textproduct: State College

WHAT HAS CHANGED

Increasing confidence in significant heat and humidity next week (July 1-4).

KEY MESSAGES

1) Rain tapers off across the south this morning with mostly cloudy skies and generally dry conditions through the afternoon.

2) Steady, light to moderate rain tonight into Saturday; lingering risk for a pop up shower on Sunday.

3) Increasing heat risk builds through the middle and latter part of next week with a chance for daily afternoon storms.

DISCUSSION

KEY MESSAGE 1: Rain tapers off across the south this morning with mostly cloudy skies and generally dry conditions through the afternoon.

A cold front will stall out along the Mason-Dixon Line today, with mostly cloudy skies to the north. Steady rain showers mainly along and south of I-76 will taper off this morning giving way to a mostly dry day. High temperatures will range from the mid 70s in the higher elevations across the northern mountains and Laurels to the mid 80s in the Lower Susquehanna Valley. There will still be sufficient upper level energy to support a few showers this afternoon, though they are mostly likely confined to south of the Turnpike.

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KEY MESSAGE 2: Steady, light to moderate rain tonight into Saturday; lingering risk for a pop up shower on Sunday.

A wave of low pressure will drift along the front tonight, bringing a more widespread shield of stratiform rain with embedded/trailing convection Friday night into Saturday, similar to what we saw on earlier this week on Monday. The best upper level energy in the form of a shortwave will drift overhead around daybreak Saturday, which is when the rain will be most widespread. As the shortwave moves east of the region, the coverage of rain should slowly taper off into the middle of the day and afternoon on Saturday. The combination of persistent clouds and widespread rain (at least in the morning) will keep temperatures on the cool side, generally in the 70s across the entire forecast area. Dewpoints will surge into the low 70s in the south, which will feel rather uncomfortable.

Storm total rainfall tonight into Saturday could exceed an inch in a few spots, especially across southern PA along the Mason- Dixon Line. Rainfall amounts should generally decrease with northern extent. Showers should become more scattered during the afternoon and the risk for lightning appears rather low at this juncture. By Sunday, high pressure will begin to move into northern PA, helping ensuring dry weather there along with slightly lower dewpoints. But a few showers could develop during the afternoon in south central PA closer to the front and best moisture.

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KEY MESSAGE 3: Increasing heat risk builds through the middle and latter part of next week with a chance for daily afternoon storms.

A deep trough settling into the Pacific Northwest and Intermountain West this weekend will likely result in a downstream upper ridge developing over the eastern US by early next week. Blended ensemble guidance driving the prototype probabilistic Heatrisk is showing a signal for a significant warmup for Central PA to end June and start July. With increasing confidence in an upper level ridge over the Ohio Valley, high temperatures will likely exceed 90 degrees over a large portion of the area towards the end of the forecast period (Tue- Fri). Height and temperature fields are progged to be 2 to 3 standard deviations above normal, placing them above the 99th percentile. In addition to anomalous heights and temperatures, low level moisture will also be at least a couple standard deviations above normal. This combination of anomalous heat and humidity will push heat index values over 100 degrees by mid to late week, especially in the lower Susq Valley. The Climate Prediction Center has highlighted all of Pennsylvania in a High Risk for Extreme Heat on Friday 7/3 and a Moderate Risk on Saturday 7/4.

Depending on the exact positioning of the ridge, there may also be some potential for clusters of showers and thunderstorms moving in from the northwest in a classic 'ring of fire' pattern. Machine learning guidance highlights potential for daily afternoon storms Wednesday through Friday, with relatively lower probabilities until then. This type of pattern could also support some higher end severe potential in the form of mesoscale convective systems riding along the ridge. We'll consider this a low probability-high impact event at this point. Anyone overseeing Fourth of July weekend events should continue to monitor the forecast closely for the potential of impacts from both heat and severe weather.

AVIATION /08Z FRIDAY THROUGH TUESDAY/

IFR/LIFR conditions are locked in at KBFD and should expand to KIPT through 09Z. Highest probability of IFR remains focused across the northern airspace where skies have cleared after yesterday's rainfall. VFR is likely to hold at KMDT/KLNS with passing light showers through 09Z. The remaining 3 terminals (KJST/KAOO/KUNV) are very susceptible to MVFR/IFR through daybreak, but confidence is lower vs. the other TAF sties.

Sub-VFR flight cats improve by mid/late morning with VFR expected through the afternoon. Another period of rain is most likely over the southern tier airspace tonight with areas of fog and low clouds into Saturday morning.

Outlook...

Sun-Tue...AM fog; otherwise VFR. Hot with isold PM t-storms psbl.

CTP WATCHES/WARNINGS/ADVISORIES

None.