Area Forecast Discussion
Issued by NWS State College, PA

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756 FXUS61 KCTP 271433 AFDCTP Area Forecast Discussion National Weather Service State College PA 1033 AM EDT Tue Sep 27 2016 .SYNOPSIS...
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Mostly sunny and pleasant conditions are expected today. A storm system digging southward across the Great Lakes today 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 /THROUGH TONIGHT/...
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Benign conditions to last through tonight...with no sig changes in the near term. Main wx focus is on potential heavy rain event midweek. A well- defined dry slot/below normal PW air overspreads the area behind departing cold front and upper level low dropping south from the Northern Great Lakes. Look for a pleasant afternoon with max temperatures within a degree or two of late September climate normals 65-75F. The model guidance is in good agreement in taking the upper low to a position near the southern tip of Lake Michigan by the end of the period/12z Wed. Not expecting any showers tonight but the trend in rain probs will start to edge higher into midweek.
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&& .SHORT TERM /WEDNESDAY THROUGH THURSDAY NIGHT/... *Increased risk of heavy to excessive rainfall and flooding Wednesday night into Thursday. Primary threat area is south- central Pennsylvania. The aforementioned upper low will cut off from the mean flow and drift south across the OH-TN Valleys Wednesday into Thursday. The guidance has been coming into better agreement with this feature over the past few days with the ECMWF only 100 miles west of the consensus (near eastern KY) by 12z Friday at 500mb. The exact evolution and location of the upper low will play a significant factor in the magnitude and location of potential heavy rainfall axis during the period. At this time, the favored location appears to be over south-central PA. The risk for heavy rain ramps up after 00z Thursday as the upper low digs and takes on a slight negative tilt, resulting in increasing upper level diffluence. 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. && .LONG TERM /FRIDAY THROUGH MONDAY/... 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 /15Z TUESDAY THROUGH SATURDAY/...
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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.
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&& .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Steinbugl NEAR TERM...DeVoir/Steinbugl SHORT TERM...Steinbugl LONG TERM...Steinbugl AVIATION...DeVoir/Gartner

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