Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 181137 AFDCTP Area Forecast Discussion National Weather Service State College PA 737 AM EDT Wed Apr 18 2018 .SYNOPSIS... A weak ridge of high pressure at the surface and aloft will slide east across Pennsylvania today. A rather compact, but potent surface low and upper level trough will move east from the Ohio Valley with the surface low likely crossing Southern Pennsylvania tonight and early Thursday. Periods of rain or showers will occur across much of Central and Southern PA tonight, while a mixture of rain and wet snow falls throughout the valleys of northern PA. A few inches of snow will accumulate at higher elevations along the PA/NY border late tonight and Thursday morning. Gusty west to northwest winds will impact the southern half of the commonwealth beginning during the early to mid morning hours Thursday. Wind gusts of 35 to 45 mph will be common near and to the south of the Route 22/322 corridor, while lighter winds will be found north of Interstate 80. Blustery northwest winds and unseasonably cold temperatures will persist Thursday night and Friday morning with occasional rain and snow showers in the valleys and snow showers with light accumulation across the mountains of Northern and Western PA. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... The cold and gusty northwest flow that brought a feel of mid winter temps and snow showers to the region Tuesday, will relax (briefly) for later this morning through around dusk. The extensive albeit shallow and rather warm strato cu deck, still producing some scattered light flurries early today, will gradually dissipate/retreat north later this morning through the mid afternoon (Central and West) and until close to sunset over the east. Despite a very cold April to date, the growing season has technically begun over the lower Susq Valley. With lows around freezing or even a few deg colder early today, a Freeze Warning is in effect through 12Z for the counties from Franklin east to Lancaster and north to include Dauphin and Lebanon. Elsewhere the this morning`s lows will average about 25-30 deg. The lighter wind and at least several hours of sunshine today will allow temps to rebound sharply into the 55-60F range across the valleys of central and southern PA respectively, while maxes reach the upper 40s to low 50s across much of the northern and western PA. These noticeably milder readings will still be about 5 or 6 deg F shy of normal high temps for the date. A thick layer of mid clouds will advance quickly east across the state late today and early tonight and gradually lower overnight as the nose of a potent, 3 sigma (45-55kt) 850 mb swrly jet heads our way from the Ohio River Valley. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/... The meat of the active weather (until at least early next week) occurs roughly within this 24 hour period, though Thursday night and Friday morning won`t afford us with any big improvement. The action will come in the form of a vigorous sfc low, sharp upper trough and strong core of winds through a deep layer - near and just to the south and west of the surface low track. 00Z Operational models, ensembles and hourly high res model guidance has shifted the track of this low (that will impact our region tonight into Thursday) a little further south. This slight shift in the track now brings the potential for a few to several inches of snow accumulation tonight and Thursday across the northern tier of PA. Initial weak to mdt isentropic lift and borderline Wet Bulb temps in the 920-850 mb layer will lead to a rain/wet snow mix to start up north late this evening, while notably warmer Wet bulb temps across the Central and South mean just plain rain for the first half of the upcoming event. During the early to mid morning hours Thursday across the north (and mid to late morning hours across the Central Ridge and Valley Region of PA) Wet Bulb temps throughout the boundary layer gradually slide to below zero, which will support a changeover to all snow at elevations AOA 1500 ft MSL, while the deeper valleys see the rain mix with or even fall as wet snow at times (with no accum expected in the valleys). Periods of stronger FGEN forcing beneath the left exit region of 90+ KT SWRLY jet AOA 500 mb will likely lead to some mesoscale areas or narrow East-West oriented bands of heavy rain/snow across the northern third of PA between about 05-11Z Thursday. Painted a general 1-3 inches of snow across the Northern Tier counties of PA, with a sharp gradient to nearly nothing through 12Z Thursday near and to the south of Interstate 80. In addition, CSI banding and some slantwise-wise low-topped TSRA are likely near and just to the north of I-80 for a 2-3 period tonight as the nose/axis of