Cashiers / Highlands

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Forecast Discussion



... A vigorous cold front will cross the region from the west tonight through Tuesday morning. Dry and very cold high pressure will then settle over the area Tuesday night through Wednesday, and warm only slightly through Thursday. An upper-level low pressure system will develop over the Deep South late in the week, with an associated surface low pressure center developing over the coastal waters off the southeast coastline. Moisture from this system could brush Piedmont parts of our region Friday into the weekend.

NEAR TERM /THROUGH TUESDAY/... As of 215 PM: As Arctic high pressure continues to expand across the Plains, a strong cold front will continue to push in our direction thru the early morning hours Tuesday. In the meantime, aided by an offshore sfc high, seasonably mild conditions will continue this aftn and most of tonight, though it will be breezy at times as the height gradient strengthens surrounding the deep frontal trough. While the lower elevations will see winds subside somewhat after sunset, high elevations will be exposed to continually better gradient flow as the night continues. That said, little in the way of prefrontal isentropic lift is seen on model progs, so PoPs tonight initially reflect southwesterly upslope low-level winds into the Appalachians. The best frontogenetic forcing occurs just before dawn in the mtns, reaching the Piedmont by mid-morning.

Prog soundings reveal dramatic cooling in the PBL, promoting a rapid drop in sfc temps beginning about daybreak along the Tenn border. Deep saturation implies ice nuclei will already be present. While the soundings also suggest a warm nose will persist above the PBL for up to a couple of hours, the unusually strong front combined with the potential for evaporative cooling of said layer suggests only a very brief transition through sleet to all snow. Thus we will advertise rain/snow based on sfc temp. Hourly temp guidance was used thru the day; as precip tapers off east of the mtns and as skies clear by midday, some insolation will limit cooling but overall max temps will occur prior to the diurnal peak. QPF rapidly falls off as the front exits, but light NW-flow snow showers are possible in the most preferred areas thru most of the afternoon. The presence of the warm nose should negatively impact snow ratios; plus, the majority of the QPF will fall just prior to the cold air. Expecting the average ratio to be around 8:1, the net result is snow accums generally below 2 inches in populated areas, but in the 2-4" range on the ridges.

While wind gust potential is not especially great with this front, gusts are expected to top out close to our usual Wind Advisory criteria in the higher elevations. By late afternoon tomorrow, significant portions of the mountain zones will see wind chills below zero, but the area hitting Wind Chill Advisory levels is too small to warrant an advisory on its own. The wind chill risk will continue into tomorrow night. Furthermore, the very fast arrival of the cold air coupled with precipitation could result in wet roads freezing over by the end of the day Tuesday, which given the temps Tue night will remain treacherous into early Wed. Combining all of the relatively small impacts, and considering how early in the season it is, we have issued a Winter Weather Advisory for all areas NW of the Blue Ridge Escarpment from late tonight through tomorrow night.

SHORT TERM /TUESDAY NIGHT THROUGH THURSDAY/... As of 200 pm EST Monday: Transitory surface high pressure crossing the Appalachians Tuesday night through Wednesday will bring the coldest air of the season thus far. Anticipate Wednesday morning minimum temperatures in the teens across the mountains and 20s elsewhere, generally falling to within 3 to 4 degrees of record lows at most places. Highs will not get out of the 40s east of the mountains Wed. afternoon, and plenty of 30s max temps will be possible across the mountains.

Meanwhile, zonal flow will briefly set up over the eastern CONUS Wednesday, with varying degrees of northern and southern stream height falls developing over the western U.S. There is increasing consensus on a positively-tilted wave digging into the mid- Mississippi River Valley on Thursday, but return flow moisture at lower-levels remains quite uncertain through late day. Anticipate weak modification in the cold airmass in place, with lows still exhibiting teens along the higher peaks and 20s elsewhere Thursday morning. Afternoon highs could touch 50 in the southern piedmont, but will generally run in the 40s once again.

LONG TERM /THURSDAY NIGHT THROUGH MONDAY/... As of 215 pm EST Monday: There is growing medium range consensus on a positively-tilted trough moving east of the MS River Valley and deepening into a cutoff low pressure system over the Deep South Thursday night through Friday. The GFS is a touch deeper and farther south with this system than the ECMWF, but nearly all solutions now have the associated surface reflection moving from the northeast Gulf of Mexico Thursday night to the Carolina coastal waters Friday night. This will provide a relatively close shave on the deeper moisture north and west of the surface low, with it passing just to our southeast. The main concern is that fairly chilly and dry conditions could well still be in place across the region with any precipitation onset Thursday night into Friday. Will thus feature slight chances of rain/snow in the southeast part early Friday, but transition to mainly a cold rain, if anything, the rest of Friday into Friday night. Fortunately, the colder air in northwest parts of the forecast area and the deeper moisture in southeast sections do not appear to have any appreciable overlap with this event.

Depending on how quickly the coastal low pivots away over the Carolina offshore waters, frontogenetical forcing could linger in or near our eastern piedmont. This could once again provide a slight chance of rain/snow mixing into the forecast around the piedmont along and east of I-77. At present, it appears that this vigorous southern tier system will close off and deepen far enough south that impacts for our area should not be appreciable. But it certainly bears watching. If the system cuts off a bit farther west, and upglide moisture returns sooner into the lingering dry air, wintry precipitation types would be possible for the forecast area.

Very shallow ridging may briefly return on Sunday behind the departing coastal low before prominent southern tier height falls develop once again on Monday. However, moisture return ahead of this second system is highly uncertain at present for early next week. Temperatures will remain 5 to 10 degrees below climo through the period.