449 
fxus61 klwx 212017 
afdlwx 


Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia 
317 PM EST Friday Nov 21 2008 


Synopsis... 
a weak low pressure system will move across the middle Atlantic this 
evening. High pressure will build over the region for the weekend. A 
strong cold front will move through on Monday...bringing the next 
chances for precipitation. High pressure builds for the middle to 
end of next week. 
&& 


Near term /until midnight tonight/... 
convective-type cumulus scattered about the middle Atlantic this afternoon...W/ the 
most concentrated areas of conv snow showers over the I-95 crdr and 
sliding east-southeast over Delaware-Maryland-Virginia. 


But the main feature of interest is a mesoscale-vorticity sliding down central 
PA. Heavy banding wrapped around the northwestern side of the low...W/ 
little likelihood of occlusion before sliding somewhere in between District of Columbia 
and phila /likely directly over the balt metropolitan area/. 


See wbcspslwx for more details on the evening snowfall possibilities 
for the District of Columbia/balt areas drng the evening rush. 


District of Columbia area not a likely candidate for much more than a dusting...W/ 
isld areas of a half inch or a general light coating. After the 
initial band asctd west/ the mesoscale-vorticity pushes se/wrd...more convective 
and isld snow rain showers/S will push through...again west/ only isld areas of 
accumulation near or under a half inch. Back NE up the I-95 
crdr...slightly better chances for accumulating snow near an 
inch...espc west/ areas that received a coating earlier this morning/early 
afternoon. 


Winter weather headlines still out for the higher elvns...W/ a warning 
for the wrnmost zones and adzy for the ptmc hilands. Only an adtnl 
couple of inches exptd for the evening hours. 


Winds will stay gusty thruout the evening...gusting just under 30 miles per hour 
areawide. This will also keep accumulate lighter across areas receiving 
any snowfall. Surface temperatures still relatively warm...and temperatures at this 
hour...into the M/u30s west/ only a slight rise from that this afternoon. 
Overnight...wind chill values will drop into the single digits and 
even a few negative values for the mountains 
&& 


Short term /midnight through 6 PM Saturday/... 
cold and windy. Highs only a few degrees above freezing at peak 
heating. Wind chill values drng the day will likely be steady in the 
l/m20s. Wind speeds will be sustained in the 15-20mph range west/ gusts 
near 30mph drng the afternoon. Partly cloudy skies and very dry. 
&& 


Long term /Saturday night through Friday/... 
zonal flow aloft will be the rule for the weekend. Strong shortwave 
energy and an upper low begin to dig southeast out of central Canada 
on Monday. The negatively tilted trough axis looks to come through 
overnight on Monday. Broad troughing will remain near the eastern 
Seaboard through most of next week...as several pieces of shortwave 
energy dive southward. 


High pressure at the surface on Saturday night...will shift eastward 
out to sea on Sunday as a cold front approaches from the west. The 
front is expected to move through on Monday afternoon and evening. 
Upslope snow showers should begin in earnest Monday afternoon and 
evening...and last through much of Tuesday. High pressure will build 
back over the area for the middle of the week. 


Not many changes to the ongoing forecast package were needed. 




&& 


Aviation /20z Friday through Wednesday/... 
likely an interesting day at area airports...espc around the balt 
metropolitan west/ scattered snow bands moving over eastern Maryland from early this morning. 
Another area of steady snow bands across central PA will likely make 
for an interesting evening as well. The most intense bands will push 
through likely between 22-00z across the I-95 crdr west/ a general 
dissipation afterwhich. After the snow chnces have ceased late 
tonight...the only weather concerns will be the steady northwest/rly winds for 
much of Saturday. 


VFR ceilings/visibilities are likely through the weekend as high pressure 
settles over the area. Sub VFR ceilings/visibilities are anticipated Monday 
into Monday evening as a cold front moves through. Low ceilings could 
persist into Tuesday. High pressure will build back over the area 
for middle week with improving conditions. 
&& 


Marine... 
a tightening pressure gradient offshore from a developing low off the 
Carolina coast...and wind gusts near 30kt have warranted a Gale 
Warning for the Bay and tdl ptmc for the evening hours. A Small Craft Advisory will again 
take over in it/S place for the overnight hours...lingering through all 
of Saturday into Sunday morning. Strong winds will continue to push 
across the waters from a passing upper level trough axis...which will 
keep it/S influence over the Bay through Sat. 


Small craft conditions are expected to continue through Saturday 
night. Winds will dip below criteria on Sunday as high pressure 
settles over the area. Small craft conditions are expected once 
again Monday and Tuesday as a cold front moves through the area. 
&& 


Lwx watches/warnings/advisories... 
District of Columbia...none. 
Maryland...Winter Storm Warning until 10 PM EST this evening for mdz501. 
Winter Weather Advisory until 10 PM EST this evening for 
mdz502. 
Virginia...Winter Storm Warning until 10 PM EST this evening for vaz021. 
WV...Winter Storm Warning until 10 PM EST this evening for wvz054- 
501-503. 
Winter Weather Advisory until 10 PM EST this evening for 
wvz050-055-502-504. 
Marine...Small Craft Advisory from 8 PM this evening to 4 am EST Sunday 
for anz530>537. 
Gale Warning until 8 PM EST this evening for anz530>537. 


&& 


$$ 
Gms/listemaa