Alert For Increasing Showers And Thunderstorms Thursday Into Friday As The Remnants Of Harvey Approach And They Transform Into An Extratropical Storm System ( i.e., With An Increasingly Cold Core )
While the heaviest, widespread rainfall totals look most likely across western & central portions of Kentucky-Tennessee the potential exists for orographically enhanced rainfall amounts along the Blue Ridge and within favored upslope areas of the Cumberland Mountains & Cumberland Plateau.
The Storm Prediction Center Has Issued A Slight Risk For Severe Thunderstorm Development As Remnants Of Harvey Impact The Southern Appalachians
The main storm threat will be wind damage, to include an isolated tornado risk due to strong low-level wind shear driven by Harvey’s remnant circulation. A notable & increasing temperature gradient will develop late Friday into Saturday as an increasingly cold core develops with transformation into an extratropical cyclone.
Storm Prediction Center Forecast Severe Weather Risk Regions
Very chilly conditions are expected to develop over the mountains by Saturday as extratropical Harvey draws unseasonably cool air into the Cumberland & Allegheny mountain ranges, which look to experience the burnt of chilly air Saturday into Saturday Night.
Temperatures Saturday will struggle in the 50s at the elevations of Norton-Wise and may hold in the 40s to near 50 degrees at upper elevations within the High Knob Massif during the day, where conditions will feel even cooler due to wind chills.
Overnight Into Wednesday Morning
Partly to mostly cloudy. Areas of valley fog. Light winds. Temperatures in the 50s to around 60 degrees, except the upper 40s to low 50s within colder high valleys at upper elevations in the High Knob Massif.
Wednesday Afternoon
Partly to mostly cloudy. More humid. A chance for showers and thunderstorms by mid-late afternoon into the evening. Winds generally light and variable. Temperatures varying from upper 60s to lower 70s at upper elevations to the upper 70s to lower 80s.
Wednesday Night Into Thursday Morning
Showers & thunderstorms becoming likely by morning. Downpours possible. Winds variable at generally less than 10 mph. Temps varying from the upper 50s to the middle 60s. Areas of dense fog, especially at high elevations.
Thursday Afternoon
A chance of showers. Thunder possible. Humid. Winds SE-SSE at 5-10 mph, with higher gusts, on mid-upper elevation mountain ridges. Temperatures mainly in the 60s to near 70 degrees ( around 60 degrees at highest elevations ).
Thursday Night Into Friday Morning
Showers & thunderstorms. Locally heavy rainfall totals. Humid. Winds SE to S and increasing to 10-20 mph, with gusts over 30 mph at high elevations, by morning. Low clouds with dense fog at upper elevations. Temperatures widespread in the 60s.
Friday Afternoon
Showers & thunderstorms. Locally heavy rainfall totals. Humid. Winds SSE-SSW at 15-25 mph, with higher gusts. Temperatures from the upper 60s to the upper 70s ( coolest at the highest elevations ). Low clouds and dense fog at the highest elevations.
Friday Night Into Saturday Morning
Gusty with rain and showers. Turning much cooler. Winds becoming S-SW at 10-20 mph, with higher gusts, on middle to upper elevation mountain ridges-plateaus. Low clouds with dense fog across the higher elevations. Temperatures dropping into the low-mid 40s to mid 50s ( coolest highest elevations ). Warmer south into the Great Valley.
Saturday Morning Through Afternoon
Low clouds with dense fog at high elevations. Chilly. A chance of drizzle or light showers. Winds SW-W at 5-15 mph, with higher gusts. Temperatures near steady or only slowing rising in the low-mid 40s to mid-upper 50s, coldest at upper elevations. Wind chill factors in the 30s & 40s to around 50 degrees, coldest at highest elevations.
A very complicated extended range period is upcoming as a deep, autumnal trough digs into the USA with potential for new tropical development in the Gulf of Mexico. Meanwhile, a Cape Verde born disturbance may become a major Hurricane as it approaches the Leeward Islands and Caribbean in a week to ten days.
Weather Discussion ( Wild Times )
Thursday Evening Update
A very humid air mass has now engulfed the mountains, with orographic clouds forming along the High Knob Massif all day ( afternoon temperatures hovering in the 60 to 62 degree range on Eagle Knob ).
