Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 071705 AFDCTP Area Forecast Discussion National Weather Service State College PA 1205 PM EST Thu Dec 7 2017 .SYNOPSIS...
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A prolonged period of mainly dry and cold weather will continue into next week.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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The single, intense, Lakeshore parallel LES snowband will remain near, or just to the north of the I-90 corridor in far NW PA until late today when a sfc (and weak upper level) trough will slide east across the region. Slight veering of the mean boundary layer flow in the wake of these features will help to shift the single snowband south and into NW Warren county for a few to svrl hour period late today/early tonight. Painted in a tight gradient of snow probs and accums across the NW half of Warren county, where much of the county will see less than an inch of snow, while the highly rural NW part of the county could see between 2-3 inches. There is some uncertainty with respect to the southern extent of this snowband between 23Z Thu and 05Z Friday, so will refrain from issuing a Winter Weather Advisory for the LES at this point. prior to, and after that 6 hour window, the single LES band should be over the I-90/NYS Route 17 corridor and of little or no consequence to our NW snowbelt. In the meantime, weak low-mid level isent upglide ahead of the sfc trough will support a rather compact area of very light snow/flurries spreading NE across the Laurel Highlands and Central Mtns, where vsbys briefly dipping into the 4-6 SM range could result in a light coating of snow (up to 2-3 tents of an inch) across the Laurels and along the Allegheny front just NW of the I-99 corridor. Little or no precip (aside from a brief flurry) is expected across the Lower Susq Valley and much of the NW mtns through 23Z Thursday. Clouds will be clearing out in the evening. Wind will still be gusty today - sustained between 10-15 mph, with gusts of 20-25 mph.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM FRIDAY/...
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The mean boundary layer flow backs to the SW again by shortly after 05Z Friday, causing the persistent band of LES to shift north again into SWRN New York. As previously noted, tonight will be the best chance for snow accums across the far NW zones. Will let it top out at 2 or 3 inches in NW Warren Co, less than inch elsewhere in the NW mtns. Elk county may stay flurry- free as the fetch is rather poor to get any bands there. Any flurries in the SW/C counties in the early evening will go away rapidly as the forcing moves off to the east. No additional accums are expected there. Clouds should break up. Mins should be into the teens west and L-M20s elsewhere, but will not drop too drastically with some wind still going through the night. It does seem like the gradient relaxes some on Friday, so there will be a little less wind. Maxes will be 4-7F colder than Thurs. The old front off to the SE may continue to push cirrus over the SErn cos, but few other clouds will be found.
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&& .LONG TERM /FRIDAY NIGHT THROUGH WEDNESDAY/... Global model and ensemble guidance are in excellent agreement on the overall persistence of a highly amplified upper flow pattern over the next 7-10 days across N. America with differences arising in the precise timing and amplitude of individual short waves and sfc front digging SE/rotating around the deep vortex situated across northern Quebec. There is high confidence for a prolonged period of below average temperatures through the second full week of December with reinforcing shots of arctic air likely late this weekend, and another during the Mon night-Wednesday timeframe. Each of these events will be accompanied and followed by widespread flurries and snow showers focused primarily across the nrn and wrn mtns of the state. Any significant Lake-effect snow through late this week should be pinned close to, and parallel with the south shore of Lake Erie as mean sfc-10 KFT AGL winds remain from the WSW for the bulk of the time. Occasional snow showers (with overall light accums of just an inch or two in any given 12-hour period) will occur Thursday night through Saturday morning. In addition, there appears to be multiple waves of low pressure lifting north along a western Atlantic front from late week into the early weekend timeframe, with guidance varying on specifics. There is a possibility one of these waves could clip the eastern part of the area with some light snow, but right now the mostly likely outcome is for precip to stay east of the area or off the coast. A rather well-defined clipper will slide east across the Lower Great Lakes late Saturday through early Sunday. Any morning sunshine Saturday will favor the eastern part of the CWA, followed by thickening, layered clouds for the afternoon and evening hours Saturday. Steepening mid level lapse rates will support sct- numerous snow showers with a few inches of snow likely across the northern and western high terrain and perhaps several inches of LES across the snowbelt of NW Warren county. Weak and brief ridging at the sfc and aloft Sunday night through the first half of Monday should help to consolidate significant LES back north near the I-90 corridor before shifting south with the passage of the aforementioned second shortwave Mon night and Tuesday and a trough of even colder, sub-510 DAM thicknesses. A reinforcing shot of arctic air is looking more likely next week along with a more favorable northwest flow lake-effect pattern. The core of the winter cold should grip the area during the second week of December with some signs of moderation during week 3. && .AVIATION /18Z THURSDAY THROUGH TUESDAY/...
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A large area of high based strato cu clouds (mainly between 3.5 and 6 kft AGL) will stream ENE across the region as a result of weak low to mid level warm advection and gentle lift. The one exception may be across the Laurel Highlands, where an upslope flow is resulting in varying/borderline MVFR deck between SCT025 and BKN025. A weak cold front will cross the area this afternoon, perhaps accompanied by a brief vis reduction to 4-6SM from a passing snow shower at either KJST or KAOO. Otherwise, widespread VFR conditions are anticipated this afternoon through tonight. Bufkit soundings support west- southwest winds this afternoon gusting mainly between 20-25kts, with some highest gusts to around 30 kts possible for a few hours late this afternoon (INVOF KJST/KAOO) with the passage of the upper shortwave and descending left rear segment of a 150KT upper jet max - currently located across northern WVA. Another ridge of high pressure will build in behind the front tonight, bringing diminishing westerly wind, and resulting in VFR conditions across most, if not all, of the forecast area. Will have to watch KBFD and KJST, where upslope flow could potentially yield tempo MVFR cigs tonight into Friday morning. Outlook... Fri...No sig wx expected. Sat...Evening snow showers/vis reductions poss Allegheny Plateau. Sun...AM snow showers/vis reductions poss Allegheny Plateau. Mon...PM light snow possible, mainly northwest Pa.
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&& .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Dangelo/Lambert NEAR TERM...Dangelo/Lambert SHORT TERM...Dangelo LONG TERM...Lambert AVIATION...Fitzgerald/Lambert

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