ALERT For A Continuation Of Strong SW Winds At Mid-Upper Elevations During Thursday Night Into Friday, With Increasing Gusts Into Lower Elevations During The Overnight-Predawn Of Friday
A strong SW flow of air will continue in advance of an approaching cold front through Thursday Night into the morning hours of Friday before a wind shift to W then N directions occurs during the day Friday. Local power outages will remain possible.
The leading edge of this front may be marked by a line of heavy rainfall, followed by widespread rainfall of lesser intensity. Remain alert for possible water level rises.
An ALERT For DENSE Fog Is Likely For Late Friday Into Friday Night With Dropping Cloud Bases On Northerly Upslope Flow Along & North Of The High Knob Massif And Tennessee Valley Divide ( A Limited Period )
Hydrology ALERT For Significant Rainfall Amounts
A moist low-level air mass will combine with strong SW winds to produce rain showers at times through coming days. Downpours will be possible, especially within the upslope corridor along and southwest of the High Knob Massif & Tennessee Valley Divide.
Large Region of Significant Rainfall Possible During Next 7 Days
A large region with significant rainfall will be possible during the next 5-7 days, with heaviest amounts expected to again fall along and west of the Appalachians from the mountains to Ohio River.
Due to saturated conditions in wake of recent flooding persons living along streams and in low-lying, flood prone locations will need to remain alert for the possibility of water level rises.
Former ALERT
ALERT For Dense Fog Across Mid-Upper Elevations And Within The SW Flow Funneling Zone Along U.S. 23 At Powell Valley Overlook
Dense fog was present throughout the daylight hours of Wednesday along the flanks of the High Knob Massif and highest elevations, as well as within the Head of Powell Valley where SW flow rises to funnel through Little Stone Mountain Gap ( on either side of Powell Valley Overlook ).
This has been causing a sudden reduction is visibility along U.S. 23 such that caution is advised for those traveling between the Town of Big Stone Gap and City of Norton.
Overnight Into Wednesday Morning
Cloudy and windy. Rain developing with a chance of downpours. Low clouds with widespread dense fog at upper elevations. SSW to SW winds 10-25 mph, with higher gusts. Temperatures widespread in the 40s to around 50 degrees.
Wednesday Morning Through The Afternoon
A chance of rain showers. Local downpours possible. Low clouds with dense fog at high elevations. SSW-SW winds 10-20 mph with higher gusts. Temperatures in the upper 40s to middle 50s.
Wednesday Night Into Thursday Morning
Rain showers. Downpours possible. Low clouds with dense fog at mid to upper elevations. SW winds 10-20 mph, with higher gusts, on mountain ridges-plateaus below 2700 feet. SW-WSW winds 15-25 mph, with higher gusts, on mountain ridges above 2700 feet. Unseasonably mild with temperatures mainly in the 50s.
Thursday Morning Through The Afternoon
A chance of rain showers. Downpours possible. Low clouds with dense fog at upper elevations and in SW upslope flow sites at mid elevations ( e.g., Powell Valley Overlook ). SSW-SW winds 15-25 mph, with higher gusts. Unseasonably mild with temperatures from the low 50s to the low 60s.
Overnight Into Friday Morning
Mostly cloudy. Windy. Unseasonably Warm. Chance of showers developing toward morning. SW Winds 15-30 mph, with higher gusts. Temperatures in the 50s to lower 60s.
Friday Morning Through The Afternoon
Morning rain, heavy at times. Windy. Turning colder with rain tapering to showers or a mix. Rain changing to snow at the upper elevations by mid-late afternoon before ending, or becoming flurries. Morning temperatures in the 50s to lower 60s dropping sharply during mid-late afternoon into the lower 30s to lower 40s ( coldest highest elevations ). A drop in cloud bases with dense fog formation for a period during late afternoon-early evening at elevations above 2500-3000 feet. Developing mid-later afternoon wind chills in the 20s & 30s.
Friday Night Into Saturday Morning
Low clouds & fog giving way to mostly cloudy skies. Areas of dense fog possible at low elevations. Winds becoming light & variable. Temperatures varying from low 20s to lower 30s ( coldest in upper elevations ).
*Another wave of rain, with locally heavy amounts, is expected to overspread the area during Saturday. This could cause strong rises on creeks.
The currently forecast upper air pattern into next week continues to look most favorable for another high water event, with deep tropical moisture set to generate additional heavy to excessive rainfall. See my updated discussion below for more details.
Weather Discussion ( Wet Pattern )
Thursday Night ( Feb 15 ) Update
A couple more rain producing systems will be crossing the mountain area into this weekend, with strong SW winds in advance of the first system into Friday morning.