High Knob Massif Webcam – University Of Virginia’s College At Wise
Heavy showers, with torrential downpours, occurred during the afternoon with 0.50″ to 1.00″ amounts in local areas. At one point rain was falling so heavily along U.S. 23, from the Powell Valley Overlook area into Norton, that some had to pull off the highway. A signal that upcoming rain will have a tropical, downpour nature as the main system arrives.
High Knob Massif Webcam – University Of Virginia’s College At Wise
The forecast appears to be on target, with the first round of organized activity expected to overspread the mountains by the predawn-morning hours of Friday as winds increase at higher elevations, then winds increase across the entire area during the daylight hours of Friday.
HRRR Model Future Doppler At 4:00 AM on Friday – September 1, 2017
Any breaks Friday in the overcast could help fuel formation of strong-severe thunderstorms before the focus shifts to a influx of increasingly cool air, low clouds, showers-drizzle into Saturday.
One aspect to note for Friday is that narrow bands of convergence, much like feeder bands that come ashore from such a system, are likely to develop with cross-isobaric ( ageostrophic ) flow due to both Harvey’s large-scale circulation and friction ( terrain driven ) to support zones of higher impact. It is not possible to tell exactly where these will form until they begin to develop, so remain alert and tuned to NOAA Weather Radio and your favorite media out-lets for any warnings or advisories that may be needed.
High Knob Massif Webcam – University Of Virginia’s College At Wise
Meanwhile, enjoy a tropical skies filled sunset.
High Knob Massif Webcam – University Of Virginia’s College At Wise
Previous Discussion
A wild weather roller coaster ride is upcoming through the next 1-2 weeks as all sorts of havoc tries to play out across this region of the world. The mega-disaster of Harvey being the start of what appears to be an extreme weather pattern for this time of year in the eastern USA.
National Hurricane Center Forecast Track of Harvey Remnants
There has been a gradual but important eastward shift in the forecast track of Harvey’s remnants into Friday and Saturday, with clear indications now that it will become incorporated into the jet stream and transformed into an extratropical rain storm; thus, the increasing amount of chilly air that will develop due to dynamically forced cool-ing with rising air and advection of cool air to the north that will wrap around the circulation by Saturday.
Cool here means REALLY cool for this time of year, with temperatures Saturday having the potential to remain in the 40s all day in the high country of the High Knob Massif and struggling to rise through the 50s in Wise ( milder at lower elevations, below 2000 feet, and to the south into the Great Valley of eastern Tennessee and southwest Virginia ).
Spaghetti Model Cluster Forecast For Harvey – 2 AM Model Run Wednesday
Enough spread in the model spaghetti cluster remains for there to continue to be an eastward shift as the remnants reach the latitude of Kentucky and this region.
NAM 12 KM Model Total Precipitation Forecast – Next 84 Hours
Regardless of the exact track, increasing air flow into the mountains will allow for orographic forcing to develop and this will help to increase rainfall amounts by Thursday into Friday. Given a wet August in the Cumberland Mountains, this will bear close watching. The windward slopes of the Blue Ridge will be favorable for the heaviest orographic rains, with several inches or more being possible.
European Model Mean Sea Level Pressure Forecast – 8 PM September 6
The European Model has been on a “hot streak” lately, to the great agony of everyone in southeastern Texas, so it is hard to deny the operational and many ensembles which show a new low developing in the Gulf by the early-middle parts of next week ( this has now been shown for a couple days ).
European Model 850 MB Temp Anomalies Forecast: Days 6-10
At the same time, an anomalously deep upper air trough for early September has now been shown for days and days to dig deeply into the central-eastern USA by the middle to later portions of next week.
Collectively, a wild setting being shown. Although early September is the climatological peak of Hurricane season, it is also the beginning of Meteorological Autumn, a season that climatically is the driest within the Mountain Empire.
With luck this big trough could dig deeply enough to deflect any developing tropical systems away from the USA, but if not then big problems could arise for more than just coastal locales. It is simply too early to know, but certainly something that all forecasters will be closely following into next week.
Thoughts & prayers to our Texas friends.
*While Tropical Storm Claudette still holds the continental USA record for the most rain to fall within 24-hours ( 42.00″ fell on Alvin, Texas just south of Houston in July 1979 ), Harvey now appears to have shattered the record for the most rain to fall during a single storm event ( with a preliminary storm total of 51.88″ reported in the heaviest location ).
