Overview (from 8:45 p.m.): The much anticipated late season winter storm has already sent us periods of rain this afternoon and evening, but the main show is still to come. Temperatures have fallen into the 30s and low 40s across the area this evening. However, much colder air crashes south in the overnight period. Don’t let the current wet conditions fool you, even main roads will probably turn impassable for a time during the morning when snow is falling at a decent clip and temperatures fall well into the 20s.
Our timeline and detailed analysis of the storm still appears on track, with northwestern parts of the area falling below freezing over the next few hours and the freezing line progressing south and east thereafter. A short period of rain and freezing rain is possible initially as the next batch of precipitation arrives as we head toward midnight. Precipitation quickly changes over to sleet and eventually snow in the hours after midnight. Snow then persists into the midday with temperatures well below freezing.
Note: See our 11:00 p.m. forecast update below for modifications to snowfall total expectations.
11:55 p.m. update: Rain is now falling across much of the area, but cold air is starting to spill in more efficiently, both at the surface and aloft. North and west of the area, Hagerstown has gone over to snow, and without any period of sleet or freezing rain. This trend should continue south and east over the next several hours. It’s possible any freezing rain or sleet is very brief. The HRRR, shown below, has temperatures aloft quickly cooling, which should support a changeover to snow as we get through the hours past midnight.
Surface temperatures range from near freezing north and west to the upper 30s south and east. We’ll see these cool at a decent clip over the coming hours as well. Before sunrise, everyone should be below freezing. Recent radar, seen below, highlights the transition from rain to a period of sleet then snow. Note the yellow band between Frederick and Hagerstown on the right side of the image. To the north of that, it’s snow. That zone is steadily progressing south and east now.
Conditions are likely to rapidly deteriorate in the pre-dawn period, and that lasts into the morning. If you don’t have to travel during that time, it is suggested you don’t at least till things start to wind down.
This concludes the live blog for the evening. We’ll be back between 4 and 5 a.m. with live updates. We’ll update on Twitter and Facebook for a while longer as needed (see Twitter feed above).
11:00 p.m. update: Given tonight’s models and radar plus observational trends, we’ve made a slight update to our forecast map. The main change is to bump up totals in the zone we had highlighted earlier as a greater potential for a forecast boom. So, much of the southern half of the area — basically cutting Fauquier, Prince William, Fairfax, and Prince George’s counties in half — now sits in a 7-11″ zone. Everyone else remains in the 5-9″ forecast area.
We still expect the worst of the storm to hit in the 4-11 a.m. period, maybe lasting till about noon or 1 p.m. over southernmost spots. The heaviest snow could make roads impassable for a time, and cause very low visibilities. It’s also worth noting that a small shift in the maximum zone of snowfall could have a significant impact on totals, and that’s why our boom and bust percentages remain rather high.
The model information below should not be considered a forecast.
10:45 p.m. update: The GFS is a little colder than prior runs, and focuses its maximum snowfall in the area. The max zone may still be a bit south of D.C., despite the snow map in the update below zeroing in on D.C. itself. Just a brief period of mixed precipitation, then over to snow in the pre-dawn period. Precipitation ends by midday or early afternoon. It’s about 0.70″ liquid equivalent in D.C. after the changeover, so a 6-9″ type snowfall in after some sleet, verbatim. Very cold tomorrow, as we’ve pointed out numerous times. (note, the map here is probably a little too cold… but even adding 5 is still cold!)
10:30 p.m. update: The GFS is in, and it keeps the idea going of a snow max in the local area and south. Here’s a snow map. More in a few.
10:25 p.m update: NWS has mentioned the risk of isolated thunderstorms tomorrow during the snow. This system has already produced quite a bit of thunder and lightning in cold air. Tonight’s U.S. lightning map shows a fair amount of activity still, including in Arkansas and Tennessee, where temperatures are in the 20s and 30s. Something to listen for early tomorrow.
— WilliamNobleScherer (@WilliamMScherer) March 2, 2014
10:00 p.m. update: The GFS model will be running in the next 30 minutes. But while we’re in the pause between the main models this evening, a look at regional radar shows some light rain and drizzle over the area, and the forward flank of the main event approaching from the west and southwest. This batch moves in over the next few hours. Temperatures are now closing in on freezing over northern and western portions of Frederick and Loudoun counties. Those areas may see freezing rain and sleet early in the next batch, with other places south and east following along in the hours after midnight.
9:45 p.m. update: NWS Baltimore/Washington produced a nice graphic earlier this evening explaining their forecast and the transition to snow expected tonight. It’s pretty close to our thinking.
9:35 p.m. update: CWG’s Jason Samenow sent an e-mail making a good point that the NAM raw output is currently running a little cold. Earlier runs “thought” we’d be in the 30s in D.C. by 7 p.m., and we’re still sitting at 41 as of 9 p.m. Not a huge difference, and the model output statistics (MOS) was much closer to actual, as is often the case.
So, we probably want to assume that there may be some lag between snow starting and snow accumulating in any meaningful fashion, though sleet beforehand may help us out there. Still, it appears that we’ll be close to freezing — and close to snowing — somewhere around 4 a.m. in the city. If we cut a little bit of the liquid off the roughly 0.75″ as snow noted below, we’re looking at NAM snow totals pretty much in line with our 5-9″ forecast for the area, but perhaps on the high end.
9:18 p.m. update: The short clip below is of a simulated radar loop from the high resolution NAM. It’s intense. It suggests that from around sunrise, then through much of the morning, we see very heavy snow rates pass through (ignore that it thinks there’s rain on the north end, it’s wrong there). 1-2 inch per hour rates are possible at the peak of this event. The type of snow that destroys roads.
9:10 p.m. update: A look at soundings, which map temperatures up into the atmosphere among other things, shows D.C. likely is all snow by about 9z (4 a.m.). Below is what falls after that time. At least 0.75″, which in a simple calculation would be about 7.5″ of snow. In this case we probably trend from about 10:1 ratios toward 15:1 ratios, so this run suggests about 8-10″+ of snow even in the city.
9:00 p.m. update:The good thing about being close to storm time: we don’t have to wait long for the models to run to it. The NAM is out, and it looks like a big hit across the area. It’s north of the last run too.
8:45 p.m. update: While we’re pretty much in “nowcast” mode (watching radar, observations, and other short term guidance) at this point, the main set of evening models are about to run. While we wait, take a look at this sprawling storm system on radar and water vapor satellite. It stretches about 1,500 miles — the long cat of winter storms?