Marine Weather and Tide Forecast for Mercersburg, PA

KEY MESSAGES
1) Mainly dry conditions with variable amounts of high based stratocu and altocu clouds will persist through Saturday with any precipitation chances confined to the southern tier of the state.

2) Sunday will bring a return to wet conditions with showers and thunderstorm chances returning.

DISCUSSION
KEY MESSAGE 1: Mainly dry conditions with variable amounts of high based stratocu and altocu clouds will persist through Saturday with any precipitation chances confined to the southern tier of the state.

A northwesterly upper level jet core will extend from southern Ontario to the New England Coast. As the right entrance region of this jet drifts over western and southern Pennsylvania this afternoon it will interact with a remnant MCV from convection that is currently ongoing across Illinois and Indiana.

This area of more focused ascent should lead to another round of clouds and some shower development in the southern tier of the state later today.

Main timing looks to be from the late morning to mid-afternoon.
The ascent may allow for some spotty afternoon showers elsewhere, but dry air filtering in from the north should keep things fairly isolated.

In the wake of upper level features, drier air filters in from the north and skies should start clearing this evening into Saturday. Noticeably less humid conditions will accompany the start of the weekend with any isolated rain chances looking to remain in the southwest.

KEY MESSAGE 2: Sunday will bring a return to wet conditions with showers and thunderstorm chances returning.

A low pressure system moving through Canada will allow a stalled frontal boundary to move through Pennsylvania as a warm front on Sunday. This will bring a moist airmass, with PWATs possibly exceeding 2" into Pennsylvania. With a secondary low likely forming closer to Pennsylvania at the intersection of the warm front and a trailing cold front, this will invigorate development of showers and thunderstorms. With a cold front draping across Pennsylvania Sunday night, multiple rounds of showers/thunderstorms may be possible into the night.

As it stands right now, ensemble guidance has the northern tier of Pennsylvania targeted for highest rain probabilities and totals. Exact timing of this activity and how fast/slow the cold front moves through will likely become clearer as we approach Sunday. With such high PWATs and potential for multiple rounds/training activity, flooding potential will need to be watched. Should the cold front stall/move slower, activity may linger into Monday morning. After that passage, the start of next week is showing signs of high pressure moving over the region.

AVIATION /18Z FRIDAY THROUGH WEDNESDAY/
Light rain around KJST through 21Z with MVFR cig/vis restrictions possible. The rain will shift south of KJST into the evening.
Elsewhere, high confidence in VFR into tonight. Model guidance trended bearish on fog potential particularly across the northern airspace so adjusted to VCFG with the 14/18Z scheduled issuance. VFR prevails through Saturday afternoon with increasing clouds into Saturday night. Showers could arrive by early Sunday morning. Sub VFR conditions are probable at times throughout Sunday with periods of rain and scattered heavy thunderstorm downpours possible.

Outlook...

Sat...AM fog possible, otherwise VFR.

Sun...Restrictions probable in showers/thunderstorms.

Mon-Tue...VFR most of the time. Chance of showers/tstm Monday.

CTP WATCHES/WARNINGS/ADVISORIES
None.