051517 Forecast

Mountain Area Forecast ( May 15-17 )

Overnight Into Monday Morning

Mostly clear.  Areas of dense river valley fog.  Large vertical temperature spread between colder mountain valleys and milder ridges-plateaus.  Winds SW-WNW at 5-15 mph, with higher gusts, along mid-upper elevation mountain ridges and exposed plateaus.  Temperatures varying from lower-middle 40s in colder mountain valleys to mid-upper 50s.

Monday Afternoon

Partly to mostly sunny.  Winds NW-N at mostly less than 10 mph.  Temperatures varying from upper 60s to lower 70s in upper elevations to the upper 70s to lower 80s.

Monday Night Into Tuesday Morning

Mostly clear.  Large vertical temperature spread between colder mountain valleys and milder ridges.  Light winds.  Temperatures varying from 40 to 45 degrees in the colder mountain valleys to mid-upper 50s.  Areas of dense river valley fog ( widespread along the Clinch & Cumberland ).

Tuesday Afternoon

Partly to mostly sunny.  Unseasonably warm.  Winds SW at 5-10 mph, with some higher gusts.  Temperatures varying from 70s in upper elevations to the middle-upper 80s.

Tuesday Night Into Wednesday Morning

Partly cloudy during the evening with increasing high clouds overnight into morning.  S-SW winds 5-15 mph, with higher gusts, along mid-upper elevation mountain ridges.  Temperatures varying from 50s in cooler valleys to the 60s along exposed mountain ridges-plateaus.

An increasing chance for hit-miss showers and thunderstorms will be observed during late week into the upcoming weekend.  Heavy rainfall will be possible by this weekend into next week ( note that any thunderstorm at this time of year can cause heavy local rain ).  Stay tuned for later forecast updates on this potential.


Weather Discussion ( Warmer )

Although chilly nights will continue for a couple more days in mountain valleys, the pattern is shifting toward much warmer and gradually more humid conditions.  Eventually, by later this week into the upcoming weekend, this will set the stage for showers and downpours in thunderstorms.

View From Powell Valley Overlook On Saturday – May 13, 2017
Temperatures into Mother’s Day morning dipped into the lower 40s in Clintwood and Norton and well down into the frosty 30s within high mountain valleys in the High Knob Massif ( other locations with 30s included Burkes Garden and Shady Valley ).

Large temperature differences between chilly mountain valleys and milder ridges & exposed plateaus will continue during morning hours of Monday and Tuesday mornings.

Whitewater Gushes In South Fork Gorge of High Knob Massif ( PM of May 13 )

Within complex mountain terrain surface dewpoints in valleys with access to the higher terrain are not that important during conditions favorable for drainage.

*Dewpoints and the cooling potential of air at the top of drainage basins are more important under this type of condition.  That is why nocturnal temperatures can drop below dewpoints observed in lower-middle elevation valleys, and how high wetland valleys that may quickly reach near 100% RH can continue to cool during the night as lower dewpoint air aloft drains downward from high mountain ridges ( radiational cooling + lower dewpoint air = lower valley temps amid complex mountain terrain than often forecast ).
The time of year for widespread river valley fog is nearing, as folks living within the Clinch River Valley of Scott and Russell counties and the Powell River Valley, from East Stone Gap-Big Stone Gap into Lee County, know well.  The drainage of cool air downward out of the High Knob Massif and Tennessee Valley Divide working to enhance fog formations ( the latent heat that releases with condensation adds warming to the air, countering some of the cooling to make these river valleys milder than their cold air drainage sources** ).  A similar setting being applicable to the Cumberland and Russell Fork-Levisa Fork river valleys. 
**There also appears to be a compressional warming factor after air vertically drops 1500 to 2000+ feet, even with drainage flows, as noted by valleys beneath the Black mountains in southeastern Kentucky which tend to be significantly milder than those on the Virginia side.  More will be learned as a University Of Virginia’s College At Wise microclimatology field research project continues through coming seasons and years. 

Like water, which gushes out of the high country, air drains into upper elevation valleys ( where it chills most ) and then continues downward through middle into lower elevations under the continuous force of gravity when conditions are favorable for nocturnal temperature inversions and development of cold air drainage flows.

European 51-Member Ensemble MEAN 500 MB Heights Forecast: Days 1-5

While a digging western USA upper trough will pump up or raise heights across the eastern USA ( warming air expands such that the height of any given isobaric surface is raised upward in the atmosphere ) during the next 5 days, a trend toward an eastward shift of storminess within the “clash” zone between the western trough-eastern ridge will occur.

European 51-Member Ensemble MEAN 500 MB Heights Forecast: Day 6-10

Hit or miss afternoon showers and thunderstorms will initially become possible due to daytime heating and the increase of instability; however, by this weekend into next week ( especially ) it appears that more widespread and organized showers-thunderstorms will become likely.

European 51-Member Ensemble MEAN 850 MB Temperatures: Days 1-5

At this time of year, as already observed this month, the positioning and potential stalling of any frontal zone will have to be monitored closely.

European 51-Member Ensemble MEAN 850 MB Temperatures: Days 6-10

Check back later this week for a better idea of timing on any potential heavy rainfall setting that may be upcoming.