... As a large area of low pressure slowly tracks through the Great Lakes over the weekend, a cool northwesterly flow across the region will produce periods of snow showers in the North Carolina mountains through Monday. High pressure builds into the region leading to drier conditions Tuesday. Low pressure is forecast to approach our region from Texas later in the week.
NEAR TERM /THROUGH SATURDAY/... As of 955 PM: In a surprise turn of events, a weak stationary boundary north of I-40 east of the mountains has spawned isolated rain and snow showers with some sleet/graupel mixed in. It seems that weak warm air advection near over top the boundary and cold/dry air advection aloft supported fairly strong mid-level lapse rates (~6.5 C/km), which combined for weak instability on the order of 100 to 200 J/Kg of instability. There have been some reports of light slushy accumulations in brief, but heavier showers. These showers should continue to weaken and lift north of the area in the next hour or so. Fairly widespread dense fog has formed across much of the NC Piedmont into the far eastern Upstate of SC where a Dense Fog Advisory has been issued through 4 am. Low lying areas, especially across the NW Piedmont may experience at least a brief period of freezing fog late tonight. This will be monitored closely.
As of 640 PM: The cold front has pressed east of the area. Weak high pressure nosing into the NC Piedmont has allow surface winds to weaken this evening. This combined with the lingering moisture from the earlier light rain is now looking like it will support fog development across much of the area. Will issue an SPS shortly to highlight the threat of at least patchy dense fog developing across portions of the NC piedmont/foothills. We are beginning to see mainly high based moisture associated with the leading edge of the upper low push into the southwest mountains. We are probably seeing some flurries at high elevations, but it's probably too dry in the valleys for anything to make it to the ground for now.
Otherwise, the upper low will bring deeper moisture to the NC mtns enhanced by a general w/ly h92-h85 flow. The hires models have been backing off on the snow initiation time across wrn mtns, but believe by 06z there will be -snsh developing across the higher elevations. Due to the w/ly llvl flow, upslope lift will not be maximized and thus expect mainly sct rounds of -snsh thru the day Sat. Nonetheless, the NC/TN spine and higher ridges shud get abt 1-3 inches by the time the snow tapers off Sat evening...with far wrn Graham Co possibly receiving 4-6 inches. Will go ahead and issue a high elevation winter weather advisory for the NC mtn spine counties including nrn Jackson and Macon Co. Outside the mtns, precip will be nil, except perhaps across the NC piedmont, where some measure of instability may materialize as mlvl LRs increase with the passing upper low. Temps will be too warm for any snow by the time this happens arnd 18z, but the far nrn NW NC piedmont could see a -ra/sn mix for a couple hrs with no accum.
Min temps tonight drop a couple degrees below normal and max temps Sat will be held a couple cats below normal as a cP airmass mixes in thru the day.
SHORT TERM /SATURDAY NIGHT THROUGH MONDAY/... As of 200 pm EST Friday: Deep-layer northwesterly flow and upslope moisture will continue behind the departing trough axis on Saturday evening, but with the flow turning more westerly through the overnight hours as heights fall again over the upper Midwest. Thus, any additional snow accumulations in and near the Tennessee border counties Saturday night will be mainly during the evening hours. Anticipate breezy to windy conditions along the higher terrain as the pressure gradient briefly tightens up Saturday evening as well.
The complete end of any residual upslope showers will be hard to determine since low levels never really dry out across eastern TN through Sunday. But, the increasingly west to southwest flow component should greatly cut down on the upslope forcing. Meanwhile, the next upstream system will deepen over the Ohio River Valley through the day on Sunday. The associated strong trough passage will likely occur Sunday night into Monday, but with somewhat limited moisture return. Any deeper moisture should impact mainly the northern tier of the forecast area, albeit with persistent low-level moist upslope continuing along the spine counties through Monday. Mid-level lapse rates will also be fairly steep at times with the trough passage, so the precipitation could be showery with some embedded heavier rates. Mountain accumulations continue to appear sub-advisory early Monday morning. Chilly weather will continue through the period, with subfreezing overnight mins and daytime temperatures mainly in the 40s, except high mountain 30s, on Sunday and Monday.
LONG TERM /MONDAY NIGHT THROUGH FRIDAY/... As of 210 pm EST Friday: Surface high pressure briefly returns to the southeast Monday night into Tuesday, with zonal flow aloft. Another fropa appears slated for Tuesday/Tuesday night and this will be accompanied by a brief shot of moisture, confined mainly to the mountains. The boundary should clear the region to the southeast, but there is some potential for the front to stall near the area if the upper level flow quickly becomes zonal. Otherwise, dry profiles and chilly thicknesses will briefly settle back into the region on Wednesday.
Pacific moisture transport may return quickly to the southern Appalachians Thursday through Friday, but with a great deal of model uncertainty on the location and timing of the salient features. There is decent agreement that the recently stalled frontal boundary will likely lift back northward as a warm front on Thursday as associated upglide moisture returns. The trailing cold front may then settle southeast back into the region Thursday night into Friday, but with much less confidence on the boundary location late week to provide any further focus for heavy rainfall. After robust PoPs Thursday, will thus cap values in the chance range for Friday.