Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 252315 AFDCTP Area Forecast Discussion National Weather Service State College PA 715 PM EDT Tue Jul 25 2017 .SYNOPSIS... A large area of high pressure will slide southeast from the Great Lakes to blanket most of the Middle Atlantic Region with an airmass consisting of slightly below normal temperatures, comfortably low humidity, and abundant sunshine for most places on Wednesday. Low pressure and an attendant cold front will approach from the Ohio Valley for later Thursday into Friday bringing a few periods of showers and scattered thunderstorms. Another refreshingly drier and cooler airmass will overspread the commonwealth this coming weekend. && .NEAR TERM /UNTIL 6 AM WEDNESDAY MORNING/...
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Some sunshine at times, but still a lot of clds. Even a hint of perhaps some drizzle across the far northeast. Adjusted cloud grids some. Earlier discussion below. Low level thermal trough was situated from northcentral PA into the New England states, while warmer air was advecting into the region in the mid levels. This increasingly stable vertical thermal profile will help to maintain plenty of mainly shallow strato cu with sprinkles scattered across the central and nrn mtns. A few brief showers could drop a quick 0.01 or 0.02 of an inch, but that will be about the extent of it in the QPF dept until Thursday afternoon or night. The clouds should slowly break from west to east (but temporarily in some places) late this afternoon. High pressure will gradually build in from the Great Lakes Region late today and tonight, as the coolest air aloft shifts east of the CWA with llvl downsloping helping to dry out the boundary layer SE of the I-99/RT 220 corridor. Across the east, an inverted sfc trough/marine layer boundary will be located mainly to the east of the Susq Valley through early tonight, bringing with it a slight enhancement to llvl theta-e convergence and the chc for a brief/light shower in some spots. Skies should become mostly clear across the western mtns for most or all of the overnight, and clear out temporarily across the Central Mtns through around midnight. Afterward, the HRRR looks quite realistic is pushing a layer of 1-2kft agl stratus/stratocu wwd across the Susq Valley between 05-08Z Wed and reaching a KELZ to KUNV and KMRB line around or shortly before sunrise. Prior to the arrival of the low clouds, a 20-25F air/stream delta T will lead to patchy valley fog (mainly across nrn PA) where min temps will dip into the 50-55F range. Elsewhere, mins early Wednesday will upper 50s to lower 60s.
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&& .SHORT TERM /6 AM WEDNESDAY MORNING THROUGH 6 PM WEDNESDAY/... High pressure will build over the region Wednesday, bringing clear to partly cloudy skies and dry conditions across the NW mtns and Laurel Highlands. There is some concern that whatever lower clouds spread wwd into the Susq Valley and Central Mtns may have a difficult time eroding through the morning hours under very light NE to SE breeze, and a stable llvl profile with subsidence aloft placing a lid on any appreciable vertical mixing. Something to watch closely, but not to jump into with both feet just yet. For now, we`ll play the middle road and have the morning clouds across the east transition into a mix of sun and sct-bkn flat strato cu for the afternoon across the Susq Valley and points east. Highs Wednesday will be slightly below normal across the eastern half of the CWA, and near normal across the Western Mtns. && .LONG TERM /WEDNESDAY NIGHT THROUGH TUESDAY/... Increasing dual to multi-layer clouds advect east into the state later Wednesday night through Thursday morning with little more than some scattered and brief showers moving East to NE. Deep layer warm advection and a return to a notable higher PWAT airmass will occur in advance of a cold front and another upper trough headed our way from the Great Lakes region. Anomalous PWATs on a Sw to NE axis will top out between 1.5-1.75 inches over the state Thursday afternoon and night before likely settling just south of the state on Friday. It appears that a highly anomalous North-Northeast flow in the sfc-850 mb layer may be sufficiently strong to push much of the precip (forming beneath the left exit region of a potent upper level speed max) to the southern half of PA. Precip chcs and QPF display a distinct north-south gradient for Friday from under 40 percent just NW of KBFD to over 70s percent in the far SE near or just north of the path of a 1002-1005 mb sfc low. Some 12Z model solutions linger the precip accompanying this wave of low pressure into the first part of Saturday as weak high pressure building over Lake Michigan (and the surge of drier air) may be too far west to nudge the baroclinic zone to our east. Conditions eventually dry out later Saturday and Saturday night as high pressure builds in. Sunday and Monday appear dry at this time. && .AVIATION /23Z TUESDAY THROUGH SUNDAY/... Axis of a mid and upper-level trough overhead late this morning will lift off to the east as the afternoon progresses. At the surface, high pressure already building in from the west will take control through midweek. Dealing with a slow breakup of the lingering post-frontal NW flow stratocu cloud cover as a SE building surface high squeezes moisture under a strengthening and gradually lowering subsidence inversion, creating low-end VFR Cigs over the west/north (and even a few light showers around BFD) and much higher based/VFR stratocu across the central mtns and Susq Valley. Winds will be from the NW around 10mph early, shifting to the north at 5-10mph late today/early tonight before diminishing to light NE or easterly in direction. Could see a return of restrictions with a stratus/stratocu deck (between 1-2 kft agl) advecting westward from the Susq Valley to the Central and Northern mountains late tonight (and maybe a lower stratus deck edging into the Susq Valley from the east). Elsewhere, we`re only expecting some valley fog to develop tonight as the air/stream water temp difference approaches 25F (very favorable for fog formation under initially clear to ptly cloudy skies with light wind). This fog will likely impact several terminals toward sunrise Wed with MVFR to possibly IFR. Otherwise, the midday and afternoon hours of Wednesday should bring good improving flying weather with VFR conditions and light winds turning around to the east then south as high pressure slides off to the east. Numerous showers and isolated-scattered thunderstorms return for Thursday afternoon and night. .OUTLOOK... Fri...Scattered showers north (mainly in the morning), with more numerous and heavier showers and scattered TSRA across the south. Sat and sunday...No sig wx. && .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Lambert NEAR TERM...Lambert/Martin SHORT TERM...Lambert LONG TERM...DeVoir/Lambert AVIATION...Lambert/RXR is the U.S. government's official web portal to all federal, state and local government web resources and services.