* Winter storm warning for far northern and western suburbs | Winter weather advisory west of I-95 (map) *



Warnings and advisories for Wednesday (National Weather Service)

Wednesday’s weather in Washington is sloppy, and in some places quite snowy, so expect slow-going and delays throughout the entire region.

The areas likely to see the most substantial snow and hazardous driving conditions are from northern Montgomery County through eastern Loudoun and northern Fauquier County and to the north and west.

In the immediate metro area along and west of I-95, rain mixes with and changes to snow during the course of the morning. Although roads are probably more wet than white due to above freezing temperatures (33-36 degrees), we cannot rule out some slush build-up during heavier bursts of snow, and visibility may well be compromised.

East and southeast of I-95, a driving rain, mixing with snow at time, is sure to slow things down.



Second call for Wednesday snow amounts, issued 11:30 a.m. Wednesday (Capital Weather Gang)

KEY POINTS

Areas likely to see the most snow – 3 inches or more – are roughly north and west of a line from Reisterstown and Germantown in Maryland, and Ashburn and Warrenton in Virginia. In addition to the possibility of snow-covered roads in these areas, the build-up of heavy wet snow on trees could cause some power outages.

This will be an elevation-dependent snowfall. If you live below 200 feet in elevation (near sea level), snow will have a hard time accumulating. Between 200-500 feet, slushy accumulation – especially on grassy areas – is possible. More substantial accumulations are favored in areas above 500 (and especially 1,000) feet.

In D.C.’s western suburbs to the I-95 corridor, we expect the changeover from rain to snow will occur between about 8 and 11 a.m. from northwest to southeast, reaching the I-95 corridor last. A coating to 2 inches is most likely along the I-95 corridor, with 1-3 inches to the north and west, but we cannot rule out more or less.

Throughout the region, the worst traveling time is probably from mid-morning to mid-afternoon, when wet snow may reduce visibility and has the best chance to accumulate, especially in our colder locations.

TIMELINE

2-5 a.m.: Rain develops in metro area. Temperatures cool from the mid-40s to near 40. In our colder areas (upper Montgomery, Loudoun, northern Fauquier and north and west), a rain-sleet-snow mix or wet snow is possible, with temperatures 35-40.

5-8 a.m.: Cold rain in the metro area, possibly mixing with snow north and west of the Beltway. Temperatures cool from near 40 into the mid-to-upper 30s. In our colder areas, any mixed precipitation changes to snow with temperatures in the low-to-mid 30s.

8 a.m.-11 a.m.: Rain mixes with and changes to sleet and then snow from northwest to the I-95 corridor, with more rain south and east of I-95. Temperatures cool from the mid-to-upper 30s into the low-to-mid 30s. In our colder areas, expect snow, heavy at times, with tempreatures in the low 30s.

11 a.m.-2 p.m.: Wet snow for most of the western half of the metro region, except a rain-snow mix mostly east and southeast of the I-95. Precipitation may be heavy, with reduced visibilities. Temperatures in the low-to-mid 30s.



High resolution NAM model simulation of precipitation at 1 p.m. Rain-snow line is right around the I-95 corridor (WeatherBell.com)

2 p.m.-5 p.m.: Snow and mixed rain and snow tapers off from southwest to northeast. Temperatures in the low-to-mid 30s.

5-8 p.m.: A 30 percent chance of lingering light snow or rain – but worst should be over. Temperatures in the low-to-mid 30s.

AMOUNTS

We think a coating to 2 inches or so is most likely along the I-95 corridor. However, more snow cannot be ruled out if temperatures cool faster than expected and the snow is heavy – in which case 2 to 6 inches would be possible. If temperatures are slow to cool and/or the precipitation intensity is lighter, no snow accumulation could result.

Just north and west of the I-95 corridor, from roughly Columbia to Rockville to Fairfax to Manassas, 1 to 3 inches of snow is most likely. The boom scenario (amounts are higher than forecast) here is 3-8 inches, and the bust scenario (amounts are less than forecast) would be less than 1 inch.

