Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 290458 AFDCTP Area Forecast Discussion National Weather Service State College PA 1258 AM EDT Wed Mar 29 2017 .SYNOPSIS... An area of low pressure will move off the Mid Atlantic coast overnight. High pressure will build down out of southern Canada and bring cooler and drier weather for Wednesday and Thursday. A new storm system will approach for the end of this week with the potential for more inclement weather. && .NEAR TERM /UNTIL 8 AM THIS MORNING/...
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Drier air slowly filtering southward over central PA early this morning. Low clouds persist across central and northern areas in subsidence inversion and areas of fog have formed across the northern tier. Should see some of these start to break up towards daybreak but will need daytime mixing to more fully dissipate. Lows will drop to around freezing over the north, and range to the mid 40s near the MD border. This will still average some 5-10 deg warmer than normal.
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&& .SHORT TERM /8 AM THIS MORNING THROUGH 6 PM THURSDAY/... High pressure will bring a mix of sun and clouds for Wednesday over northern areas, and a bright sunny day over southern areas. Highs in the 40s and 50s will be near to slightly above normal. It will be breezy out of the north between 10 and 20 mph. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... A nearly north-south ridge of high pressure with dry air, and normal to slightly below normal 850 hPa and 925 hPa temperatures will be overhead Wednesday night, creating nearly optimal conditions to radiate strongly/cool off into the 20s to low 30s. The one factor to watch closely that could impact min temps will be the timing of thickening mid/high clouds that will be spilling over the top of the upper ridge just to our west. For Thursday...should the timing of thickening clouds occur during the morning (prior to the late March sun having much of a chance to warm the sfc) then the much cooler NAM temps (ranging from the 40s to around 50F in the south) may be in store for us, accompanied by a light southeasterly flow. 12Z GFS/EC and their respective ensembles are in good agreement with the onset timing of warm frontal rain that streaks quickly east from the Ohio Valley Thursday afternoon and evening. Evaporational/dynamic cooling of the airmass helps 925-850 mb temps dip to around or slightly below 0C across our NE counties Thursday night and Friday as the steadier light-mdtly heavy, Warm Advection rain begins there. Included mention of a rain/sleet and snow mix to the North and East of a KIPT and KSEG line (mainly across elevations AOA 1800 ft msl, where even a light coating of snow/sleet is possible by daybreak Friday). There`s a low prob for a light coating of snow on the high terrain across Sullivan County. Model consensus is also for a quasi Miller-B type of low track heading just north of the Ohio River during the day Friday, then a pressure jump and slight intensification of the sfc low (to 996 mb) off the southern NJ and New England Coast Saturday. Under normally colder winter-time circumstances, this low track would be pretty favorable for a significant snowfall across much of central and northern PA. However, a quite warm boundary layer (and 850 mb temps) to begin with, coupled with the lack of a stronger anchoring high over southeast Canada and New England will mean a widespread/soaking rain evening with 24-36 hour rainfall at the majority of places here in Central PA coming in between 0.75-1.00 inch. This rainfall will add a healthy spike to the flow and levels on smaller streams and creeks across the region with broader mdt rises on rivers, but no significant threat for flooding as headwater flash flood guidance is averaging 2-2.5 inches for a 12 and 24 hour period respectively. Periods of light rain, drizzle, and ridge shrouding dense fog will persist Friday night into Saturday as slightly colder air wraps around behind the storm. Some breaks in the cloud cover should occur Saturday afternoon, esp across the Lower Susq Valley where drying downslope flow will be enhanced. Forecast temps Friday may be a bit warm considering the aforementioned storm track just to our south, and amount of rain expected. Max temps Saturday will near to a few deg F above normal in many locations. High pressure building over the region (coupled with the offsetting effect of abundant sunshine and GEFS mean 850 mb temps cooling by 3-4 deg C imply max temps close to what we`ll see Saturday, which is upper 40s to lower 50s across the nrn mtns...and mid to upper 50s elsewhere. Dry and slightly milder conditions are in the forecast for Monday as the ridge of high pressure slides just to our east and ens mean 850 mb temps rebound by a few to svrl deg C (atop a light serly sfc flow). The next, rather potent and moisture laden southern stream wave heads our way for Mon night and Tuesday, bringing a likelihood of a widespread rain. && .AVIATION /06Z WEDNESDAY THROUGH SUNDAY/... The last of the showers are slowly drifting down over the SERN 1/3 of the CWA. HRRR shows this activity being gone by 03Z/11PM with drier air expected to begin filtering in for the remainder of the overnight. But considerable areas of MVFR and IFR will persist over western and northern mountains into the overnight (esp at BFD/JST), while central mountains gradually improves from MVFR to VFR overnight, and downslope flow brings the SE to VFR late this evening. Any lingering restrictions over the N/W will improve by mid/late morning, with widespread fair weather and VFR conditions tomorrow as high pressure builds in. NW winds will pick up to 10-20 mph during the day however. Outlook... Wed...early A.M. restrictions; becoming VFR. Thu...VFR/No sig wx. Thu Night-Fri...Sub-VFR restrictions/rain likely. Sat...Gradually improving conditions. Sun...VFR/No sig wx. && .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...La Corte NEAR TERM...Grumm/DeVoir/La Corte SHORT TERM...Grumm LONG TERM...Lambert/Gartner AVIATION...Grumm/RXR

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