Area Forecast Discussion
Issued by NWS State College, PA
Issued by NWS State College, PA
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499 FXUS61 KCTP 230546 AFDCTP Area Forecast Discussion National Weather Service State College PA 146 AM EDT Mon Sep 23 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 TODAY/... A veil of mid and high clouds has increased over central and eastern portions of central PA early this morning. Lower cigs are found over the NW/WC Mountains and Laurel Highlands and now spilling eastward into the NC and C mountains, with light rainfall moving into the NW Mountains. Regional radar mosaic and gage reports indicate most of the initial reflectivity returns are aloft and only T amounts reported south of Erie in the past 1-2 hours. Clouds will continue to increase, thicken and lower overnight. Temps should again stay much warmer than normal tonight, with most places at least 10F above normal. The approaching sfc frontal system runs into the CAD in place over Central PA and stalls, so eastward progression of whatever precip does make it into the Alleghenies overnight will be a chore. Still, moisture transport aloft is continuous overnight and rain/showers will eventually spread across much of the western half of our CWA through sunrise. 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. Highs Monday will be mainly in the low to mid 60s in most places, except for around 70F across the Lower Susq Valley. && .SHORT TERM /TONIGHT THROUGH 6 PM TUESDAY/... 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 NIGHT 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 systemoverhead co 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 range, have maintained numbers close to NBM guidance which continues to paints SChc PoPs (~15-25%) or below across the western PA with some enhancement of precipitation along the Laurels. && .AVIATION /06Z MONDAY THROUGH FRIDAY/... 06Z TAFS sent. Not a lot of change from the earlier TAF package. Band of showers coming in a bit faster. The rain will tend to lower conditions during the day, but time of day will tend to offset the lower conditions a bit. By sunset I would expect conditions to lower again. The main feature on the weather map early this morning is a weak low over western Lake Erie. Trend for the next few days is for the frontal system to remain near the area. Thus I would expect similar conditions each day for most of the week. While VFR conditions may occur at times, I would expect this to be for brief periods of time. 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/DeVoir/Dangelo NEAR TERM...Lambert/DeVoir/Dangelo SHORT TERM...Lambert/Dangelo LONG TERM...Lambert/NPB AVIATION...Martin