Winter Storm Update (21 JAN 2016)
POSTED ON: Thursday, January 21, 2016 12:27 PM by A.R. Davies
Almost all major global circulation models are dumping a potentially historic amount of snow on the greater Baltimore-Washington-Annapolis metro area over the weekend. This is the result of a potent amount of "mid-level" atmospheric dynamics from out of Canada combining with a slug of moisture from the Gulf of Mexico. The two forcing mechanisms will merge to form a powerful nor'easter near the mouth of the Chesapeake Bay late Friday night/early Saturday morning. The storm will move up the coast on Saturday. Meteorologically, we refer to this scenario as explosive or "bomb" cyclogenesis because the coastal low will deepen and intensify at a rate we do not often see associated with mid latitude weather system (the rate of intensification may be equivalent to a rapidly intensifying tropical cyclone).
Even as the global models have reached a consensus on the large scale atmospheric processes, the subtle details still need to be refined and may not be fully understood until the day of the storm. However, we are now in the forecast window where our short range, high resolution models (in this case the NAM 4km model) can help fill in the details including: timing of snow onset, precipitation type, and specific wind conditions.
Snow Onset Timing:
The Figure below shows the 12z NAM/Wrf 4km simulated radar and through 2200 on Friday. The simulated radar suggests the leading edge of precipitation may be upon Annapolis between 1400 and 1600 on Friday. However, this is tricky because the initial snow will be light and often evaporates before reaching the ground (called virga). Once the atmosphere is saturated, snow will reach the ground and begin accumulating on the cold ground.
The Figure below shows the 12z NAM/Wrf projected snowfall accumulation through 2200 EST on Friday. This suggests snow will not begin accumulating until after 1800 on Friday in Annapolis (faster to the south and west). Once the snow begins falling, it could come down at a rate of 0.5 to 1.0 inches per hour. The onset of accumulation in this model matches what the large scale models project. However, if the atmosphere becomes saturated quicker than the models project or if the precipitation field arrives quicker (somewhat common with these systems), it is possible snow could begin falling and accumulating earlier.
Probabilistic forecast for snowfall onset in Annapolis:
- 20% chance between 1300 and 1600
- 40% chance between 1600 and 1800
- 30% chance between 1800 and 2200
- 10% chance after 2200
The Figure below shows the latest snowfall forecast. I've increased the totals from yesterday because I have more confidence in the overall storm track and timeline. I think the most likely snowfall range is between 15.0” and 20.0” in Annapolis. This range is a bit less than other forecasters/models are projecting for Annapolis because I am concerned areas in pink may mix over to snow, sleet and/or freezing rain for a period of time on Saturday. As the system rapidly intensifies, easterly winds in the mid-levels of the system will pull in a lot of moisture and warm air off the Atlantic Ocean. The sea surface temperatures approximately 100 miles offshore range 50-55 degrees. My concern is that the models are not fully realizing this dynamic which could result in heavier and "wetter" snow combined with a mix of sleet/freezing rain during the morning on Saturday; these factors could slightly reduce overall snowfall totals. Even if Annapolis does "mix" a bit Saturday morning/early afternoon, it will return to all snow Saturday evening. At this time, I anticipate the snow will come to an end around 0400 on Sunday.
Note that at times late Friday night and through the day on Saturday, the snow could fall at rates of 1.0-2.0 inches per hour.
Probabilistic snowfall forecast for Annapolis:
- 25% chance of < 15" (more mixing than I anticipate)
- 45% chance of 15-20"
- 30% chance of > 20" (less mixing and more snow than I anticipate)
The Figure below shows the 12z NAM model wind speed forecast between 0100 Saturday and 1300 Saturday. It is important to note that this forecast is at a height of 10 meters in the air (common meteorological height) and hence the surface values will be slightly less. The model forecast also does not include wind gusts which will be 10+ mph higher than the values shown. Expect sustained northeast winds between 25-35 mph with gusts to 50 mph in Annapolis beginning predawn on Saturday and continuing into the afternoon. The heavy wet snow/mix is likely to accumulate on trees and power lines. This combination could result in widespread power outages throughout the region.
Furthermore, the wind will blow the falling snow and the already accumulated snow; this could create white out conditions between 0000 on Saturday and 0600 on Sunday. Travel is almost totally out of the question late Friday night until sunrise Saturday morning... and during that period of time and plowing efforts will be challenging. In addition, expect wind chill temperatures in the teens throughout the event (perhaps even in the single digits Saturday morning).
For these reasons, the National Weather Service Baltimore/Washington has issued a Blizzard Watch. Here is a link to their advisory:
Behind the storm:
Sunday will clear out with mostly sunny skies. Northwest winds 10-15 mph will persist through the day. Expect a high near 37 degrees. The overnight low on Sunday night/Monday morning will be around 20 degrees so expect a lot of refreezing.
Disclaimer: This is not an official U.S. Navy weather forecast and should not be confused with the official Navy weather forecast provided by the Fleet Weather Center in Norfolk, VA. Furthermore, the official U.S. Government forecast for the greater Annapolis area is issued by the National Weather Service Baltimore/Washington Forecast Office in Sterling, VA.
Category: General Interest