Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 281203 AFDCTP Area Forecast Discussion National Weather Service State College PA 803 AM EDT Fri Jul 28 2017 .SYNOPSIS... A slow moving low pressure system, located over the Lower Ohio Valley this morning, will strengthen as is tracks south of Pennsylvania later today through Saturday. Heavy rain will fall across southern portions of central Pennsylvania this afternoon through Saturday morning. Behind this system, high pressure over Great Lakes will slowly build into the region into early next week. && .NEAR TERM /UNTIL 6 PM THIS EVENING/... Sfc front appears to have settled along the Mason Dixon Line early this morning. Large scale subsidence took care of earlier isolated showers over my southeast zones, which produced an inch and a half of rain around Harrisburg overnight. Areas of fog producing 2-4sm vsbys across the Lower Susq River Valley and Laurel Highlands, which locally thicker fog was seen farther north (KIPT currently 1/4sm) Main focus this morning is on heavy rain potential this afternoon through Saturday morning over the southern third to half of central PA. Overnight guidance showed a slight southward shift in heaviest precipitation axis, with locally 3 to 4"+ expected over much of the southern third of central PA by Saturday afternoon. Unusually strong upper trof digging SSE from the Western GLAKS this morning will drop into central Ohio by this evening, and then to the WV/MD panhandles by 12z Saturday. A long duration moderate to heavy rain is expected from the Laurel Highlands and South Central Mountains and perhaps into the Lower Susq River Valley, where Flash Flood Guidance is perhaps the lowest in the state given recent heavy rainfall. As the upper low digs into central OH this afternoon, a surge of deep layer moisture lifts across south central and central PA with PW of 1.5" to 1.75" across much of the southern half of the state. 850 mb circulation over western VA will produce anomalous ESE flow aiding upslope enhancement. Current Flash Flood Watches in good shape for now. Expect showers to develop over western and central areas mid to late morning, with moderate intensity developing mid to late afternoon across the Laurel Highlands and South Central Mountains. && .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM SATURDAY/... Periods of heavy rain will persist for much of the night across the southern third of central PA. Stayed the course with previous thinking despite a very slight southward shift in guidance from 00z cycle. Some areas could exceed 5", but that is looking more likely to the south of the Mason Dixon line at this time. Steadiest and heaviest rainfall may be over by 12z Saturday, with lingering but decreasing showers retreating southward throughout the morning as drier air works into northern and central sections. Max temps Saturday will almost certainly be several Deg or more lower than most MOS guidance given the widespread moderate rain and a trajectory of the anomalously strong llvl wind from the New England Coast in contrast to the Mid Atl Coast. && .LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... Weather highlights are front-end loaded this period with rare (unusually strong for late July) rainstorm continuing to impact southern portions of the area on Saturday. Model and ensemble guidance shows an anomalously strong minus 3.5-4.0 sigma below normal closed 500mb low over the southern Mid Atlantic by the weekend. Organized heavy rainfall is expected along and north of its associated surface wave which is projected to consolidate and gradually amplify as it tracks east- northeast along frontal zone from the OH Valley across northern VA/D.C. into the Delmarva Friday night-Saturday morning before shearing and weakening off the coast Saturday-Sunday. A secondary low is fcst to develop and lift north off the Mid Atlantic coast Mon- Tue but at this time does not look to impact the wx here in central PA. Steadier rains early Saturday across the southern tier counties should generally taper off from northwest to southeast later Saturday into Saturday night. High pressure and accompanying push of abnormally dry/below normal PW air will work into the area by Sunday with dry and pleasant wx continuing early next week. A stray shower to T`storm is possible Tue-Wed as warmer temperatures and higher dewpoints return to the area. NAEFS and ECENS favor a continuation of the western ridge/eastern trough pattern which should allow for one or more weakening cold front to push southeast from the Great Lakes later next week. Temperatures will start off well below average on Saturday with fcst departures on the order of -10 to -15 degrees. Following the unseasonably cool start, a gradual warming trend is expected with max/min temperatures returning to seasonal normals heading into August. && .AVIATION /12Z FRIDAY THROUGH TUESDAY/...
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Dense fog at MDT and LNS will slowly improve over the next few hours before lifting by 14Z. Have put tempo groups in for BFD, MDT and LNS to account for the variances as the fog lifts/dissipates. The next restriction will be from the widespread showers and storms with heavy rain expected later today into Saturday. Expect west to east convection late this afternoon and into the overnight period as rain showers could persist through 09Z Saturday. The northern TAF sites should improve after 00Z Saturday with UNV following between 06Z to 09Z. The southern TAF sites could see periods of MVFR into 12Z saturday. .OUTLOOK... Friday night-Sat morning...Scattered SHRA north. Widespread showers and heavier SHRA/TSRA across the southern airfields. Saturday afternoon...improving conds from west to east through the evening. Sun...Areas of fog and low cigs Sunday morning. Mon-Tue...No sig wx.
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&& .CTP WATCHES/WARNINGS/ADVISORIES... Flash Flood Watch from 2 PM EDT this afternoon through Saturday afternoon for PAZ024>028-033>036-056-057-059-063>066. && $$ SYNOPSIS...DeVoir NEAR TERM...DeVoir/Fitzgerald SHORT TERM...DeVoir/Lambert/Martin LONG TERM...Steinbugl AVIATION...Ceru/Gartner

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