Weather

381 
fxus61 klwx 211835 
afdlwx 


Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia 
235 PM EDT Thursday Aug 21 2008 


Synopsis... 
high pressure will drift off the coast this weekend. The next 
cold front will move into the region early next week and is 
expected to stall out over or just south of the area for the 
middle of the week. 
&& 


Near term /through tonight/... 
cumulus field holding up across blurdg-shen valley. Guidance has been premature to 
erode the moist layer...and would think that those trends should continue west/ a 
continued east/southeast fetch across County warning forecast area. Synoptically...County warning forecast area under infl of hipres 
surface through 500 mb. Therefore...sensible weather will be controlled by the 
mesoscale. 


Clouds should prtly dspt this evening...supported by WRF h10-9 relative humidity fields. 
The big qstn then becomes what to do west/ the overnight forecast. WRF relative humidity 
suggests at a marine layer advecting inland. NAM hinting at simlr 
solution...especially across central Virginia which is closer to a deep satd layer. Not 
sold on marine stratus...and suspect ongoing clouds will interfere west/ 
fog. But...moisture does exist west/in column...and its trapped by 
subsidence invsn. Since it will be reinforced by continued light Ely flow... 
dont feel comfortable going west/ sky clear as advertised by most guidance. 
Thus...forecast will receive heavy influence from obsvd weather today...painting a 
stripe of broken clouds shd/cho northward alng ridge to iad/jyo. 
&& 


Short term /Friday/... 
dvm from hipres will still be in place...and big forecast challenge remains 
what to do west/ residual moisture. Since forecast deviates from guidance at the 
onset...that will have dwnstream effects on rest of day. 
Synoptically...we should be sunny. Will be a bit more pessimistic than 
that based on today/S outcome and concern about Ely fetch surface through 800 mb. 
Moisture will continued to be deeper across central Virginia. Impact of txt forecast will be 
ptsun in the morning...otherwise mosun. Not the hiest confidence at this time. Will 
see if mesoscale guidance can catch onto going trends. 
&& 


Long term /Friday night through Thursday/... 
500mb height/anomaly pattern during the past five days shows a ridge 
has worked east across southern Canada...allowing an upper low to 
settle into the Pacific northwest. Flow across the Pacific Ocean 
remains zonal with short waves embedded in the westerlies across the 
northern Pacific Ocean. Water vapor loop shows Tropical Storm Fay 
off the Atlantic coast of northern Florida. Meanwhile...a trough is 
shearing north over the plains...with some of the energy lifting 
around the ridge toward the Great Lakes. 12z surface map showed a 
1027mb high over the eastern Great Lakes...wedging south along the 
Appalachians. A baroclinic zone extends west across the 
Carolinas...then northwest into the middle Mississippi River valley. 
An inverted trough was noted from the plains northeast into the 
eastern Great Lakes. 


Easterly flow off the Atlantic is in place Friday night. With dew 
point depressions over the water being somewhat high...think that 
easterly flow will support upslope cloudiness along the Blue 
Ridge...rather than a full scale maritime status event. 


Cold front pushes into the forecast area from the northwest 
Monday. This pattern of late has failed to produce much in the way 
of precipitation...although forecast precipitable water values are 
higher this time around. Have represented these chances with a 
silent 20 pop for now. 


Much of the extended forecast revolves around Tropical Storm Fay. 
This forecast is based on the official track guidance from TPC 
advisory number 24 from 11 am today. Current forecast brings the 
center somewhere into the lower Mississippi River valley region by 8 
am Tuesday...with the deterministic forecast center in south central 
Mississippi. 


From midweek Onward...have chosen to follow the European depiction 
from the last several runs...with remnants of Fay advancing 
northward into the Ohio Valley. Arrival of deeper layer moisture 
should increase chances of convection Wednesday across western 
zones...and across the entire area by Thursday. 
&& 


Aviation /19z Thursday through Tuesday/... 
kept VFR forecast through 18z/22. But...lingering low level moisture trapped beneath 
subsidence invsn a concern. It has led to broken cumulus taftn...which should 
dspt toward evening. However...the moisture will remain...making Ely morning low 
stratus deck or fog development a possiblity /contingent on how much cloud cover 
lingers/. At this time...do not have the confidence to introduce flgt 
restrictions. But they do remain a possiblity...which will need to be 
addressed in 00z pkg. Deepest moisture will reside near cho...and will be 
most scant BWI/mtn. 


East/southeast flow may advect lower clouds later Friday night into 
early Saturday. Otherwise...no significant weather is expected 
through Sunday. A cold front will move through Monday from the 
northwest. 
&& 


Marine... 
no flags expeceted next 24 hours. Light onshore flow will attain southerly channeling 
tonight...likely 10-15 knots. Channeling will dmnsh aftr midngt. East/southeast flow 
will continue tmrw. 


Southerly flow Sunday evening will need to be watched for possible 
Small Craft Advisory conditions. A cold front will move across the 
waters Monday from the northwest. 
&& 


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


&& 


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