NAM 12 KM Model Total Precipitation Next 60 Hours
A general 1.00″ to 2.00″ of rain will be possible with these two waves ( heaviest amounts centered on the morning to early afternoon hours of Friday and again Saturday afternoon-early Saturday evening ).
GFS Model Total Precipitation Forecast Next 60 Hours
Due to orographic clouds capping high elevations, and moderate to strong lift, locally higher rain amounts will be possible. What do I mean by orographic clouds?
High Knob Massif Webcam – University Of Virginia’s College At Wise
Once higher level clouds dissipated the orographic capping clouds became visible. Note how the cloud bank stretched across the High Knob Massif ( above ) is stationary over time ( below ), indicating that it is being supported by orographic lifting on strong SW flow ( in this case ).
This is part of an important seeder-feeder precipitation process in which rain falling out of a higher altitude seeder cloud falls down through the moisture rich environment of the feeder cloud and becomes enhanced, with more rainfall subsequently reaching the surface than falls out of the seeder clouds ( and more than is able to be detected by Doppler radar whose beam shoots over top the orographic feeder clouds capping the high country ).
This is part of the reason why rainfall is always under-estimated for the High Knob Massif area, with my climatology showing that this process is important throughout the year but especially in the orographic forcing season ( November-April ) when strong winds generate long-lived orographic cloud masses.
High Knob Massif Webcam – University Of Virginia’s College At Wise
People living along streams, and in low-lying poor drainage locations, will need to remain alert for water level rises into Friday afternoon and again later Saturday into Saturday night. Given general 5.00″ to 10.00″+ amounts already observed this month, it will not take as much rain to generate water level rises.
As noted above, I continue to think next week will offer a more serious threat to the mountain region with more heavy to excessive rainfall becoming likely.
GFS Model Rainfall During The Next 2 Weeks
Although specific rainfall forecasts vary from run to run and among models, and no single model run should be taken as being accurate ( like above ), composite ensemble means for the European Model group and GFS are again showing a strong signal for heavy to excessive rainfall amounts.
European 51-Member Ensemble Mean 500 MB Height Anomalies ( Feb 19-23 )
A western Atlantic High, analogous to a summer-like Bermuda High, in combination with a deep upper air trough anchored by Gulf of Alaska ridging is truly an ominous signal for heavy-excessive rainfall from the mountains west to the Mississippi River.
Previous Discussion
A wet pattern will continue to dominate the mountain region through coming days, with a strong SW air flow helping to enhance the potential for showers and local downpours. While a change in the flow is expected to briefly occur by late week-early this weekend, an even more potent flow with a deep, tropical connection may develop by next week. A truly worrisome pattern.
General Rainfall Pattern During Next 5-7 Days
A general 2.50″ to 3.00″ of basin average rainfall is now expected from the High Knob Landform west & northwest during the next 5 to 7 days. Like during the previous event this is a broad generalized composite of model forecasts and does not fully account for orographic forcing.
*Low clouds ( widespread fog in orographic clouds ) currently engulfing much of the high country are expected to be a notable feature through coming days, signaling that an efficient seeder-feeder precipitation process will be in place to help enhance rainfall amounts with orographic forcing on strong SW flow.
GFS Model Total Rainfall Forecast During Next 5 Days
All forecast models under-estimated rainfall amounts during the previous event, with the GFS Model being the closest to final storm totals ( it was still up to 3.00″ lower than MAX amounts observed in the High Knob Massif ).
European Model 850 MB Wind Analysis At 7 PM Tuesday – February 13, 2018
The current pattern has Gulf of Mexico & Atlantic moisture inflow ( as observed by the 7 PM Tuesday 850 MB chart ) plus a connection to the tropical Pacific Ocean as seen on this full view of the Earth ( Western Hemisphere ).
GOES-16 Low-Level Water Vapor At 3:15 AM on February 14, 2018
Since I created this website I have had an Earth Flow Field Tool Embedded, and if you have not been using it then you have missed a great resource to view the big picture.
Note this tool is interactive. The Earth can be rotated in any direction and different parameters can be selected for viewing by clicking the word EARTH. Once selected, click the word EARTH again to remove the selection panel for complete viewing.
Next week it currently appears that the flow field will become even more conducive to tapping deep tropical moisture from the Pacific + Gulf of Mexico, as we now refer to as an Atmospheric River. We must hope this changes, and that rainfall during the next 3-5 days will be manageable.
This February is on pace to become the wettest on record. Reference The High Knob Landform for a look at the wettest.