the southerly LLJ passes across Central and Southern PA, and lifts the much milder and moist air up and over the cold air dome across the Central and Northern Mtns. We`re seeing this same feature leading to quite active TSRA and TSSN with intense snowfall rates across Northern Iowa and Southern Minn early today. Granted, this area of Instability will weaken by the time it gets here, but there will likely be some fireworks by mother nature tonight with the cold rain/wet snow across the northern half of PA. Will also have to watch for a 1-3 hour burst of snow across the Endless Mtns NE of KIPT and the Western Poconos southwest of KHZL between about 12-16Z Thursday. Boundary layer temps should be a bit warmer in that area after sunrise, so accums of the wet snow should be limited to an inch or less. The other very notable aspect will be the quick onset of quite strong westerly, then NW winds in the tight pressure gradient across Southern PA and associated subsiding left entrance region of the aforementioned strong mid-upper level jet. Latest High Res Ensemble Forecast guidance paints a belt of 40-50 kt westerly winds at 850 mb across the southern 1-2 layers of counties (beginning across the Laurels about 05-06Z Thursday, then expanding east to the Lower Susq Region by 07-09Z Thursday. As the deeper, colder air moves into the Laurels around daybreak Thursday (10Z), 55 to 60kt winds at 850 MB will likely be mixed down to the surface in gusts around 40 KTS (if not 5 kts or so higher). The wet ground from Sunday nights 1-2 inches of rain may mean some downed trees from the strong winds. Will allow the upcoming day shift to better assess the degree and extent of this potential strong wind event before issuing a 2nd period Advisory or High Wind Watch. Thursday will appear very similar to the mid winter-like conditions we just experience yesterday (Tuesday) with gusty northwest winds of 25 to 35 mph, and frequent snow showers with localized 1-2 inch (additional) accums possible across the NW Mtns and Laurel Highlands. Min temps tonight will range from 30-32F across the far north, to the mid and upper 40s - near or just south of the sfc low track and brief warm sector. Highs Thursday will struggle to reach the mid to upper 30s across the high terrain of the north and west, while the Central and Southern valleys nose into the mid 40s to mid 50s respectively during precip-free periods. Any rain/wet snow showers racing through the ridge and valley region Thursday will quickly drop temps by 6-10 deg F in the span of several minutes. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... Shallow stratocu and flurries will persist across the NW half of the state Thursday night, with partly cloudy skies in the SE. Winds should stay up enough through daybreak Friday to prevent a frost or freeze in the SE zones, while lows elsewhere dip into the mid 20s to low 30s. Friday and beyond, there will be a welcome break of lighter wind, sunshine and milder temps as a large area of high pressure moves out of the midwest. Yet another upper low in this busy pattern looks to drop into the deep south toward the end of the period. This may spread some precipitation northward toward our region for the beginning of next week. && .AVIATION /12Z WEDNESDAY THROUGH SUNDAY/...
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MVFR ceilings across the northwest 2/3 of the airspace Wednesday morning will be slow to improve to VFR. Meanwhile, sct-bkn low VFR ceilings will prevail across the southeast 1/4. The next system moving quickly east from the Midwest through the OH Valley will bring rain into the airspace 00-04Z Thu with MVFR restrictions a good bet across the western 1/3. Colder air wrapping into the backside of the system will turn rain to snow across the northwest airspace into early Thursday morning with IFR likely at KBFD and possibly KJST later Tuesday morning. Wind gusts >30kts are possible across the southern 1/2 of the airspace Tuesday. Rain/snow will taper to snow showers over the western 1/3 Thu night with sub-VFR restrictions lasting into Friday. Outlook... Fri...MVFR cigs nw 1/3; VFR elsewhere. Sat-Sun...VFR/no sig wx.
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&& .CLIMATE...
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A daily record snowfall of a trace (T) was set at Harrisburg on April 17th. This tied the old record of T snow set in 2001, 1951, 1936, 1929, 1928, 1911 and 1905.
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&& .CTP WATCHES/WARNINGS/ADVISORIES... Freeze Warning until 8 AM EDT this morning for PAZ036-057-059- 063>066. && $$ SYNOPSIS...Lambert NEAR TERM...Lambert SHORT TERM...Lambert LONG TERM...Lambert/Gartner AVIATION...Steinbugl CLIMATE...Steinbugl

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