Area Forecast Discussion
Issued by NWS State College, PA
Issued by NWS State College, PA
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266 FXUS61 KCTP 210535 AFDCTP Area Forecast Discussion National Weather Service State College PA 135 AM EDT Sat Sep 21 2024 .SYNOPSIS... A ridge of high pressure through our region will make for another pleasant afternoon and evening. An approaching warm front on Saturday will bring scattered showers and thunderstorms, a few of which may be severe. Drier weather returns for Sunday, before an approaching trough brings the next chance of rain from late Sunday night through midweek. && .NEAR TERM /UNTIL 8 AM THIS MORNING/... Patchy mid and high clouds moving across the state early thus morning; however, patchy valley fog is still possible mainly across the Susquehanna Valley into the early morning hours on Saturday. Currently coverage of any lowering visibilities or lower cloud ceilings is sparce at best. Surface winds tonight out of the south southeast will bring increased strato cu into the eastern PA overnight while an approaching shortwave to the west will allow for building stratus over our western zones. && .SHORT TERM /8 AM THIS MORNING THROUGH MONDAY NIGHT/... A warm front approaching from the west will struggle to push east into the cooler, more stable easterly flow over east central PA through the day on Sat. Nevertheless, the approach of the left exit region of a relatively strong upper level jet on Saturday will combine with moderately high sfc dewpoints in the low to mid 60s (with corresponding PWAT of around 1-1.25 inches) to spark scattered showers/TSRA first through the Alleghenies and northwest mountains Sat morning, then expanding eastward into the afternoon. Given the progged 1000+ J/kg of SBCAPE and NWrly 0-6 km shear near 40 kts, there is a MRGL risk for severe weather, mainly in the form of localized damaging straight line wind gusts and hail during the late afternoon/early evening hours across west central PA. Although tornadoes are not mentioned in the SPC outlook, given substantial veering of the albeit weak winds in the lowest 1 km and LCLs less than 1000 m, a brief, weak tornado can not be ruled out where there is sufficient instability (Laurels and northwest mountains). In general, forecast QPF is less than 0.50 inch, but localized higher amounts are possible in thunderstorms, and amounts near 0 certainly possible for places that get missed given the scattered nature of the convection. No flooding issues expected given how dry it has been over the past month. The timing of cloud cover in the morning hours Saturday will play a significant role in the max temps and amount/eastward extent of CAPE to fuel the convection in the afternoon and evening. Early cloud cover (especially if the clouds move in right around sunrise) will greatly flatten the temp curve and subsequent intensity of convection that develops. Model guidance indicates the shortwave and weak surface low pass east of the region Sat night, then a cold air damming scenario begins to develop between a stalled warm front along the OH/PA border and high pressure over the Canadian Maritimes. Forecast soundings indicate an upsloping southeast flow will result in persistent stratus and perhaps a bit of fog or drizzle over the Central Mtns early Sunday morning, with partly sunny skies possible over Eastern PA. The cloud cover and flow off of the Atlantic should result in markedly cooler afternoon temps Sunday than on Saturday, with highs likely stuck in the 60s over the Central Mtns. A weak shortwave and associated plume of higher pwats overrunning the warm front could produce scattered showers late Sunday night into Monday. && .LONG TERM /TUESDAY THROUGH FRIDAY/... Upper trough energy and wavy frontal systems will be kicked from the Plains/Midwest into the Great Lakes/Ohio Valley and northern Mid Atlantic next week. An associated threat of rain/showers will accompany these systems with some potential for heavier rainfall within an amplifying mid/upper level upstream flow pattern during the second half of next week. Given model variability/growing uncertainty by later next week, we did not identify any actionable targets of opportunity and therefore weighted forecast basis heavily toward the National Blend of Models (NBM). These data indicated a modest cooling trend from the 9/21 weekend warmth trending closer to climo (near or slightly below average) for daytime highs in the 60-70F range. Meanwhile, overnight low temps continue to run above the historical average for late September in the 50-60F range. && .AVIATION /06Z SATURDAY THROUGH WEDNESDAY/... Looking at just some mid level clouds overnight for the most part. Some lower CIGS and fog possible across the east for a short time, especially IPT. A few showers and storms possible today, mainly across the west, where the combination of instability and forcing will be better. Left storms out for now, later shifts can time them better, once something starts to form. Earlier discussion below. Meanwhile, a warm front transitioning to an occluded front approaching from the west will trigger scattered SHRA and TSRA across mainly west central PA, pushing eastward into the evening. As the frontal system moves east tonight, areas of fog and -DZ will be possible across central and western PA into Sun morning. Some guidance also hints at MVFR cigs sticking around for much of the day on Sunday for portions of central PA in upslope SE flow. Outlook... Mon-Wed...Scattered showers. && .CLIMATE... The first day of autumn 2024 (Fall Equinox) begins at 8:44 am EDT on Sunday September 22nd. && .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Colbert/Bowen NEAR TERM...Colbert/Tyburski SHORT TERM...Lambert/Colbert/Bowen LONG TERM...Steinbugl AVIATION...Martin/Gartner CLIMATE...Steinbugl/NPB