Wisp Resort / McHenry

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McHenry, MD

Forecast Discussion

Summary

SYNOPSIS

... Rain will increase from west to east this evening, changing to snow overnight with the passage of a cold front. Snow accumulations of around an inch are expected for most, with the usual areas near I-80 and higher elevations receiving upwards of 4 inches. Record-breaking low temperatures may occur tomorrow night.

NEAR TERM /UNTIL 6 AM TUESDAY MORNING/... Evening update...

Adjusted timing of PoPs slightly for the next several hours to account for slightly earlier onset across western zones. Temperatures were refreshed with current obs and high-res blends to adjust timing of falling temperatures behind the front. Otherwise, the overnight forecast is on track.

Previous Discussion...

GOES Water Vapor imagery shows a significant upper-level low digging across the north-central CONUS. At the surface, a frontal boundary currently extends from the Pittsburgh area southwestward through the Ohio Valley, with precipitation across eastern OH and just beginning to push into western PA.

Mesoanalysis shows that a frontal wave has developed and is currently positioned in the Ohio Valley, somewhere east of Columbus. As the aforementioned upper trough digs into the Great Lakes, cyclogenesis of this wave is anticipated under the right-entrance region of the upper jet as it shifts northeastward up the valley.

A strengthening temperature gradient and frontogenetical forcing in conjunction with the upper-level divergence will promote a growing precipitation field along and on the west/upstream of the frontal boundary and the developing surface low. Precipitation should overspread eastern Ohio and far NW PA by this evening, eventually reaching Pittsburgh in the late evening, and ridges shortly thereafter.

Precipitation type will initially be rain at the onset of precipitation, but will transition to snow as very cold air filters in behind the departing and strengthening low as it ejects to the Northeast. Forecast soundings suggest that the surface will see snow when the surface/low-levels can support snow... so expect a transition when the surface temp gets within a few degrees of the freezing mark. This will likely be in the late evening for Franklin southwest towards Zanesville, midnight or early AM for the Pittsburgh metro, and by 3am or so for everywhere else.

Snowfall will come from two mechanisms. The first of which is the aforementioned frontal-precip. The second is that of lake-effect snow through the day Tuesday. Snowfall accumulation associated with the frontal-precip will likely end by dawn Tuesday as the system departs. Accumulations of somewhere between 0.5 and 1 inch is expected for lower elevations and south of I-80, while areas north of I-80 and the ridges may see upwards of 2 inches by this point. Snowfall accumulation at onset may be delayed by warm ground temperature, but should quickly begin to accumulate as the very cold, arctic air filters in. The second wave or mechanism, the lake-enhanced snowfall, will be talked about in the undermentioned short-term section of this AFD.

The bulk of frontal precipitation will exit shortly after the thermal profile becomes cold enough to support snow. In fact, some parameters suggest that there will only be a very short window for snowfall associated with the frontal system. For this reason, only expecting an inch or so at most in portions of eastern Ohio and have not issued a winter weather advisory for those counties.

SHORT TERM /6 AM TUESDAY MORNING THROUGH WEDNESDAY/... By Tuesday morning, most snowfall associated with the frontal system will have cleared the CWA. Strong cold air advection of an arctic airmass will be underway, leading to a very cold (potentially record- breaking) subsequent 48 hours across the forecast area, with lake- enhanced showers persisting through the first half of that period.

Strong northerlies carrying -15 to -16 degrees Celsius air at 850mb over a ~10 degrees Celsius Lake Erie will create steep lapse rates, yielding CAPE in excess of 500 J/kg. This will promote robust and fairly widespread lake-effect / lake-enhanced convection into the area through much of Tuesday.

Between the frontal precipitation and subsequent lake-enhanced snowfall, accumulations of around an inch are to be expected area- wide. The outer snowbelt counties near/north of Interstate-80 may receive between 2 and 6 inches, while the higher elevations from the Laurels south through West Virginia will likely receive between 2 and 4 inches. Therefore, a Winter Weather Advisory has been issued for these counties tonight into tomorrow. A few bands of isolated higher amounts throughout the CWA are possible with some of the more robust snow streamers that mesoscale models are hinting at. Due to low confidence in exact placement, have went with a general North to South gradient (and elevation gradient) in snowfall accumulation, ignoring any possible mesoscale snow streamers.

Shower coverage will decrease Tuesday night as the upper trough axis passes and high pressure builds into the lower levels. The resultant backing of wind from NW to W will quickly end snow shower coverage for the CWA.

Highs on Tuesday will struggle to reach 30 before dropping into the teens by Wednesday morning. A few cities may break or tie their record daily low temperature.

Cold weather will continue on Wednesday as the low-level ridging becomes centered across the CWA. Expect another day where the temperature remains below the freezing mark w/ increasing upper clouds moving in from the west.

LONG TERM /WEDNESDAY NIGHT THROUGH MONDAY/... Another quick-moving shortwave will move north of the region Thursday. With the path of the wave and a dry atmosphere in place, appears any light snow showers will remain north of the region.

Looking ahead, temperatures will gradually moderate as the synoptic- scale wave pattern becomes less amplified. The next chance of widespread precip will likely be at the end of the weekend or early next week.

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