Marine Weather and Tide Forecast for Harrisburg, PA

KEY MESSAGES
1) Light upslope and lake enhanced snow showers will bring a few inches of total snow accumulation (with locally as much as 4 or 5 inches along the summit of Laurel Ridge and Mount Davis in Somerset County) late today through Thursday, mainly across Warren and Mckean Counties and the Laurel Highlands.

2) Winter weather threat for Sun-Mon timeframe continues to trend very slightly lower.

DISCUSSION
KEY MESSAGE 1: Light upslope and lake enhanced snow showers will bring a few inches of total snow accumulation (with locally as much as 4 or 5 inches along the summit of Laurel Ridge and Mount Davis in Somerset County) late today through Thursday, mainly across Warren and Mckean Counties and the Laurel Highlands.

Vertically growing/cooling boundary layer (with the upper half of the cloud layer falling within the DGZ this evening through Thursday) will initially compete/be offset by large scale subsidence beneath the left entrance region of an upper level jet.

Rather steady, albeit light snow will develop across primarily the NW Mtns and Laurel Highlands late this afternoon and evening, continuing overnight. FLurries or more brief, isolated snow showers will occur across the Ncent Mtns and Central Ridge and Valley region, where a light coating to one half of an inch is possible on the higher terrain near and just to the NW of the I-99 and Route 220 corridors.

Based on this larger scale subsidence aloft and the lack of any significant cross-haired signature within the cloud layer on model time-height cross sections ala BUFKIT, we feel that the snow will be slow to accumulate with rates under 1/2 inch per hour.

Otherwise, the wind stays gusty for the rest of this afternoon and tonight - likely generating some gusts up near 40KT on the highest ridges (mainly the Allegheny Front/Babcock Mtn). We'll continue hold off on a wind advy at this point.

KEY MESSAGE 2: Winter weather threat for Sun-Mon timeframe continues to trend very slightly lower.

A wave of low pressure is set to develop/deepen over the desert SW and TX on Fri-night. The upper feature which is expected to drive this development is still off the CA coast. Thus, upper air sampling/initialization is not infused with any actual sounding data. Old forecaster once told me to not trust anything still over the Pacific at model runtime. Advances in model init with sat data and better first-guesses have improved our long- range forecasting since then (along with the extinction of the dinosaurs). Yet, I can't shake the large amount of doubt that this upper low/trough brings to a forecast for >3000SM away and ~5 days in the future.

While many deterministic model solutions now take all of the precip associated with the low to the south of PA, all of the ensemble systems continue to keep a relatively high (50-60pct)
chc of measurable precip falling over PA sometime Sun into Mon.

Thus, the main message here is that a winter storm - or at least a meager amount of light precip - remains possible.
Progged temps on Sat night and Sun morning look sub-freezing over all of the forecast area as a high pressure/cold dome moving down from the N/NE could make a wedge of CAD. However, Sunday may hold a warming up to 35F N and 42F S. So, arrival time of (any) precip would be key to (any) precip type. With so much uncertainty, we've only shown a slight nudge downward in PoPs Sun-Mon by about 10-15pct from the previous model cycle, leaning toward the drier forecast that the deterministic model solutions continue to trend toward.

AVIATION /06Z THURSDAY THROUGH MONDAY/
Northwest flow over the region continues to produce scattered lake effect snow showers west of UNV; however, coverage has dwindled across much of the Laurel Highlands and stayed more confined to NW PA as of 12/06Z. An uptick in snow shower coverage is expected overnight, thus have highlighted mentions via PROB30s based on 30% confidence of SHSN at JST overnight.
MVFR ceilings will persist at BFD/JST/AOO overnight, gradually expanding to UNV as well although guidance remains slightly uncertain with respect to timing on lower ceilings. VFR conditions at IPT/MDT/LNS appear most likely based on HRRR/NBM model guidance overnight. Low potential for LLWS overnight at JST/AOO/MDT/LNS with increased winds at 2000ft AGL; however, based on current observations this appears less likely than before so have nixed mentions in the 06Z TAF package.

Ceilings at AOO and UNV quickly rise to VFR during the morning, while BFD/JST likely remain MVFR through the 06Z TAF period due to lower ceilings between 1500-3000ft AGL. Some guidance does show the potential for MVFR ceilings to redevelop at UNV and IPT during the afternoon, but continued VFR appears to be the more likely outcome.

Outlook...

Fri-Sat...Improving conditions late week into the weekend

Sun-Mon...Monitoring potential for winter weather on Sunday and Monday. Most likely restrictions are in southern PA at this time.

CTP WATCHES/WARNINGS/ADVISORIES
None.