Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 271901 AFDCTP Area Forecast Discussion National Weather Service State College PA 301 PM EDT Tue Sep 27 2016 .SYNOPSIS...
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Dry conditions will prevail tonight. A storm system digging southward across the Great Lakes tonight will bring an elevated risk for heavy to excessive rainfall and flooding Wednesday night into Thursday over south central PA. Unsettled weather will continue into the weekend with periods of showers.
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&& .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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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.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/...
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*Increased risk of heavy to excessive rainfall and flooding Wednesday night into Thursday. Primary threat area is south central/east central Pennsylvania. Still absorbing guidance which is narrowing in on the heavy rain potential for Wednesday night and Thursday. A widespread 2 to 4 inch rainfall appears likely for many with the potential for higher amounts...but details remain unclear this far out. 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. 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.
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&& .LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/...
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A pronounced convergence axis is progged to develop to the east of the low and tap 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 likely 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 differing slightly at each 6hr interval. Therefore, used WPC QPF which shows a broad area of 2-4" across the south-central and southeaster counties with localized amounts of 4-6" possible. That said, based on trends in the latest 27/00z guidance, WPC will introduce a SLGT and MDT excessive rain risk for Wednesday and Thursday. Included chance for +RA into wx grids during this time and will highlight in HWO. Flood watches may be considered with later shifts. 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.
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&& .AVIATION /19Z TUESDAY THROUGH SUNDAY/...
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Now that morning fog has burned off...VFR conds will prevail through tonight. Wednesday should be the last good VFR day before more showery and cloudy conditions develop later in the week into the weekend with widespread reduced conditions likely. Outlook... Wed...Showers/cig reductions possible late. Thu-Sat...Sct showers/cig reductions possible, esp w mountains. Sun...Reductions possible north with shra/cigs...fair elsewhere.
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&& .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...DeVoir NEAR TERM...DeVoir SHORT TERM...DeVoir LONG TERM...Steinbugl AVIATION...DeVoir/Gartner

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