106 
fxus61 kctp 211929 
afdctp 


Area forecast discussion 
National Weather Service State College PA 
229 PM EST Sat Nov 21 2009 


Synopsis... 
high pressure will provide fair weather through Sunday night. 
Stubborn cloudiness will finally give way to sunshine over most 
sections on Sunday ahead of a weather system that promises to 
bring rain statewide on Monday. This system will be followed by a 
series of disturbances which are expected to bring cooler weather 
to the region for the Holiday and through the Holiday weekend. 




&& 


Near term /through tonight/... 
NAM BUFKIT profiles continue to show a classic boundary layer 
inversion around the 296k to 298k levels across central and 
western PA between 3k and 5k feet this afternoon. At the same time... 
as center of 1023 mb surface high moves only slowly eastward across the 
Ohio Valley...core of the coldest air at 850 mb associated with 500 mb trough 
aloft is sinking southward towards The Laurels. Any breaks in the 
overcast that do form from the central and south central mountains... 
laurels...extending northward across the northern tier quickly fill in 
as a result of the low level instability. However...there is 
enough downsloping in the north-northwest flow across my southeast counties to 
keep p/c conds in place through tonight. 


As the cold air departs tonight...NAM forecast soundings indicate 
the inversion weakens...although will have to deal with the 
typical diurnal surface cooling and shallow overnight inversions early 
on Sunday before better mixing and brighter skies prevail for 
most. 


&& 


Short term /Sunday through Sunday night/... 
morning patchy fog will dissipate by middle to late morning...and 
with continued ridging surface and aloft...expect skies to become 
partly to mostly sunny everywhere with a milder afternoon. 
Naefs gives higher mav MOS temperatures...ranging in the l/m50s most 
spots. 


Expect increasing low level moisture from deep east/southeast flow 
Sunday night as surface ridge tracks towards Newfoundland bringing a 
large over water fetch into the Middle Atlantic States. At the same 
time...a decaying upper wave impinging upon the upper ridge from 
the west over the Ohio Valley will spread deeper layer moisture 
across the region by late Monday. The result will be a slight 
chance of light rain by dawn Monday...with much better chances 
later Monday as the cad is well established below warm air advection aloft. 


&& 


Long term /Monday through Friday/... 
an active split flow pattern is forecast to continue through much of the 
period by long term guidance. Several upper level troughs are forecast to 
traverse the Continental U.S. In both the northern and southern stream flow with 
varying levels of influence on PA. Overall however...most of the 
next week looks to be dry with continued above normal temperatures. 


The first trough...in the southern stream...is forecast to move across the Gulf 
Coast today...then lift NE along the middle Atlantic coast late Sunday. 
The wave will be lifting into the northern stream mean upper ridge with 
high pressure in place at the surface. Model trends are for a 
smaller...farther east solution of the surface low asscd with the southern 
trough and forecasts drier than pvs runs. GFS appears an outlier 
with a sig wet forecast for Monday across much of PA...and such 
have lowered probability of precipitation for this time period following the multitude of 
other long term guidance. 


Highest ensemble probability of precipitation are Monday during the day into Monday 
night. Some timing issues. Lowered the probability of precipitation into the 30-50 percent 
range Monday during the day and upped them Monday night into 
Tuesday due to the uncertainty in the timing. Pretty weak system 
and the rain should be very light. 


Amplification in the northern stream is forecast to occur over the Great 
Lakes by Tuesday...with a weakening surface front reaching western on Wednesday as 
supporting closed upper trough lifts into southeastern Canada. A stronger upper 
trough/cold front is forecast to move through PA Thanksgiving night...if 
current timing holds...with a noticeable cool down in store for 
shoppers next Friday. Any shower activity over northwest PA could be in 
the form of snow Thursday-Friday as 850 temperatures fall below 0c. 


&& 


Aviation /19z Saturday through Thursday/... 
moisture trapped under subsidence inversion will keep some MVFR 
ceilings for several hours into this evening and overnight across 
central and western sections. VFR will be the rule in the east 
with slight downslope component in light westerly winds. Clouds 
will increase later Sunday and Sunday night as deep easterly flow 
combined with overrunning moisture from the Ohio Valley bring MVFR ceilings 
late Sunday into Monday morning...followed by widespread MVFR/IFR 
ceilings and chance of r- through Monday night. 


Outlook... 
Monday...MVFR local IFR. Possible rain showers in the afternoon. 
Tuesday...MVFR/IFR ceilings. Rain showers in the am. 
Wednesday...MVFR to VFR. 
Thursday...MVFR with chance -shrasn. 


&& 


Ctp watches/warnings/advisories... 
none. 


&& 


$$ 
Synopsis...devoir 
near term...devoir 
short term...devoir 
long term...grumm/gartner 
aviation...devoir/rxr