Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 211020 AFDCTP Area Forecast Discussion National Weather Service State College PA 620 AM EDT Wed Jun 21 2017 .SYNOPSIS... A weak ridge of high pressure will slide south of Pennsylvania this morning. A trough of low pressure at the surface and aloft will push southeast across the region this afternoon, accompanied by a round or two of showers and isolated to scattered thunderstorms. Some of the storms could contain gusty wind and hail. However, most of the time with be fair and dry. A deep upper level trough will approach the region from the Northern Plains and Upper Great Lakes Region later this week and draw a plume of deep tropical moisture north from the Gulf Coast region. This will bring increasing showers and thunderstorms back to the state from late Thursday into early Saturday. Drier and cooler conditions will return later Sunday into next Monday. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... An area of high based strato cu and some altocu clouds was nestled within an area of relatively steep low-mid level lapse rates on the order of 6-6.5C/km. Lapse rates will increase markedly across the much of the region late this morning through the afternoon hours, as the primary upper trough axis across eastern Ohio drifts over the CWA and the left exit region of a potent 110 kt upper speed max (now over the Upper Midwest) drops sewd. SPC has painted much of the CWA (with the exception of the far NW counties) into a Marginal Risk for severe tsra this afternoon, and this looks quite reasonable given moderately strong deep layer bulk shear values, meso-B scale lift via the left exit region of the aforementioned upper level jet, and decent llvl convergence along and ahead of a sfc trough migrating SE across the state. Relatively high convective LCLs/cloud bases will help to produce isolated and brief, strong downburst winds in the 18-23Z window. Basin average rainfall today should stay under 0.20 of an inch. A push of low PWAT air in the wake of this sfc/upper level disturbance should result in a dry afternoon across NW PA. Max temps this afternoon will be in the mid to upr 70s across the mtns, around 80F across the Central Mtns, and near 85F across the Lower Susq Valley. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM THURSDAY/... Convection dies down pretty quickly early this evening, and initially mainly clear skies and quickly diminishing wind (combined with wet damp ground from one or two showers/TSRA in many areas) will lead to areas of valley fog during the early- mid morning hours. Lows early Thursday will be on the chilly side and in the 40s across the NW mtns, while the rest of the CWA will see a min temp in the 50s to lower 60s. Thursday starts out partly cloudy and comfortable in the temp and humidity Dept as a bubble of high pressure slides SE and over us for late morning. The low level flow will become swrly during the afternoon and evening hours Thursday, and another flat upper trough (and sfc warm front) will move east across the region and trigger showers and scattered TSRA over the Central and Southern portion of PA. PWATs will increase from below normal and only 15-20 mm this morning, to over 40 mm (or 1-2 sigma) by 00Z Friday just to the west of the approaching warm front. && .LONG TERM /THURSDAY NIGHT THROUGH TUESDAY/... The medium range flow pattern looks to be mainly dominated by a broad quasi-zonal flow between the subtropical ridge off the SERN US and a broad low over SRN Canada. By Friday the tropical airmass will have re-established itself with dewpoints well up into the 60s and lower 70s over the entire region making for a sticky end of the work week. Friday is also the trickiest part of the extended rain-wise as moisture from newly minted Tropical Storm Cindy in the Gulf gets funneled up the west side of the SERN upper ridge. The question remains how far north this moisture can get before being shunted east under the fast quasi-zonal flow over the northern US. As it stands, it looks like the best chance for widespread showers/thunderstorms will be during the day Friday into Friday night. The 12Z guidance suggests the heaviest rains stay south of our forecast area, but being days away yet, need to be monitored. The GEFS brings drier air in by early Saturday and even shows some below normal PWAT air in place by Sunday. The zonal flow is forecast to become more cyclonic during the weekend, which warrants the mention of small chance POPs, but less humid with just some spotty pop-up showers. By early next week there is disagreement between the broad cyclonic flow aloft in the GFS/GEFS and a stronger upper trough/low advertised in the ECMWF for later Monday into Tuesday. As of this cycle we are keeping an optimistic forecast to start next week. After a couple of warmer than normal days early in the period, temperatures are forecast to return to slightly below normal by the weekend lasting into next week. && .AVIATION /10Z WEDNESDAY THROUGH SUNDAY/...
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VFR conditions prevailing across Central PA in all but a few spots across the north this morning. A weak ridge of high pressure will slide south of Pennsylvania this morning, replaced by a weak trough of low pressure. This feature is being represented by some meandering showers north of I-80 at 10z. Impacts of this trough this afternoon will shift southward, with isolated to scattered showers/tstms developing which may bring brief gusty winds and localized reductions. Otherwise it will remain VFR today with a SW wind picking up to around 10 mph gusting to 15-20 mph. Any convection dies down pretty quickly by early evening, and initially mainly clear skies and quickly diminishing wind will likely lead to areas of valley fog during the overnight hours (mainly where rain falls today). Warm front lifts across the Ohio Valley Thursday, triggering showers and scattered TSRA over the NW half of CWA. .OUTLOOK... Thu...Sct SHRA and Iso TSRA N. Fri...Cig restrictions likely west. SHRA likely / sct TSRA. Sat...Chance for SHRA/TSRA south. Restrictions poss early. Sun...Mainly VFR. Chc SHRA/TSRA.
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&& .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Lambert NEAR TERM...Lambert SHORT TERM...Lambert LONG TERM...La Corte/Lambert AVIATION...RXR is the U.S. government's official web portal to all federal, state and local government web resources and services.