Marine Weather and Tide Forecast for Thornburg, PA

NEAR TERM /THROUGH SUNDAY/
KEY MESSAGES:

- Snow returns tonight, transitioning to rain Sunday - 1-3 inches north of I-80, locally up to 4 inches in spots - Glaze of ice accumulation possible in ridges of SW PA and northern WV, where a Winter Weather Advisory is now in effect from 3am until Noon Sunday.

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Surface high pressure is exiting to our east today as low pressure deepens over the Upper MS Valley, crossing into the Great Lakes tonight. Precipitation chances will initially overspread the area from the west late this evening and overnight -- initially as snow for most before transitioning to rain and a rain/snow mix with strong warm advection Sunday morning and afternoon. There are two primary areas of concern with this system: 1) accumulating snow along/north of I-80, and 2) potential for freezing rain in the ridges of southwest PA and northern WV.

Regarding concern number 1, at this time guidance continues to suggest the most likely snowfall amounts range from 1-3 inches along and north of the I-80 corridor with lower totals farther south. Ensemble probabilities for exceeding 3 inches continue to decrease, with the NBM now showing a less than 10% chance.
Additionally, the NBM now only shows a 20-30 percent chance of exceeding 2 inches. That said, hi-res guidance continues to suggest the possibility for some convective snow showers Sunday morning before transitioning to rain or ending completely.
Therefore, a few localized amounts up to 4 inches can't be entirely ruled out, but given such low probabilities and the isolated nature of these higher totals (should they occur), the decision now is to continue holding off on issuing any winter headlines north of I-80.

Regarding concern number 2 from above, investigation into model soundings in the ridges of southwest PA and northern WV suggest that supercooled water content in clouds and near- surface thermal profiles may be supportive of freezing rain mixing in with rain/snow, primarily during overnight and early morning hours Sunday. Therefore, a glaze of ice accumulation will be possible and a Winter Weather Advisory has been issued for these areas from 3am until Noon Sunday.

Widespread precipitation should end through Sunday afternoon as the cold front crosses. Lingering rain and snow showers, favored north of Pittsburgh, will transition back to all snow as temps cool behind the front towards sunset Sunday evening. Any additional snowfall amounts would be light.

SHORT TERM /SUNDAY NIGHT THROUGH MONDAY NIGHT/
KEY MESSAGES:

- Snow north of Pittsburgh trails off Sunday night - Dry and cold Monday - POPs rise again by Monday night

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Snow showers linger north of Pittsburgh in NW flow early Sunday night. An additional 0.5-1" will be possible along the I-80 corridor overnight. POPs trail off overnight as dry advection takes over and short lived high pressure noses in across the Ohio River Valley.

High pressure continues dry conditions through the day Monday but few breaks in the cloud cover are favored behind our exiting system and ahead of our next system. High temps remain well below average for this time of year, peaking in the low to mid 30s.

Another shortwave builds through the flow and moves eastward towards the Great Lakes by Monday afternoon. At the SFC, this drags a low up from the Gulf Coast and through the Southeast, positioning us on the far northern edge. As such pops begin to increase quickly overnight Monday night with snow favored to begin before sunrise.

LONG TERM /TUESDAY THROUGH SATURDAY/
KEY MESSAGES:

- Active pattern continues - Snow and rain return Tuesday, then again Thursday/Friday - Below normal temperatures through the period -------------------------------------------------------------------

The exact storm track through the southeast and off the Atlantic Coast will determine the exact progression of precipitation types for Tuesday. At this time models have shifted southward over the last 24 hours putting more of the region in the cold air and raising chances for accumulating snowfall.

Stout warm advection up the ridges hints at the chance to see some freezing rain in our WV ridges overnight Monday. Ice accumulations of a glaze up to a few hundredths could be possible by Tuesday morning.

As temperatures rise during the day areas mainly south of Pittsburgh can see a changeover to a rain/snow mix, with areas north likely to stay all snow. Snowfall totals are likely highest north of Pittsburgh and especially along the I-80 corridor, where probabilities for advisory level snow are the highest and probabilities for warning level snow peak between 10-20%.

Uncertainty increases closer to Pittsburgh as the exact location of the transition zone will largely factor into how much snow areas see. The noted southern shift in modeling over the last 24 hours would favor more snow in the metro and currently paints the relative minimum of snow across the Mon Valley. If this trend continues, in a colder and snowier solution, we could end up needing a Winter Weather Advisory for much of the forecast area outside of the Mon Valley. However, we will have to wait as fluctuations in exact track will shift the transition zone and totals over coming forecast cycles.

POPs begin to trail off by Tuesday night as high pressure enters the Ohio Valley. This returns dry conditions for Wednesday before snow chances pick up again late week with another passing system. Temperatures are likely to remain below average through the period favoring daytime highs in the low to mid 30s.

AVIATION /18Z SATURDAY THROUGH THURSDAY/
Increasing mid-level cloud cover is overspreading the area from the west ahead of an approaching low pressure system, which arrives tonight and brings widespread precipitation chances to the area as well as increasing probabilities for MVFR to IFR restrictions. Thermal profiles suggest initial precipitation may fall as snow and have some potential for accumulation (favoring ZZV/BVI/FKL/DUJ) before warm advection aides in a changeover to rain Sunday morning. Precipitation ends with the passage of a cold front Sunday afternoon, but a few post-frontal rain/snow showers may linger at FKL/DUJ into Sunday evening.

Outlook...
VFR returns to the area on Monday as high pressure moves across the area. This is quickly followed by yet another low pressure system, which moves into the area and brings another round of wintry mix and associated restrictions on Tuesday.

PBZ WATCHES/WARNINGS/ADVISORIES
PA...Winter Weather Advisory from 3 AM to noon EST Sunday for PAZ074-076.
OH...None.
WV...Winter Weather Advisory from 3 AM to noon EST Sunday for WVZ512-514.