Area Forecast Discussion
Issued by NWS State College, PA

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384 FXUS61 KCTP 280311 AFDCTP Area Forecast Discussion National Weather Service State College PA 1111 PM EDT Tue Sep 27 2016 .SYNOPSIS... A storm system digging southward across the Great Lakes tonight will settle over the OH valley and bring a prolonged period of heavy or even excessive rainfall Wednesday night into Thursday over south central PA. Unsettled weather will continue into the weekend with periods of showers. && .NEAR TERM /UNTIL 8 AM WEDNESDAY MORNING/... Temps have dropped off well over the nrn mtns where it has so far been clear. A few clouds sneaking in from the west look like they may graze the area but the short wave trough helping to make the clouds will be off to the NE soon. Air is pretty dry. Will hold onto fog mentions. But, only knock off a deg or two from mins - mainly in the Laurels where Bedford (KHMZ) is into the u40s already and a little ahead of the current curve. Newest NAM continues to generate a few shra over the Laurels by 12Z but skies are still clear well down to the south along the Blue Ridge. KCHO is the first location with a lower cloud deck. Prev... Mid clouds and some low clouds nearing the NW but otherwise clear at 23z. Lower cloud deck should start to form south of PA and may nudge into the SC mtns before sunrise Wed. Temps have dropped 3-6F in the last hour with dewpoints very low (30s) in the W/NW, but in the 50s in the eastern valley cities. No big changes to the near term grids. Also reviewed the lastest WPC excessive rainfall guidance and the threat area has slid southward just a bit - with the higher threat to the south of the MD border. Will continue to hold off on flood watch at this point. Prev... Dry conditions prevail one more night before several days of unsettled and quite soggy weather beginning late Wednesday. Weak high pressure remains overhead tonight keeping skies mainly clear for most with patchy river valley fog. No sig changes in the near term as main wx focus is on potential heavy rain event midweek. In the pre dawn hours...winds will begin to shift to the southeast signalling the beginning of deteriorating weather over the commonwealth. Narrow ribbon of sub-0.5" PW will be sandwiched from the west and east by deeper layer moisture advecting into the region. Mins will range from the lower 40s north to the lower 50s southeast. && .SHORT TERM /8 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/... For Wednesday...southeast flow will bring increase in cloudiness from southeast to northwest...with a chance of showers arriving late Wednesday. The risk for heavy rain ramps up after 00z Thursday. Highs will range from the upper 60s northwest to the lower 70s southeast. && .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/... *Increased risk of heavy to excessive rainfall and flooding Wednesday night into Thursday. Primary threat area is southern third of Pennsylvania. Latest guidance is narrowing in on the heavy rain potential for Wednesday night and Thursday, with growing confidence of at least a small shift southward for heaviest rain potential. Still looking at high likelihood for a widespread 2 to 4 inch rainfall (with the potential for higher amounts across the south). What is certain is that a favorable cluster of parameters come together and remain focused on central PA for a 24 to 36 hour period from Wednesday evening through early Friday...bringing the greatest heavy rain potential we`ve seen in several months to the region. A pronounced convergence axis is progged to develop to the east of an upper low that parks over the Ohio Valley, tapping into deep moisture/high PW air off the Atlantic via anomalous east- southeast low-level winds. The models also indicate some marginal instability (a few hundred J/KG of CAPE) along the moisture convergence axis, suggesting some embedded convective elements are possible within the rain axis. The combination of strong forcing aloft, impressive low level moisture convergence, magnitude of 850mb wind anomalies, terrain enhancement and slow overall movement suggest an axis of heavy to excessive rainfall with potential flooding is likely to evolve over south-central PA Wednesday night into Thursday. The main question of course is exactly where with model QPFs continue to differ somewhat at each 6hr interval. Therefore, generally followed the latest WPC QPF which slightly shifts a broad area of 2-4" a bit further south than previous runs (across mainly the south- central and southeastern counties with localized amounts of 4-6" possible. WPC continues the SLGT and MDT excessive rain risk for Wednesday and Thursday. This is highlighted in the HWO and possible heavy rain will be mentioned in grids/text products. Idea of a flood watch was tossed around a bit, but event still 30+ hours out and evolving. Most likely area would be southern tier/lower susq unless later runs continue to shift heavy rain axis southward. The other wildcard is the recent dryness, but given the favorable synoptic set-up and magnitude of the anomalies the risk is warranted. The upper low is fcst to wobble back to the west/NW toward the OH Valley on Friday before slowly moving to the ENE across the Lower Great Lakes through the weekend. This will keep the pattern unsettled with periods of showers - but we are not expecting the heavy rain risk to persist beyond Thursday. Temps still holding close to seasonal normals as we close out September and head into October. && .AVIATION /03Z WEDNESDAY THROUGH SUNDAY/...
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Still mainly clear as of 11 PM. Current satellite trends would take the showers and storms to the south of PA as of 11 PM, east of our area by sunrise. Thus no major changes to 03Z TAFS needed. Earlier discussion below. A clear early fall evening outside. Airmass quite dry now, not expecting fog overnight. Wednesday should be the last good VFR day before more showery and cloudy conditions develop later Wednesday Night into Thursday with widespread reduced conditions likely. For the 00Z TAF package, did bring in some mid level clouds and a gusty southeat breeze during the day on Wednesday. The only site I have MVFR conditions in before 00Z Thursday is JST. Outlook... Thu-Fri...Widespread showers/cig reductions. Sat...Reductions possible with a chance of showers. Sun...Improving conditions.
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&& .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...DeVoir NEAR TERM...Dangelo/DeVoir SHORT TERM...DeVoir LONG TERM...RXR/Steinbugl AVIATION...Martin

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