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FXUS61 KCTP 182239 AFDCTP

Area Forecast Discussion National Weather Service State College PA 639 PM EDT Sat May 18 2024

SYNOPSIS
An upper trough overhead will slide east through the evening with showers decreasing by late tonight. High pressure will start to build into the region on Sunday accompanied by generally rain- free conditions that are expected to persist through the middle of next week. A storm system should arrive Wed, and perhaps generate severe weather and heavy rain.

NEAR TERM /UNTIL 6 AM SUNDAY MORNING/
Scattered open cellular showers continue to roam areas from the Finglerlakes to the WC Mountains early this evening as upper low tracks off the coast and drier air slowly works in aloft behind it. Beyond some brief slow moving downpours, these present little concern given weak shear and marginal bouyancy. Lighter showers over my far southeastern counties are slowly dissipating and moving east as well, so it's fair to say that slow improvement is in progress across the area.

Showers will taper by late evening, and as the sky begins to clear under the influence of NVA/rising heights, fog will form esp where it rained this aftn and eve and some could become locally dense in the predawn hours. Lows will range from the mid 50s to around 60.

SHORT TERM /6 AM SUNDAY MORNING THROUGH SUNDAY NIGHT/
High pressure rolls in for Sunday. Clouds/fog in the AM clear out nicely. However, just enough instability develops and enough moisture lingers to pop a couple of SHRA. Literally just a few with the highest probability (30pct) over the SErn hill tops.
Maxes in the 70s will be pretty normal. We are likely to see some areas of fog again Sunday night with the sky clearing out better, but dewpoints not dipping out of the m50s.

LONG TERM /MONDAY THROUGH SATURDAY/
No real changes or updates from the previous long term discussion. Monday and Tuesday will be dominated by high pressure ridging and dry conditions before the next system makes its way into the region by Wednesday for the return of the unsettled pattern.

Previous discussion...
A dry start to the extended period will be likely with high pressure ridging building over the mid- atlantic region and a deep surface low parked over the central plains. High pressure and dry air will stay in place through Tuesday before the upper level trough pushes them both out by Wednesday afternoon. 850mb temps of near 16C along with surface warm air advection suggest high temperatures could reach the mid 80s on Tuesday and Wednesday.

By the middle of next week, upper troughing will move across Pennsylvania and support a better chance for showers and thunderstorms. At the surface, there remains plenty of model uncertainty about the timing of a cold front. The mean timing is Wednesday afternoon, but individual models bring precipitation as early as Tuesday evening and as a late as Thursday morning.
If the front moves through Wednesday afternoon or evening, it could produce the first threat for severe weather in some time given the aforementioned WAA bringing greater instability.

After the front moves through, considerable uncertainty exists in terms of how much and how frequent rain will be. A chance of showers remains in place through the end of the week despite zonal flow aloft providing little in the way of synoptic forcing. The next chance for a widespread soaking rain will come during the weekend with a center of low pressure developing across the Ohio Valley by Friday afternoon.

AVIATION /00Z SUNDAY THROUGH THURSDAY/
21Z Update...Recent HRRR/LAMP guidance has suggested airfields across SE PA (MDT/LNS) going towards low-end MVFR to high-end IFR CIGs overnight compared to previous guidance. Recent RAP soundings also outline this potential (mainly after 03Z Sunday, where winds could become calm) with increased low-level moisture. Have introduced lower cloud decks in the 03Z-12Z Sunday timeframe given multiple runs of GFS LAMP guidance and recent HRRR guidance trending downwards, although there is low- to-moderate (30-40%) confidence at this time.

Prev...
Lots of SHRA popped over the Alleghenies, and are moving only slowly. MVFR cigs in the SErn half of the area will linger into the night but probably break up some more. Rain showers should mostly be over by the early evening hours as the upper level trough drifts east. Remaining low level moisture and night time cooling will make for fog development for Sunday morning likely. Most sites will likely drop to IFR for a time as low clouds and fog take hold of the region through the early morning hours Sunday.

Outlook...

Sun...Morning low cigs/fog possible. Becoming VFR by afternoon.

Mon...Patchy AM fog possible. Otherwise, becoming VFR.

Tue...Predominantly VFR, w/ no sig wx expected.

Wed...Sct to widespread SHRA/TSRA return.

Thu...Chc of TSRA.

CTP WATCHES/WARNINGS/ADVISORIES
None.

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