Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 280543 AFDCTP Area Forecast Discussion National Weather Service State College PA 143 AM EDT Wed Sep 28 2016 .SYNOPSIS...
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Heavy rain remains possible across portions of south-central and southeastern Pennsylvania tonight through Thursday, as moisture increases ahead of an anomalous upper low digging from the Midwest to the Tennessee Valley. Unsettled weather will continue Friday and into the weekend with periods of showers.
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&& .NEAR TERM /UNTIL 8 AM THIS MORNING/...
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Minor changes for the 1AM update - mainly to hourly temps and sky cover. 05z obs were coming in below previous fcst with temps dropping off rather quickly with some spots down into the low 40s. Expanded patchy fog over the central ridge/valley region in the wx grids. IR satellite shows low clouds starting to develop into the Lower Susq. Valley and therefore adjusted sky cover upward but perhaps not enough. HRRR is trending more bullish with shower activity spreading into south-central/southeast PA by mid morning.
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&& .SHORT TERM /8 AM THIS MORNING THROUGH 6 PM TODAY/...
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Southeast flow will bring in clouds with a chance of showers increasing from south to north through the day. The risk for heavy rain ramps up after 00z Thursday. Highs will range from the upper 60s northwest to the lower 70s southeast.
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&& .LONG TERM /TONIGHT 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 /06Z 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 southeast 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...Steinbugl NEAR TERM...Steinbugl SHORT TERM...Dangelo/DeVoir LONG TERM...RXR/Steinbugl AVIATION...Martin

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