4:10 p.m. update: Reports of flurries or very light snow are starting to show up, mainly in higher elevation locations, and radar shows a “hole” over the D.C. area starting to close as virga (precip evaporating before it hits the ground) gets nearer the surface. This means light precipitation might become somewhat widespread toward the early side of the initial window below. As such, we’ve widened it from 5-8 p.m. to 4-8 p.m. It’s also possible rain will be limited or nonexistent to start as well, especially west of I-95, though temperatures will remain well above freezing for a while yet.



Updated CWG snowfall forecast as of 2:30 p.m. March 16.



Radar & lightning: Latest regional radar shows movement of precipitation and lightning strikes over past two hours. Refresh page to update. Click Latest regional radar shows movement of precipitation and lightning strikes over past two hours. Refresh page to update. Click here or on image to enlarge. Or see radar bigger on our Weather Wall

From 2:30 p.m.: Coming off a 70-degree day on Saturday, and with temperatures nearing 50 degrees in D.C. today, it’s hard to believe in snow. But a moisture-packed storm is now just hours away from beginning. Much colder air is also ready to filter in, and we’ve seen dew points fall into the teens and twenties which will help us cool off nicely as precipitation begins. Following a brief period of rain to start, most spots transition to snow fairly quickly, and we expect 2 to 4 inches to be most common when the storm winds down tomorrow.

Key points

Worst of the storm happens around and after midnight through the pre-dawn period. Total accumulation of 2 to 4 inches is expected in much of the area, with 3 to 6 inches to the west in higher elevations. In downtown D.C. and Potomac River lowlands, 1 to 3 inches is most likely.

Despite a springlike start to the weekend, the bulk of storm comes overnight along with cold temperatures. Some accumulation is pretty close to a sure thing.

Travel impacts are expected, but retained road warmth and precipitation lessening in intensity after sunrise should limit odds of lengthy impassibility.

Monday will be unusually cold for this time of year as temperatures struggle to get past freezing. But, that strong March sun should aid in melting on roads at the least.

Timeline

4-8 p.m.: (note: window enlarged to 4 p.m. from 5 p.m.) The main body of light precipitation moves into the area from southwest to northeast, perhaps mainly as rain to start but transitioning to a wintry mix or snow fairly quickly. There could be a shower or two prior, and higher elevations may start as snow. Temperatures: Upper 30s to mid-40s, and falling.

(note: window enlarged to 4 p.m. from 5 p.m.) The main body of light precipitation moves into the area from southwest to northeast, perhaps mainly as rain to start but transitioning to a wintry mix or snow fairly quickly. There could be a shower or two prior, and higher elevations may start as snow. Temperatures: Upper 30s to mid-40s, and falling. 8-11 p.m.: Wintry mix, turning to all snow. Accumulation on grass and elevated surfaces begins in lower elevations, and increases to the west in higher elevations. Snow intensity picks up by late in the time period, especially south and west. Temperatures: Mid-30s to near 40, falling toward freezing.

Wintry mix, turning to all snow. Accumulation on grass and elevated surfaces begins in lower elevations, and increases to the west in higher elevations. Snow intensity picks up by late in the time period, especially south and west. Temperatures: Mid-30s to near 40, falling toward freezing. 11 p.m. through 6 a.m: Snow, moderate to potentially heavy at times. Accumulation likely on all surfaces during heavier bursts, and grass throughout. Roads become slick, especially untreated ones. Temperatures: Near or just above freezing, falling into upper 20s to near 30.



NAM simulated radar at 2 a.m. (College of DuPage)

6-9 a.m.: Snow tapers in intensity from west to east, additional light accumulation possible. Briefer heavier bursts may also still occur. Temperatures: Fairly steady in the 26-30 degree range.

Snow tapers in intensity from west to east, additional light accumulation possible. Briefer heavier bursts may also still occur. Temperatures: Fairly steady in the 26-30 degree range. 9 a.m through 2 p.m: Snow showers may persist as the storm pulls away, but winding down overall. Roads improve despite temperatures remaining cold — thanks to March sun angle. Temperatures: Upper 20s to low 30s and slowly rising.

SchoolCast

2.5 apples: Numerous delays and cancellations are a decent bet, but it doesn’t look like everyone will be home all day for this one.

FedCast

2.5 domes: Better than even chance of unscheduled leave policy and/or delay. Less than 50/50 chance of shutdown.

Frequent questions

What does Winter Weather Expert Wes Junker have to say?

Nothing much has changed since yesterday. The models continue to forecast accumulating snow, but also suggest that accumulations around the city will be tough until after midnight.

