Area Forecast Discussion
Issued by NWS State College, PA

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382 FXUS61 KCTP 251153 AFDCTP Area Forecast Discussion National Weather Service State College PA 753 AM EDT Sat Mar 25 2017 .SYNOPSIS... A cold front will sag southward across Pennsylvania today and tonight, before returning north as a warm front late Sunday and Sunday night. Very mild conditions are expected early today and again Sunday night into Monday as we enjoy the warm sector south of the frontal boundary. However...thickening clouds and southeast flow are expected on Sunday as the boundary sags southward as a backdoor cold front...bringing cooler conditions and eventually some patchy drizzle or light rain. Unsettled conditions will last from late Sunday through Tuesday as a series of disturbances interact with the front. No heavy rain is expected during this timeframe however. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Weak surface frontal boundary is along the Pennsylvania...New York border early this morning, and will be drifting slowly to the south through this afternoon. The cold front should make it to northern Virgina and West Virginia before stalling out once again. Warm temps will persist this morning. However, the warmup will be short-lived, as high pressure building into New Eng pushes a backdoor cold front through central Pa during the mid afternoon and evening hours. A good deal of cirrus will hold temps down a bit, but even just mixing to 925mb supports max temps in the upper 60s and low 70s south of I-80. A earlier arrival of the front by just a few hours will result in a cooler and cloudy day across the northern tier counties, where southeast flow and nearly saturated soundings in the sfc- 850mb layer suggest patchy drizzle is possible. Further south, there could be scattered afternoon showers in the increasingly warm and unstable air over southern Pa...but most will stay dry. Scattered light showers and areas of drizzle will be along and just to the north of this boundary today into this evening. The locations most likely to receive some rain today and tonight will be the northern mountains of the state. Expect less than one tenth of an inch of rain today in most places as short range/high res models indicate that there will be a shallow dry layer between the southward advancing stratus and stratocu...and the alto cu and cirrus deck. SREF`s prob for LIFR stratus ceilings behind the front will be limited to primarily the mountains to the North and Northeast of KIPT. Areas farther south stand to see the least cloud cover. Temps around sunrise today should range from the upper 30s to mid 40s in the east, to the very mild mid 50s to lower 60s across the region near and to the west of the RT 99 corridor. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SUNDAY/... Frontal boundary sags back against the Alleghenies of western PA, pushed by a high pressure area sliding SE across S Ontario on Sunday as a low approaches from the Ohio Valley. This will advect cooler and more moist llvl air into the region. Light showers...with areas of drizzle/ridge top fog are likely through much of the day Sunday. Local experience causes us to lean greatly away from blended model guidance for max temps in such llvl cold air damming situations, as warmer outliers and the mean temp displayed by such blends/ensembles typically masks the amount of shallow cold air that gets funneled southwestward through the deep valleys of Central PA. Went primarily with the latest...25/06Z NAM blended about 25 percent of the way with the Nat`l Blend of Models. Forecast NDFD Temps Sunday will be on average about 8-10 deg F below NBM guidance (which also collaborates well with neighboring offices and continuity). && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/...
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The main precip event impacts central PA Sun night into early Mon as low moves through. QPF looks to be around 0.50 inch. Above average temperatures (especially nighttime) appear very likely through early next week, as GEFS indicates PA remains beneath a mean upper level ridge and plume of anomalous PWATs. The chance of showers again spikes with passage of next shortwave Tuesday into Wednesday and another one to start the weekend. U.S. 25/00Z and 06Z GFS and GEFS differs drastically with the 25/00Z EC and its ensembles during the second half of the week, as the EC guidance is dominated by stronger southern stream troughs and associated northward advection of warmer air and higher 500 mb heights. In contrast, the GFS and its ensemble mean appears to maintain a several decameter deeper and more persistent upper low over nrn New England and the Canadian Maritimes, while also latching onto a piece of northern stream energy that breaks away from a potent upper low that will be digging SE across the Four Corners area Tuesday, before heading east across the High Plains of E New Mexico and West Texas later Wednesday and Wed evening. The presence of this aforementioned northern stream short wave has big implications for maintaining/reinforcing colder NW flow aloft across southern Canada and the NE U.S., while shearing its initially potent southern stream "parent" low harmlessly east (for us here in Central PA) across the Carolinas. Considering the several-run persistence in the GFS, along with the amount of snow cover across intact parts of the nrn Mid Atlantic and New England States, (not to mention the persistence of cold air there and points north across Ont and Quebec), I slightly favor the GFS/GEFS solution at this point in time with a more extended period of drier and cooler conditions for Wed- Sat. The northern edge of the precip shield associated with the shearing srn stream wave and mdtly strong forcing via the R.E. region of a 100-120 kt upper jet across the Lower Great Lakes and Mid Atlantic region may support a period of cold rain...mix...or even a light accumulation of snow across the southern half of PA late in the week. The "Less likely" EC solution (at this point) would help to develop a stronger upper ridge across the eastern U.S. for the second half of the week with the upper low lifting NNE to the upper midwest by 00Z Sat with warmer air at the surface and aloft surging northward through the Ohio River Valley and Appalachians.
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&& .AVIATION /12Z SATURDAY THROUGH WEDNESDAY/...
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Frontal boundary just north of the PA/NY border early this morning will be pushed southward through the airspace toward the PA/MD line by 25/00z as strong high pressure builds over eastern Canada. Increasing low level moisture and wind shift to the east should translate into a lower trend in ceilings and visibility from north to south with widespread MVFR to IFR restrictions likely by tonight into Sunday. Scattered light rain showers will be possible today especially near the southward-moving frontal zone with pockets of fog/drizzle overnight into early Sunday morning. The front should lift back to the north on Sunday which should allow for at least some marginal improvement especially over portions of the southern airspace. Outlook... Sun...VFR-MVFR western 1/4 with low risk for thunderstorm in the evening. MVFR-IFR central and east with drizzle/fog. Mon-Tue...Sub-VFR with periods of rain. Wed...Becoming VFR. No sig wx.
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&& .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Lambert NEAR TERM...Lambert SHORT TERM...Lambert LONG TERM...Lambert/RXR AVIATION...Steinbugl

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