textproduct: State College

WHAT HAS CHANGED

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

KEY MESSAGES

1) Periods of light to briefly moderate rain tonight into Saturday; lingering risk for a pop up shower on Sunday.

2) Increasing heat risk builds through the middle and latter part of next week with a chance for thunderstorms rising into the end of the week.

DISCUSSION

KEY MESSAGE 1: Periods of light to briefly moderate rain tonight into Saturday; lingering risk for a pop up shower on Sunday.

A wave of low pressure will drift east along the stalled out front to our south tonight, bringing periods of light to moderate rain. 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 diminish during the midday and afternoon hours.

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 rise into the low 70s in the south, which will make it feel rather uncomfortable humidity-wise.

Storm total rainfall tonight into Saturday could exceed an inch in a few spots across southern PA, while much lower amounts are expected north of Interstate 80 where less than 0.30 of an inch is expected.

Showers should become more scattered during the afternoon and the risk for lightning appears rather low. By Sunday, high pressure will begin to move into northern PA, bringing slightly lower dewpoints and drier conditions. But a few showers or thunderstorms could develop during the afternoon mainly along and south of I-80, closer to the front and best moisture.

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KEY MESSAGE 2: Increasing heat risk builds through the middle and latter part of next week with a chance for thunderstorms rising into the end of the week.

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 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, some of which tending to last into the overnight hours. 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 /01Z SATURDAY THROUGH WEDNESDAY/

Ceilings will lower west to east tonight as a low pressure system approaches, with scattered -RA/-SHRA moving in mainly after midnight. Intermittent IFR visibility restrictions are possible in the steadiest showers (PWATs increase to around 1.5" overnight, improving rainfall efficiency), though most showers will be light with VFR-MVFR favored. Confidence is low regarding the northern extent & onset of this activity as a result of drier air near the surface to start, with the HRRR bringing patches of steadier rain farther north (KBFD/KIPT) & most other guidance farther south. Best forcing for a more organized batch of rain is late tonight into Saturday morning.

Steady rain in the morning will give way to scattered showers heading into Saturday afternoon, with moderate confidence in timing of this transition. The probability for lightning is low.

Outlook...

Sun...Fog and low cigs possible in the morning, then scattered afternoon showers and storms.

Mon-Wed...AM fog; otherwise VFR. Trending hotter with isold PM t-storms psbl.

CTP WATCHES/WARNINGS/ADVISORIES

None.