textproduct: State College

WHAT HAS CHANGED

*Decreased rainfall totals for tonight across northwest PA. *The potential for severe thunderstorms on Tuesday has increased for most of central PA. *Lowered high temperatures for Wednesday.

KEY MESSAGES

1) Multiple chances for showers and thunderstorms through the end of the week. Severe thunderstorms possible on Tuesday.

2) Very warm temperatures continue through the middle of the week. Uncertainty in the temperature forecast increases for Thursday and Friday.

DISCUSSION

KEY MESSAGE 1: Multiple chances for showers and thunderstorms through the end of the week. Severe thunderstorms possible on Tuesday.

Weak warm advection will result in a few showers developing across the region through the afternoon. Better forcing arrives tonight with a 40 to 50 knot southwesterly low-level jet, which should help increase the coverage of showers and thunderstorms across northern PA. Recent guidance suggests that the heaviest rainfall through Tuesday morning will remain north of the New York border, but we could still see up to half of an inch of rain across parts of the northern tier of central Pennsylvania.

After a lull in the precip for the first half of the day on Tuesday, a few rounds of showers and thunderstorms are expected for the afternoon and into the evening. There is some uncertainty regarding the coverage of storms for the early afternoon as model soundings show a strong capping inversion over most of the area. If storms do form, the environment appears favorable for severe weather. Temperatures in the mid 60s and 70s and dewpoints in the mid to upper 50s should allow for at least some destabilization, and the HREF shows surface- based CAPE in the 500-1000 J/kg range. 0-6km bulk shear of 30 to 45 knots will be supportive of organized convection. Model hodographs show decent curvature in the low levels, with 0-3 KM SRH in excess of 200 m2/s2. Damaging winds appear to be the primary threat with these storms, but a conditional hail and tornado threat is not off the table. SPC has expanded the slight risk to include the northern half of Central PA.

A second round of thunderstorms is expected to move into the region in the late evening ahead of a slow moving cold front. These storms will weaken as they move into a more stable environment, but will at least initially be capable of producing strong to severe wind gusts. Heavy rainfall will also be a concern with these storms and isolated instances of flooding may be possible.

The cold front will slowly progress southward across Central PA on Wednesday, bringing additional showers and thunderstorms to the region. The severe weather threat appears low at this time. The front begins to lift northward again on Thursday, keeping precip chances in the forecast, though confidence is lower on timing and placement as it will depend on the movement of the front.

KEY MESSAGE 2: Very warm temperatures continue through the middle of the week. Uncertainty in the temperature forecast increases for Thursday and Friday.

Strong southerly flow will allow for temperatures to remain well above normal through the middle of the week. Highs on Tuesday will be in the upper 60s across the north to near 80 along the Mason-Dixon Line. Warm temperatures are expected again on Wednesday for areas south of I-80, but areas north of the cold front likely stay stuck in the 50s. The front attempts to lift northward again on Thursday, but some guidance suggests a backdoor cold front may delay the return of warmer temperatures across much of Central PA. The NBM has a 20 to 25 degree spread for high temperatures on Thursday across the region, so it is certainly possible that our current forecast, which is closer to 50th percentile, ends up being too warm. If this does end up being the case, the cooler temperatures should be short-lived as Friday will likely see highs in the 70s for all of central Pennsylvania.

AVIATION /01Z TUESDAY THROUGH SATURDAY/

Rain showers have developed across portions of northern PA this evening as west-southwesterly flow ahead of an approaching low pressure system brings mild, moist air across the region. Prevailing VFR is expected across most of central PA through 00z Wednesday, though flight restrictions are possible across the Northern Tier (KBFD) and the Allegheny Front (KJST). Moreover, LLWS is expected regionwide through 12z-16z due to a 40-50 kt LLJ developing overnight. LLWS chances diminish by midday Tuesday as daytime heating mixes out the boundary layer, with 10-15 kt winds gusting to 20-25 kts expected regionwide in the afternoon.

For the Northern Tier, proximity to a stationary front draped across Upstate NY and southern New England will be a focus for moisture convergence and shower/thundershower activity through the morning hours. The latest LAMP guidance signals high confidence (60-80%) in MVFR ceilings at KBFD after 03z, with a window for IFR (30-50%) between 10z and 14z, associated with a potential round of showers during that time (as shown by CAMs such as the HRRR, WRFs, and 3km NAM). Moreover, with MUCAPE progged to increase to around 300 J/kg overnight, an isolated thunderstorm cannot be ruled out.

Across the Alleghenies, a combination of moisture advection and orographic lift may contribute to lower ceilings overnight, with the latest LAMP bringing 30-50% probabilities for MVFR ceilings at KJST from 10z to 16z. While the HREF suggests MVFR ceilings might linger after 16z, it seems more probable that daytime heating and southwesterly winds of around 15 kts would raise ceilings to VFR through mixing, as suggested by the HRRR.

Elsewhere, VFR is expected through 00z Wednesday, along with lower rain shower coverage as the best forcing for ascent is positioned farther north. With that said, mean SBCAPE values on the HREF rise to around 1000 J/kg Tuesday afternoon, which suggests at least a conditional risk for showers and thunderstorms. The HREF has 1-hr PoPs fairly low (10-20%) for much of central PA Tuesday afternoon/evening, owing to a lack of organized forcing during that timeframe, with higher PoPs (30%+) primarily after 00z Wednesday.

Outlook...

Wed-Sat...Numerous SHRA with restrictions possible, especially on the cool side of a frontal boundary that oscillates across central PA during this timeframe.

CTP WATCHES/WARNINGS/ADVISORIES

None.