Partly-mostly clear. Areas of fog. Light winds. Temps varying from 50s in cooler mountain valleys to the mid-upper 60s on exposed middle elevation ridges-plateaus.
Partly cloudy & warm. Chance of hit or miss showers and thunderstorms. NW-NNE winds 5-10 mph. Temperatures varying from 70s in upper elevations to the low-middle 80s ( hotter south into the Great Valley ).
Wednesday Night Into Thursday Morning
Becoming mostly clear following any early evening activity. Light NE-ENE winds generally less than 10 mph along mid-upper elevation mountain ridges. Areas of valley fog. Temps varying from upper 40s to lower 50s in cooler mountain valleys to the upper 50s-lower 60s.
Partly cloudy. Small chance of a hit-miss shower or storm. Light NNE to ENE winds. Temperatures varying from 70s in upper elevations to the lower-mid 80s ( hotter south into the Great Valley ).
Thursday Night Into Friday Morning
Partly-mostly clear. Winds ESE-SSE 5-15 mph along mid to upper elevation mountain ridges. Temperatures varying from 50s in cooler mountain valley to the mid-upper 60s.
Partly-mostly cloudy. Chance of hit-miss showers-storms. SSE-SSW winds 5-10 mph, with some higher gusts. Temps varying from 70s in the upper elevations to lower-middle 80s ( hotter south into the Great Valley ).
Weather Discussion ( Weak Front )
An increase in the chance of showers & thunderstorms is expected into this weekend, following a drier than average first half of September 2016.
The northern woods of the High Knob Massif remain damp, and if one did not know this area they would never know that September had been drier than average.
*The lower than normal lake level at Big Cherry Lake, due largely to much release during Summer 2016 to augment the Powell River, reveals that the regional pattern has been drier than average.
A simply HUGE contrast in conditions were observed during September 14 as afternoon temps in the valley dropped into the 60s, with clouds and local downpours, versus lower 90s at the same time in the Tri-Cities of the Great Valley.
Another large spread in conditions were observed during the afternoon hours of September 15, despite partly cloudy skies above the High Knob Massif.
*Data courtesy of Field Studies In Microclimatology at the University Of Virginia’s College At Wise using highly accurate, National Weather Service calibrated temperature sensors.
The NAM Model is showing the potential for some heavy rainfall by Sunday into Monday of September 18-19. Stay tuned for updates on this possibility.
The breaking news at the time of this update was the upgrade of Invest 93L to Tropical Storm Julia by the National Hurricane Center.
The forecast going into Wednesday is for a weak front to move into the mountains and trigger hit-miss showers and thunderstorms into the afternoon. A wind shift then allows lower dewpoint air, at mid-upper elevations especially, to invade the Cumberland Mountains into Thursday morning.
Although Julia is not expected to directly impact the local area, it could slow the weak boundary and/or aid sinking aloft and development of drier air depending upon its strength and movement. This is the uncertainty in the current forecast period that may need to be updated*.
*Temperatures into Thursday morning may not be as cool as I am currently forecasting in mountain valleys if the boundary is not able to push south-southwest of the High Knob Massif-Tennessee Valley Divide. On the other hand, if deep convection continues around Julia and it moves more east than north then sinking air aloft could reinforce the development of drier air over the area into Thursday-Friday ( dissipating the boundary and enhancing easterly to northeasterly flow into the area ).
In the big picture, on the synoptic-scale, nothing has really changed from my previous outlooks with an above average temperature pattern ( especially by day ) continuing. If the Tri-Cities do not reach 60 days of 90+ degree heat it will not be because this pattern is not favorable, especially given an increasing precipitation deficit across the Great Valley.
500 MB heights remain well above average during the next week to ten days, decreasing only slightly toward 588 dkm on the VA-TN border during the 6-10 day period ( September 19-23 ).
Although sun angles are decreasing and nights growing longer, the persistence of these anomalous heights over dry ground ( especially in the Tennessee Valley ) will continue to keep temperatures above model guidance.
Exceptions being on days that feature more cloudiness and/or showers-thunderstorms. Rainfall, for the most part, remaining limited into at least the beginning of this weekend as it looks now.
The MEAN of the 51-Member European Model Ensembles is for temperatures to decrease a little, with emphasis being on little, amid the 6-10 day forecast period of September 19-23 ( below ). Looking at individual days; however, it would be no surprise for another day, or two, to hit 90 degrees in the TRI next week! If their ground can moisten then this threat may finally end.
As I noted previously, it will likely be late September-early October before the southern Appalachians have a chance to actually feel a true push of autumn air.
As noted by the rapid spin-up of Julia, things can change quickly so stay tuned for updates. In this case, the synoptic-scale pattern has been rather stable to increase confidence in the persistence of this big picture setting for a while longer.