textproduct: State College

WHAT HAS CHANGED

* Added more detail about potential severe weather today and on Wednesday.

KEY MESSAGES

1) Late summer surge of heat and humidity brings multi-day severe thunderstorm and excessive rainfall threat to kickoff September

DISCUSSION

KEY MESSAGE 1:

A series of mid level impulses tracking eastward along the northern periphery of an expansive 500mb ridge will interact with anomalous moisture/instability and strong WNW flow aloft to promote a multi-day severe storm and excessive rain risk across portions of CPA today through Thursday.

A surge of late summer heat (max temps 80-90F range) with dewpoints in upper 60s to mid 70s will result in a very moist boundary layer with impressive MLCAPE values in the neighborhood of 1500-3000 J/kg given mid level lapse rates around 7 C/km. Forecast soundings also show robust deep layer/effective shear values and appreciably curved low level hodographs. As such, a couple of supercells will be possible. From a large scale lift perspective, rising motion may be at least partially suppressed by the right exit region of an upper level jet max through much of today until at least mid afternoon. Convection initiation is more likely in the 4pm-9pm time frame as the boundary layer RH increases and the right entrance region of the jet max approaches.

Increasing MUCAPE values late tonight through Wed morning suggest storms/severe risk likely won't be tied to the typical diurnal cycle. Any preexisting MCS upstream could be maintained by the unstable atmosphere with us Wednesday morning. Hires model guidance also shows an easterly flow across our eastern zones associated with a back door cold front. Along that boundary, models depict very large hodographs associated with a strongly veered wind profile, with 0-3 km SRH on the order of 350 m2/s2 and 0-3km wind shear approaching 40 kts. If supercells can ride southeastward on the unstable side of the boundary and ingest this highly sheared inflow, a couple stronger tornadoes can not be ruled out Wed afternoon into Wed evening.

The severe risk will be focused more across the southern tier of central PA by Thursday, and south of the PA-MD border on Friday. An upper trough passage suggests some lingering shower activity into the weekend, otherwise it looks mostly dry.

The late summer heat is forecast to peak today and Wednesday with very warm nights +10-20F above the historical average and max heat index values just short of Heat Advisory criteria (100F). The exception will be across the eastern zones Wed when the back door cold front could hold max Ts down a few to several degrees. The upper ridge is forecast to retrograde by the end of the week which allows for increased troughing and a return to more seasonal temperatures.

AVIATION /12Z TUESDAY THROUGH SATURDAY/

Warm frontal boundary extending from the Eastern Finger Lakes region, south through the Western Poconos and SE PA will meander a bit to the south and west over the next 24 to 48 hours. This airmass boundary will provide an avenue to a few rounds of showers and afternoon/evening thunderstorms to track southeast along.

The most likely places to see fog/low CIGS through the mid morning hours today will be at IPT and to a lesser degree at MDT/LNS.

BFD could see brief periods of MVFR ceilings early today, but IFR seems unlikely.

Any fog or stratus will quickly mix out in the 12-13Z period. Attention will then turn towards afternoon convection.

Isolated to scattered showers and thunderstorms are expected for the mid afternoon to early evening hour today, but there remains uncertainty as to the exact timing and placement given the lack of any larger-scale, deep-layer forcing and the presence of the subsiding, thermally indirect branch (right exit region) of a 40-50 kt mid/upper level jet max embedded in the NW flow. Some guidance would suggest storms in the northwest as early as 15Z and others suggest convection may not arrive until 23Z. Overall, expect the potential for brief periods of active convection at all terminals across central PA this afternoon/evening, with a hedge toward the airfields across the eastern third of PA. Brief reductions to visibilities and ceilings to low end MVFR or IFR are possible in the 18Z Tue-02Z Wed period.

Outlook...

Tue-Fri...Scattered SHRA/TSRA with brief restrictions possible; morning fog possible.

Sat...SHRA/TSRA trending less likely as cold front moves southward across the Commonwealth; morning fog possible.

CTP WATCHES/WARNINGS/ADVISORIES

None.