Mountain Area Forecast ( May 28-30 )

ALERT For High Water Levels And Flooding In Low-lying Locations Along The South Fork of the Powell River and Big Stony Creek of the Clinch River

More than 3.00″ of rain fell over the Big Cherry Lake basin and adjacent locations into Saturday evening ( May 27 ).  ROARING water and dangerously high levels will continue into Sunday.

The Storm Prediction Center Has Issued A Marginal Risk For Severe Thunderstorm Development Sunday

Thunderstorm development Sunday will largely be dependent upon cloud coverage and if the stabilizing overnight atmosphere can recover ( become unstable ) during the day.

*Downpours Of Flooding Rain Will Remain Possible Throughout The Memorial Day Holiday Period

Due to a super-saturated surface layer, and high rates of evapo-transpiration ( a positive moisture feedback ), showers and any thunderstorms which develop will remain capable of producing heavy rainfall and possible localized flooding.  

Overnight Into Sunday Morning

Low clouds.  Areas of fog.  A chance of showers.  Thunder possible.  Winds SW at 5-10 mph, with higher gusts, along mid-upper elevation mountain ridges.  Temperatures from mid-upper 50s to the lower 60s.

Sunday Afternoon

Mostly cloudy.  A chance of showers and thunderstorms.  Any storm could become strong-severe.  Local downpours.  SW winds 5-10 mph, with higher gusts.  Temperatures in the 60s to lower 70s.  Dense fog possible at upper elevations ( i.e., orographic clouds ).

Sunday Night Into Memorial Day Morning

Mostly cloudy.  A chance of showers and thunderstorms.  Local downpours possible.  SW-W winds 5-10 mph, with higher gusts, along mid-upper elevation mountain ridges.  Temperatures in the 50s to lower-mid 60s ( coolest at the highest elevations ).  Dense fog in upper elevations, with areas of fog at middle-lower elevations.

Memorial Day Afternoon

Mostly cloudy.  A chance of showers and thunderstorms.  Local downpours possible.  SW-WSW winds generally less than 10 mph.  Temperatures varying from 60s in upper elevations to the lower-middle 70s.

Monday Night Into Tuesday Morning

Mostly cloudy.  A chance of showers.  Thunder possible.  SW-W winds 5-10 mph, with higher gusts, along middle to upper elevation mountain ridges-plateaus.  Temperatures varying from 50s to middle 60s ( coolest at highest elevations ).

 

Weather Discussion ( Saturated )

The mountain landscape is not only saturated, it is super-saturated, in wake of a wet Spring 2017 and torrential rain producing thunderstorms on Saturday ( May 27 ).

Cumulonimbus clouds & orographic mountain waves, as well as low-level orographic clouds ( across Little Stone Mountain Gap and the Powell Valley Overlook area as seen at lower right of below image ), formed as severe thunderstorms with flooding rains, damaging winds, and vivid lightning developed Saturday afternoon-evening.

Rapid convective development occurred during the mid-late afternoon hours of Saturday, with overshooting cloud tops and gravity waves, indicative of vigorous upward motion, clearly seen on new GOES-EAST Imagery ( below ).

Although the amount of convective formation is uncertain during Sunday, the situation is now so saturated that a wet feedback for rain shower development means that it will take little upward motion to trigger new activity ( i.e., even showers without thunder will be capable of downpours ).

Any breaks within the overcast will make showers-storms nearly certain to develop amid this “jungle-like” setting.

It is hoped that Memorial Day afternoon will be dry, but for now I have left a chance of showers-storms in the forecast until it is seen if somewhat drier low-level air can actually reach the Cumberland Mountains ( stay tuned for updates ).

The continuation of mean upper air troughing + wet ground means that this unsettled pattern will continue into early June, with dry periods to be savored in between showers and thunderstorms.

While generally humid, the cooler than average temp regime observed since May 21 will also continue.

Although this pattern can certainly change, wet ground and abundant evaporation + transpiration heading into summer often develops a feedback that helps to maintain a setting which is unsettled via input of abundant moisture into the atmosphere ( latent heat energy for storm formation ).

*Positive and negative feedbacks are an important part of every summer season and are driven largely by soil-vegetative moisture inputs ( or the lack thereof ) into the overlying air.  Seasons with abundant moisture input into the lower atmosphere often trend wetter and cooler than average versus those which feature dry ground and reduced evapotranspiration ( ** ).

**Cooler by day, especially, since drier summer seasons in the mountains can feature anomalous nocturnal coldness at times ( with frost-freezing temperatures having been observed during every month of the year amid high mountain valleys, sometimes this may be due to low dewpoint air behind strong summer fronts…but in seasons like Summer 1988 it was largely due to developing drought conditions ).