Area Forecast Discussion
Issued by NWS State College, PA
Issued by NWS State College, PA
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270 FXUS61 KCTP 200614 AFDCTP Area Forecast Discussion National Weather Service State College PA 214 AM EDT Sat Jul 20 2024 .SYNOPSIS... An upstream shortwave over the Lower Mississippi Valley will track northeast across Pennsylvania late Saturday. High pressure will build into the state behind this feature Sunday, then an advancing warm front will stall out in the vicinity of Pennsylvania for much of next week. && .NEAR TERM /UNTIL 6 AM THIS MORNING/... Late evening satellite imagery shows clear skies across Northern PA, where a high pressure ridge and low-pwat air resides. Some of the coolest mesonet observations in this area have already dipped into the upper 50s as of 02Z. Further south, sct-bkn cumulus are noted over Southern PA, associated with low level moisture advection and modest height falls ahead of an upstream shortwave over the Miss Valley. Can`t rule out an isolated late night shower across the southwest part of the forecast area, as large scale forcing increases ahead of this feature. Efficient radiational cooling and warm water temps will likely result in late night fog in the deep river/stream valleys of Northern PA. Elsewhere, increasing cloud cover should limit the fog potential, despite increasing low level moisture. Morning lows on Saturday should range from around 50F in the coolest valleys of the NW Mtns, to the upper 60s across the Lower Susq Valley. && .SHORT TERM /6 AM THIS MORNING THROUGH 6 PM SUNDAY/... The focus Saturday will be on the upstream shortwave, which is progged to lift northeast across the Central Appalachians late in the day, then pass over Central PA during the evening. Model RH profiles indicate warm advection aloft ahead of the shortwave will spread of veil of cirrus across all but perhaps the extreme northwest part of the forecast area, which will limit diurnal heating. However, a southerly flow and associated low level moisture advection, combined favorable large scale forcing beneath the right jet entrance region, should support scattered convection by afternoon. The focus of the showers and isolated tsra should be in the vicinity of a weak surface trough running along the spine of the Appalachians from Somerset County northeast to Sullivan County. Latest ensemble output suggest most places will receive less than a tenth of an inch, with isolated pockets exceeding 0.5 inches possible. Weak shear and very modest instability will almost certainly preclude any severe weather or flooding concerns Saturday. Passage of the shortwave should result in any showers ending by late evening over the northeast part of the forecast area. High pressure building into the state behind the shortwave should bring clearing skies and light winds late Sat night. Latest SREF/NAMNest indicate the likelihood of patchy late night valley fog, mainly in the favored deep river/stream valleys of the Alleghenies, but also in any spots the receive rain Sat PM. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Fair and warm weather with light wind and moderate humidity is expected Sunday, as surface ridging and associated low-pwat airmass builds across the state. Ensemble mean 850mb temps near 17C supports max temps ranging from the low 80s over the N Mtns, to around 90F across the Susq Valley. Medium range guidance all point to a showery period with high humidity and more clouds than sun for much of next week, as an advancing warm front stalls out in the vicinity of PA. The 500mb pattern features upper ridging off of the East Coast and a persistent trough over the Grt Lks. A deep southwest flow at the interface of these features will import GOMEX moisture northward, interacting with the front to promote multiple days of diurnally enhanced showers/thunderstorms and potentially beneficial rainfall. The southern tier of central PA could really use some rain to lessen the impacts of moderate to severe drought conditions (D1-D2 drought monitor). Ensemble plumes indicate highs Monday will be several degrees above average with only a limited chance of PM convection across primarily Southern PA. After that, plenty of cloud cover and more numerous showers/tsra should hold daytime temps to near seasonal norms. Surging pwats along the frontal boundary should translate to very warm/muggy nights. Model guidance begins to diverge toward the end of the forecast period with some models tracking the upstream trough north of PA, which pushes the front south of the area and introduces drier weather by Friday. However, the latest GEFS/EPS look a bit less progressive, suggesting the fropa and return to drier weather takes until Saturday. && .AVIATION /06Z SATURDAY THROUGH WEDNESDAY/... High cirrus will be on the increase overnight as an upper level disturbance moves north out of the southern Appalachians. Most airfields will remain VFR through the night, although patchy valley fog is expected once again in the northern valleys. There is some signal for MVFR cigs at BFD closer to sunrise (after 10Z), thus have shown some deterioration in this timeframe with low-end MVFR to high-end IFR vsbys in the 10Z-12Z Saturday timeframe with moderate (50-60%) confidence. Scattered rain showers and potentially a few areas of stratiform light rain will develop early Saturday and continue throughout the afternoon and evening hours mainly east of BFD/JST. A few thunderstorms are possible as well, but low CAPE will be a limiting factor. After SHRA tapers off, winds are expected to become light with clearing skies potentially allowing for fog formation Saturday night into Sunday morning. Outlook... Sun...Primarily VFR. Patchy late night/early morning fog and low ceilings are possible. Mon...Primarily VFR. Patchy early morning fog possible. Scattered afternoon SHRA/TSRA. Tue-Wed...More widespread PM SHRA/TSRA could lead to at least brief restrictions. && .CLIMATE... The average temperature from July 1 to July 18 at Harrisburg is 81.6F which is the warmest July month-to-date. && .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Fitzgerald NEAR TERM...Fitzgerald SHORT TERM...Fitzgerald LONG TERM...Fitzgerald/Steinbugl AVIATION...Colbert/NPB CLIMATE...Steinbugl