Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 200643 AFDCTP Area Forecast Discussion National Weather Service State College PA 243 AM EDT Thu Jul 20 2017 .SYNOPSIS... Pennsylvania will be on the periphery of the subtropical ridge through the weekend. Temperatures will be above normal and there is the risk of an MCS in the ring-of-fire moisture plume. The heights are forecast to fall over the weekend into early next week suggesting cooler, dry weather next week. && .NEAR TERM /UNTIL 8 AM THIS MORNING/...
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06Z satellite loop showing patchy fog has already formed in the deep valleys north of I-80. Expect the valley fog to become more widespread by dawn, affecting primarily those locations which received rain yesterday between the Pa turnpike and I-80. A couple isolated showers still noted over southern Pa at 06Z, but coverage extremely sparse and continued cooling/stabilizing of boundary layer should result in these showers falling apart between 06Z-09Z, so will not mention precip in the grids overnight. Temps on track to bottom out from the low 60s over the northern mountains, to the low 70s across the Lower Susq Valley.
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Fog should burn off by 13Z-14Z based on trends of recent days and latest LAMP guidance. Main focus will then shift toward a potential MCS tracking across the Grk Lks toward northwest Pa. Latest convection-allowing model guidance, including NCAR ensemble and SPC SSEO support likely POPs across the northwest mountains during mid to late afternoon, with a diminishing chance of showers/storms as convection progresses southeast into the rest of central Pa this evening. Focus of the strongest tstorms may be just north of Pa at nose of anomalous low level jet. However, moderate CAPES and 0-6km shear of around 40kts are favorable for an organized severe weather threat across the northwest half of the forecast area. Other concern will be potentially dangerous heat indices across the Lower Susq Valley, where combination of heat and humidity should push heat indices to near 100F. Not sure we will achieve the criteria of widespread 100F for 2+ hours, so have held off on an advisory for now.
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&& .LONG TERM /THURSDAY NIGHT THROUGH WEDNESDAY/... The latest thinking is that upstream convective activity should evolve east-southeast across the Lower Great Lakes during the afternoon and reach PA by late Thursday afternoon into Thursday night. The details remain somewhat unclear at this time given the prospects of new storm development in advance of initial /Day 1/ MCS and potential for additional storms out ahead of outflow boundary or cold front. Global model QPF may be too far north with QPF max which is a typical bias in MCS/ring of fire patterns. The slight risk was adjusted southward to align better with projected MCS movement and axis of best instability. Showers and storms should weaken and dissipate late Thu night into Friday morning. Model data shows large scale subsidence over the area during the day on Friday with convective activity re-focusing over the Midwest/Great Lakes Friday night and propagating east-southeast toward central PA by Saturday morning. The Day 3 MRGL risk extends into west-central PA on the eastern extent of upstream organized convective zone. Over the weekend, pcpn pattern looks to reach a relative peak with max POPs based on multi-model blend. Exact timing and location of pcpn will depend on details not well resolved until the very short term time frame. Therefore the later periods will keep a 40-60 percent POP given lack of confidence in highlighting any specific areas/timeframe with concentrated pcpn potential. Medium range models generally agree that a continued/gradual weakening and east-southeast shift of the persistent central U.S. upper ridge can be expected with time, as shortwave troughing shifts slowly southeast out of the Canadian Prairie across the northeast quarter of the country by early next week. The other concern is for a several-day period of hot/humid conditions punctuated across the Lower Susquehanna Valley with blended guidance showing 2-3 days of 90+F max temps with max heat index potential approaching 100F. The heat risk remains in the HWO. A heat advisory may be needed for at least 1 day based on NDFD max HI and WPC D3-7 HI probs. The hot temperatures should be eroded from the NW with time, as the frontal boundary gradually sinks to the south in response to mean troughing developing late in the period. Above average temperatures (+5-10 degree max T departures) should trend toward mid to late July climo numbers by next Tue-Wed. && .AVIATION /06Z THURSDAY THROUGH MONDAY/... Another round of showers and thunderstorms moving through south this evening. All convection is over for the evening and VFR is currently dominating. with high minimum temperatures and high dewpoints, another round of patchy fog/mist and low stratocu is possible tonight into tomorrow. Best chances will be along the south where it rained, especially MDT, IPT, AOO and JST. Expect any restrictions to improve between 12z to 14Z. Tomorrow will bring an increased chance showers and thunderstorms through NW PA Thursday afternoon with better chance tomorrow night in the central and southeastern areas .OUTLOOK... Thu...AM valley fog. Cig restrictions poss NW. TSRA PSBL NW Thu aft, scattered TSRA central Mtns Thu eve. Fri...AM valley fog/cig restrictions likely NW half. Isolated TSRA. Sat...AM valley fog poss. TSRA likely. Sun...AM valley fog poss. TSRA likely north/scattered south. && .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Gartner NEAR TERM...Fitzgerald SHORT TERM...Fitzgerald/Gartner LONG TERM...Steinbugl AVIATION...Grumm/Ceru is the U.S. government's official web portal to all federal, state and local government web resources and services.