*Weather Headlines
ALERT For The Potential Of Heavy Rain Developing Late Wednesday Afternoon And Wednesday Night
Strong rises on streams will be likely Wednesday night into early Thursday if heavy rainfall develops, with flash flooding possible. Residents living or driving along streams and in low-lying, flood prone locations will need to remain alert for the possibility of rapid water level rises. Mud and rock slides will also be possible given antecedent wetness.
Significant rainfall is expected. This will continue to push April totals toward record levels, with amounts currently within the 10.00″ to 11.00″ range in wetter portions of the High Knob Massif (marking the third consecutive double digit precipitation month).
April totals of 6.00″ to 8.00″ have been widespread across the region surrounding the High Knob Massif, enhancing the risk of high water levels with any additional heavy and/or prolonged rainfall.
A couple of issues related to the next weather system will help determine rainfall amounts and potential for strong-severe convection.
Convective Outflow Boundary
A). A strong-severe line of thunderstorms moving across the Mississippi River Valley will dissipate into early Wednesday, with an outflow boundary being generated. Some models suggest redevelopment along this outflow boundary will focus west of the Virginia-Kentucky border while others predict it to become active toward the Tennessee-North Carolina border.
Interactive Surface Streamline Flow
Cold Front Becoming Parallel To Upper Flow
B). The tilt of an upper-level trough axis will become nearly parallel to the system cold front at the surface for a time late Wednesday into early Thursday, slowing down the eastward progression of the front and associated band of potentially heavy rainfall and thunderstorms.
Monday-Tuesday (27-28 April 2020)
Skies became clear toward morning, following some snowflakes that fell from late Sunday afternoon into the early overnight period on High Knob, marking the 5th event in April to produce at least some flakes (heavy snow fell at the start of April). More snow has fallen in April than during March 2020 (April has also been colder, especially by night, than March).
*A rapid drop in temperature is expected into this evening, especially in mountain valleys, with a large vertical temperature spread developing between colder valleys and exposed mountain ridges.
Big Cherry Basin
(An Appalachian Classic Cold Air Formation Basin)
Reference my focus on research for information about nocturnal cold air formation within classic Appalachian cold air collecting basins. These represent one end of a spectrum in contrast with thermal belts where coldest air is generated by advection and adiabatic upslope cooling.
Increasing clouds overnight will likely slow or reverse temperature drops in mountain valleys as readings rise on mountain ridges with increasing wind and warm air advection on SW-WSW flow (rising ridge temps).
*A chance of localized showers Tuesday will give way to rain and thunderstorms Wednesday in advance of another upper-level low.
*Air turns colder again on Thursday, with low clouds and showers (mixed snow at the highest elevations). Temperatures will again hover in the 40s, with 30s at highest elevations, in locations along and northwest to north of the High Knob Massif-Tennessee Valley Divide. Wind chills will make conditions feel colder.
Upper-Level Low Pattern
(Above average precipitation)
The formation of upper-level lows is relatively common during the spring season, especially in patterns that possess repeatability.
Troughing in the mean, with individual upper-level lows, will continue to dominate the eastern USA through the first week of May 2020.
Although temperature fluctuations will certainly occur, this type of pattern favors below average temperatures to continue in the mean into early May, along with above average precipitation.
A continuation of below average temperatures will be supported by a pattern that has developed with break-down of a strong polar vortex that dominated Winter 2019-20, keeping temps above average.
Mid-latitude cooling has been ongoing for a couple weeks, with the potential for a couple more weeks of below average temps across eastern North America which means (if it verifies) a slow development of spring at upper elevations (especially).