989 
fxus61 klwx 201842 
afdlwx 


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


Synopsis... 
high pressure will move off the Atlantic coast late Sunday...ahead 
of a low pressure approaching from the Southern Plains. This a 
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 /through tonight/... 
Bay breeze will continue through early this evening along western 
shore of the Chesapeake...with temperatures there struggling to 
reach 60 with southeast surface flow. 


GOES-8 satellite imgry at 18z shows scattered-broken high clouds streaming across 
County Warning Area. 12z NAM continues to hold firm to scattered-broken middle clouds 
associated with 700 mb 40kt jet and warm air advection as evidenced by 12z NAM 850 mb-700 mb 
layer relative humidity. 


Added a couple of degrees to adjusted mav for min temperatures tonight as 
cloud cover will inhibit full radiational cooling...with low temperatures 
tonight 10-15 degrees above normal in the middle to upper 40s outside 
of the downtown areas and the lower 50s for downtown District of Columbia and 
Baltimore. 


&& 


Short term /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. 


&& 


Long term /Sunday night through Friday/... 
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 /18z Saturday through Thursday/... 
no flight restrictions expeceted through taf period. Lamp maintains a 
chance of mifr br to terminals early Sun morning between 6 and 8 
am with moisture advection underneath the inversion...but have not 
introduced it in the tafs since probability is below the threshold 
necessary to make a categorical forecast. 


Scattered light rain will begin to move into the middle Atlantic region 
late sun into the early morning hours Monday. 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 slacken tonight...and then will increase across the waters 
with heating tomorrow afternoon. Have raised the winds tomorrow 
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. 


&& 


$$ 
Synopsis...gms 
near term...Lee 
short term...Lee 
long term...gms 
aviation...Lee/gms 
marine...Lee/gms 

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