Cloud bases will be dropping and temperatures turning chilly during Wednesday, with latest model trends being suppressed toward the south with rain. Locations along and to the south of the High Knob Massif-Tennessee Valley Divide will have better rain coverage than locations to the north. Temperatures should remain above freezing; although, some places in the high country of the High Knob Massif may get close to freezing the coverage of any freezing rain should be localized.
A more significant precipitation producer will develop by Friday into this weekend in advance of much colder air. A transition to snow will be possible by the Christmas Holiday, but it remains too early to forecast amounts.
Overnight Into Tuesday Morning
Partly-mostly cloudy. Windy at higher elevations. Areas of fog. SW winds 5-15 mph, with higher gusts, along mountain ridges-plateaus below 2700 ft. SW-WSW winds 10-25 mph, with higher gusts, on mountain ridges above 2700 feet. Temperatures in the 30s to low-mid 40s. Wind chills in the 20s & 30s at upper elevations.
Tuesday Afternoon
Partly-mostly cloudy. Gusty. SW-WSW winds 10-20 mph, with higher gusts. Temperatures varying from the 40s in upper elevations to the mid-upper 50s ( around 60 degrees possible in downslope locations from Pound to Grundy ).
Tuesday Night Into Wednesday Morning
Cloudy with rain developing toward morning. SW winds shifting N-NNE at 5-15 mph, with higher gusts, overnight into morning. Temperatures dropping into low-mid 40s.
Wednesday Morning Into The Afternoon
Rain & showers, especially along and south of the High Knob Massif & Tennessee Valley Divide. Low cloud bases. NE-ENE winds at 5-10 mph, with higher gusts. Temps dropping or nearly steady in the 30s to lower 40s ( coldest within upper elevations ).
Wednesday Night Into Thursday Morning
Partly-mostly cloudy. Chance of evening showers-drizzle. Light winds. Temperatures mainly in the 30s, except locally colder in high valleys at upper elevations.
The summation of multiple waves may lead to a major precipitation event developing across portions of the southern Appalachians and Tennessee Valley into this weekend.
An update tonight has removed the threat of freezing rain from upper elevations on Wednesday with most temps now expected to remain above freezing. The heaviest and most widespread rain is expected to remain south of the area, with locations along and south of the High Knob Massif-Tennessee Valley Divide expected to have best coverage.
NAM 12 KM Model Total Precipitation Forecast Next 24-Hours
Model runs Tuesday night have come into better agreement with the rain shield associated with this initial wave.
GFS Model Total Precipitation Forecast Next 24-Hours
A much stronger and more important wave will begin to impact the mountain region Friday into this weekend in advance of cold air. Stay tuned for updates on what is looking like a heavy rainfall event with orographic enhancement of amounts.
Previous Discussion
A very stormy pattern is taking shape through the rest of December as a major surge of arctic air sinks southward into the United States, setting up a strong temperature gradient and baroclinic zone ( storm track ).
High Knob Massif Webcam – University Of Virginia’s College At Wise
A persistent layer of capping pilatus ( orographic clouds ) were observed Monday across upper elevations in the High Knob Massif.
NAM Model Forecast At 1:00 PM Wednesday – December 20, 2017
The first in a series of important upper air waves will begin impacting the mountain region today into Wednesday, with gusty SW winds giving way to developing rain and chilly air with a wind shift to the N-NE.
NAM Model Forecast At 7:00 PM Wednesday – December 20, 2017
Cooling with both advection and upslope flow in low-levels will have to be closely monitored Wednesday for the chance that rain will become freezing rain at elevations above 3000 feet, with possible lowering of the freezing level by the late afternoon-evening. This would be most likely in locations along and north-northeast of the High Knob Massif and Tennessee Valley Divide where air will be forced to rise.
NAM 12 KM Model Total Precipitation Forecast
The European and NAM models are showing a potential for some snow to develop where the air may become vertically deep enough to support snow toward the Greenbrier & Roanoke valleys. In this area a layer of above freezing air looks to hold aloft as the lower levels chills on NW-ENE upslope flow.
European 51-Member Ensemble Mean 500 MB Height Anomalies: Days 5-9
By later this week into Christmas week a highly amplified upper air flow pattern will be developing as another surge of positive WAF ( wave activity flux ) perturbs the polar vortex and helps to develop cross-polar flow into the continental USA ( above ).
European Model 850 MB Temps & Anomalies For December 23-27, 207
A huge north-south temperature gradient will develop as brutally cold air drops southward through southern Canada into the northern Plains and upper Midwest, setting up a battle zone to the south. This spells trouble for many folks planning on traveling for the Christmas Holiday period.
GFS Model Total Precipitation Forecast – Next 7 Days
The summation of multiple waves has the potential to generate a major precipitation event over time. While some of this will eventually be in frozen precipitation, the positioning of the rain-snow line is yet to be determined.
