Sugar Mountain

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Sugar Mountain, NC

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Forecast Discussion

Summary

SYNOPSIS

... An upper low pressure system will support rain and snow showers as it passes through the region Friday afternoon into Friday night. An arctic air mass building in behind the system this weekend will support well below normal temperatures. Temperatures rebound early next week ahead of our next system.

NEAR TERM /UNTIL 6 AM FRIDAY MORNING/... As of 250 PM : Quiet weather will persist through tonight. Zonal flow aloft will give way to developing southwesterly flow tonight as we see height falls ahead of our next system. Surface high pressure will continue weaken and slide east of the area, supporting light south winds developing by this afternoon and continuing into tonight. Increasing mid to high clouds tonight along with weak warm advection will support overnight lows 5 to 10 degrees warmer than last night, approaching normal for this time of year.

SHORT TERM /6 AM FRIDAY MORNING THROUGH SATURDAY/... As of 230 PM EST Thursday: Guidance continues to indicate that a midlevel trough will sharpen over the Tennessee Valley Friday evening, even as a 120kt upper jet shifts over the Carolinas, amplifying into a split stream pattern aloft. This allows upper divergence to blossom off the NC coast overnight, and paired with deep-layer DPVA streaming into the Carolinas, this will result in rapid, potent cyclogenesis off the central Georgia coast. Both deterministic and ensemble guidance continues to favor the eastward, just-offshore track for the developing surface low, which suppresses a robust warm conveyor belt across the NC/SC Piedmont and inhibits development of a warm nose aloft, allowing for a rare rain-snow-only forecast for the NC Piedmont and SC Upstate.

From perhaps 00Z Saturday onward, strengthening NW flow across the mountains will result in a widespread rain-snow mix beginning Friday evening, transitioning to all-snow by midnight. Lee troughing east of the mountains appear to result in the development of an additional circulation over the Piedmont, depicted in the HREF as a small-scale low pressure system which skirts just north of the NC-SC border. As a result, a changeover from rain to snow is expected across the NC Piedmont and at least the northernmost part of the SC Upstate, before profiles dry out leading up to daybreak Saturday. So, at this point, the NC Foothills/Piedmont and SC Upstate are forecast to see small accumulations, below advisory criteria; the NC mountains will see more appreciable snowfall. A Winter Weather Advisory has been issued for Buncombe and northern Jackson Counties, while an elevation-dependent Winter Weather Warning-Advisory has been issued for counties along the NC-TN border.

After daybreak Saturday, the rapid intrusion of dry cP air on the backside of the departing system will sap profiles of their ability to produce precipitation, even as Z500 heights rebound. Gusty winds embedded in post frontal CAA are likely across the mountains and, more sporadically, the NC/SC Piedmont. Clear, dry conditions will maximize radiative cooling Saturday night, allowing temps to drop into the mid- to low-teens across most of the forecast area.

LONG TERM /SATURDAY NIGHT THROUGH THURSDAY/... As of 250 PM Thursday: An arctic air mass in place by Sunday morning will lead to well below normal morning lows in the 10s to near 20 degrees. Rebounding low level heights and thicknesses will support a warming trend Sunday with highs in the 40s replaced by mainly 50s by Monday. Increasing moisture and clouds Tuesday followed by rain with our next system Wednesday into Thursday will limit any additional warming for the daytime highs. Daytime lows will continue to climb thanks to insolation effects of the clouds and increased low level mixing, leading to lows mainly in the 40s by mid-week. Precipitation may start out as a wintry mix across the mountains into the I-40 corridor Wednesday morning, before quickly turning over to rain as warm air advection overcomes the lingering cold air mass. Precipitation falling into the retreating cold air mass may set up a hybrid wedge, potentially protecting much of the area from surface based instability and any severe weather threat on the front side of the system. The bigger threat may be excessive rainfall as deep layer southwesterly flow set up over the region Thursday into Friday with a long fetch of moisture originating off of the Gulf of Mexico with high water vapor transport resulting in precipitable water values surging to around 1.5 inches which would be near record value for this time of year. The prolonged southwest flow might support a couple of waves of low pressure, maintaining periods moderate to heavy rainfall across the region for a day or so. Fortunately, antecedent conditions will be dry, so unless copious amounts of water fall from the sky, the rainfall will mostly be beneficial in putting a dent in the ongoing drought while reducing any fire weather concern.

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