Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 180719 AFDCTP Area Forecast Discussion National Weather Service State College PA 219 AM EST Sat Nov 18 2017 .SYNOPSIS... An intensifying storm system will move through the eastern Great Lakes Saturday. followed by a strong cold front for Saturday night. Cold gusty winds and some lake effect snow will then affect the area Sunday into Monday. High pressure will then bring improving conditions through the middle of next week. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... Clear skies will give way to increasing high clouds overnight with the first of the rain forecast by the latest HRRR to be entering my western stripe of counties right around sunrise. Temperatures will continue to fall this evening, with readings already freezing or below over the northern tier. While the clouds will eventually put a brake on the fall as they thicken up, some of the normal chilly nooks could be vulnerable to a short period of spotty light freezing rain as the precip overspreads the CWA. Right now the confidence is too low to issue an advisory, but the potential will continue to be monitored as the rain approaches. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SUNDAY/... The storm system currently emerging from the front range of the Rockies will deepen on its track into the eastern Gr Lakes Saturday and Saturday evening. The warm front south of the state will be very slow to advance into the central ridge-valley region Saturday with the models showing a strong cold air damming scenario setting up. With the right entrance region of an upper jet streak sliding by just to the north of the area, the thermally indirect vertical circulation will support a strong NELY component to the low level ageostrophic flow out of the north-northeast. With this strong northerly component to the low level flow and precip falling into an initially dry airmass contributing to further stabilization through evaporational cooling, it points to a chilly rain for the forecast area during the day, with the steadiest stripe of rain likely over about the northern 2/3 of the CWA. Another concern for Saturday will be for an elevated thunderstorm. SPC has us in its general outlook, but it`s hard to find even a hint of elevated instability in the face of the intense cold air damming. While I won`t rule it out in the strong warm advection pattern that is expected to develop, I think the probability is low enough to not mention at this point. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... One-two punch of gusty West to Northwest wind, then a quick transition to the first significant bout of Lake Effect Snow this season across the NW mtns of PA in the wake of a strong cold frontal passage Saturday night. The synoptic set-up of a deepening low moving down the St Lawrence River valley and a potent short wave trough aloft swinging around the base of the longer trough is quite favorable for strong winds. The center of an area of very sharp pressure rises (10 mb/3 hours) moves ENE right over the region during the mid to late morning hours Sunday. We could easily see Wind Advisory Criteria with this type of fall/rise couplet passing directly overhead, and possibly even a High Wind Watch/Warning scenario with west to northwest gusts in the 50-60 mph range. It`s still far enough out to look for consistency in this feature and decide whether to go the High Wind Watch route at ~4th period. The second punch is the deep/strong cold air advection with -10C air at 850 mb crossing Lake Erie Water temps in the mid-upper 40s, which results in an extreme Lake-850 mb Delta T of 18C Sunday afternoon and evening across the perennial snowbelt of NW PA. Winter Weather Advisory criteria will likely be reached from this upcoming bout of LES, considering anticipated 4-8 inch forecast snow amounts over an extended 24-36 hour period. Elsewhere, much lighter snow amounts of a coating to 2 inches will occur from sinuous bands of snow showers and briefly heavy squalls that should make it well into the Central Ridge and Valley Region and portions of the Susq Valley based on fairly strong 35 to 45 kt nw winds in the deep and cold boundary layer. High pressure sliding by to our south will influence the weather to start Thanksgiving week, bringing a couple of cool- dry days. Temperatures will begin to moderate Tuesday with a gusty SW flow expected to develop as potent sub-1000 mb low pressure passes by across southern Ontario and Quebec. Tuesday will be the lone day in the Mon-Fri period when 850 mb temps will poke a few to several deg C above zero. Though the jet stream will remain north of the region, a quick moving trough will brush the region on Wed followed by a weak shortwave Wed night, which will bring chance for periods of snow showers (with light accums of a coating to 2 inches) back into the NW zones, along with a return of slightly cooler air. Flurries or brief snow showers will occur just SE of the Allegheny Front. 12Z GFS and EC have trended into better agreement on a deepening upper low across southeastern Canada or the NE U.S. over the Thanksgiving weekend. Surface and 850 mb temps could be a few deg colder than our upcoming cold outbreak later this weekend. && .AVIATION /07Z SATURDAY THROUGH WEDNESDAY/...
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Widespread VFR noted across the region at 06Z, but deteriorating flying conditions are expected late this morning, as rain and lowering cigs overspread the area in advance of low pressure lifting out of the Mississippi Valley. Rain is expected to overspread the airspace between 10-16Z with a few pockets of freezing rain possible on the leading edge over the north- central airspace. Confidence is low on fzra risk and it would only last for a short duration. Periods of rain will continue through the day with predominantly IFR/LIFR cigs over the northern mountains (KBFD), where the southerly flow ascends the high terrain. Elsewhere, model soundings support predominantly MVFR cigs, with brief IFR vsby possible in heavier showers. Model soundings indicating borderline LLWS over the western portion of the airspace early today. However, there appears a better, more widespread chance of LLWS this evening, as a core of strong winds aloft lift over the region. Southerly winds may become gusty late this evening over the western and southern airspace, as strong winds aloft begin mixing to ground level. Latest NAM/HRRR suggests gusts in the 20-30kt range are possible between 00Z-06Z Sunday. Outlook... Sun...Windy with frequent gusts 30-40kts from 270-300 degrees. IFR snow showers NW trending MVFR central to VFR east. Mon...AM shsn/reduced vsbys possible NW mountains. Tue-Wed...No sig wx expected.
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&& .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...La Corte/Gartner NEAR TERM...La Corte/Gartner SHORT TERM...Dangelo/Gartner LONG TERM...Lambert AVIATION...Fitzgerald

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