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fxus61 klwx 211409 aaa 
afdlwx 


Area forecast discussion...updated 
National Weather Service Baltimore Maryland/Washington District of Columbia 
1009 am EDT sun Mar 21 2010 


Synopsis... 
high pressure will move off the eastern Seaboard today...ahead of 
low pressure approaching from the Tennessee Valley. This low will 
move toward the area tonight and Monday...crossing the region 
Monday night...and off to Cape Cod by Tuesday. High pressure will 
then build in for middle week...ahead of another complex low pressure 
system. 


&& 


Near term /through tonight/... 
a weak front still stationary over the Mason-Dixon line this 
morning. Not much translating from this other than calm conditions 
near the boundary and increasing southerly winds well south /across southern- 
central VA/. Temperatures on the rise - currently in the upper 50s and 
expected to top-out into the low 70s...a few locales in the m70s 
where skies can remain clear longer. The I-95 corridor has been 
under a fairly dense layer of middle level stratocu that is beginning 
to disperse. More middle-upper clouds will begin to stream into the 
area from the SW...just ahead of the strong cut-off low over the 
lower Mississippi Valley. 


&& 


Short term /Monday through Tuesday/... 
deep cutoff 500 mb low will approach County warning forecast area Monday. Area will be west/in thte 
ridge...W/ relatively high precipitable water air /arnd 1.25 in/. Surface cyclone 
wont be all that deep...but warm front should make it through during the 
morning...placing County warning forecast area west/in warm sector. Dewpoints at or above 50f. H10-8 low level jet 
/close to 40 knots/ will provide moisture feed and nose of upper jet will 
provide upper divgnc. Hence...xpctg a period of rain showers. Given a hint of 
instability per Li/cape fields cant rule out a rumble of thunder 
either. Consequence of this for grids...Cat probability of precipitation/MDT rain showers/chance 
thunder. MOS depicting quite a spread for some sites. Gnly found 
middle grnd but leaning toward higher end. Despite clouds and rain...warm 
humid air mass suggests the higher numbers better...especially since it wont 
be a day long washout. 


As surface low mvs toward New Jersey and upper low approaches Monday night...cooler drier air 
advects from the west. That will support a dry slot that should cut off most 
precipitation. But...leaning upon 800 mb-5 lapse rates wont remove probability of precipitation entirely. 
As heights/tkns lower...temperatures in the mountains drop near or just below freezing. 
While probability of precipitation will be in the chance range...will need to hold onto mixed precipitation 
if not snow for the higher terrain...especially Highland/pndltn counties. 
Measureable precipitation highly conditional. 


Any continued precipitation Tuesday will be a result of upper low/steep lapse rates. 
Dont think covering will be greater than scattered...and will primarily be 
found in Maryland/wva counties. Precipitation chances lessen during afternoon...a result of 
incrsg westerly dwnsloping winds...the subsidence of which may support 
breaks in overcast. Also of note will be the temperatures...it be decidedly cooler than 
the prvs several days...only in the 50s. 


&& 


Long term /Tuesday night through Saturday/... 
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 /13z Sunday through Thursday/... 
increasing cirrus clouds today and tonight...with some middle clouds moving in 
after midnight. Precipitation and associated visibility/ceiling restrictions should 
hold off until after 12z. 


Light winds this morning will become southerly at 5 to 10 knots...and remain at similar 
speeds tonight. 


Poor flying conds Monday-Monday night due to passing lopres. Terminals will 
be on north side of storm...leading to east/southeasterly winds bringing in plenty 
of low level moisture. Areas IFR expeceted. In addition...could have an elevated 
rumble of thunder. A 40kt low level jet centered near 2 thsnd feet should support 
low level wind shear as well. 


Winds veer Monday night as lopres passes nearly ohd. Impvg conds Tuesday as 
westerly winds increase. Should be VFR by Tuesday night...continuing through rest of period. 


&& 


Marine... 
dry conds with southerly winds increasing to near 15 knots this afternoon as lopres 
slowly approaches the waters. 


Lopres continues its approach Monday. Low level jet just off the deck. While 
uncertain if full mixing will transpire /mdl soundings suggest that it 
may not hppn/...there will be just too much wind too cls to surface-- 40 
knots centered about 2k feet and 20 knots only several hundred feet off deck. Any rain showers 
could mix that down. Will raise Small Craft Advisory for day...continuing into Monday night. 


Center of low will pass nearly ohd Monday night. Expect a lull as winds veer. 
Low wont pass east of waters until midday Tuesday...permitting an increase in westerly 
flow Tuesday afternoon. Small Craft Advisory possible Tuesday ngt-Wed...in a pttn more conducive to 
mixing. 


&& 


Lwx watches/warnings/advisories... 
District of Columbia...none. 
Maryland...none. 
Virginia...none. 
WV...none. 
Marine...Small Craft Advisory from 9 am to 10 PM EDT Monday for 
anz530>543. 


&& 


$$ 
Synopsis...heights 
near term...gms 
short term...heights 
long term...gms 
aviation...gms/sbk/hts 
marine...gms/sbk/hts