Marine Weather and Tide Forecast for Chambersburg, PA

KEY MESSAGES
1) After a lull tonight and early Sunday, gusty winds return Sunday afternoon through Monday, with the potential for strong to severe thunderstorms Monday afternoon or early evening.

2) Following Monday storminess, a period of sharply colder temperatures is expected through mid-week, with some snow possible Monday night, as the cold air first builds in.

DISCUSSION
KEY MESSAGE 1: After a lull tonight and early Sunday, gusty winds return Sunday afternoon through Monday, with the potential for strong to severe thunderstorms Monday afternoon or early evening.

As of late Saturday afternoon, skies continue to clear from south to north across the Commonwealth, heralding the approach of a narrow surface ridge axis. Fair and seasonably chilly conditions are foreseen overnight and early Sunday.

By mid to late afternoon Sunday, as the above mentioned ridge axis shifts offshore and a significant storm tracks from the Plains towards the Upper Midwest, a return S-SE flow will start to increase across PA. As this occurs, we also expect lower stratus clouds to begin to overspread much of the region, along with periods of light rain and drizzle Sunday night.

Monday continues to be a period of high focus, as model consensus zeroes in on the potential for significant convective development. At this early juncture, confidence is quite high on the synoptic-scale evolution (the approach of a negatively tilted upper-trough, an associated sharp cold front, and impressively strong wind fields at all levels). More uncertainty still has to be dealt with, though, in terms of smaller-scale factors, such as: exact timing of the cold frontal passage, can any breaks develop in the overcast conditions that should prevail to start the day (which would enhance instability)?, and convective mode (will any discrete cells form ahead of a probable intense squall line?). Bottom line, though, ultimately we have increasing levels of confidence in high wind producing thunderstorms Monday afternoon, with the best potential for damaging gusts near and south of I-80, as well as east of I-99.
Please stay tuned for later updates over the next couple of days.

KEY MESSAGE 2: Following Monday storminess, a period of sharply colder temperatures is expected through mid-week, with some snow possible Monday night, as the cold air first builds in.

On Monday evening, once the surface cold front pushes through, colder air should rush in. At issue is how much steadier precipitation will linger behind the boundary. Given favorable upper jet placement and the probability that some wrap-around moisture will get steered into the Commonwealth, there is the potential for a quick 1-3" of snow in some areas Monday night, particularly over the higher terrain and northern sections of the state. Again, this will come down to smaller details, so it is advised that you stay tuned for later forecast updates.

Tuesday looks windy and much colder, with temperatures a good 25-35 degrees lower than what will be experienced on Monday (highs no better than the upper 20s-mid 30s in many locales).

Wednesday should be less harsh, but still quite chilly for mid- March, with highs in the 30s-lower 40s for much of central PA.

AVIATION /00Z SUNDAY THROUGH THURSDAY/
VFR conditions are expected at most Central PA airfields through the night with the exception of BFD where light snow showers may bring a period of IFR visibility between 07Z and 10Z. LLWS will be a concern overnight and through the day on Sunday at both BFD and JST as a 40 to 50 knot low-level jet moves in from the southwest. A brief period of wind shear is possible at AOO as well during the morning, before surface winds begin to increase. The low-level jet strengthens and moves eastward Sunday evening, resulting in very strong LLWS at all airfields into Monday.

After a VFR start to the day, expect low clouds to gradually move in from the south during the late morning and early afternoon. MVFR ceilings will first develop at AOO, MDT, and LNS before 00Z Monday, and will reach the other sites by 03Z.
Expect ceilings to lower further to IFR overnight.

Outlook...

Mon...Very strong LLWS will occur before a VERY strong cold fropa. Pre-frontal SHRA/TSRA with IFR possible. Then turning blustery and colder.

Mon night...Snow possible (eastern terminals) behind a departing cold front with MVFR/IFR restrictions across much of Central PA.

Tue...Blustery. Sct SHSN northwest. IFR possible at BFD and JST.

Wed...Warm fropa. -SN or -RA possible northwest.

Thu...Sct showers over northern PA with restrictions possible.

CTP WATCHES/WARNINGS/ADVISORIES
None.