Area Forecast Discussion
Issued by NWS State College, PA

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387 FXUS61 KCTP 281135 AFDCTP Area Forecast Discussion National Weather Service State College PA 735 AM EDT Wed Sep 28 2016 .SYNOPSIS... A multi-day heavy rain event is probable across far south-central Pennsylvania beginning later today and continuing tonight through Thursday. Flooding impacts are likely across the southern half of central PA...but impacts may be lessened to some extent by dry antecedent conditions. Unsettled weather with occasional lighter rain showers will persist into the weekend with a gradual drying trend by early next week. && .NEAR TERM /THROUGH TONIGHT/... IR satellite trends continue to show low clouds advancing northward across south-central PA. The IR loop also reveals patchy fog in the river/stream valleys across the northern Alleghenies. Therefore will continue to make upward adjustments to sky cover and speed up northward cloud expansion early today. Low POPs along the PA/MD line still look good to account for shower activity shifting northward from Northern VA. Backing low to mid level flow downstream of southward moving closed upper low over the OH valley will begin the large scale set-up for what should be a multi-day/36+hour heavy precipitation event focused over far south-central PA into Northern VA/MD. Precip probs will increase through the day with moderate showers likely reaching the Lower Susq. Valley by 29/00Z. Model guidance still shows a surge in rain coverage and intensity later tonight into Thursday which corresponds with arrival of stronger dynamical forcing and strengthening E-SE upslope low level flow. Max QPF amounts derived from WPC blend are located over the Lower Susq. Valley along/south of I-81. && .SHORT TERM /THURSDAY THROUGH FRIDAY NIGHT/... *Flood Watch in effect for the Laurel Highlands, South Central Mountains and southern portions of the Central Mountains midnight tonight through Friday morning. *Heavy rainfall event probable over far south-central Pennsylvania on Thursday *Flooding is likely however high impacts may be mitigated by dry antecedent conditions Heavy rain event is expected to be ongoing over south-central PA on Thursday. An enhanced convergence axis/frontal zone is progged to develop on the east side of an upper low meandering over the OH/TN Valley, tapping into deep moisture/high PW air off the Atlantic Gulf Stream via anomalous east/southeast low level jet. The heavy rain axis is progged to gradually pivot to the southwest with time oriented within moderate excessive rain outlook area from southwest PA near the Laurel Highlands southeast into Northern VA/D.C. area. The strength of the upslope flow and high PW transport along with some elevated instability should provide an environment for efficient rainfall processes. WPC QPF blend again favored a slight shift in QPF to the southwest but maintained a broad area of 2-4" with localized 4-6" possible. Keep in mind these amounts are over a 36+ hour period with preceding dry conditions and near to below normal streamflows. Therefore the dry antecedent conditions may limit more significant flooding/runoff issues. Still expect flooding to result from long duration event...and have issued Areal Flood Watch for Laurel Highlands, South Central Mountains and southern portions of the Central Mountains for late tonight through Friday morning. Will continue to highlight potential flood risk in HWO elsewhere. The upper low is fcst to drift back to the west/NW into the OH Valley on Friday before slowly moving to the ENE across the Lower Great Lakes through the weekend. We should continue to see periods of showers on Friday but rainfall amounts will be much lower and less organized. && .LONG TERM /SATURDAY THROUGH TUESDAY/... Model guidance still suggests unsettled conditions over the weekend with a gradual decrease in the chance for showerThe most likely watch area would be from Cambria/Somerset southeast to York County.s. Temps still holding close to seasonal normals as we turn the calendar to October. && .AVIATION /12Z WEDNESDAY THROUGH SUNDAY/... Expect VFR conds to continue overnight across most of the airspace. Exception will be far southeast airfields, KLNS/KMDT, where low clouds/IFR cigs have crept in ahead of developing weather system over the MidAtlc region. Sub VFR cigs should dissipate after sunrise. 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. Outlook... Thu-Fri...Widespread showers/cig reductions. Sat...Reductions possible with a chance of showers. Sun...Improving conditions.&& .CTP WATCHES/WARNINGS/ADVISORIES... None. && .CTP WATCHES/WARNINGS/ADVISORIES... Flood Watch from late tonight through Friday morning for PAZ024>028-033>036-056-063>065. && $$ SYNOPSIS...Steinbugl NEAR TERM...DeVoir/Steinbugl SHORT TERM...DeVoir/Steinbugl LONG TERM...Steinbugl AVIATION...Gartner/Martin

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