Area Forecast Discussion
Issued by NWS State College, PA
Issued by NWS State College, PA
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932 FXUS61 KCTP 291539 AFDCTP Area Forecast Discussion National Weather Service State College PA 1139 AM EDT Sat Jun 29 2024 .SYNOPSIS... Low pressure will track north of Pennsylvania this weekend, with the associated warm front lifting through the state today and the trailing cold front pushing through on Sunday. A large high pressure system will build into the region early in the upcoming week, then a dying cold front will likely push in from the Great Lakes and stall out over the area late next week. && .NEAR TERM /UNTIL 7 PM THIS EVENING/... Rain/showers across the nrn tier have been generally light with obs there only in the hundreths in the Elk-Tioga Co corridor, but nrn Warren and McKean Cos have had a little over half an inch in the last 6 hrs. High PWATs make efficient rainfall. Showers over SWrn PA are moving into our CWA at about 40KTs. No really deep convection in those yet, though, with only a couple ltg strikes embedded. The clouds are thinning in the central mtns, and briefly cleared in the SW. Elsewhere only tiny breaks in the overcast thus far. Mixing should continue to make more breaks/thin spots occur and allow temps to get warm enough to trigger additional showers and storms by noon. The vort max over roughly eastern LE should continue ENE, and the lull/slight suppression of convection currently over ern and central OH could delay development of deep convection over PBZ`s area over the next 1-2 hrs and could help our wrn highlands keep convection weaker, too. But, the wave of showers in the Alleghenies right now could assist in the development over our south-central counties shortly. But, the earlier breaks in the clouds in our SW have closed up for the most part and the thicker lower clouds in the E will help keep things suppressed (not totally) before late aftn or early evening there. Minor tweaks to near term sky/PoPs were all that were made with this update. See the hydro section below, too. Prev... A low amplitude upper trough will approach from the Grt Lks this weekend, with the associated surface low passing well north of PA through Southern Ontario/Quebec. The attendant warm front is progged to lift through Central PA this afternoon and the trailing cold front should slice southeast across the forecast area during the daylight hours of Sunday. A developing and relatively strong southwesterly low level jet (focused across the Northern counties of PA this morning and mainly across the Susq Valley later this afternoon and evening) and surging pwats ahead of an approaching mid/upper level shortwave will overrun warm front, slowly advancing NE across the middle and upper Ohio river Valley. This will bring the best chance of showers and a slight chance of tsra to the N Mtns between throughout much of the remainder of the morning hours today. The area of greatest concern for scattered strong to potentially severe TSRA with the threat for heavy rain and flash flooding will spread south and east later this morning through this afternoon as the axis of the LLJ slides east and the favorable RR quad of an approx 2 sigma upper level jet and its enhanced MESO-B lift moves overhead. (Moved hydro talk below to a hydro section) With high pressure passing off of the New England coast the increasingly moist low level south to southeast near surface flow and moderate to strong low level speed and directional shear will bring favorable hodographs and low LCLs to support the threat of discrete TSRA cells and short line segments with their long axis oriented at a large angle to the deep layer shear vector, creating the potential for BKN-S signatures and transient/weak QLCS tornadoes formed by descending rear inflow jets and even some fairly strong and persistent mesocyclones with the more discrete cells. As detailed by the previous fcst discussions - Model soundings indicate there will be a good deal of cloudiness today, especially over the north-central mountains, where model RH profiles indicate stratus could linger into the early afternoon ahead of the approaching warm front. However, even modest diurnal heating should yield capes in the neighborhood of 1000 J/kg Sat afternoon per latest ensemble output. The moderate instability, combined with progged 0-6km shear of 40-50kts, is more than sufficient to support organized strong/severe convection Sat PM with a few supercells and an isolated tornado possible, especially across the northwest half of the forecast area, where HREF UHEL values are highest and STP values peak near 2. Temps will start the day in the 60s. Highs will range from the low to mid 70s over the Ncent Mtns to the mid and upper 80s in the Southern Valleys. && .SHORT TERM /7 PM THIS EVENING THROUGH 6 PM SUNDAY/... Latest model guidance indicates the bulk of the convection will shift east of the region late tonight, as the lead shortwave and plume of highest pwats exit the state. However, passage of the surface cold front will bring the chance for another round of convection Sunday. Locations most likely to see potential strong/svr t-storms are over the southeastern part of the CWA due to a fropa coinciding with peak heating of the late afternoon. Lows tonight will be very mild and in the mid 60s to mid 70s or 10 to 15 deg F above normal. Max temps Sunday will be a few to several deg F higher than today (Sat) in all locations. && .LONG TERM /SUNDAY NIGHT THROUGH FRIDAY/... Once the cold front clears, the door will be open for another seasonably cool/dry