477 
fxus61 klwx 181854 
afdlwx 


Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia 
254 PM EDT Friday Jul 18 2008 


Synopsis... 
high pressure will move into the western Atlantic by Saturday. A 
cold front will work across the region Monday. Typical Summer heat 
and humidity will prevail through the period. 
&& 


Near term /until 6 am Saturday morning/... 
water vapor loop shows a ridge from the southwest Continental U.S. East into 
the middle Atlantic. A system was off the Georgia coast...with the mean 
flow residing along the Canadian border. Acars 400-250mb wind 
observations show a 80-120kt jet from the Pacific northwest east 
northeast over the upper Great Lakes and New England. Light winds 
resided over the southeast Continental U.S.. 11z surface analysis placed a 
1012mb cyclone off the Georgia coast. 1021mb anticyclone dominated 
the central Appalachians...well ahead of the cold front across the 
upper Midwest. 


Subsidence with ridging aloft will yield deep mixing with 
temperatures in the middle 90s this afternoon. While the deep mixing 
should mix some drier air to the surface...locally pooling of higher 
dewpoints near the cities and along the water is possible. Heat 
index will near 100f in some locations through the remainder of the 
afternoon. Expect middle level cap to remain in tact. 


Deep mixing is allowing dewpoints to fall into the 50s and lower 60s 
west of the Blue Ridge. Believe this will mean less of a fog threat 
tonight compared to previous nights...but a few areas may see patchy 
fog...especially along Blue Ridge...and perhaps lower southern 
Maryland as higher moisture advects in ahead of cyclone off Georgia 
coast. Otherwise clear skies and light winds present good 
radiational cooling conditions...with temperatures cooler than 
previous nights especially out west. 
&& 


Short term /6 am Saturday morning through Monday night/... 
while cant completely rule out isolated convection Saturday 
afternoon...position between westerlies to the north and cyclone to 
the south should keep region in subsistent zone. With temperatures 
similar to today...and moisture increasing from the coast ahead of 
the cyclone...heat index values may once again near 100f along and 
east of Interstate 95. Best cumulus growth expected across southeast 
zones with recovering moisture...and along the terrain out west. 


Cyclone off Carolina coast will be the wild card in the sun-Monday forecast. 
NAM keeps it well defined...but curves it NE into the Atlantic. GFS 
solution absorbs low across County warning forecast area into appchg cold front. Obviously...NAM solution 
would be much drier for County warning forecast area...as area would be in ring of subsidence. 
Until trends can be determined /in 12-24 hrs/...would prefer to stay 
remain on the dry Road. That/S not to say that an isolate thunderstorms and rain or two 
cudnt develop sun...but cannot add value in terms of its placement. 
Either way...by Monday cold front should be nearby...and that would provide a 
trigger for thunderstorms and rain development. Will tie probability of precipitation to the diurnal cycle /aftn- 
eve/. Maxt will be typical for Summer...but should be edging down a cpl 
degf by Monday due to eroding heights/increase clouds. 


&& 


Long term /Tuesday through Friday/... 
what to do west/ the cold front remains the issue through the extended forecast as 
well. GFS drives it offshr...but then brings a potent WV across Wednesday. 
European model (ecmwf) much slower...keeping cold front around in shadow of 500 mb trough axis. Will 
go somewhere between the two. Ridging should be able to shove boundary east/southeast of 
County warning forecast area...allowing enough dry air to advect in. Again...there will be enough 
typicall summertime heat for diurnally driven shra/tsra. At this time Tuesday 
afternoon/evening looks like the best bet. Thereafter...confidence in any one 
solution wanes considerably. So...a dry forecast will rtn until trends can be 
determined. 
&& 


Aviation /19z Friday through Wednesday/... 
patchy fog tonight...most likely not as prevent as previous nights 
as dew points mix out this afternoon. Otherwise clear skies and 
light southerly winds through the taf period. Locally...mtn/BWI 
onshore flow should back to light southerly this evening...before 
reforming tomorrow. 


MVFR-local IFR possible in vicinity of afternoon-evening thunderstorms and rain Mon-Tue. 
&& 


Marine... 
southerly channeling regime is forecast to develop later this 
afternoon and evening. Wind speeds are forecast to remain below 
Small Craft Advisory criteria at this time. Southwest winds late 
tonight become southerly again tomorrow afternoon. 


No flags expeceted through end of forecast period. Traj southerly through sun...and may 
briefly increase in channeling lt day-Ely evening. Afternoon-evening thunderstorms and rain Monday-Tuesday 
in vicinity of cold front may bring lclly higher wnds/wvs. 
&& 


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


&& 


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