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fxus61 klwx 210135 
afdlwx 


Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia 
935 PM EDT Sat Mar 20 2010 


Synopsis... 
high pressure will continue to move off the eastern Seaboard 
tonight and Sunday...ahead of a low pressure approaching from the 
Southern Plains. This strong upper-level disturbance will move 
toward the area late Sunday and into early Monday. The upper low 
will cross the area by late Monday...and off the middle Atlantic 
coast on Tuesday. High pressure will then build in for middle 
week...ahead of another complex low pressure system. 


&& 


Near term /until 6 am Sunday morning/... 
hipres will continue to move off the eastern Seaboard but remain in 
control of the regional weather pattern tonight. Bay breeze 
circulation has weakened this evening with the loss of daytime heating. 


Quiet weather night with main forecast challenge tonight being how far 
will middle-hi level moisture ahead of the southern Mississippi Valley low 
stream north and eastward. Hi scattered to broken cirrus deck will continue across 
the area but have very little effect on radiational cooling. Middle- 
level cloud deck will likely reach the western slopes of the Allegheny 
Front and Potomac Highlands late tonight...allowing for temperatures to stay 
above freezing across even the coldest locations out west. With 
weak warm air advection...lows temperatures will be a few degrees warmer than last 
night. Lows will range from the mu30s in the Potomac Highlands to 
m40s east of the Blue Ridge/around 50 in the cities. 


&& 


Short term /6 am Sunday morning through 6 PM Sunday/... 
one more fair weather day before the rain comes Sun night. More 
high clouds than we saw across the region today are forecast for 
Sunday...which will shave a degree or two off of todays maximum temperature. 
Believe all rain showers activity will hold off until Sunday night...so 
have updated the forecast to remove the chance-to-slight chance probability of precipitation west 
of Blue Ridge. 


I am also optimistic that significant sky cover will hold until 
very late in the afternoon...later than previously forecast as 
model relatively hum/clouds show a strong gradient from the low pressure that 
will impact US Sun night through Tuesday. However...a broken to overcast cirrus 
deck will move into the area during the afternoon. 


&& 


Long term /Sunday night through Saturday/... 
a little wobble in upper low and now models have begun to trek the 
feature much closer to the middle Atlantic. This strong and anomalously 
cold upper low is currently diving into the Red River valley of 
TX/OK...and will continue to become cut-off from over the next 24 
hours. But as the feature drifts eastward into the lower Mississippi 
Valley...it will become out of phase west/ the upper wave over central 
Ontario - far to the north. This will cause a bit of a stalling-out 
over the deep south on Sun afternoon. Another affect of the cut-off 
becoming independent of the northern-stream wave will be that it can move 
further north than originally anticipated. The system will begin to 
jog northeastward by early Monday...after passing over the I-20 corridor and 
eventually up the southern Appalachians - toward the middle Atlantic. 


A decent plume of low level moisture will stream northward up the eastern 
Seaboard from early Sunday through late Monday...ahead of the system. 
The best chances for precipitation across the County Warning Area will be centered around 
the daytime hours on Monday - just into the overnight hours as well. 
With this latest trek...there will be more of an onshore southeasterly flow 
from Monday afternoon through late Monday night - when the wind back around toward 
the SW...a function of the complex storm system /sfc low to the 
north - upper low to the south by early Tue/. Along west/ some decent 
rainfall for the region on Monday aftn-evening...a period of higher 
waters levels in the Maryland Chesapeake Bay/tidal Potomac will be 
possible. 


A fairly pronounced dry-slot will likely move-in directly behind the 
large moisture plume late Monday night - that pushes the rain northward and 
stretches from central Ohio...through PA and off the New England coast. 
Only scattered shower activity expected afterward...as the upper low 
/which normally has at least a smaller inset of light precipitation/ 
moves directly across southeastern Virginia and off the middle Atlantic coast by midday 
Tuesday. This will mean mainly a cloudy/breezy-cool day for Tuesday - as 
well as the relatively cooler day on Monday...from the rainfall 
activity. Some of the colder air associated west/ the upper low may 
work down toward the higher ridges of the central Appalachians...and mix 
in a few snow showers overnight Monday into early Tuesday. 


Directly behind this system /as it exits parallel toward the New 
England coast middle week/ will be a quick upper wave that pushes 
mainly upslope moisture into the central Appalachians again on Wednesday /little 
if anything will make it east of the mtns/. An uncertain end to the 
week...as another upper wave will likely drift down the Front Range 
of The Rockies on Wednesday...into the central/Southern Plains on Thursday. Then...the 
system will move across the southeast into next weekend - and depend 
heavily on its connection to the upper low southern Hudson Bay...to what 
affect it will have on the East Coast. 


&& 


Aviation /02z Sunday through Thursday/... 
VFR conditions with light/variable winds expected through middle-Morning Sun. Southerly 
flow will increase during the day sun with winds around 10 knots 
during the afternoon. 


Scattered light rain will begin to move into the middle Atlantic region 
late sun into the early morning hours Monday. Rain showers possible at cho in 
the evening but should hold off across the other terminals until aftr 
midnight. 


Showers will become more widespread as the day progresses...W/ 
the heavier activity coming late aftn/evening. A complex upper low 
will approach the area late Monday...moving through and off the coast 
into Tuesday afternoon. Only light rain expected from Monday night into Tuesday 
afternoon. Winds will be a complex in the lower levels during this 
time...W/ a steady southeasterly flow ahead of the low...then shifting to 
southwesterly and then northwesterly as the low exits off the coast. High pressure 
for the middle part of next week. 


&& 


Marine... 
winds will become light later this evening...and then increase across 
the waters with heating Sun afternoon. Have raised the winds 
in the afternoon right below small craft criteria. This should 
only be realized near shore...as the warm air aloft will have 
difficulty breaking the inversion setup Middle-Bay with the water 
temperatures still hovering around 50f. 


Southeasterly winds will increase overnight sun into Monday. Small Craft Advisory conditions will 
become more likely into the daytime hours Monday morning...as a complex 
low pressure system approaches from the SW. The low will move over 
the region late Monday into Tuesday...W/ steady rain showers on Monday - 
mainly ending by early Tuesday. Northwesterly winds behind the system...keeping 
Small Craft Advisory conditions over the waters until late Tuesday. Another upper wave 
pass through on Wednesday - creating more marginal Small Craft Advisory conditions. High 
pressure will then build into the region until late Thu-Fri. 


&& 


Lwx watches/warnings/advisories... 
District of Columbia...none. 
Maryland...none. 
Virginia...none. 
WV...none. 
Marine...none. 


&& 


$$ 
Update...Klein 
previous disc...Lee/gms 

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