Sugar Mountain

Forecast Discussion

Summary

SYNOPSIS

... Cool high pressure will set up north of our region today. Another low pressure will bring widespread rain Wednesday night into early Thursday. Freezing rain and sleet also will develop where temperatures fall near or below freezing. Cool and dry high pressure moves over our region for the weekend.

NEAR TERM /THROUGH THURSDAY/... As of 640 AM: Under confluent upper flow, a strong sfc high is expanding out of the Midwest and Ohio Valley toward the East Coast. At the leading edge of this airmass, a cold front extends across the Southeast. A cutoff upper low is located over the ArkLaTex region and will move slowly northeast over the next 24 hours. With a weak frontal wave developing to our west, moist upglide will redevelop over the cold high and lead to another CAD event setting up by tonight. The forecast is complicated both by the remarkably deep moisture producing a heavy rain threat, and by temps being cold enough for some of that rain to change over to sleet and/or freezing rain early Thursday in parts of the area.

PoPs this morning have been lowered somewhat with radar trends indicating smaller chances of accumulating rain, though a mention of isolated sprinkles has been added through midday. As upper divergence increases with time, rain will spread back in from S to N. Thick cloud cover will keep temps generally in the upper 30s over the mountains and lower to mid 40s in the Piedmont. Rainfall rates and coverage increase more sharply late in the day through early Thursday morning as strong isentropic lift develops in advance of the frontal wave. Winds in this layer are progged to be as strong as 70 kt, and ideally oriented for upslope lift along the Blue Ridge Escarpment as well. PWAT values again are fcst to exceed the 90th percentile of climo. Runoff response from the heavy rain earlier this week exceeded expectations, and with that event priming hydrologic conditions ahead of this round, it is appropriate that we are carrying a Flood Watch. Its extent looks fine for now. QPF numbers will stick close to WPC values tonight, with some minor adjustments made to timing.

Temps are expected to fall somewhat rapidly with the onset of steady precipitation this afternoon and evening. We have the usual challenge of deciding how far they will fall. Dewpoints will be on the moist side, which will limit the wet-bulb temps. However, it still looks likely that temps will be at or below freezing tonight in a fairly contiguous area along and east of the Blue Ridge, remaining there until late tomorrow morning. The warm nose aloft will be a limiting factor for snow or sleet occurring at the sfc. It is progged to be quite deep, at about 150 mb. Though models disagree as to how warm temps get in that layer, in most areas even a model blend would suggest the ice nuclei will melt. Thus we will feature rain or freezing rain, depending on sfc temp, except in a portion of the northern mtns where the temps aloft still support some sleet. With our robust fcst QPF late tonight and early tomorrow, any of the various methods we use to fcst ice accretion result in warning-criteria numbers at elevations above 2500 feet along the Escarpment and in the adjacent ranges (the Balsams in particular). The northern mountains could result with a notable amount of sleet, up to around an inch, though that is of little impact compared to a heavy coat of ice. With a little sleet possible, and more sleet expected in Blacksburg's CWFA to our north, we are upgrading the watch to a Winter Storm Warning. This will be in effect for all of our northern mountain zones, plus the high elevations of the zones southwest of the Black Mountains. The lower elevations of those zones, plus the northern foothills and far NW Piedmont, will receive a Winter Weather Advisory for lesser ice accumulations mainly causing travel issues. With some ice possible on the ridges of far northern Greenville County, the Greenville Mountains zone is included in the advisory. One note: while the elevation-split headlines are technically in effect for all areas above 2500 feet in this event, the Smoky Mtn portion of Haywood County is not expected to see accumulating ice; the warning was issued specifically to cover the Balsams.

Precip will taper off from SW to NE Thursday morning, and by aftn temps will warm into the upper 30s in the areas affected by the ice/sleet. The upper low will be to our west by then and low-level flow will reorient to westerly along the mountain spine, setting up for a northwest flow snow event later.

SHORT TERM /THURSDAY NIGHT THROUGH SATURDAY/... As of 300 AM EST Wednesday: Short term begins Thursday night with upper low lifting into the Upper OH Valley/central Appalachians, and precip lifting north as well. At some point early in the period, as the cold core upper low moves over the region and column winds shift all W/NWerly, topographic lift combined with increasing lapse rates will result in a changeover from frozen to freezing precip, with the event ending as a few hours of NW flow snow in the mountains. Snow accums look pretty light given the lack of moisture depth in the column, but a quick 0.5-1.5" certainly possible especially across the northern mountains and maybe the Smokies. Overnight lows with the CAA will drop into the 20s across the mountains, near or below freezing elsewhere. Decent day on Friday with the damming completely scoured, and high temps rising markedly into the mid 50s in the Piedmont, still int eh 40s at higher elevations in the mountains.

LONG TERM /SATURDAY NIGHT THROUGH TUESDAY/... As of 100 AM EST Wednesday: Quiet and generally dry through the extended (a welcome change). This weekend looks exceptionally nice (we'll deserve it for sure), with high pressure in place as a (mainly) dry front sweeps through the Midwest and OH Valley. Temperatures both Saturday and Sunday look to be flirting with 60 across the Piedmont, overall just a few degrees below normal, with nearly full sun on Saturday. As the front approaches Sunday, should see an increase in upper cloudiness, and guidance is squeezing a little (slight chance) pops out over the mountains on Monday as the front moves through. The steering shortwave will dissipate as the system moves east, and the post-frontal CAA isn't too strong, but still looking at a couple degree drop in temperatures Monday and again on Tuesday