Area Forecast Discussion
Issued by NWS State College, PA

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857 FXUS61 KCTP 170936 AFDCTP Area Forecast Discussion National Weather Service State College PA 436 AM EST Sun Dec 17 2017 .SYNOPSIS...
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High pressure will push east of the region today. A weakening wave of low pressure will move in from the midwest late today, spreading mixed rain and snow showers over central Pennsylvania. Scattered snow showers will be in store for the west during first part of the week. Fast west to east flow aloft will keep the weather unsettled.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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GOES 16 shows broken to overcast skies covering NE third to half of central PA this morning thanks to shallow warm advection above the boundary layer, while SW 2/3 is clear to pc. High clouds are starting to increase from the Ohio Valley ahead of an upper trof over the central and southern plains which will track towards the region this afternoon and evening. Clouds in the NE third should begin to break up this morning, but just in time for a new batch of warm advection to ride in from the west. Models have backed off on POPs and precip amounts from this wave, but we still expect some very light precip to break out in the SW/SC counties by nightfall. The temps will get into the 40s in the south with sun. without much sun in the NE, it will likely stay below 35F.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM MONDAY/...
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The aforementioned upper trof over the Ohio Valley will shear and weaken as it rides over the flat upper ridging along the eastern seaboard tonight. Still, there will be sufficient forcing and moisture to keep low chance of rain/snow showers across central and southern areas primarily. Much of the precipitation will be hard pressed to measure. Clouds will be widespread keeping mins up tonight...with most areas remaining above freezing. Mild southwest boundary layer flow will persist on Monday, as another shearing trof drops towards the region from the central and eastern GLAKS. Slgt to chc pops are again favored on Monday in this scenario, with best chc along and west of the Alleghenies which will have slightly enhanced Pops due to upslope zonal flow. Highs will range from the upper 30s north to the mid to upper 40s south.
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&& .LONG TERM /MONDAY NIGHT THROUGH SATURDAY/... Fast westerly zonal flow will turn a bit more SW by Friday as a trough digs over the upper Plains. A western trough kicks a strong shortwave out across the Plains Sunday into Monday. The associated low center will move northeast across the Great Lakes staying to our north. This system will drag in some warmer air in southwest flow ahead of it. However, we may be cold air dammed near the surface so the +1 to -2C air at 850 hPa may not translate to a warm boundary layer. The warm advection should bring snow and snow to rain in the south but QPF amounts are light. Will have to watch for potential for some ice also Sunday night. Then through much of the week, we move to fast moving pattern of weak shortwaves and chances for precipitation (mainly over the northern mountains) through the week. As a northern stream wave zips by to our north by Monday night, it will bring cold air back into the region Tuesday into Wednesday. The northwest flow triggers snow in relatively high POPS in NW PA due to LES. This cold surge should be relatively fleeting and high pressure and a potential surge of relatively warmer air follows Thursday. The warm advection and approaching cold front will likely produce increased potential for snow/ice in the west Thursday night into early Friday. As precip works in on Friday, precipitation looks to transition through mixed to light rain. Temps could fall below guidance over the east Thu night if skies remain clear as exiting surface high pressure will leave a dry airmass in place. A strong frontal boundary will likely push another surge of cold air into the region Friday into Saturday as the eastern long wave trough tries to re-establish itself. GFS and ECMWF diverge by this time, with the EC moving surface low slower and thus forecasting higher pops than GFS. && .AVIATION /10Z SUNDAY THROUGH THURSDAY/... Looking at the latest satellite nighttime microphysics skies are continuing clearing. Most regions are remaining fairly dry, though MDT and LNS and BFD have dewpoint depressions 3 degrees or less. BFD restrictions have been dropping over the last 3 hours and IFR cigs have finally developed. Expect these restrictions at BFD to continue through mid morning before finally lifting. Downstate the forecasting challenge remains in determining when and where any fog and lower stratus may form. Sunday, any areas of low clouds and fog may be stubborn to lift given light wind conditions. Warm advection on developing southerly wind will bring increasing mid and high clouds from the Southwest. Expect a brief period of mainly VFR conditions across the region, before precipitation spreads in later in the day into the overnight. Outlook... Sun...Areas of morning MVFR/IFR conditions, most likely across the north, otherwise VFR. Lowering conditions moving in from the southwest late in the day. Mon...Mainly VFR. Spotty SHSN poss W. Tue...Mainly VFR, but possible restrictions in the western and northern mountains in snow showers. Wed-Thu...No sig wx. && .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...DeVoir NEAR TERM...Watson/DeVoir SHORT TERM...DeVoir LONG TERM...RXR AVIATION...Jung/Ceru

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