Caution For Areas Of Fog Developing Friday Night Into Saturday Morning At Middle-Lower Elevations

Areas of fog, drizzle, and light rain showers will continue into the overnight hours of Saturday.  Orographic clouds will be widespread at upper elevations.

Former Alerts

Weather ALERTS ( December 27-30 )

ALERT For Strong SSE to SW Winds Developing Thursday Into Friday ( December 27-28 )

While strongest general wind speeds will be experienced at middle to upper elevations, mountain waves are expected to produce powerful wind gusts in localized locations where waves break downward on northwestern sides of major mountain barriers.

A strong pressure gradient will drive strong SSE-S winds across the mountains beginning early Thursday at upper elevations and working downward into middle to lower elevations during Thursday afternoon into Friday.

A period of strong SW winds are expected to develop behind a frontal passage on Friday into the Norton-Wise area and mid-upper elevations along the High Knob Massif and Tennessee Valley Divide.

ALERT For Heavy Rainfall, Strong Rises And Possible Flooding On Streams During Friday Into Friday Night

The first of multiple waves will spread heavy rainfall across the mountains Friday.  A Flash Flood Watch Is Now In Effect Through Friday Afternoon.

Updated_Friday Afternoon_December 28

Water levels rises are expected to remain safely below flood stage through Friday into Saturday.

A general 1.00″ to 1.50″ of rain, with locally higher amounts, was less than predicted by models and this will keep stream levels below flood stages ( 0.50″ to 2.50″ being the general rainfall extremes ).

Downstream convection, with up to 12.00″+ of rain along the Mississippi-Louisiana border, was partly responsible for less total rainfall to the northeast.

Melting of high water content snow cover at upper elevations will be adding to run-off from rainfall during Friday into Friday night within places along and downstream of the High Knob Massif, Black Mountains, and other high mountain locations impacted by the December 21 winter storm.

Mountain Area Forecast ( Dec 27-29 )

Thursday Night Into Friday Morning

Rain showers early, then rain developing.  Windy. Downpours likely.  Thunder possible.  Winds SSE-SSW 15 to 35 mph, with higher gusts ( gusts 40-50+ mph at upper elevations and mountain wave zones ). Temps varying from low 40s to low 50s.  Areas of dense fog developing overnight into early morning.  Wind chill factors in the 30s along high mountain ridges ( locally below freezing on highest peaks ).  

Friday Morning Through The Afternoon

Rain, heavy at times.  Thunder possible.  Rain tapering to showers during the afternoon.  Winds shifting SSW-SW at 10-25 mph, with higher gusts.  Temperatures in the 40s to low-mid 50s ( coolest in upper elevations and mildest in downslope areas of northern Wise and Dickenson counties ).  Areas of dense fog, widespread at upper elevations and in the SW flow upslope zone of the High Knob Massif-Tennessee Valley Divide.

Friday Night Into Saturday Morning

Low clouds.  Chance of drizzle.  Areas of dense fog, widespread at the upper elevations.  Turning chilly.  Winds shifting W-NW at 5-15 mph with higher gusts. Temperatures varying from around 30 degrees at highest elevations to around 40 degrees.  Wind chills in 20s & 30s, except 10s in gusts at high elevations.

Saturday Afternoon

Partly cloudy.  Light & variable winds, except SSW-WSW at 5-10 mph and gusty at highest elevations.  Temperatures in the upper 30s to mid-upper 40s, coolest highest elevations.

Wind chill factors in the 20s to lower 30s at highest elevations.

A second upper-air disturbance will trigger more moderate-heavy rains late in this weekend and early next week.  Water level rises will again need to be closely monitored.

This has been well depicted by the 51-member ensemble mean of the European Model group. There is increasing concern for another important, heavy rain producing wave of moisture to impact the mountains late this weekend into early next week.  This is likely to NOT be the final wave in this series of storm systems, with a trend toward colder conditions late in the 7-10 day period.

Please stay tuned for later updates.

Weather Discussion ( Worrisome )

While the short-term period is a concern, of course, I am especially concerned about the big-picture of the pattern upcoming through the next 7+ days.

A positive-tilted upper trough and SW flow aloft is climatologically favorable for heavy precipitation when it changes slowly over time with multiple short waves moving through the long-wave flow regime in this part of the world.  In addition, this is especially true for transitional periods when the synoptic-scale flow pattern is changing from warm to cold ( or cold to warm ).  In this case, the pattern will be undergoing a transition from milder to colder conditions over time.

Add to this anomalously wet antecedent conditions and the ingredients are present for serious problems to develop at any point from now into next week.

Short-range Model Predictions

While strong mountain waves are running on SSE flow in the low-levels, there will be a considerable SSW-SW flow developing at mid-upper elevations into the overnight-morning hours of Friday.

This type of strong change in wind speed and direction ( shear ) enhances convergence in the lower troposphere and often means rainfall amounts are heavier in locations west of the Blue Ridge versus when SE-SSE flow extends upward ( is vertically deeper ) through middle-upper elevations of the mountains.

The high resolution NAM model has been one of the most consistent with placement of the heavy rainfall axis, which will largely dictate what zones will have the greatest high water risk through the short-term.

The GFS Model has been tending to shift the axis of heaviest rainfall toward the northwest, along and northwest of the Cumberland Mountains.

This is similar to the high-resolution NAM.

The European Model has also been relatively consistent through past days, with a tendency toward increasing and shifting the main axis of heaviest rains to the northwest along the front range of the Cumberland Mountains and adjacent plateau.

Given a convective tendency with this system on the synoptic-scale plus an anomalous surge of tropical moisture in advance of an approaching upper wave the placement of heaviest rains can change due to convective influences downstream.

If the axis of heaviest rain does not develop where the high-resolution NAM Model predicts it is not because the model has not been consistent.

The High-Resolution Rapid Refresh ( HRRR ) model is generally in line with the high resolution NAM.

The crunch-time window will be Midnight to Noon for counties along the Cumberland Mountains and VA-KY border, with water level rises expected to continue for a period after rain diminishes during Friday PM.

The new run of the GFS has shifted a little southeast with the axis of heaviest rain, but continues to have it located across far southwestern Virginia into the adjacent sections of Tennessee.