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fxus61 klwx 211943 
afdlwx 


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


Synopsis... 
high pressure will remain over the region tonight before settling 
over New England Sunday. An area of low pressure will develop off 
the middle-Atlantic coast Monday before slowly moving up the 
eastern Seaboard Monday night into Tuesday...bringing rain. High 
pressure will briefly build in late Tuesday into Wednesday...but 
another cold front will pass through late in the week. 
&& 


Near term /through tonight/... 
current analysis shows an area of high pressure over the Ohio Valley 
that extends into the mid-Atlantic. A split jet continues across the 
central and eastern Continental U.S.. an upper-level low in the northern branch 
is moving into Pennsylvania this afternoon while an upper-level low 
in the southern branch tracks into the Gulf Coast states. 


Surface high pressure will continue to bring dry and seasonable 
conditions tonight. The strato cumulus deck associated with the 
upper-level low over PA should dissipate across most areas this 
evening with the loss of daytime heating. However...clouds may hang 
on across the northwestern County Warning Area into the Potomac Highlands where 
there is a little more low-level moisture. Min temperatures will range from 
the lower and middle 30s in portions of the Potomac Highlands into the 
Shenandoah Valley...to the middle 40s in downtown Washington/Baltimore. 
&& 


Short term /Sunday/... 
the upper-level low currently located over Pennsylvania will slowly 
track south into our region while the surface high builds toward New 
England Sunday. The southern branch system will track into the 
southeastern Continental U.S....but it should remain far enough to our south 
not to have any impact on our weather through Sunday. 


A low-level north-northeast flow around high pressure to our north 
will keep dry conditions in place along with sunshine. Maximum temperatures 
will top off in the middle to upper 50s across most locations...but 
upper 40s in the higher elevations. 
&& 


Long term /Sunday night through Saturday/... 
guidance trending much sloer...holding off precipitation Sun night as strong hipres 
over new englnd rdgs down Atlantic CST. Emphasis of rafl now Monday-Monday 
night...as a weak WV in the southern stream spills moisture northward...and eventually 
spawns coastal low. Warm air advection on southern doorstep sunset sun...and clouds will 
thicken/lower through evening. Will bring probability of precipitation into grids lt sun evening/sun 
overnight...especially southern counties which is nearer triggering mechanism. 


By Monday-Monday night...energy will Transfer off CST. Precipitation will be in response 
to developing low...mostly in deformation zn. Have lingered a chance into 
Tuesday...especially northern counties. However...cold air advection should be able to mix out the deep 
moisture. But...flow wont go westerly...and as such am afraid will hold onto a 
prtl marine deck into midwk. Per extended dscn 2 paragraphs down...forecast 
at this pt becomes a bit more uncertain...and gridded details become 
broader in scope. 


Per temperatures...have limited diurnal swings in response to cldcvr next cpl 
days...going on high side for min-T...and low end for maxt. Did 
shade dwnwd Monday night since County warning forecast area will be affected by cold air advection by then. 


We/ve entered the time of year when split flow wrecks havoc on model 
solutions. It appears we/re entering such a period. The main forecast challenges 
revolve around shortwave energy digging into trough across the plains...likely 
cutting off an upper low...and then traversing the Continental U.S. As addtl 
pieces of shortwave energy dive into the trough axis. GFS and ggem deeper 
and quicker west/ 500 mb low...which is almost contradictory. Will take a 
highly ensembled approach...leaning on steadier European model (ecmwf). Regardless... 
will have an upper trough in the neighborhood /prbly centered over the Great Lakes/ 
this thxgvg...which means the extended forecast will start out warm...then have 
a cold front passage cool County warning forecast area off. There also will be the support for precipitation-- rain at 
1st...but heights/tknss/h8 temperatures would support at least some snow...especially 
for the mountains by Thursday night..which may continue into Friday across western upslope 
areas. Pop forecast low confidence at this time...but will continue to be monitored 
due to potl trvl impacts. The western upslope aspect to the forecast the most 
solid. 
&& 


Aviation /20z Saturday through Thursday/... 
patchy br may bring a brief period of MVFR visibilities early Sunday 
morning...otherwise VFR conditions are expected tonight through 
Sunday. Light northerly winds are expected tonight...winds should 
remain below 10 knots Sunday with the directions veering toward the 
northeast. 


VFR sun evening...as clouds thicken/lower from S-N. Flgt restrictions possible 
overnight as rain spreads in. Restrictions prevalent Monday-Monday night as lopres 
passes east of terminals. 


Should be a break Tue-Wed...though marine layer could potentially hold on. 
Then...anthr storm system mvs in Wednesday ngt-Thu...fllwd by cold air by 
end of weak. 
&& 


Marine... 
high pressure over the waters tonight will build toward the north 
Sunday. Northerly winds tonight will veer toward the northeast 
during the day Sunday. Wind speeds will remain below Small Craft Advisory criteria. 


Weak WV of lopres tracks across southeastern Continental U.S. Sun night...and 
transfers/deepens off Delaware-Maryland-Virginia Monday-Monday night. Winds increase during this 
period...W/ Small Craft Advisory conds possible. 


In wake of this low and in advc of next storm systm...north winds veer 
Ely. More precipitation will spread in midwk...but Small Craft Advisory not expeceted at this time. 
&& 


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


&& 


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

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