082716 Forecast

Mountain Area Forecast ( August 25-28 )

Wednesday Night Into Thursday Morning

Partly cloudy.  Areas of dense valley fog.  Winds S to SW at 5-10 mph, with higher gusts, along mid-upper elevation mountain ridges.  Temperatures in the upper 50s to mid-upper 60s.

Thursday Afternoon

A chance of hit-miss showers & thunderstorms.  Partly cloudy.  Hazy.  Winds S-SW at 5-10 mph, with some higher gusts along highest ridges.  Temperatures varying from the low-mid 70s in upper elevations to the lower-middle 80s.

Thursday Night Into Friday Morning

Partly to mostly clear.  Areas of dense valley fog.  SW winds becoming northerly into morning at 5-10 mph, with higher gusts, along mid-upper elevation mountain ridges.  Temps varying from lower 60s to around 70 degrees.

Friday Afternoon

Partly to mostly sunny.  Hot.  Light N to NE winds.  Temps varying from middle-upper 70s in upper elevations to the middle to upper 80s ( hotter south into the Great Valley ).

Friday Night Into Saturday Morning

Mostly clear.  Areas of dense valley fog.  Light E-SE winds.  Temperatures varying from 50s in the coolest mountain valleys ( mid-upper elevations ) to the mid-upper 60s.

Saturday Afternoon

Partly sunny.  Hot & humid.  Chance of a hit-miss shower or thunderstorm.  Light SSE-SSW winds.  Temps varying from mid 70s to around 80 degrees at highest elevations to upper 80s to around 90 degrees ( hotter into the Great Valley ).

Saturday Night Into Sunday Morning

Partly cloudy.  Humid.  Areas of dense valley fog.  Light southerly winds along mid-upper elevation ridges.  Temps in the 60s to around 70 degrees.


Weather Discussion ( More Summer )

A heat ridge centered near the Great Smokies is well known climatologically as a heat maker for the Mountain Empire, even in the final days of the Meteorological Season.

Sinking Air Beneath Late Summer Heat Ridge
Sinking Air Beneath Late Summer Heat Ridge – Northern Dickenson County
Although some afternoon cumulus formed Thursday and Friday, especially along major lifting zones like the High Knob Massif, the atmosphere was clearly being dominated by subsidence aloft.
Mathematically deriving the Law Of Conservation Of Mass, from which various forms of the Continuity Equation is obtained, it is easy to develop a basic Subsidence Model for the atmosphere that clearly illustrates the relationship between rising & sinking air.
Essentially, low-level rising in one region of the atmosphere must be compensated for by large-scale sinking within another region.  While we often focus on convection that forms in a ring around high pressure centers it is seldom noted how air spreading out aloft above the convection acts to support and reinforce, via  compensative subsidence, the adjacent high pressure.
Spaghetti Plots For Invest 99L
Model Spaghetti Forecast Track For Invest 99L
So even as Invest 99L struggles, and models struggle with what it will eventually do, there will be significant low-level convergence across the Gulf of Mexico into the Upper Midwest through coming days that will continue to support the large-scale subsidence of air that is dominating the Mountain Empire.  The result being heat, humidity and little rain ( only localized storms in places where orographics and/or instability may briefly overcome sinking ).

Eventually, as is true of all things, this will change but for now this is the Bottom Line of the Big Picture.

Model Spaghetti Intensity Forecast For Invest 99L
Model Spaghetti Intensity Forecast For Invest 99L
*While rising-sinking ( outside of that orographically forced ) is not as coupled in mid-latitudes on the synoptic-scale as it is amid the tropics, the principle still applies and regions where synoptic high pressure is favored can be ( and often are ) aided by these processes ( since mass can be neither created nor destroyed, only rearranged in space & time and/or transformed in ways that still result in the total mass remaining unchanged ).


Previous Discussion

Following a short-stretch of simply gorgeous conditions that featured a few 40s in high mountain valleys, the heat of Summer 2016 is returning.

Pristine Water In Big Cherry Lake Basin
Pristine Water In Big Cherry Lake Basin – August 22, 2016
The lazy, hazy, crazy days of August also feature plenty of noise courtesy of Annual Cicadas ( below ) that tend to be plentiful at this time of year.
These are in a different genus from Periodical Cicadas that erupt in cycles of 13-17 years, forming one of the greatest natural events on planet Earth which is endemic to eastern North America.
Annual Cicada ( Neotibicen spp. ) - Cumberland Mountains
Annual Cicada ( Neotibicen spp. ) – Cumberland Mountains – August 23, 2016

National Weather Service stations in Wise and Clintwood have not recorded a day above the low 80s during the past seven days, since August 17 ( the MAX being recorded for the 24-hours ending during the AM of August 18 ).

National Weather Service Station - UVA Wise
NWS Station – UVA Wise – Observers Bob Vangundy & Layton Gardner
The morning of August 23 has been the coolest, with 52 degrees on the plateau at the University Of Virginia’s College At Wise and 40s in adjacent valleys from the City of Norton upward into the high country of the High Knob Massif.  A brief preview of Autumn!
Dense fog prevented the MIN from dropping below 54 degrees in Clintwood and most valleys across the lower elevations ( below 2000 feet elevation ).
NWS Station In Clintwood
NWS Station In Clintwood – Observer Wayne Browning

August rainfall of exactly 3.56″ in both Wise and Clintwood might suggest that rains had fallen evenly during the past 3 weeks; however, that is not the case as exemplified by 5.59″ measured in the official rain gauge at the City of Norton Water Plant by Andrew Greear & Staff, and more than 7.00″ measured by Jerry Rose in the Crab Orchard community near Guest River Gorge of the High Knob Massif.

August rains have also varied significantly across the Big Cherry Lake Basin, with 4.38″ at Big Cherry Dam being in contrast to more than 6.00″ across the basin head near the High Knob peak.
NAM 12 KM Model Precipitable Water Forecast
NAM 12 KM Model Precipitable Water Forecast At 8 AM Saturday – August 27

Following a chance for hit-miss showers & thunderstorms on Thursday, drier air aloft with subsidence will lower the total precipitable water values in the atmosphere Friday into Saturday to aid late season summer heating.

From August 25 onward through the remainder of the year only 4 days have ever been able to break 90 degrees in Wise, such that the window for this type of heat is closing with shorter days, longer nights and lowering sun angles.
In order to break 90 degrees from this point forward, in the middle elevations of the Cumberland Mountains ( between 2000-3000 ft ), everything must fall into perfect alignment ( mainly…either dry ground + few to no clouds + sinking air and/or normal wetness + few to no clouds with strongly sinking air ).

So the key to perhaps hitting 90 again ( for the 2nd time ) in either Wise or Clintwood will be for the air to dry out aloft with subsidence to prevent cloud formation.  It is difficult to do on the heels of an average to above average rainfall summer, but Friday-Saturday will attempt this feat.

Meanwhile, temperatures in the Great Valley of eastern Tennessee will blaze under conditions like this with downslope flow in lower levels combined with subsidence aloft over ground that remains drier than average in wake of a very dry June 1-August 15 period.
Spaghetti Model Tracks For Invest 99L
Spaghetti Model Tracks For Invest 99L

The main focus from this weekend into next week will be on Invest 99L and its potential for development into a tropical storm and hurricane.  Will it impact the Appalachians in a direct manner, with heavy rainfall?  That is impossible to tell at this point in time.  Stay tuned for updates.

However, some impact is almost certain as even if the storm’s rain shield passes well to the east, a zone of strong subsidence will be to its west and help prolong the heat of Summer 2016 for places that get under its ring of sinking air ( via the Law of Conservation of Mass ).