This morning’s model output statistics (MOS) from the NAM keep the temperature at Reagan National Airport at 36 at 11 p.m. and only drop the temperature to 31 by 2 a.m. Monday morning. Dulles airport is forecast to still be 34 at 11 p.m. The latest European model also suggests we could lose what in winter what might equate to a couple of inches of snow to warm surface temperatures.

The low level temperature issues argue that accumulations across most of the area inside the beltway and for points south and east will probably occur after midnight. For that reason, we still think a general 2-4 inch snowfall across the region is the most likely outcome except near the river in the heart of the urban heat island, where 1-3 inches might be more the norm. A band of heavier snow accumulations is possible farther to the west where temperatures should be a little cooler.

How will snow stick after Saturday’s warmth and today’s above freezing conditions?

As with a number of other storms this winter, despite temperatures being well above freezing before the storm starts, a good cold air source is just to our north and west. Winds from the north have fed in drier air at the surface already, which will aid in bringing the temperatures down via evaporational cooling as precipitation starts. Cold air is expected to continue to bleed into the region throughout the storm and temperatures reach the 20s most spots tomorrow morning.



Temperature analysis at 12 p.m. EDT. Cooler than normal, but well above freezing in the D.C. area, with much colder air just north and northwest. (Weatherbell.com)

When will road conditions begin to deteriorate?

Road temperatures remain pretty warm (even near 60 in parts of Md.), and they’ll cool considerably slower than most other surfaces. Even though we had clouds overhead today, sky cover was pretty thin through early afternoon, so roads took on more solar energy. We’ll probably need to wait for heavier snow in the overnight period before roads might head downhill en masse. Bridges and overpasses will freeze up quickest, and that could happen by late evening.

How about the morning commute?

Given that the most intense part of the storm seems to occur from late night into pre-dawn, coincident with the coldest temperatures, it’s fairly likely the commute is going to be tricky. We don’t anticipate heavily snow-packed main arteries though. With temperatures below freezing, icy spots are certainly a concern, particularly on any untreated roadways.

And the ride home?

We think snow will wind down at least several hours before the evening commute, and much of the activity during the day should be light in intensity. Add in the March sun angle and it shouldn’t be the worst commute ever — especially if less people are on the roads — but some impacts are likely to remain.



Gridded MOS (model output statistics) forecast for 5 p.m. tomorrow, near daytime highs. May be a smidge cool, but indicated we won’t see much warming on Monday. (Weatherbell.com)

Will air travel be impacted?

Plan on moderate to major delays and cancellations by Monday morning. Through this evening, local airports should be able to continue operations fairly close to normal, but delays may increase after dark as temperatures fall closer to freezing. So far, there aren’t any widespread cancellations showing up for tomorrow, but deicing operations as well as trickle-down impacts from other spots should ensure this won’t be the smoothest travel day ever.

Why are your numbers lower than the National Weather Service’s?

We anticipate a fair amount of the first half of the storm to have trouble accumulating well, especially in and around D.C. Also, a big storm this time of year is a few inches, and that’s about what we expect. Our numbers and the NWS numbers have become a little more closely aligned today, with them lowering expected amounts in parts of the area (mainly south of D.C.) and us bumping the lower end of expectations slightly.



Snow forecast from NWS Baltimore/Washington as of 1:45 p.m. (NOAA)

Should we worry about the northern edge?

Over recent days, some models have put a sharp northern edge across the immediate area. In the last day or so, that’s moved back north. It does seem likely that there will be a tight gradient from measureable snow to very little or none in the broader Mid-Atlantic, and that’s why counties up near the Pa. border in Md. are under winter weather advisories instead of winter storm warnings. We’ll need to watch where this sets up, but it appears to want to mainly hang north of the area. As with March 3rd, pressing cold air may limit the time of heavier bands.



Total liquid equivalent precipitation as forecast by this morning’s high resolution NAM. (Weatherbell.com)

What would cause more snow than expected to accumulate?

If surface temperatures cool a little quicker than expected it would help reach the high end or higher in the forecast. Though temperatures are much colder than normal, this is still a marginal situation, especially early in the event. Should readings fall near or below freezing instead of right above, it can make a big difference. While we don’t anticipate lengthy snow banding, any bands that develop can cause local maximums as well.

What would cause less snow than expected to accumulate?

At this point, we feel pretty confident that the storm won’t entirely bust. If temperatures end up slightly warmer than expected, better accumulation may take longer than planned to get underway. The strongest part of this storm doesn’t appear to last very long, so the odds of huge totals are not that high. Even though it’s coming at night, if the main part is not consistent with heavier rates, that could also cut down on accumulation risk.

Luck of the Irish: Will the snow be green?

So far, there are no large-scale indications of colored snow falling from the sky. There is a chance that localized areas will see this phenomenon. If you happen to catch that, please let us know! Just remember to stay away from yellow snow.