Area Forecast Discussion
Issued by NWS State College, PA

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799 FXUS61 KCTP 280028 AFDCTP Area Forecast Discussion National Weather Service State College PA 828 PM EDT Sat May 27 2017 .SYNOPSIS... Low pressure over southeastern Ohio this afternoon will slide by just to the south of the Mason/Dixon line late today and tonight, while weak high pressure just to our north supplies dry weather and light wind across practically all of Central PA. A warm front will try to work to the north as a cold front approaches the region on Sunday. The cold front will pass through on Monday morning. Low pressure will stall over northern Ontario mid-week and create a slightly cooler than normal and unsettled stretch of weather. && .NEAR TERM /UNTIL 6 AM SUNDAY MORNING/... A weak upper trough accompanied by multi-layered clouds (and a small area of light to mdt showers was scooting ESE acrs the far southeastern part of our CWA this afternoon. Weak ridging at the surface and aloft will build across most of the state for this afternoon and tonight bringing dry and milder temps than recent days. The 1010 mb sfc low and associated weak south-swrly WAA out ahead of it may try to re-fire isolated-scattered late day/early evening showers near and to the south of the Mason/Dixon line where we`ll maintain the low...15-25 percent through early this evening, before decreasing to less than 15 percent in far southern PA tonight. The chc for rain across the nrn half of the CWA will be nearly zero over the northern half of the state that will be close to the weak bubble of sfc high pressure. Will still keep a watchful eye on our far southwest zones late this afternoon and evening for the potential of some strong storms. However, recent runs of the HRRR and trends in sfc obs and radar point to a lower chc of anything near strong convection - especially as SPC trimmed the nrn edge off their MRGL area that brushed southern Somerset and southern Bedford counties. Winds will be light and variable for the rest of this afternoon and tonight as weak sfc ridging noses south across the CWA. 18Z temps are within 2-3 deg F of their fcst maxes in all areas and hourly values look in good shape for the upcoming 12-16 hours. The current bkn/Flat cu and strato cu should dissolve for several hours late today and this evening...allowing for a moderate drop in temps early tonight with mins in the u40s to lo 50s. Elsewhere, any brief clearing will be quickly replaced by a consolidating/slowly nwd advancing deck of stratus/stratocu within a well-defined area of higher 925-850 mb Theta-E air. Areas of fog possible late tonight through the mid morning hours Sunday, especially on the east facing slopes of the Alleghenies where a light serly, upslope flow will be developing and could even support some patchy drizzle after midnight. Min temps across southern PA will be about 6-10 deg F milder than across the northern...ranging through the 50s. Will maintain almost nil tonight and Sun AM to the north of the PA turnpike. && .SHORT TERM /6 AM SUNDAY MORNING THROUGH 6 PM SUNDAY/... The mean sfc-850 mb flow becomes more southeasterly, then southerly in advance of the storm and sfc cold front approaching from the west. The chance for storms increases on Sunday afternoon. The destabilization will be easiest in the east where less low clouds are expected, but the deeper moisture is off to the west and the dynamics are even farther to the west. PWAT goes above 1.5 inches in the west by the end of the day Sunday. Will ramp POPs up in the west through the aftn, leaving the east in chc or less POPs. Due to the expected extent of low clouds to start the day...followed by thickening mid and high clouds/and increasing chc for showers/sctd tsra, we leaned away from the Nat`l Blend and more toward the cooler NAM for high temps - in the u60s to low 70s. && .LONG TERM /SUNDAY NIGHT THROUGH SATURDAY/... A quick transition to increasing deep layer moisture and higher PWAT air of 1-1.25 inches will occur late Sunday. A large-scale trough and increasing upper level divergence (associated with a lead upper shortwave in the west/SW flow) will lead to a fairly large area of moderately strong uvvel and a widespread moderate rainfall along and ahead of a cold front. GEFS Probs for GE 0.50 inch of rain/24 hours ranges from 50-80 percent across the wrn mtns of generally less than 50 percent across the eastern half of the state. This translates into very high POPs for measurable rain (90 percent or more) across the western half of the CWA, to between 70 and 90 percent across the south-central mtns and Susq Valley. 12Z EC and 12z GEFS are in very good agreement on the timing/location of the north/south sfc cold front during the afternoon hours Monday across the eastern half of the state with weak deep- layer ridging and drying for Monday afternoon and night. Dry conditions expected Tuesday morning before significant cooling aloft accompanies a pair of short waves late Tuesday and later Wednesday. POPs during this midweek period will be painted as chc - translating to scattered showers with preference for the majority of the QPF (albeit quite light at that)to be across the typical west-nwrly flow upslope prone areas of the Laurel highlands and NW mtns. The chance for any thunder will be very minimal Tuesday through Thursday. Generally dry NW flow occurs late in the week into next weekend as a 1020 sfc high over the upper midwest Friday evening builds SE twd the region. Pops Thur night through Fri night will be 30 percent or less. After our bout of coolish weather with temps several Deg F below normal Sunday and Monday, GEFS mean temps rebound back to near or slightly above normal (even under slightly cooling 850 mb temps) for Tuesday through at least Thursday. ECENS mean temps are several deg warmer than the GEFS and Nat`l Blend of Models temps. leaned closer to the cooler solutions based on the few-svrl deg C of cooling aloft with near to slightly below 850 mb temps. && .AVIATION /00Z SUNDAY THROUGH THURSDAY/... A weak ridge of high pressure will build across the region tonight, resulting in a calm wind and conditions favorable for fog formation. Evening dewpoint depressions and latest model guidance both indicate southern Pa will be most susceptible to IFR/LIFR conditions, potentially beginning as early midnight in vicinity of JST/AOO/LNS and later tonight further north. At this point, would categorize IFR/LIFR conditions as likely at JST/AOO/LNS overnight, around 50/50 at UNV/MDT and slightly less than 50/50 at BFD/IPT. Early low clouds/fog will begin to lift after 12Z, with VFR conditions expected in most locations by afternoon. However, there is at least some potential of a moist sse flow into the mountains creating a MVFR stratus deck along the east slopes of the Appalachians from BFD south through UNV,AOO and JST. The other area of concern will be isold tsra impacts across the western airfields associated with a warm front pushing into the region. .OUTLOOK... Mon...AM showers/low cigs possible. Tue...No sig wx expected. Wed...Slight chance of showers/reduced cigs. Thu...No sig wx expected. && .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Lambert NEAR TERM...Lambert SHORT TERM...Lambert LONG TERM...Lambert/Ceru AVIATION...Fitzgerald is the U.S. government's official web portal to all federal, state and local government web resources and services.