Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 160844 AFDCTP Area Forecast Discussion National Weather Service State College PA 444 AM EDT Mon Apr 16 2018 .SYNOPSIS...
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A strong storm accompanied by a plume of deep moisture will will move east through the region this morning followed by much below normal temperatures with gusty westerly winds over 30 mph at times this afternoon and again during the day Tuesday. The cool conditions will persist through the latter half of this week.
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&& .NEAR TERM /UNTIL 6 PM THIS EVENING/...
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The nearly north/south axis of anomalously high PWAT air (30-35mm and 2-3 sigma) will continue to slide gradually east across the region this morning, while pivoting to a more SSE/NNW orientation through 15Z across the eastern CWA. The Flash Flood Watch issued early this morning looks in good shape based on the current region of heaviest rain and projected amounts by the HRRR and SPC HREF over the next 6 hours, with embedded elevated TSRA streaming north across the PA/MD border. KLWX Storm total precip over the past 6 hours indicates a swath of 1.5 to 1.9 inch amounts centered over Franklin County in Scent PA where 6 hourly FFG values are currently 2 to 2.5 inches. The heaviest rain will be shifting to the Lower Susq Valley and Western Poconos for the mid morning hours, before breaking up and exiting across far eastern PA by early this afternoon. Expect some minor flooding problems to develop along some small streams and throughout poor drainage areas as areas of rainfall exceeding 3 inches occurs across the region near and to the SE of I-81. High Shear (40-55 kt southerly flow at 925 mb) and low cape of a 200-400 J/kg fueling the TSRA between KDCA and KTHV will advance NE across the KMDT...KLNS and KMUI areas in the 12-14Z time range right along and ahead of the cfront. Will have to keep a close eye on this area for low-topped mini supercells/QLCS broken-S signatures that could produce brief weak tornadoes and/or concentrated downburst winds. Further north and west, rainfall will average between 1 and 2 inches with little in the way of flooding problems expected. The higher terrain of Central and Eastern PA (mainly AOA 1800 msl) being impacted by the strong LLJ will cause some scattered downed trees and power lines through the late morning. Winds in the stable llvl air will still be anomalous strong from the east at 15 to 25 kt with gusts of 35-40 kts. Cloudy skies, with a gusty west wind over 30 mph at times, and showery conditions will continue in the wake of the cold frontal passage later this morning. The air aloft becomes cold enough for a mix with, then changeover to snow showers across the Laurel Highlands late this morning, and during the early to mid afternoon over the rest of the Western Mtns. Some wet snow flakes will mix in over the Central Mtns later this afternoon/evening as the main cold pocket of air in the 700-500 mb layer swings ENE across the region. Highs in the 40s over most of the area will be some 10-15 deg colder than normal, though readings this morning in the mid to upper 50s will fall just several deg f shy of normal afternoon highs in the greater Harrisburg/Lancaster areas.
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&& .SHORT TERM /6 PM THIS EVENING THROUGH 6 PM TUESDAY/...
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Well aligned NW flow, an anomalously cold boundary layer air being lifted across the NW Mtns and Laurel Highlands will lead to occasional snow showers and the likelihood of a light, 1-2 snowfall tonight/early Tuesday. A few of the ridge tops near and to the West of Rt 219 could see around 3 inches by later Tuesday morning. Flurries and a few brief snow showers will be found throughout the Central Ridge and Valley Region and perhaps a light coating in some locations late tonight through 12z Tuesday. Lows will vary from the upper 20s across the northern and western mtns, to the mid and upper 30s in the SE. Coolest daytime temps and another day of gusty westerly winds over 30 mph at times will occur Tuesday with some intervals of sunshine across the Susq Valley and mainly cloudy skies with flurries and isolated-scattered snow showers across the NW half of the state. Highs Tuesday will be about 15F colder in most areas. Afternoon temps will vary from the mid to upper 30s across the Northern and Western mtns to the Upper 40s across much of the Middle and Lower Susq Valley.
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&& .LONG TERM /TUESDAY NIGHT THROUGH SUNDAY/... On the back side of the strong front crossing the state Sunday night and early Monday, we will see yet another return to colder than normal weather and even a couple of opportunities for snow showers. The first chance for snow showers will be Monday night and early Tuesday as the upper low swings through to our north. Another system may bring more snow showers Wednesday night into Thursday. We don`t expect any significant accumulations, but as we enter mid April, the prospects for snow are less than welcome. Past Thursday we foresee a break of dry weather into early next weekend as energy reloads over the southwestern U.S. allowing weak upper ridging to build over the eastern part of the country. Yet another upper low in this busy pattern looks to drop into the deep south toward the end of the period...possibly spreading some precipitation toward our region for the beginning of next week. && .AVIATION /08Z MONDAY THROUGH FRIDAY/...
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Gusty winds and LLWS will be the main problems overnight, as a tight pressure gradient is present between a cold high pressure system to the northeast and low pressure to the west and south. Very strong frontal inversion remains in place. Some bands of rain to the west, will continue to move eastward overnight. Small band of thunderstorms over central VA, but very cold air at the lowest 5000 feet, hard to see much thunder in our area, so left out of the 00Z TAFS. Conditions should improve later on Monday, as the cold front moves east of the area. Outlook... Mon...Rain will taper to showers W to E. MVFR/IFR early trending VFR PM central/eastern 2/3. Rain/snow showers and sub-VFR cigs continue western 1/3 into Mon night. Tue...MVFR ceilings western 1/3 with lingering rain/snow showers. VFR elsewhere. Wed-Fri...Chance of rain/snow showers with sub VFR most likely NW 1/2.
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&& .HYDROLOGY...
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1 to 2 inches of rain is expected through this morning and will result in significant rises on area rivers and streams. Some 2.5 to 3 inch amounts are likely across the region near and to the SE of Interstate 81. However, FFG values were initially the highest in that location. High res models/ensembles still confirm show the potential for localized max amounts of 3+ inches which would cause some minor flooding problems along small streams and throughout poor drainage areas. Several river points are forecast to reach action/caution stage, however, minor flooding appears unlikely.
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&& .CTP WATCHES/WARNINGS/ADVISORIES... Flash Flood Watch until noon EDT today for PAZ036-056>059- 063>066. && $$ SYNOPSIS...Lambert NEAR TERM...Lambert SHORT TERM...Lambert LONG TERM...La Corte/Lambert AVIATION...Steinbugl HYDROLOGY...Lambert

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