Marine Weather and Tide Forecast for Mercersburg, PA

NEAR TERM /THROUGH TONIGHT/
In the very near term, radar trends show a belt of rain showers lined up from near Cleveland, OH into the Laurel Highlands through the predawn hours. Patchy dense fog in the eastern portion of the Lower Susquehanna Valley should dissipate as the westerly flow picks up through sunrise.

Below normal 500mb heights and 925-700mb temperatures within a mean cyclonic/westerly flow aloft will spell another mostly cloudy, breezy and unseasonably cool day across CPA with passing rain showers. Highs generally in the 50-60F range are 10 to 20 degrees below the historical average for late May. We favored a high POP/low QPF solution leaning "wetter" on the margin (QPF <0.10" in most places) with more low clouds into tonight based on the blended hires model guidance. Some breaks in the clouds are likely tonight particularly downwind of the mtns along the southern tier of CPA. Max POPs after 00Z should be focused over the north central Alleghenies. Low temps in the low to mid 40s are below climo with the largest departures on the order of -5 to -10F found along the MD line.

SHORT TERM /SATURDAY THROUGH SUNDAY NIGHT/
Overall, not much change in the large scale pattern with broad cyclonic flow through Memorial Day weekend. Coverage of rain showers should be reduced to some extent Saturday and especially Saturday night as shortwave ridging briefly builds over the area in advance of a reinforcing trough. This feature should drive a ramp up in shower activity by Sunday afternoon.

High pressure finally brings and end to the rain showers Sunday night. Any clearing could provide some downside risk to minimum temps and possibly result in patchy frost over the western highlands with lows currently fcst in the upper 30s to near 40F.
This pattern evolution appears to salvage at least one nice (Memorial) day over the extended holiday weekend.

LONG TERM /MONDAY THROUGH THURSDAY/
Best chance for a dry day will be Memorial day and perhaps most of Tuesday.

Similar pattern to what we had yesterday (Wednesday) sets up by next Wednesday again. Another sfc low stalls over SW Ontario, as a coastal low reforms off the VA coast. No real sign of the pattern changing. Some model variation to where the upper level trough is by next Saturday, which would determine how far west the area of rain lingers. Cooler than normal temperatures expected again. A very amplified pattern for late May.

More information below.

If you squint really hard at the upper air charts for Sunday, you might be able to convince yourself that upper level flow is becoming slightly less trough-y, but that's probably just wishful thinking. We are growing less optimistic to see any marked improvement before Monday. The recent blocking pattern has proven to be rather persistent and in no hurry to unravel.
The cyclonic flow and cool temps aloft should translate into a good deal of cloud cover with POPs peaking during the afternoon hours coincident with diurnal heating.

Precip probs should bottom on Monday before ramping higher into Tuesday-Wednesday. Models are coming into better agreement in tracking the southern stream wave farther to the north. There are even signs that the briefly split-stream flow will try to close off into another upper low in the Great Lakes/Ohio Valley region by the middle of next week. So it appears the cool and wet pattern will continue into the end of the month with no signs of summer weather on the horizon (meteorological summer begins on June 1).

AVIATION /12Z FRIDAY THROUGH TUESDAY/
As W-NW flow continues to take over, we've settled into a cold- season regime with mainly MVFR ceilings along with scattered light showers over the NW half of PA and VFR conditions in the Susq Valley. By midday, all terminals except BFD should be VFR.
Expect winds to become breezy with frequent gusts of 15-25 mph before shower chances increase areawide this afternoon with brief local restrictions.

Still a chance of a shower Saturday into Sunday, as weak systems rotate around a deep upper level low. Best chc for dry weather will be Memorial Day.

Outlook...

Sat-Sun...Still a chance of a shower.

Mon...Dry with VFR conditions.

Tue...Widespread showers expected again.

CTP WATCHES/WARNINGS/ADVISORIES
None.