Area Forecast Discussion
Issued by NWS State College, PA

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168 FXUS61 KCTP 200549 AFDCTP Area Forecast Discussion National Weather Service State College PA 1249 AM EST Fri Jan 20 2017 .SYNOPSIS... High pressure overhead will retreat and a warm front will push rain across the state on Friday. Relatively mild conditions will persist for the next several days. A very complicated storm system may bring a prolonged period of rain from Sunday into early next week. Snow or mixed precipitation is also possible from that same storm early next week. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... H5 ridge axis lies over the Allegheny Plateau late this evening. High clouds area increasing over eastern OH in the veering WSW flow aloft...while stable air along and east of the plateau promotes fog formation where low clouds had yet to fully scour this afternoon. The overnight will see an increase of clouds throughout, with pockets of near or slightly below freezing temps over mainly eastern and northern areas. Onset of precip expected after temps warm above freezing Fri morning. Mins will range overall from near 30F in northern cold spots to the mid 30s southwest && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SATURDAY/... Warm frontal precip moves in Fri morning just after sunrise in the Laurels and spreads a band of 3-6hr light rainfall across the entire area during the daylight hours. Timing is very solid amongst all models and forecasts. Those places which dip below freezing overnight will have a few hours of daylight to warm. Also, the wet-bulb effect could present a minute chance of it freezing up - mainly over the NE - but again, those areas should have the chance to warm up enough to keep the fzra threat low. Maxes/diurnal swing will be held down once again by the cool air trapped and not quite get into the 40s over a good portion of the area - with mid 30s the expectation in the NE and the central mtns. Even MDT and LNS may not eclipse 40F. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... The upper ridge is forecast to re-establish itself over the eastern U.S. over the upcoming weekend, bringing mild and mainly dry conditions Saturday, through at least the mid morning hours on Sunday. Daytime temperatures across most of Central Pennsylvania will be well above normal, but the departures for overnight lows should be greater by several degrees more throughout the entire extended period. The ridge axis will shift to the Hudson Valley and Eastern Seaboard early Sunday as a sfc backdoor cold front drifts swwd into PA and stalls out as a potent and moisture laden nearly stacked low lifts NE from the deep south to the Delmarva region by 12Z Tuesday. Temps could be marginal at the start of pcpn overnight Sunday for some ice or snow, and soundings via the GFS/GEFS and EC appear cold enough for Sunday night through Monday night for a mix of rain/snow (or even periods of all wet snow at times across the northern mtns of PA). However, the bulk of pcpn across the Central Ridge and Valley Region looks to be rain at this point through Monday afternoon. The trend in most operational models and ensembles has been further to the south and east with this particular late weekend/early week system. This places parts of the fcst area in the sweet spot for potentially several inches or more of heavy, wet snow (especially later Monday/Monday night across the nrn and wrn mtns of PA at elevations of 1400 ft or higher), as the mean 850 mb low in the GEFS and EC tracks NE along the I-95 corridor in VA and adjacent SERN PA/Southern NJ. U-wind anomaly at 850 mb peaks at -4 to -5 sigma during much of Monday as a 45-55kt easterly llj develops between this approaching low, and the aforementioned colder airmass pushing swwd from SERN Canada and the New England states. Rainfall amounts are expected to be in the 1 - 1.5 inch range across roughly the SE half of PA with lesser amounts to the NW. Another short wave ridge will slide east from the Ohio Valley and move over the Commonwealth Wednesday, accompanied by light wind and fair/generally dry conditions. && .AVIATION /06Z FRIDAY THROUGH TUESDAY/... 06Z TAFS sent. Some spots have low cigs and dense fog already, as strong cooling continues. Earlier discussion below. Satellite showing a few breaks in the low cloud cover at BFD at 03Z, which has resulted in fog development. GLMP MELD suggests some improvement is possible at BFD after about 07Z, as a southeast breeze develops, causing fog to disperse a bit. However, confidence in significant improvement is low and will continue to forecast IFR/LIFR conditions through dawn at BFD. Elsewhere, VFR conditions currently observed across southern PA at 03Z. However, a calm wind and mclear skies have allowed temps to fall close to the dewpoint late this evening. Thus, feel radiation fog will form over much of central/southern Pa between 04Z-07Z. Can`t rule out a brief dip to IFR visibilty before thickening mid and high clouds arrive late tonight, diminishing the fog threat. Based on latest SREF and downscaled NAM, believe IPT stands the best chance of significant vis reductions overnight. An approaching warm front will bring a period of rain and low CIGs to the entire region on Friday. Based on current model data, arrival time of rain should range from 12Z-13Z at JST to 17Z-18Z at IPT. The rest of central PA should see rain/low CIGs develop in between those times. The steady rain will exit the area between 20Z-00Z. However, low clouds/drizzle are expected to linger into the evening over most of central PA. Outlook... Sat...Low CIGs likely, mainly in the morning. Sun...IFR/MVFR CIGs likely. Rain advances fm S-N. Mon...Rain/low CIGs likely with snow possible central Mtns. Tue...AM rain/snow and low CIGs possible. && .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Dangelo/Lambert NEAR TERM...DeVoir SHORT TERM...Dangelo LONG TERM...Lambert/Gartner AVIATION...Fitzgerald/Martin

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