Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 091609 AFDCTP Area Forecast Discussion National Weather Service State College PA 1109 AM EST Fri Dec 9 2016 .SYNOPSIS... A broad and very cold upper level trough over the Great Lakes and southeastern Canada will remain in place for the next few days. This weather feature and a well-aligned westerly low level flow off the lower Great Lakes will bring a prolonged period of lake effect snowfall and significant snow accumulations to northwest PA today through Saturday. High pressure will briefly build across the commonwealth late Saturday and Saturday night causing the Lake Effect snow bands to lift north of the region with a brief period of clearing elsewhere. Then, a weak low pressure system is likely to track east from the Ohio Valley and impact Pennsylvania with another bout of wintry weather Sunday into Monday. Unsettled weather will persist next week with additional storm chances during the mid...and late week periods. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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An anomalously cold cross-lake flow will maintain a long-duartion, multi-band lake effect snow event across the northwest PA. Model soundings continue to display a deep, moist boundary layer with an inversion height around 700MB (and 850mb temps of around -11 to -14C), sufficient to maintain moderate to heavy banding from relatively warm lake waters of around 50F. Multiband event continues late this morning...and passage of a shortwave currently over northwest PA will support lake effect snow streamers extending well downwind of the lakes into the central mountains...with perhaps a quick coating possible in the ridge/valley region of Central Pa and possibly even into parts of the Susq Valley...although this is not imminent per regional radar mosaic or from high resolution HRRR. Multiple bands of moderate to heavy Lake Effect Snow will continue to stream southeast and well inland from Lake Erie through tonight as the sfc-850 mb flow stays well aligned from about 285-290 deg and the wind in the cloud bearing layer averages around 30 kts. Snow/water ratios will average at a minimum...17-20:1 and the Laurels and North-Central mountains will experience periods of moderate-briefly heavy snow, and a quick 1-3 inches of snow accum in some of these narrow LES bands (and favored areas for orographic snow to the west of rt 219 in the Laurels). Will have to keep a close eye on persistence/snowfall intensity of the snowbands across the Laurels with respect to the possible issuance of a Winter Weather Advisory. Will maintain the Lake Effect Snow Warning (Warren County) and Lake Effect Snow Advisory (Mckean County) through 17Z Saturday, before the llvl mean wind gradually backs to about 250 Deg carrying the LES bands north of the PA/NY border Saturday afternoon. Little change to forecast storm total snow amounts as 12-18" with locally 20"+ is expected over the NW third-half of Warren County by Saturday. A light coating to one inch will occur in select areas of the central Ridge and Valley Region of PA today...with some spots receiving additional light accums tonight and Saturday. Skies in this region of the state will stay mostly cloudy to cloudy. Across the Mid and Lower Susquehanna Valley Region, it will be partly to mostly cloudy with generally just some scattered flurries. A brief heavier snow showers is possible near and to the NW of the I-81 corridor especially across the higher terrain of Schuylkill County...where a coating to one inch is possible. Highs this afternoon will be in the mid to upper 20s across the Mtns, lower to mid 30s in the Central Valleys, and mid to upper 30s in the Susquehanna Region.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... Slow backing of the mean Low-level wind will occur after midnight tonight with an expected quasi-single heavy LES snow band continuing to impact parts of nrn Warren and Mckean counties. Additional significant accumulation will occur across these 2 NW PA counties...while some areas of light additional accumulation is possible across the west-central mtns and Laurel Highlands. Partly to mostly cloudy skies will mainly flurries will persist SE of the I-99/RT 220 corridor. though one or two long/narrow LES bands could snake their way well into central PA with localized 1-3 inch amounts. 09/07Z RAP indicates the likelihood of one single and rather intense band with a good upstream connection to moisture off the full length of Lake Huron and central Lake Erie, aimed near or just to the north of a KFIG/KPSB and KUNV line late today/early this evening. Gradually improving conditions heading into Sat across the NW Mtns as lake effect snow bands become more scattered and slowly wind down. Still potential for a renegade snow band (with Lake Huron connection) to stretch into the central mtns Fri night with localized impacts though as trough lifts out. However, WSW fetch off the lakes should continue to produce snow showers until Sat evening across the N Mtns. Breezy winds abate Fri night, with clearing skies across Southern Pa late Friday night and Saturday as surface high pressure builds in from the Ohio Valley. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... High pressure will bring a brief period of tranquility Sat evening. However, a developing overrunning scenario is expected Sunday into Monday, as weak low pressure lifts into the Eastern Grt Lks and high pressure remains lodged over northern New Eng. Enough cold air is in place to support a light snowfall across much of area by Monday AM, with GFS bringing light precip into the area on Sunday earlier than the ECMWF. Model consensus low track west of PA should bring in enough warm air to cause snow to change to rain (or a period of more problematic freezing rain) across Southern PA, and esp the Lower Susq Valley, a before precip tapers off later Monday. A bit cooler than normal temps indicated early next week, but longer range models indicate a significant cold front passage Wed with noticeably below normal temps filtering in for the rest of next week. && .AVIATION /16Z FRIDAY THROUGH TUESDAY/...
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Cold W-NW flow pattern downwind of the Great Lakes will continue to support a prolonged period of MVFR/IFR conditions over the western 1/3 of the airspace into the weekend, with frequent lake effect/upslope snow showers at KBFD/JST. MVFR cigs and ocnl vsby reductions in -shsn are possible over the central terminals with VFR prevailing across the eastern sites. Expect sfc wind gusts 20-25+kt from 270-300 degrees for most of today before winds subside after 10/00z. Lake effect snow showers should decrease in coverage and intensity by later Saturday as area of high pressure briefly slides over the airspace. Low pressure tracking from the lee of the Rockies to the Ohio Valley will bring a broad area of snow/rain to the area Sunday into early Monday. Outlook... Sat...MVFR/IFR western 1/3 with snow showers decreasing. Sun-Mon...Widespread MVFR/IFR in snow/rain. Becoming breezy Monday night with snow showers over the wrn 1/3. Tue...VFR to MVFR cigs. Snow showers ending.
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&& .CTP WATCHES/WARNINGS/ADVISORIES... Lake Effect Snow Advisory until noon EST Saturday for PAZ005. Lake Effect Snow Warning until noon EST Saturday for PAZ004. && $$ SYNOPSIS...Lambert NEAR TERM...DeVoir/Lambert SHORT TERM...Lambert LONG TERM...Lambert/RXR AVIATION...DeVoir/Steinbugl

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