textproduct: State College

WHAT HAS CHANGED

* Eastward expansion of the SPC Slight risk for Monday * Increasing confidence of strong to severe storms, but timing and placement confidence remains low

KEY MESSAGES

1) Multiple rounds of showers and thunderstorms possible on Monday, heavy downpours, gusty winds, and hail possible.

2) The unsettled pattern continues after Monday with daily afternoon storms chances through mid week.

DISCUSSION

KEY MESSAGE 1: Multiple rounds of showers and thunderstorms possible on Monday, heavy downpours, gusty winds, and hail possible.

The remainder of Sunday will remain dry and pleasant with fair weather cumulus clouds overhead. The forecast becomes tricky after today though with multiple rounds of storms possible for monday, but low confidence in the evolution and placement of the large scale forcing.

An MCS developing over Lake Michigan will slowly continue to progress eastward through the rest of this afternoon and into tonight. This feature and how it evolves with time will play a key role in thunderstorm development over central Pennsylvania tomorrow. The timing of the MCS in the morning as if moves over Lake Erie along with multiple 850mb shortwaves will serve as the catalysts for storm initiation.

Current model solutions depict two main rounds of storms for tomorrow. One is in the morning along the outflow of the MCS. Morning CAPE in northwestern PA will be between 500-1000J/kg and available bulk windshear will be around 25kts. These storms will favor a linear storm motion and could contain strong to severe wind gusts as they propagate along the MCS cold pool. While the environment that early in the morning is not entirely favorable for stronger storms, the possibility is still there. The second round of convection would be most likely to initiate in the mid to late afternoon as a second shortwave comes across the Allegheny Front. The environment at this time would more supportive of severe storms if they can initiate. Widespread CAPE between 1500-2000J/kg is being depicted in the mean 12Z HREF solution along with area wide 30kts of bulk shear. This is the solution with the highest confidence as of right now, but the question will come down to where storms initiate and the eastward extent of those storms. If the MCS cannot maintain its current strength and speed overnight then Monday could look entirely different than what was just described.

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KEY MESSAGE 2: The unsettled pattern continues after Monday with daily afternoon storms chances through mid week.

After Monday we can expect continued daily afternoon chances for showers and thunderstorms at least through Thursday. Repeated short waves rotating around a larger upper level low, along with moisture injections from the gulf, and daily highs in the upper 70s to mid 80s will provide ample instability and lift for afternoon storms. It is too early to say with any confidence whether or not these storms will pose a significant severe threat, as these will mostly be typical afternoon summer time storms. However, the SPC has painted an area of 15% probabilitiesfor severe weather on Wednesday that includes portions of southwestern PA.

While severe weather confidence remains low for Tuesday through Thursday, there is likely going to be a daily threat for isolated to scattered flash flooding. Model mean PWATs are between 1.5-2" meaning these storms will likely be efficient rain producers, and given current antecedent conditions it would not take a ton of rain to cause problems in areas that have seen repeated flash flooding over the past week.

AVIATION /18Z SUNDAY THROUGH FRIDAY/

VFR conditions will continue through the rest of the day and into the overnight hours. The forecast then begins to become much more uncertain. Most guidance shows a line of weakening showers and storms moving in from the northwest late tonight into early Monday morning. BFD, JST, UNV, and IPT are the most likely to see any impacts from these storms, potentially seeing brief drops to MVFR or IFR. A period of MVFR ceilings will be possible behind this precip at BFD and possibly JST as well.

The forecast for the afternoon is highly dependent on the evolution of this morning convection. Possibilities range from scattered showers and thunderstorms with minimal impact, to a line of strong to severe thunderstorms moving across most of central PA. JST and AOO would be most likely to see thunderstorms prior to 18Z, so PROB30s have been included at those two sites for now.

Outlook...

Mon-Thu...Scattered showers/storms. Restrictions possible.

Fri...Mainly VFR. A few showers or storms possible over southwestern PA.

CTP WATCHES/WARNINGS/ADVISORIES

None.