textproduct: State College

WHAT HAS CHANGED

* A Heat Advisory has been issued for south central PA valid 11 AM to 8 PM Thursday.

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

A series of mid level impulses tracking eastward along the northern periphery of an expansive 500mb ridge will continue to interact with anomalous moisture/instability and strong WNW flow aloft to promote severe storm and excessive rain risk today and tomorrow.

The first cluster of storms is already poised to move off of Lake Erie into our NW zones between 7 and 8 AM. This cluster has near continuous lightning and was severe warned along the lake shore by the Cleveland WFO. Inland from the lake shore, the sfc boundary layer is still rather stable this morning with near 150 J/kg of SBCIN, so this will at least somewhat offset the severe wind potential across our NW zones. However the wind threat could increase to the south and east IF the cluster is maintained until mid-morning, by which time morning sunshine will have eroded the sfc stable layer. Climatologically, however, these morning MCSs rarely survive the nocturnal to diurnal transition, and several CAMs do show it falling apart before the boundary layer destabilizes.

The mesoscale severe weather setup will be complex this afternoon into this evening, with key players being the presence of a north-south stationary front / backdoor cold front across the eastern zones, and whatever remains of the cold pool from the morning MCS. These boundaries will likely be the focus for new convection initiation this afternoon. Forecast vertical soundings depict large, clockwise turning hodographs associated with a strongly veered wind profile, with 0-3 km SRH on the order of 200-350 m2/s2 and 0-3km wind shear approaching 40 kts. Combined with MLCAPE on the order of 1500-3000 J/kg, this setup will likely yield both supercells and bowing line segments. If supercells or southeast-bulging bow echos remain on the unstable side of the stationary boundary and ingest this highly sheared inflow, a couple stronger tornadoes and large hailers can not be ruled out this afternoon into this evening. There will be a large CAPE gradient across the stationary front, with limited severe potential to its east.

Increased southwest to westerly flow on Thursday will push the backdoor cold front out of the region, ushering in higher temperatures and dewpoints. The Heat Index will make another run at 100-104F across the southern zones, similar to what we saw on Tuesday. We issued a Heat Advisory valid 11a-8p Thu for this likely threat.

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 additional showers and storms on Friday, but fortunately the Labor Day weekend is trending drier. The upper ridge is forecast to retrograde by the end of the week which allows for a return to more seasonal temperatures.

AVIATION /12Z WEDNESDAY THROUGH SUNDAY/

A brief period of highly localized heavy rain occurred INVOF KUNV between 0430Z and 0530Z overnight as an outflow boundary pushed wwd from earlier, widespread convection over the Middle Susq Valley and points east. Two more, brief waves of SHRA/ and potentially a few TSRA occurred through Altoona and now KJST as the boundary pushed steadily Southwestward. Expect some areas of LIFR/VLIFR until around 12-13Z tdy, esp in areas that had rain yesterday and experienced some clearing/radiational cooling resulting in fog formation.

The aforementioned frontal/outflow boundary and some mid and upper level jet streak lift will combine to blossom areas of strong to severe TSRA, the first in the form of a Mesoscale Convective System (Moving SE at 30 kts acrs NW and Central PA this morning), followed by a several hour lull, then more convection/TSRA from the mid afternoon to mid evening hours today.

PROB30/Tempo groups for -TSRA are highlighted in most terminals (exception being KIPT at this time) given a signal on CAMs for potential lines or clusters of showers/storms moving SEward across the CWA as noted above.

Outlook...

Thu-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.

Sun...VFR favored; morning fog possible.

CTP WATCHES/WARNINGS/ADVISORIES

Heat Advisory from 11 AM to 8 PM EDT Thursday for PAZ026>028- 035-036-056-057-059-063>066.