textproduct: State College

WHAT HAS CHANGED

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

KEY MESSAGES

1) Dry conditions for the rest of today with skies varying from partly sunny across the Northern Mountains to generally cloudy over the southern half of the state.

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: Dry conditions for the rest of today with skies varying from partly sunny across the Northern Mountains to generally cloudy over the southern half of the state.

The cold front that pushed through the state last night remains stalled out near the Mason-Dixon Line.

Dewpoints were generally in the 50s to mid 60s respectively from north to south across PA, while readings were in the mid to upper 60s across Northern MD, VA and WVA just to the south of this boundary.

High temperatures today will range from the low to mid 70s over the higher terrain of the Northern Mountains and Laurel Highlands to the mid 80s in the Lower Susquehanna Valley.

There still could be a few showers sneaking just over the PA/MD border in Scent PA and the Lower Susq Valley early this evening, but rainfall amounts will be severe hundredths of an inch or less through dusk. --------------------------------------------------------------

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

A wave of low pressure, centered over Northern Missouri this afternoon, will drift east along the stalled out front tonight, bringing a more widespread shield of stratiform rain with embedded/trailing convection tonight 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 diminish during the midday and afternoon hours 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 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 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 /18Z FRIDAY THROUGH WEDNESDAY/

VFR will prevail this afternoon as mid-high clouds overspread the region ahead of a weak low pressure system over the Ohio Valley. A quasi-stationary front to our south will maintain drier air across our area through this evening (SPC Mesoanalysis shows PWATs generally <=1.0"), with any shower/thunderstorm activity favored south of the Mason-Dixon line where greater moisture is in place. Otherwise, westerly winds of around 10 kts this afternoon will trend light/variable this evening as the boundary layer decouples.

Ceilings will lower west to east tonight as the system approaches, with scattered -RA/-SHRA moving in 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. Patchy fog and/or low ceilings may also develop across high elevations if cloud cover is lower (via radiational fog) or showers increase low-level moisture (via low stratus).

The more organized batch of rain overnight will give way to scattered showers heading into Saturday afternoon, with low- moderate confidence in timing of this transition. Slower recovery from flight restrictions is expected where showers linger into the daytime & where more rain falls, with quicker recovery favored where conditions remain drier overnight (particularly KBFD/KIPT).

Outlook...

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

CTP WATCHES/WARNINGS/ADVISORIES

None.