303 
fxus61 klwx 210842 
afdlwx 


Area forecast discussion 
National Weather Service Baltimore Maryland/Washington District of Columbia 
342 am EST Sat Nov 21 2009 


Synopsis... 
high pressure will cross the region and settle over New England 
through the weekend. A low pressure area will move up coastal 
Carolina Sunday night into Tuesday...bringing rain. High pressure 
will return late Tuesday...though a cold frontal passage is 
expected in the middle to late portions of next week. 
&& 


Near term /until 6 PM this evening/... 
high pressure will remain across the eastern Ohio Valley today. 
Light northerly surface flow will continue...with highs ranging from 
the middle 40s in the higher terrain to upper 50s across the 
Piedmont. No precipitation is anticipated. 


Water vapor loop and 500mb analysis shows a trough across the 
central Great Lakes...with strong shortwave energy rotating through 
the base. The trough will continue to move eastward today...and set 
up along the Mason Dixon line through tonight. The GFS and NAM are 
slightly different with their solutions for the trough 
evolution...and both initialized rather well. The NAM develops a cut 
off low across the northern County warning forecast area overnight...while the GFS maintains 
a shear axis. Neither model is producing precipitation...just increased 
cloudiness. 


250mb jet streak continues to help spread cirrus across the County warning forecast area 
early this morning. Expect the cirrus to hang around most of 
today...at times becoming opaque. Have opted to go partly sunny for 
today...rather than mostly sunny for most of the County warning forecast area. The northwest County warning forecast area 
should remain cloudy/mostly cloudy in the broken/overcast stratus deck. 
Thicker cirrus could lend to temperatures being a degree or so cooler than 
guidance. Since the mav MOS has been performing rather well...have 
used those numbers and bumped temperatures down a degree or two. 
&& 


Short term /6 PM this evening through 6 am Sunday/... 
cirrus and lower stratus deck in the northwest are expected to thin out a 
bit by the evening...however moisture from the Gulf states system is 
expected to stream northward overnight. Partly cloudy/mostly cloudy 
skies are expected mainly after midnight. Mav numbers continue to 
verify slightly better than the other guidance...and have used for 
the overnight mins. 
&& 


Long term /Sunday through Friday/... 
Sunday...flow becomes northeasterly with high pressure over New England and 
an approaching coastal low off the Carolinas. Clouds in the region 
due to coastal system...so expect mostly sunny skies until cloud 
bank rolls north-Ward in advance of the low. With more sun northern 
zones...maximum temperatures Sunday could be the inverse of normal with warmest 
/around 60f/ locales in Harford/Baltimore and cooler /mid 50s/ 
Nelson/Albemarle. 


Sunday night through Monday night...high pushes off New England and 
low pushes east of Hatteras...bringing Ely flow/cad to the area. 
Having already moistened...expect light rain spreading east across 
the area with east facing slopes having the greatest precipitation 
potential due to topography. Should be quite cloudy with some 
rain...expect low diurnal range...mins middle 40s...maxes middle 50s. 


Tuesday and Wednesday...a closed low over the Central Plains/middle 
west to break cad with southwesterly flow by Tuesday afternoon /did increase 
probability of precipitation for Tuesday morning across eastern half of County Warning Area due to extended 
cad/. Warming trend Tuesday /upper 50s/ to Wednesday /around 60f/ ahead of 
frontal passage. 


Wednesday night/Thanksgiving...short wave lobe/S/ from the low to bring one 
or more cold fronts passages in middle to late week timeframe bringing 
precipitation chances across the County Warning Area. However...moisture availability for 
the middle Atlantic is uncertain. Cold air does look to follow the 
front...bringing a cool down and mountain wintry precipitation. 


Thursday night/Friday...models uncertain of low intensity and 
track...but the potential is there for an early upslope snow event 
in northwesterly flow around the backside of the low feeding Great Lake 
moisture to the Allegheny Front and possibly ridges further east. 
Will have to monitor future models for consistency/greater 
discernment. If anything...cooling trend late next week. 
&& 


Aviation /09z Saturday through Wednesday/... 
VFR conditions continue across the County warning forecast area as high pressure remains 
across the eastern Ohio Valley. Upper jet streak continues to help 
spread cirrus across the area tonight...with ceilings well above 3 kft. 
Some lower stratus has settled across kmrb...in the northerly 
surface flow...but ceilings are around 4 kft. Expect the cirrus ceilings to 
continue through the afternoon...at times broken. The lower stratus 
should continue to slowly spread along the Mason Dixon line...but 
ceilings below 3 kft are not expected. 


Onshore northeasterly flow sets up Sunday...with moistening/precipitation beginning 
by late Sunday night as Ely flow sets up across the terminals. Low 
ceilings and reduced visibilities in rain/drizzle into Tuesday morning. Southwesterly 
flow ahead of the next cold front later Tuesday/Wednesday with 
improved aviation conditions. Quantity of precipitation associated with front 
looks to be low at this time though with timing later 
Wednesday/early Thursday certainly pay attention to subsequent 
forecasts. 


&& 


Marine... 
northerly flow is expected to continue across the marine area 
thought tonight...with wind speeds 15 knots or less. 


High pressure building over New England and a coastal low moving up 
the Carolinas Sunday/night brings about northeasterly flow. Small Craft 
Advisory winds possible Sunday night through Monday night. Southwesterly flow 
ahead of the next front later Tuesday and Wednesday. One or more 
fropas later Wednesday through Friday to bring some precipitation and 
colder air. 
&& 


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


&& 


$$ 
Listemaa/baj