Area Forecast Discussion
Issued by NWS State College, PA

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931 FXUS61 KCTP 151848 AFDCTP Area Forecast Discussion National Weather Service State College PA 148 PM EST Fri Dec 15 2017 .SYNOPSIS...
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A weak upper level disturbance will swing down across the region today as a surface low forms over the Outer Banks and tracks out to sea. High pressure and a brief moderating trend in the temperature are expected by Sunday. A series of cold fronts will move across the area late Sunday into Wednesday. Windy and colder temperatures will follow.
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&& .NEAR TERM /THROUGH TONIGHT/...
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A fairly potent upper level shortwave/Alberta clipper is diving SE out of the Gr Lakes. Forcing in the favorable left front quad of the upper jet streak is helping support a brief period low level convergence/frontogenetic forcing and fairly widespread - but light snow. CAMs suggest the best chance for a coating to an inch or so of snow will be over my SERN zones where advisory criteria is small and travel issues common with even light snowfalls. At this point it doesn`t seem to warrant a headline, but we did issue a Special Weather Statement to alert for the potential of locally slippery travel. Models use the shortwave to develop a weak surface wave over coastal NC and speeding it well offshore later today and overnight bringing a quick end to the SERN snows. Moisture will also increase over the northwest throughout the day as cold front approaches. That will bring increasing snow shower chances the deeper into the day we get. As the winds shift more westerly behind the upper trough this evening and overnight, cold flow over the lakes will become more favorable for lake effect snow showers. The flow will favor a more westerly than NWLY direction suggesting the best snows will stay closer to the lake, mainly affecting NRN Warren and McKean counties. Lows in the teens and 20s will average about 3-6 deg colder than normal.
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&& .SHORT TERM /SATURDAY/...
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Lake effect snow showers will continue Saturday morning, tending to diminish during the afternoon as large scale subsidence begins to develop under rising heights aloft. It will still be colder than normal with highs ranging from the upper 20s northwest to around 40 over the south.
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&& .LONG TERM /SATURDAY NIGHT THROUGH FRIDAY/... The period starts out with below normal 500 hPa heights and below normal 850 hPa temperatures. The heights and temperatures rise to above normal values over the northeastern United States Sunday as a strong ridge retrogrades to our south. The really warm air should stay to our southeast. The heights and temperatures settle back toward normal Tuesday and Wednesday at 500 hPa and 850 hPa respectively. Models indicate a developing band of low level fgen forcing across northern Pa Saturday within left exit region of jet streak. Model soundings also indicate a nearly saturated sfc- 700mb layer with upslope flow into the Alleghenies. Thus, have mentioned the chance of spotty light snow across the entire Allegheny Plateau Saturday, with higher POPs across the northwest mountains, where best fgen forcing is indicated. WPC qpf indicate a few inches of accum are likely Fri night- Saturday across Warren/Mckean counties, with minimal amounts likely further south along the Alleghenies. Sunday into Monday a strong shortwave to our north will help bring in warmer air. 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 low and EFS based POPS are not very high in central and southern areas. As this northern stream wave zips by to our north it should drag 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 warmer air in the NAEFS/GEFS. This suggests relatively warmer on Thursday. The Warm advection and approaching cold front will likely produce increased potential for snow in NW Thursday into Friday. Farther south any precipitation would likely be mixed or rain. A strong frontal boundary will likely push another surge of cold air into the region Friday into Saturday. At this time and recent GEFS/CMCE and combined NAEFS runs keep the strong ridge suppressed to our south. This keeps the warm air from getting here. This keeps us relatively dry with all the significant QPF events suppressed to our south. There is considerable spread in the ensemble members. And we have seen run-to-run variations in the solutions with the strong southwest Atlantic ridge. The NAEFS deeps reforming vortex over east-central Canada and maintains the sharp ridge over northwestern North America. This seems to keep us in or on the edge of cold air intrusions. Thus so far single model runs of big warm ups appear to disappear in the ensembles. && .AVIATION /18Z FRIDAY THROUGH WEDNESDAY/...
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* Cloud/Ceiling info will be missing at KBFD until new part can be installed * Areas of light snow and pockets of MVFR/IFR over central and eastern terminals will continue into the late afternoon with improving conditions between about 21-00Z. Over the NW the flow will take on more of an over-lake trajectory which will bring occasional snow showers into the normal snow belt areas and Laurel Highlands. Expect mainly VFR conditions with locally IFR/LIFR under heavier bands of snow showers. Outlook... Sat...Scattered snow showers and local restrictions NW/Laurels. Sun-Mon...Mainly VFR. Tue-Wed...mainly VFR, but possible restrictions in the western and northern mountains in snow showers.
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&& .CTP WATCHES/WARNINGS/ADVISORIES...
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Winter Weather Advisory from 7 PM this evening to 4 PM EST Saturday for PAZ004-005.
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&& $$ SYNOPSIS...La Corte NEAR TERM...La Corte SHORT TERM...La Corte LONG TERM...Grumm/Fitzgerald AVIATION...La Corte

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