In the corridor north and west of Reisterstown, Germantown, Ashburn and Warrenton, 3-6 inches is most likely, except 6 inches or more is a good possibility at high elevations above 1,000 feet. The boom scenario here is 6-12 inches at lower elevations (and more than a foot at high elevations) and the bust scenario would be less than 3 inches.

Warm ground temperatures (due to recent highs above 70) and temperatures above freezing will make it hard for snow to stick (especially around the city) UNLESS the snow falls very heavily – in which case the intensity of snow could overcome the warm ground.

East and southeast of the I-95 corridor (especially south of Annapolis into southern Maryland), rain may mix with and even briefly change to snow, but we don’t expect much more than a coating.



Second call for Wednesday snow amounts, issued 11:30 a.m. Wednesday (Capital Weather Gang)

ROADS

By far, the roads most likely to be adversely impacted by snow are in the region where we’re expecting at least 3-6 inches. Roads in this area include: Interstates 81, 70, 15, 83, 66 (western portion) and 64 (western portion). Wednesday morning through mid-afternoon will be the worst travel times.

Roads like I-270, Rt-28, and Rt-29 are likely to have reduced visibility in snow and perhaps some slush-buildup, especially mid-morning to mid-afternoon, Wednesday.

I-95 and 301 are probably more wet than white but we cannot rule out reduced visibility and some slush during heavier snow bursts – mostly between late morning and mid-afternoon.

AIRPORTS

Expect crowds and delays at all of the airports due to precipitation and low visibility, not to mention backlash delays resulting from messy weather up and down the East Coast.

The potential for delays and even cancellations is higher at Dulles and BWI compared to Reagan National since accumulating snow is more likely north and west of the Beltway. It’s not out of the question Dulles and/or BWI may be forced to close for a short period of time if heavy snowfall rates materialize; there is about a 20 percent chance of this occurring.

SCHOOLCAST

Frederick, Carroll, Loudoun, and Fauquier counties (and counties to the west): (3 apples)

(3 apples) In the immediate metro area along and west of I-95, many school systems already let students out early for Thanksgiving; those that do not might given the weather forecast – although a midday dismissal may coincide with the heaviest snow-precipitation. As such, either closing altogether, or an on-time dismissal might be a better option. (2 apples)

(2 apples) East of I-95, weather-related school schedule changes will probably not be required. (1.5 apples)

FEDCAST

Unscheduled leave and telework options are a strong possibility; an early dismissal is also possible. (2.5 domes)

MODEL ANALYSIS AND FORECAST REASONING FROM WES JUNKER, WINTER WEATHER EXPERT

This forecast is highly complex and could go wrong in a lot of different ways. Yes, we now know that we will be getting quite a bit of precipitation and that the precipitation is likely to change from rain to snow from west to east. However, the timing of the changeover and, more importantly, exactly how cold the temperatures are while the heaviest snow is falling is very much in doubt.

Last night’s European model gave us plenty of precipitation, much of it as snow, but with surface temperatures staying in the mid-30s through 1 p.m. Wednesday in the District. Heavy snow at 33 will stick, but at 35 it probably wouldn’t unless the intensity is super heavy. Lighter snow won’t even stick at 33. (None of the European model’s family of simulations predict temperatures to drop below 32 east of Leesburg)

Today’s NAM model is forecasting that temperatures will fall to 34 by 10 a.m. and 33 by 1 p.m.. Its temperature profile also suggests the precipitation changes to snow by 10 a.m. and then gives the immediate area about 6 hours of snow – which could accumulate some.



NAM model snow accumulation (WeatherBell.com)

By contrast, today’s GFS predicts less precipitation and warmer temperatures than the NAM and would essentially keep any flakes in and around the city almost white rain – with no accumulation.

In short, this morning’s NAM model argues for predicting heavier amounts than our current forecast – but other modeling suggests it may be too aggressive. We’ve gone with a compromise approach.