Area Forecast Discussion
Issued by NWS State College, PA

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313 FXUS61 KCTP 121134 AFDCTP Area Forecast Discussion National Weather Service State College PA 634 AM EST Tue Dec 12 2017 .SYNOPSIS... An alberta clipper will track over northwestern PA early today with a trailing arctic cold front sweeping through the area during the mid to late morning afternoon. A deep upper level trough will remain over the eastern conus through the rest of this week with another weak clipper likely affecting the area Thursday. The upper trough will likely lift out by next weekend. A low pressure system lifting northeast from the Great Plains will bring a southwest flow of milder air for the weekend. && .NEAR TERM /UNTIL NOON TODAY/... A compact, but quite potent mid/upr level vort max moving ENE across the WRN Mtns attm will bring a brief period of rain/snow showers as mean 925-850mb temps are near to slightly above freezing across the southern tier with sfc temps in the mid-upr 30s. Near and to the north of the RT 22 corridor in Scent PA boundary layer wet bulb temps are a few deg C below freezing, which will support a few periods of primarily snow showers through 14Z. Short range, high res RAP and HRRR consensus brings a batch of warm advection snowfall to the NRN mtns during the mid morning hours with a coating to the lower terrain, and 1 to 3 inches across the Mtns north of a KDUJ to KIPT line. The mid level shear axis will be across the Susq Valley and points east between 12-14Z...accompanied by a band or two of light rain and snow showers. QPF and snowfall amounts near and to the SE of the I-99/Route 220 corridor look to be light with a few to svrl hundredths of an inch of LEQ at most and perhaps a ground coating of snow (mainly across the higher terrain near and to the north of I-80, which will be along of just to the north of the track of the aforementioned compact mid/upr level short wave. The main impacts from this system will come as the cold front ushers in the coldest air of the season into PA later today, after the arctic FROPA later this morning. Temps peak between 35 and 40 deg F late this morning/early this afternoon across the SE half of the CWA, while highs today across the western mtns (in the low-mid 30s) will occur within the next few hours, followed by falling steadily through the 20s and into the mid teens around dusk. West to northwest winds will pick up drastically in the wake of the shortwave and closely trailing arctic front, with gusts of 35 to 45 mph likely across the western mtns beginning late this morning, then developing across the central and eastern parts of the CWA this afternoon. Lake Effect Snow Warning is in effect for Warren County beginning at 12Z today, while a Winter Weather Advisory extends south from McKean County through the Laurel Highlands in Cambria and Somerset County. && .SHORT TERM /NOON TODAY THROUGH 6 PM WEDNESDAY/... The coldest air of the season is forecast to arrive behind trailing arctic cold front this afternoon and tonight with more lake effect snow lasting through midweek. Northwest Warren County should get into some of the more robust lake effect banding later Today and continuing into Wednesday. Lake Effect Snow Warning in effect for this, per collaboration with BGM, for widespread 10 to 15 inches, with local amounts up to 18 inches over extreme northwest Warren County. Model blended qpf supports a long duration advisory over Elk/Mckean counties, as well as Clearfield/Cambria/Somerset Counties where 4 to 8 inches are expected from 12z Tuesday through 18z Wednesday. Still some concern about snow squalls accompanying cold front passage between 12Z-18Z today with the best values of the snow squall parameter noted across the Lower Susq Valley for an hour or two this afternoon. However, boundary layer temps appear borderline for signif impacts from snow showers/squalls and surface temps are likely to be between 35 and 40F during the time of the best dynamics for squalls. Gusty northwest winds and snow showers will be the main issue Tonight into Wed. Warnings and advisories in effect until 18Z Wed. && .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Models show another system for Thursday. This system looks weak and EC a little further south, as been the case. Thus just have very light amts in. Maybe more of an impact for the far southeast, than system for Tue into Wed. After this the pattern supports a deep low tracking northeast from the Great Plains for the weekend, which will result in milder conditions. As been the case, the Gulf of Mexico is not really open, so looking at a mainly dry weekend. Complex pattern for Monday, perhaps a little light snow, but nothing major. Minor changes to the extended package. && .AVIATION /12Z TUESDAY THROUGH SATURDAY/...
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A compact yet potent mid level shortwave sliding across the CWA early this morning (dragging a weak occluded front along with it) will bring brief reductions in snow showers before it moves off to the NE by 13z. Then attention turns to an arctic cold front sweeping through the area during the mid to late morning afternoon. Between now and then, generally IFR reductions will persist across the NW mountains in snow showers. with reductions arriving at JST by 13z and AOO/UNV/IPT by 15z. The western sections will remain IFR for much of the day as upslope westerly flow increases and snow showers become more widespread, with MVFR spilling into the central mountains with lighter snow showers likely. Winds also increase with FROPA with gusts to 30-40 mph becoming more common by midday over the west, and 25-35 mph gusts for this afternoon over the east. There remains some concern for localized snow squalls accompanying cold front passage over NW half of CWA. Reductions continue tonight into Wed across NW half, with mainly IFR at BFD and MVFR at JST. Winds remain very gusty as well. Outlook... Wed...Snow showers with IFR NW Mtns. Sct snow showers with MVFR NW half. Otherwise VFR. Wed night...Alberta Clipper could bring light snow and more widespread reductions. Thu-Fri...Sct snow showers NW. Reductions west. Sat...Mainly VFR. Perhaps a snow shower north.
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&& .CTP WATCHES/WARNINGS/ADVISORIES...
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Winter Weather Advisory until 1 PM EST Wednesday for PAZ005- 010-017-024-033. Lake Effect Snow Warning until 1 PM EST Wednesday for PAZ004.
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&& $$ SYNOPSIS...Lambert NEAR TERM...Dangelo/Lambert SHORT TERM...Lambert LONG TERM...Martin AVIATION...RXR

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