Ward Baltimore/Washington DC Discussion April 20

Synopsis:
High pressure begins moving off to the northwest today, with a cold front expected to pass through the area on Sunday the 22nd.  A low pressure system will develop off to the southeast of the Carolinas curling back to the NNW and passing directly through the forecast area late Sunday into Monday.  This system will keep an extended period of rain, and even snow in the discussion throughout most of the forecast area into the middle of next week.

Current:
Current visible satellite imagery shows much of the fog coverage in the I-81 corridor clearing out, with some patches remaining in northern Virginia near the I-81 and I-66 region.  Current trends suggest that the upper level cloud deck in place over central Virginia and off the coast of Maryland should dissipate as the day progresses.  Current temperature observations show low 70's across much of the Baltimore and D.C. metro region with mid to upper 60's out into western Maryland and southern Virginia.  Winds are relatively light coming from the S to SE at 5-10 knots.  Water vapor imagery shows a plume of moisture coming out of the south in conjunction with the cold front in place over Illinois with fairly drier air preceding the front.
Visible satellite imagery with mesoanalysis of winds, dewpoint temp, temp, and mslp at 17:00 UTC. Some fog may still remain under the cloud deck across the northern Virginia Piedmont region.

Short Range (Friday 20th - Sunday 22nd):
The current trend shows that the cold front over Illinois right now is moving fairly slow in conjunction to the low pressure system associated with it.  The 12Z run of the NAM for April 20th has the high pressure system moving to the NE from its current location offshore of Virginia.  Today's high temperatures look to be in the mid 70's throughout much of the area with the DC metro area in the upper 70's as current southerly flow is depicting sufficient warm air advection into the forecast area ahead of this front.  As the cold front moves eastward and comes into contact with the Appalachians, sufficient moisture and upslope flow will  create slightly cloudy conditions for the region tonight keeping overnight low temps in the low 50's in higher elevations and mid to upper 50's elsewhere.  The current progress of this cold front as it comes across the Ohio River Valley is fairly slow, but should reach the western edge of Maryland by early afternoon Saturday.  Some disagreement exists between the 12Z run of the GFS and NAM with the GFS bringing the front in earlier, but later seems to be a bit more favorable.  The late arrival of this front, strong southerly flow preceding the front and advecting in moisture and warmer temperatures could allow for instability to build in Saturday morning.  The SREF ensembles have a good probability of building in some CAPE over 500 J/kg over the area and at least 30 knot shear, so organized convective storms are possible Saturday, some of which could be severe as hail and primarily winds will be the biggest threat based on the 12Z NAM model soundings for 18Z Saturday and the steep lapse rate.  Precipitation with this event will be heavy as it lasts throughout the day and into Sunday, especially in areas of convection, and strong downslope winds will be possible behind the front.  Sunday's high temperature will most likely be in the low 50's early in the day and steadily decreasing throughout the day as cold air starts to become trapped in by the low pressure system moving up from the Carolinas.  Most of the models have come together towards a solution regarding a low pressure system forming off of the digging trough over the Southeast and finally cutting off and moving up the Atlantic Coast.  This system is expected to eject off of the Florida peninsula around 9Z Sunday as a 1000 mb low and deepen rapidly as it moves northward.  This will bring southerly flow and cooler temperatures for much the region in conjunction with the front.  Heavy precipitation will be prevalent over the eastern forecast area tapering off to the west for Sunday night as cyclogenesis takes hold advecting warm air up and around the low pressure system and decent differential vorticity advection supports the deepening low, especially with the 500 mb vorticity advecting in over the surface disturbance to enhance the low.  All of this will allow for extended periods of heavy precipitation throughout the weekend, as heavy precipitation wraps around the low pressure system.

18Z Saturday SREF ensembles showing a good probability of CAPE of 500 J/kg (top) and 30 knot bulk shear (bottom) at 0Z Sunday.

12Z NAM model sounding for 18Z in northern Virginia.  Strong low level lapse rates could enhance the severe weather threat.

0Z Monday from the 12Z NAM showing the surface low off the coast of North Carolina and the 500 mb heights and vorticity advecting into the area.

Days 1-3 QPF valid through 12Z Monday.

Long Range (Monday 23rd):
Models are in generally good agreement up until Tuesday for the GEFS.  The biggest discrepancies are for precip amounts and type in the western forecast area.  As this low becomes more vertically stacked, the NAM is trending towards more snow in the western forecast area while the GFS starts snow earlier and with less accumulation for western Maryland.  The surface low appears to retrograde onto land by 12Z Monday just to the north of the forecast area, and become vertically stacked by Tuesday.  Very strong geostrophic CAA will take hold behind the low allowing for this transition from rain into snow in the upper altitudes as low temperatures into Wednesday could be near freezing.  GFS model soundings for Tuesday show sufficient saturation and cold enough layer for snow to develop and accumulate 2-5 inches. This is highly variable though, but a general agreement has been reached amongst models, even though it has taken GFS the longest to reach this solution.  This upper level trough is finally out of the region by late Wednesday and ridging takes hold across the Central Plains.  Eventually, GEFS starts showing some uncertainty as to the location of this trough-ridge-trough pattern, but the pattern does seem to take hold Wednesday into Thursday.


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