monroe may face delays due to third-rail icing.

6.2. Power Infrastructure and Vegetation

Tree Susceptibility: Northern Virginia has a high density of deciduous trees (oaks, maples) and brittle ornamental trees (Bradford Pears). While leaves are off, the surface area of branches is sufficient to hold significant ice load.

Radial Ice Physics: An accretion of 0.25″ of ice can add hundreds of pounds of weight to a single tree span. If Scenario A or C verifies, widespread tree limbs will fail.

Grid Vulnerability: Overhead power lines in older Herndon neighborhoods (downtown) are vulnerable. The combination of ice loading and forecasted wind gusts of 30-35 mph on Sunday creates a high probability of scattered-to-widespread outages.

6.3. Physiological and Human Safety

Wind Chill: The pre-storm cold (Friday night) will feature wind chills of -10°F. Frostbite can occur on exposed skin in under 30 minutes.

Carbon Monoxide: There is a heightened risk of CO poisoning from residents using improper heating sources (grills, ovens) during potential power outages or running cars in enclosed garages to charge devices.

Cardiac Strain: Shoveling heavy, wet snow (specifically the snow/sleet mix in Scenario A) dramatically increases heart rate and blood pressure, posing a severe risk for cardiac arrest in sedentary individuals.

7. Forecast Confidence and Uncertainty Matrix

7.1. Confidence Levels

Forecast Element Confidence Level Reasoning
Event Occurrence Very High (>90%) Model consensus is locked on a major cyclogenesis event impacting the Mid-Atlantic.
Precipitation Onset High (80%) Timing of Saturday evening is consistent across guidance.
Thermal Profile Low-Medium (40%) The depth and northward extent of the warm nose (850mb-700mb) remains the primary failure point in the forecast.
Snow Totals Medium (50%) Highly dependent on the duration of the sleet transition. A 3-hour difference changes totals by 4 inches.
Impact Severity High (80%) Regardless of p-type (snow vs ice), the impact on travel and safety will be major to extreme.

7.2. Key Uncertainties

The “Dry Slot”: Some model runs suggest a dry slot (intrusion of dry air into the system) could sweep over Northern Virginia Sunday afternoon, ending precipitation earlier than expected.

Mesoscale Banding: The exact placement of the heaviest f-gen (frontogenesis) bands cannot be modeled until 12-24 hours prior. If a heavy band parks over Herndon, dynamic cooling could force a Scenario B (all snow) outcome even if the broader atmosphere supports mixing.

8. Summary and Recommendations

Residents of Herndon, VA (20170) should prepare for a Major Winter Storm commencing Saturday evening, January 24, 2026. The most probable outcome is a heavy snowfall event transitioning to a significant wintry mix, resulting in 6 to 10 inches of accumulation with an icy crust.

Actionable Recommendations:

Travel: Complete all travel and supply runs by Saturday at 2:00 PM. Do not plan to travel Sunday.

Home: Prepare for potential power outages (batteries, non-perishable food, blankets). Ensure heating fuel is topped off before Friday.

Monitoring: Monitor the National Weather Service Baltimore/Washington office for updates on the Winter Storm Watch and potential upgrade to a Winter Storm Warning or Ice Storm Warning.

This report reflects the meteorological data available as of January 22, 2026.

Can AI predict weather? AI’s DC Snow Forecast

Published by Nicolas Nowinski on January 23, 2026January 23, 2026

The Prompt

Objectives

Required Research & Data Sources

Forecast Elements to Include

Probability & Confidence

Structure & Presentation

The Results

ChatGPT 5.2 Thinking Standard

Weather for 20170, VA:

1) Location overview (ZIP 20170 — Herndon, VA)

2) Model overview & comparison (and how I’m weighting them)

What each model “class” is best at

Lead-time weighting (0–96h)

Where disagreement exists (and why)

3) Scenario forecasts (probabilities sum to 100%)

Scenario A — “Plowable snow dominates, brief sleet risk late” (55%) — MOST LIKELY

Scenario B — “Warmer aloft punches north: more sleet/freezing rain, lower snow totals” (30%)

Scenario C — “Colder/track a bit south: mostly/all snow, higher totals” (15%)

4) Timeline summary (0–24h, 24–48h, 48–72h, 72–96h)

0–24h (Thu Jan 22 → Fri Jan 23)

24–48h (Fri Jan 23 → Sat Jan 24)

48–72h (Sat Jan 24 → Sun Jan 25)

72–96h (Sun Jan 25 → Mon Jan 26)

5) Key uncertainties & watch points (what could flip the forecast)

6) Bottom-line forecast confidence (operational)

7) Appendix — data/models & runs referenced

Claude Opus 4.5 Extended Thinking w/ Research and Web Search

Major winter storm threatens Herndon: 96-hour probabilistic forecast

Northern Virginia in late January: setting the stage

How weather models work and which to trust

Three plausible scenarios for Herndon’s storm

Scenario A: Cold track maximizes snow (55% probability)

Scenario B: Warm nose introduces mixing (35% probability)

Scenario C: Strong coastal intensification (10% probability)

Timeline summary: hour-by-hour evolution

Hours 0-24 (Thursday January 22)

Hours 24-48 (Friday January 23)

Hours 48-72 (Saturday January 24)

Hours 72-96 (Sunday January 25)

Key uncertainties requiring monitoring

Bottom-line forecast confidence assessment

Appendix: model runs and data sources

Numerical Weather Prediction Models Used

Ensemble Systems

Official NWS Products

Additional Sources Consulted

Meteorological Analysis and 96-Hour Winter Storm Forecast: Herndon, Virginia (20170)

1. Synoptic Overview and Atmospheric Dynamics

1.1. Hemispheric Pattern Configuration

1.2. Interaction of Northern and Southern Streams

1.3. Thermodynamic Profile and Vertical Structure

2. Location Overview: Climatology and Topography of Herndon, VA (20170)

2.1. Geographical Context

2.2. Cold Air Damming (CAD) Mechanics

2.3. Historical Analogues

3. Technical Model Comparison and Bias Analysis

3.1. Global Forecast System (GFS)

3.2. European Centre for Medium-Range Weather Forecasts (ECMWF)

3.3. North American Mesoscale (NAM) and High-Resolution Guidance

3.4. National Blend of Models (NBM)

4. Comprehensive 96-Hour Forecast Timeline

Phase 1: The Arctic Plunge (Pre-Storm)

Phase 2: Storm Onset and Accumulation

Phase 3: The Heavy Thump

Phase 4: Departure and Refreeze

5. Scenario Planning and Probabilistic Outcomes

Scenario A: The “Sleet Sandwich” (Most Likely Case)

Scenario B: The “Cold Suppression” (The Snowstorm)

Scenario C: The “Warm Nose Intrusion” (The Icing Event)

6. Hazard Assessment and Impact Analysis

6.1. Roadway and Transit Impact

6.2. Power Infrastructure and Vegetation

6.3. Physiological and Human Safety

7. Forecast Confidence and Uncertainty Matrix

7.1. Confidence Levels

7.2. Key Uncertainties

8. Summary and Recommendations

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