Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 181422 AFDCTP Area Forecast Discussion National Weather Service State College PA 922 AM EST Sat Nov 18 2017 .SYNOPSIS... An intensifying storm system will move through the Great Lakes today. A strong cold front will blast through late tonight. Cold gusty winds and some lake effect snow will then affect the area Sunday into Monday. High pressure will bring improving conditions through the middle of next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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A large swath of moderate to heavy rain has spread quickly east across much Central PA and the Susquehanna River valley over the past 2-3 hours. Temps at the sfc and aloft are just cold enough to cause a brief period of sleet and/or freezing rain in some locations for the first 30 minutes after the precip onset. A look at road temp sensors across the region indicate that most pavement is a few degrees above freezing, so no big freezing rain problems are expected for travel. Still travelers should be aware of the potential for a few icy spots on roads immediately to the north of steep slopes, where insolation is minimized and pavement temps could be around 32F for the next hour or two. Steady moderate (to breifly heavy) rain will continue into at least the early afternoon across much of the region near and to the south of Interstate 80 as strong SW winds in the 900-800mb layer (averaging 45-50 kts) advects high pwat air in excess of 1 inch up and over the shallow cooler air that`s locked-in throughout the ridge and valley region of PA. Followed the latest RAP model to track an expanding region of elevated instability from Ecent Ohio (that`s currently producing TSRA between I-70 and I-80), that will move over about the SW half of PA late this afternoon/early tonight. Included chc TSRA along with the predominant Periods of light to moderate Rain. Expanded the area of 1+ inch rainfall amounts across the region of Central PA and the Susq valley between I-80 and the RT 22/322 corridors where models show a stripe of enhanced QPF from the steepest ascent of the moist and increasing unstable air over the persistent/nearly stationary llvl boundary. The rain should become lighter and more intermittent across the southern 1-2 layers of counties late this afternoon and evening as the warm front lifts a bit to the north. The lower elevations of the central and northeast mountains will have little chance to warm up as the warm southerly wind passes right overhead. Will keep maxes in the central mtns in the lower 40s. The wrn and srn edges of the CWA have a chance to get into the lower 50s. But... I wouldn`t hold my breath.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... By nightfall, the sfc low associated with the impending storm will be over LE and deepening. The pressure rise behind the front will be 8-12mb in 3hrs, and that fast rise is usually a good signal that it will be accompanied by strong winds. BUFKIT profiles portray winds in the lowest 3kft being in the 40s tonight just before and for a few hours after the front passes. Trouble for mixing down to the sfc is that most of the area will be under a strong cold-air damming (read: stable) set up. The pressure rise is strongest over the south, and the higher elevations like the Laurels will have the strongest winds. Have posted a wind advy for the SWrn counties, eyeing top-end sustained speeds of 25-30kts and 40-45kt max gusts. The advy may need to be expanded to cover more of the forecast area if it looks just a little windier than it does currently. The gusty wind will subside but not die out during the post-sunrise morning hours on Sunday. There is a slight chance of some thunder tonight as the front nears and passes. But, the convection would be detached from the surface with the cold air in place and is not expected to pose a damage threat. Have only mentioned T in the south tonight where 8H LI`s drop below zero and CAPE is more than zilch for a few hours. Temps will be a roller coaster tonight with a rise in most places before and perhaps even slightly after FROPA, and then a hard drop as the CAA kicks in. Sunrise temps will get close to freezing in the NW. The precip left at that time may turn to snow as well in the NW. The cold air continues to drop the temp profiles evenly and snow is the dominant p-type on Sunday, but it will still be too mild anywhere but the NW mtns and Laurels to allow it to stick. At this point, the LES looks relatively tame during the daytime Sunday. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Deep/strong cold air advection with -10C air at 850 mb will cross Lake Erie. Water temps are 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 could be reached from this upcoming bout of LES, considering anticipated 4-6 inch forecast snow amounts over a 24 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. The high shear may rip the bands apart, though. With uncertainty of getting to the 12hr advy accum threshold (3+"), we will hold off on any 2nd-3rd-4th period advy. We will try to refine the time of most/heaviest accumulation. 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 /14Z SATURDAY THROUGH WEDNESDAY/... Deteriorating flying conditions are expected later this morning, as rain and lowering cigs overspread the area in advance of low pressure lifting out of the Mississippi Valley. Rain has already reached KBFD at 12Z and will overspread the rest of the airspace between 12-16Z with a few pockets of freezing rain possible on the leading edge over the north- central airspace. Periods of rain will continue through the day with predominantly IFR/LIFR cigs over the northern mountains (KBFD), where the southerly flow is forced to ascend 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. && .CTP WATCHES/WARNINGS/ADVISORIES... Wind Advisory from 7 PM this evening to noon EST Sunday for PAZ024-025-033-034. && $$ SYNOPSIS...Dangelo/Lambert NEAR TERM...Lambert/Dangelo SHORT TERM...Dangelo LONG TERM...Lambert AVIATION...Fitzgerald

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