airmass to invade the Commonwealth. This will result in a less humid and comfortably dry start to July on Monday. A fair and cool Monday night looks likely as high pressure and associated dry airmass build directly over PA, resulting in efficient radiational cooling and min temps several degrees below normal. Fair and warmer weather is expected Tuesday into at least the first half of Wednesday, as upper level ridging builds over the state and surface high drifts offshore. All medium range guidance tracks a weakening shortwave well north of PA late Wednesday into Thursday. The dying cold front is likely to sag into the state Thursday and stall out in the vicinity of PA through Friday. Falling heights/surging low level moisture ahead of the front could support scattered convection as early as Wednesday afternoon over the northwest half of PA. However, a better chance of showers/tsra areawide will come with the arrival of the front Thu into Friday. Early best guess of late week rainfall based on ensemble plumes ranges from 0.25 to 0.75 inches, with the highest amounts over the N Mtns and least in the south. Heat and humidity should peak ahead of the front Wed-Thu. Ensemble mean 850mb temps surging to near 20C indicate max temps could reach the low to perhaps mid 90s in the Lower Susq Valley Thursday, with high humidity pushing heat indices into the mid and upper 90s. Ensemble plumes suggest slightly cooler daytime temps late week, due to more clouds/showers. However, high pwats in the vicinity of the stalling front should result in muggy nights and above normal min temps the entire second half of next week. Models begin to diverge as we head into the weekend. Most guidance suggests that rain chances stick around through at least Saturday as another cold front approaches the area. && .AVIATION /16Z SATURDAY THROUGH WEDNESDAY/... Late morning update. Some adjustments to the TAFS so far, but fcst in good shape overnall. Most of the showers have been across the north overnight and into the mid morning hours. Echo tops high enough for thunder have been largely from about Route 6 northward. Cells moving from the west southwest at about 35 to 40 mph, as winds estimted by radar about 35 knots at 4000 feet (VAD winds). Earlier discussion below. A warm front crossing through the region is providing ample moisture advection in southeast flow. Low stratus has blanketed much of central PA with MVFR cigs as of 1130 UTC Sat, with the exception of JST and BFD, where conds are still VFR. LLWS will continue this morning across the northern airfields. The low stratus is expected to lift and become broken to scattered in the morning over the S, and gradually work northward a bit in the mid-day and aftn. Numerous TS/SHRA will be popping up over the NWrn third of PA in the morning and stronger thunderstorms develop later on Saturday. High moisture will make the potential for very heavy rain and very low vsbys in the TSRA. TSRA will continue pressing SE thru the evening and first half of the night Sat night. The chance for showers and storms will linger across the southeast Sunday as a cold front drags it`s feet moving through the area. There could be fog Sat night in the NW (BFD), but that will depend on the amt of clearing. IFR from overnight/AM fog (Sun morning) at BFD is only a 40pct chc at this point, and lower elsewhere as not much clearing is expected elsewhere. Outlook... Mon-Tue...Dry weather with VFR conditions except perhaps some AM valley fog N. Wed...PM TSRA moving across from W-E. && .HYDROLOGY... Over half of an inch of rain last night and this morning over the far NW has increased confidence that the precip in the N will be very efficient today. While dbZ and the amt of ltg may not impress today, the moist adibatic profiles and deep warm layer will make for heavy rain at times. Draw back to fast storm motions: 1) radar may underestimate precip as the fast cells skip along in between scans, and 2) it`s tough to get flooding when the cells have a low residence time. However the repeated showers/storms could (easily) overcome that issue. Prev... The area of greatest concern for heavy rain and flash flooding will spread south and east later this morning through this afternoon as the axis of the LLJ slides east and the favorable RR quad of an approx 2 sigma upper level jet and its enhanced MESO-B lift moves overhead. PWAT values increased steadily from west to east overnight from a little over one inch to start the night to around 1.6 inches early today. PWATS via the GEFS will peak around +3 sigma or about 2.25 inches later this morning through this afternoon (which are values that haven`t been seen so far in PA during 2024). This anomalous, deep moisture will support the threat for locally very heavy rain (in excess of 2 inches/hour) within a broad area of 1 to 1.5 inch basin average storm total rain forecast for much of Central and Northern PA. Issued a Flood Watch from 12Z today through 06Z Sunday for much of Central and Northern PA where FFG values are well under 2 inches/3 hours across parts of the Northern Mtns. && .CLIMATE... Through 6/27, June ranks as the second warmest on record at Harrisburg with an average monthly temperature of 76 degrees or +3.8 degrees above normal. && .CTP WATCHES/WARNINGS/ADVISORIES... Flood Watch through late tonight for PAZ004>006-010>012- 017>019-024>028-037-041-042-045-046-049>053. && $$ SYNOPSIS...Lambert/Fitzgerald NEAR TERM...Lambert/Fitzgerald/Dangelo SHORT TERM...Fitzgerald/Steinbugl LONG TERM...Fitzgerald/Bauco AVIATION...Lambert/Martin/Colbert HYDROLOGY...Lambert/Dangelo CLIMATE...Steinbugl