Area Forecast Discussion
Issued by NWS State College, PA
Issued by NWS State College, PA
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283 FXUS61 KCTP 222322 AFDCTP Area Forecast Discussion National Weather Service State College PA 722 PM EDT Sun Sep 22 2024 .SYNOPSIS... Welcome to Autumn! An amplifying upper level trough and slow moving frontal boundary headed our way from the midwestern states will bring several periods of rainy conditions across Central Pennsylvania beginning Monday morning and lasting through at least Wednesday. && .NEAR TERM /THROUGH MONDAY/... A slightly anomalous SSE low-level flow of moist air was supporting a broad area of upslope stratus over much of the Central mtn and Susq West Branch region between routes 219 and Route 11/15. The leading edge of any rain reaching the ground off to our west was near a KCLE to KZZV line in Ecent Ohio with weak echos further east representing some light virga. 23Z temps across the CWA were mainly in a 5 degree rain in the mid to upper 60s, though locations near and to the west of KBFD and over the Lower Susq Valley were hanging onto L70s where late day sunshine was seen. These latter, warmer locations will likely flip this evening/greatly tighten the temp spread under better radiational for several hours before thicker, layered clouds advect east over the region. Temps should again stay much warmer than normal tonight. Most places will be at least 10F above normal. We`ll hold off for now mentioning drizzle for the overnight, but there could be a patch or two on the windward slopes of the ridges. Similar to the last system, the approaching sfc frontal system runs into the CAD in place over Central PA and stalls. Moisture aloft and forcing keeps on flowing overhead, though and rain/showers should get into the western zones around or even a little before sunrise. && .SHORT TERM /MONDAY NIGHT/... The plume of moisture overhead continues eastward and hangs over the CWA most of the day on Monday. Heavy rain signal is not there, but there might be a thunderstorm or two, more probable in the SW than E. Temps on Monday won`t go more than 10F above the morning numbers. Some places may not get much more than a 5F deg diurnal swing. The 5H ridge axis doesn`t go east of our longitude, so the showers might not get to the far eastern reaches of the CWA early Mon night before fizzling. Another minor surge of WAA and moisture aloft arrives in the Laurels before sunrise Tuesday. So, the chcs for rain increase once again. PWATs get around 1.5" Mon aftn before dipping then rising again later at night. The storm motion doesn`t seem slow enough to make a widespread risk of flooding. Basin average QPF total is only 0.5-0.75" at highest. But, any storms can drop that much in a short time with PWATs in that range. Still, a big factor against any flooding is the very dry antecedent conditions, esp in the Laurels. && .LONG TERM /TUESDAY THROUGH SUNDAY/... Model guidance outlines another surface low-pressure system tracking northeastward across western portions of the Upper Ohio Valley and into southern Ontario Tuesday into Wednesday, with slightly more moisture and lift available as the warm front aligns itself north-to-south across west-central PA. Given slightly better lift with this low-pressure system coupled with enhanced moisture, have retained likely PoPs in the Tuesday to Wednesday timeframe. Given PWATs in the 1.25-1.75" range on Wednesday, locally heavy rainfall will be possible with relatively slower storm motion also possible. This threat has been outlined with marginal (level 1/4) risks in the D3 (area wide) and D4 (SE 1/2) Excessive Rainfall Outlooks from the Weather Prediction Center. Highest impacts are expected to be across areas that have recently received heavy rainfall, such as urban areas of the Lower Susquehanna Valley, along with hydro sensitive areas across the northern tier. Model uncertainty in previous packages still remain late Wednesday/early Thursday and into the end of the long-term period, so there is low confidence as we move past Thursday morning on this forecast cycle. A (potentially) slower progression of the departing low- pressure system retains chances through Thursday, with slightly lower chances as we move into Friday. The remnants of a tropical cyclone could also bring about rain chances this weekend, but uncertainty with respect to interactions with a trough that could limit chances of precipitation/increase chances as we get closer to an event. At this time, have stuck close to NBM guidance which generally paints SChc PoPs (~15-25%) across the western PA with some enhancement of precipitation along the Laurels. && .AVIATION /23Z SUNDAY THROUGH FRIDAY/... MVFR ceilings continue at AOO/UNV/IPT this afternoon with VFR prevailing at all other airfields. The bulk of model guidance continues to suggest gradual improvement over the next two hours with IPT expected to prevail by 19Z, with AOO/UNV trending VFR closer to the 20Z timeframe. Given model output, coupled with satellite observations, there is moderate (50-60%) confidence in VFR conditions prevailing by 20Z, with high confidence (> 80%) by 22Z. Once these conditions prevail, expect these conditions to continue area wide through 03Z Mon w/ high (~70%) confidence. Lower ceilings will gradually make way into the western terminals after 04Z Monday with GLAMP model guidance suggesting JST/AOO will be the first terminals to trend towards MVFR restrictions based on ceilings. There remains some signal for low-level clouds to begin making way into UNV/IPT slightly earlier; however, lower confidence on this solution so have outlined scattered low-level clouds to outline this potential after 03Z Monday. Closer to sunrise, SHRA is expected to begin entering W PA and have outlined a general west-to-east onset of VCSH/SHRA from 08Z-11Z Monday. Relatively drier air at the lower-levels could slow down timing of SHRA at MDT/LNS so have progged these conditions closer to the 14-15Z Monday timeframe with slightly lower confidence. With SHRA onset, borderline MVFR/IFR ceilings start to become a concern with the HREF/GLAMP guidance suggesting all airfields outside of MDT/LNS experiencing periods of IFR thresholds. If IFR thresholds manage to get into the airfield, generally expect these conditions to hold through the end of the TAF period. Outlook... Tue-Wed...Chc/Likely SHRA with some TSRA possible. Thu...Lingering/scattered showers possible area wide. Fri...Scattered SHRA SW PA; otherwise, trending drier. && .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Lambert/Dangelo NEAR TERM...Lambert/Dangelo SHORT TERM...Dangelo LONG TERM...NPB AVIATION...NPB