Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 271013 AFDCTP Area Forecast Discussion National Weather Service State College PA 613 AM EDT Thu Jul 27 2017 .SYNOPSIS... A cold front over the Upper Great Lakes early today will drift southeast and cross the state tonight and Friday morning. The front will likely stall along the Maryland border for Friday and an unusually strong wave of low pressure for mid summer will form and move east along this boundary, pulling abundant moisture up and over the front from the south. Heavy rain is possible Friday and Friday night, especially across the southern tier of PA. A much cooler airmass will overspread the commonwealth over the weekend and dry and slightly cooler than normal weather will last into mid- week. && .NEAR TERM /THROUGH TONIGHT/... IR satellite loop at 08Z showing fairly thick cirrus and cirrostratus streaming east across the region early today while a moist serly llvl flow off the wrn atlantic was pushing a layer of stratus across the Susq Valley and as far West as a KHGR to KUNV and KIPT line. As the surface high drifts further off the east coast, a return southerly flow will advect much higher humidity into the area by this afternoon. Model 850 temps support seasonal max temps in the upper 70s to mid 80s from north to south. Model soundings indicate that plenty of mid and high clouds will be around for much of the day today. This combined with mid level temps of 7-9C and slightly warming will likely greatly limit the severe weather potential today just a few late day/evening TSRA. Since yesterday at this time, SPC has downgraded its Slight Risk to a Marginal Risk, and this looks to be on the right track. Diurnal heating of marginally unstable air mass, combined with passage of an upper shortwave during the afternoon hours today should produce isolated to scattered thunderstorm coverage during the afternoon and evening hours. Will continue with the highest POPS across the eastern counties, where CAPE the highest, and also in the vicinity of weak frontal boundary along the NY/PA border. Despite decent 0-6km shear, lack of much instability is limiting the severe weather threat to marginal. && .SHORT TERM /FRIDAY THROUGH FRIDAY NIGHT/... The most significant weather of the next 7 days appears to fall during this approx 36 hour period as a potent, digging upper trough evolves into a closed contour 500 mb low across the Central Appalachians and forms an anomalously strong (for mid summer) 1000 mb sfc low along the quasi-stnry sfc boundary that will be draped near or just south of the Mason-Dixon Line. A strong heavy rain signal in all operational models and Ensemble Forecast Systems is apparent for much of southern PA and adjacent areas of nrn WVA and VA as very high PWAT air of 45-50 mm and elevated instability is lifted northward over the front likely producing one or two bands of convection that will move nwwd and pivot slowly into a rather sharp and strong mid level deformation zone over southern and central PA. Amazing to see such a strong 850 mb V wind anomaly in the form of a 4-5 sigma northeasterly wind during the middle of summer (outside of a tropical system transforming into an extratropical cyclone). That being said, the GEFS shows a 30-50 probability of 4 inches of rain falling in 48 hours across the Laurels, while some operational models indicate even higher totals near the extent of the Mason-Dixon line. WPC hydro outlook for Fri- Fri night is for a SLGT risk of excessive rainfall across the southern third of PA. Coordinated with regional WFOs concerning the possibility of posting a Flash Flood Watch, but we all agreed to hold off for 1-2 more model cycles to try to pin down a more precise area for the heavy rain, rather than broadbrushing a much larger area 36 hours out. Max temps Friday and Saturday will almost certainly be several Deg or more lower than most MOS guidance given the widespread light to moderate rain and a trajectory of the anomalously strong llvl wind from the New England Coast in contrast to the Mid Atl Coast. && .LONG TERM /SATURDAY THROUGH WEDNESDAY/... The trough rounds the base of a long-wave trough after it passes our longitude, and clears things out nicely. This should happen sometime Sat (most likely waiting until the afternoon or early evening in the southeast). Will linger POPs in the S for the day Sat, but go very dry and sunny/clear for the next 2-3 days. Scattered diurnal convection may then start to pop up - esp in NY state Tues/Wed. But, the risk of a shra/tsra in PA is very low until at least Wed as the humidity rises back to more- August like values. Highs Saturday will be only in the l-m 60s across the northern and wrn mtns, and a few spots may stay in the upper 50s. Temps should sneak into the lower 70s across the Susq Valley. && .AVIATION /10Z THURSDAY THROUGH MONDAY/... Widespread VFR expected for much of the overnight. Some fog possible late tonight, but clouds may limited fog to the southeast. Some chance of showers and storms on Thu, but nothing widespread. Used VCSH for now. More widespread showers and storms on Friday will be possible. .OUTLOOK... Fri...Scattered SHRA north with more numerous and heavier SHRA/TSRA across the south. Sat-Mon...No sig wx. && .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Lambert NEAR TERM...Lambert SHORT TERM...Lambert LONG TERM...Lambert AVIATION...Ceru/Martin is the U.S. government's official web portal to all federal, state and local government web resources and services.