Marine Weather and Tide Forecast for Chambersburg, PA

NEAR TERM /THROUGH TONIGHT/
GOES IR satellite shows cloudy skies across central PA early this morning. The cloud cover and persistent westerly breeze has again propped up min temps trending several degrees warmer versus blended model guidance.

Light snow and flurries lingering over the northern tier early this morning is quickly losing a connection to Lake Erie and is being driven more-so by increasing WAA and weak forcing associated with tail-end shortwave rotating through the base of the departing mean upper level trough.

Mostly cloudy and chilly conditions to start to the first weekend of December with daytime highs in the upper 20s to upper 30s or 5 to 10 degrees below the historical average.

Northern stream 500mb trough and associated clipper sfc low track southeastward from the Arrowhead of MN into the St.
Lawrence Valley this afternoon through tonight. Model consensus shows WAA snow staying mainly to the north of the PA/NY border.

Breezy conditions are expected to increase overnight with the passage of sfc trough. Hires ensemble data shows a non diurnal temperature trend in play with 12hr min temps (in the mid 20s to low 30s) likely reached shortly after 00Z followed by a neutral to slowly rising curve through daybreak Sunday.

SHORT TERM /SUNDAY THROUGH MONDAY NIGHT/
Despite a overall lack of sunshine, a noticeable warmup is on tap starting Sunday and lasting into early next week. Max temps are projected to rebound 5-10F vs. Saturday and climb into the 40-50F range (give or take a few degrees either side).

Strong WAA continues Sunday night downstream of lead southern stream shortwave reaching the middle OH/TN Valley by 12Z Monday. We remain bullish on an earlier than expected arrival of precip late Sunday night with a potential onset prior to 12Z Monday especially across the western part of the forecast area.

Key implications for onset timing include the risk of a wintry mix or freezing rain on the front end of precip shield. This mixed precip could impact the AM Monday commute as it catches up to the retreating shallow cold air. There is very high confidence in measurable precip covering all of central PA in the 12-18Z window.

The anticipated timing and evolution of precip pattern favors undercutting NBM highs for Monday with a very tight (<=3F)
max/min temp spread -- thanks to saturated low level moisture profiles and continued WAA via southerly flow.

LONG TERM /TUESDAY THROUGH FRIDAY/
10 PM Update mainly solidifies the warm up for Tuesday. That may be the last of the warm days as a wave rolling N/NE just to the east of the CWA may bring a rain-to-snow event as it pulls in colder air from the west. PoPs are highest in the east, but the potential for snow is highest in the Alleghenies and central mtns.

Prev...

The several day stretch of milder temps will last through the first half of next week, along with a period of rain Monday associated with warm advection ahead of a weakening shortwave lifting into the Eastern Grt Lks. There could initially be a brief wintry mix over portions of N Central PA depending upon the onset/arrival time, but lack of a blocking high and consensus model thermal fields point toward all rain. The low level jet and plume of highest pwats should shift east of the region by Monday afternoon and night with rain ending. Ensemble plumes indicate most areas will see rain totals of 0.25 to 0.50 inches by Monday evening. Additional warm advection ahead of an approaching warm front could support lingering low clouds and spotty showers Monday night.

EPS/GEFS still tracks a cold front across Central PA Tuesday night, which is likely to be preceded by scattered rain showers. Temperatures could potentially spike well into the 50s ahead of this front Tuesday afternoon if the region breaks into the warm sector ahead of the approaching front. It's looking increasingly likely that a changeover to snow before ending occurs on the backside of this system as the cold air catches up to the stalled baroclinic zone. A trend toward drier and colder conditions is then expected behind the front. However, an upstream mid level trough and potential secondary cold front will keep the chance of a few rain showers in the forecast Wednesday. A seasonably cold northwest flow is likely to result in scattered lake effect snow showers over the W Mtns by Thursday.

AVIATION /12Z SATURDAY THROUGH WEDNESDAY/
VFR conditions are being observed at all sited except BFD this morning, where ceilings are MVFR. Light snow showers will be possible at BFD through most of the day and may lead to brief drops in visibility. By late afternoon, most guidance suggests that all airfields will be VFR.

A fairly tight pressure gradient between high pressure south of PA and a stalled front over Southern Ontario will result in fairly gusty WSW winds today. Bufkit soundings indicate gusts up to 20kts are possible at times.

An area of low pressure passing north of the region could potentially spread a bit of light snow into extreme Northern PA Sat PM. However, odds favor it will not make it south of the NY border. A core of strong westerly winds aloft will overspread the entire region ahead of the low Sat evening. Low-level wind shear is likely at all sites as winds at 2000 feet increase to 50 to 60 knots. LLWS will first become a concern at BFD and JST around 00Z Sunday. The threat will spread eastward through the night with all sites being impacted by 09Z.

Outlook...

Sun...Low cigs possible NW Mtns.

Mon...Rain/low cigs likely.

Tue...Low cigs possible, mainly N Mtns.

Wed...Rain/low cigs possible. Possibly turning to snow over the Central Mtns.

CTP WATCHES/WARNINGS/ADVISORIES
None.