Area Forecast Discussion
Issued by NWS State College, PA

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000 FXUS61 KCTP 172043 AFDCTP Area Forecast Discussion National Weather Service State College PA 443 PM EDT Thu Aug 17 2017 .SYNOPSIS... A warm front will push northward across the region this evening and overnight and a cold front will push across on Friday. Numerous showers and thunderstorms will result, and some could be severe later tonight and during the day. After the cold front passes, the air will dry out and fair weather will follow for the weekend and into the new week. && .NEAR TERM /UNTIL 6 AM FRIDAY MORNING/...
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High pressure is now to our east and the region is in the deep southerly flow. Most of the convection this afternoon is well to our west over Ohio. Though radar shows a few isolated thunderstorms in western PA. The HRRR forecasts suggests thunderstorms will affect Warren and McKean counties this afternoon and early evening. The SPC has a slight risk out in Warren county. Cannot rule out a severe thunderstorm this evening in our northwestern counties. The HRRR gets some organized convection to central areas of Pennsylvania after midnight. Not a lot of QPF much east of State College before 7 AM. So best POPS and highest QPF are in the west. There could be areas of patchy fog in this warm humid air mass overnight.
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&& .SHORT TERM /6 AM FRIDAY MORNING THROUGH 6 PM FRIDAY/...
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There is a marginal risk of severe weather in central and eastern areas on Friday. Most of the guidance shows warm moist air over PA Friday morning. But drier air is entering from the west. This drier air and the leading edge of the cold front should sweep across Pennsylvania during the day and be to our east Friday evening. In the warm air ahead of this frontal system most forecast systems show 600 to 2000 JKg-1 of CAPE by 1800 UTC and most of this is over eastern PA and points east around 0000 UTC. So we have a good window of moist instability. However, the stronger winds and better shear are a bit ahead of the CAPE. Not the perfect alignment. Hence the marginal risk. Not surprisingly the QPF/POPS in most models is scattered about in the 1200 to 1800 UTC timeframe over central areas and peaks in the 1800 to 0000 UTC timeframe over the entire eastern half of our forecast area. Models clearly think good chance afternoon convection. But improving in afternoon in west and in the evening in the east. Most locations should see rain over between 1200 and 0000 UTC. Then things improve in the evening in the east too!
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&& .LONG TERM /FRIDAY NIGHT THROUGH THURSDAY/... We should remain relatively dry as the PW values stay near normal until perhaps Tuesday. This said there are hints of a weak wave and scattered showers perhaps Sunday. Tuesday could be rather warm as there is a surge of above normal 850 hPa temperatures and PW values above normal. Not surprisingly the GEFS shows higher POPS Tuesday afternoon into Wednesday with the wave and front. We should enter the second half of the weak in a cooler and drier air mass as currently forecast. And thus in the grids. Bottom line in this period the NCEP guidance suggests low probability of significant QPF. && .AVIATION /21Z THURSDAY THROUGH TUESDAY/... Scattered showers and storms are moving into western PA. These storms will be more widely scattered at first, but will increase in coverage through the evening and overnight. Multiple occurrences of MVFR/IFR due to TSRA are expected through the night and on Friday. The stability in between the showers overnight could generate a large patch of low clouds. This is most likely to happen over the western sites. Fog could be a problem again Sat AM with rain Fri then clearing overnight. Improving conditions for the weekend, as the cold front moves east of the area and dry air moves in under high pressure. .OUTLOOK... Sat-Mon...No sig wx. Tue...SHRA/TSRA poss. && .CTP WATCHES/WARNINGS/ADVISORIES... None. && $$ SYNOPSIS...Dangelo NEAR TERM...Grumm SHORT TERM...Grumm/Dangelo LONG TERM...Grumm/Lambert AVIATION...Dangelo

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