7:30 PM | Some evening observations

Lightning strikes over the Tennessee Valley

All systems are go for a snowstorm late tonight/Thursday for the region from northeastern Maryland (near PA border) to Maine and this evening’s analysis will focus on current observations that are providing us with some clues as to what is likely to unfold over the next 12 to 24 hours.

Current CONUS radar map; courtesy Mesonet

Temperatures
Temperatures are slowly but surely falling from the very warm levels of this afternoon.  A cold front slipped through the region early this afternoon without any fanfare, but low-level winds did shift to a northwest direction and that is the source location for a fresh, cold air mass.  Just as importantly, dew points are dropping slowly but surely to go along with the temperature drop and this is also a sign of the change in air mass from earlier today.  Temperatures have already dropped to below freezing across northwestern PA and this cooling trend from northwest-to-southeast will continue into the morning.

Water vapor imagery loop indicating upper-level energy is dropping into the Missouri Valley; courtesy NOAA/GOES

Radar/Lightning/Upper-Level Energy
Radar is showing a healthy dose of precipitation across the Ohio and Tennessee Valleys in the very beginning stages of the formation of low pressure.  The current “lightning strikes” map is also showing a lot of activity in those same areas and this is suggesting dynamics are increasingly playing a role in the overall environment.  The water vapor imagery loop indicates upper-level energy is indeed pushing across Kansas and Missouri (darker regions) and this is the key to the formation of low pressure that will intensify rapidly on Thursday in the Mid-Atlantic region and Northeast US. 

Tremendous vertical velocities predicted by the 12Z GFS for 7am Thursday over SE PA, western NJ; courtesy Pivotal Weather

The upper-level trough will deepen as it approaches the east coast and take on a “negatively-tilted” orientation (i.e., northwest-to-southeast).  This is crucial as a “negatively-tilted” upper-level trough will produce strong upward motion in the Mid-Atlantic region which will, in turn, likely lead to a burst of heavy snow and perhaps even “thundersnow” activity.  A 12Z GFS forecast map of vertical velocities for early tomorrow show tremendous action over SE PA, SW NJ and this could result in snowfall rates of 2 or 3 inches per hour for a brief time period.  Aided by the strong upward motion, low pressure will intensify rapidly on Thursday as it treks from just off New Jersey early in the day to east of eastern New England by late tomorrow.  

Precipitation should arrive in the I-95 corridor between 10pm and 2am most likely in the form of rain.  A wintry mix is then likely in the wee hours of the morning and then precipitation should change to snow snow before daybreak.  The snow will come down heavily for awhile tomorrow morning and will be of the heavy, wet variety.  Winds will increase as the day progresses and temperatures should hold at or below freezing.  Travel should be greatly impacted tomorrow morning during the brunt of the storm.  

Snowfall estimates:

DC metro region:
-Coating to 2 inches in the District and points south and west,
-2-4 inches across the far northern and northeastern sections of the metro region

Philly metro region:     5-10 inches

NYC metro region:       6-12 inches  

Meteorologist Paul Dorian
Vencore, Inc.
vencoreweather.com