ALERT Continues For Torrential Downpours In Hit-Miss Showers & Thunderstorms Through Tuesday
A local flash flood risk will continue to impact the mountain area through Tuesday amid muggy air.
A weak boundary passing across the mountains will decrease the threat of downpours from north to south during Wednesday, with much more pleasant air now expected during Thursday into Friday.
This will be a relatively short-lived break with a return of deep, tropical moisture expected to renew the threat of torrential rain producing showers and thunderstorms during the Memorial Day Holiday period.
Overnight Into Tuesday Morning
Partly to mostly cloudy with areas of dense fog. Chance of a local shower. Winds S to SW at 5-15 mph, with higher gusts above 2700 feet. Humid. Temperature varying from the upper 50s to low-mid 60s.
Tuesday Morning Through The Afternoon
Partly to mostly cloudy, warm and humid with hit-miss showers & thunderstorms. SSW-WSW winds 5-15 mph, with higher gusts. Temperatures varying from the lower 70s at upper elevations to the upper 70s to lower 80s.
Tuesday Evening Into Wednesday Morning
Partly to mostly cloudy with a chance of hit-miss showers and downpours in thunderstorms. SW winds 5-15 mph, with higher gusts, on mid-upper elevation mountain ridges & exposed plateaus. Winds shifting WNW-NW during the morning. Areas of dense fog in valleys. Temps varying from the 50s to low-mid 60s.
Wednesday Afternoon
Chance of a shower or thunderstorm, especially to the south. Becoming less humid and more refreshing from north to south, with deepening blue skies aloft. Winds NW-N at 5-10 mph with higher gusts.
Wednesday Night Into Thursday Morning
Clear & cooler. Areas of dense river valley and lower elevation fog. Light valley winds. Variable winds at generally less than 10 mph on mid-upper elevation mountain ridges. Temperatures varying from the lower-middle 40s in colder valleys to the lower to middle 50s ( coldest in upper elevations ).
*A returning tropical air mass will renew the threat of downpours during the Memorial Day Holiday Period into next week.
Weather Discussion ( Flash Flooding )
A pattern featuring torrential, hit or miss, downpours which started last week continues until a weak boundary gives the mountain area a short-lived break from late Wednesday into Friday.
Black Mountain Mesonet Data For The 1-Hour Period Ending At 5:25 PM
A 3.59″ deluge was captured by the mesonet site on Black Mountain, with 0.43″ accumulating in 5 minutes ( 5.16″ per hour rainfall rate ) during the height of the thunderstorm, late Monday afternoon ( pushing the May rainfall total to over 9.00″ ). By contrast, as has been the nature of this pattern, little rain fell only a short distance away in the City of Norton.
While the pattern since last week has possessed an easterly air flow component on numerous days, favoring the eastern side of the Blue Ridge where up to 10.00″+ of rain has locally fallen in May, it has also been characterized by a moisture feedback to further enhance the hit-miss nature of convective activity.
A convective feedback develops when heavy rainfall initially saturates a area to enhance evaporation-evapotranspiration into the overlying atmosphere over the local area. This then acts to aid new rainfall development via the local presence of extra latent heat release from moisture input into the overlying atmosphere.
At the same time, locations missed by heavy rainfall have much less moisture injection into the overlying atmosphere above them and a enhanced tendency toward having lighter rain amounts and missing torrential downpours.
These feedbacks can last for days to weeks in a persistent pattern, or change in their placement with a pattern break and shift back into a tropical-type of regime ( such as appears to be upcoming as the final days of May slip into early June ).
Black Mountain Mesonet Data For The 1-Hour Period Ending At 4:25 PM
Clintwood has been under a wet feedback in recent days with 1.60″+ of rain causing flash flooding in and around town during Monday afternoon ( pushing the May rainfall tally to over 6.00″ ).
Month To Date Rainfall Totals To 8 AM May 21, 2018
This recent pattern has generated a large difference between places like Mount LeConte ( at the summit of the Great Smokies on the western side of the mountain range with 2.24″ reported during May 1-20 ) and Mount Mitchell ( on the eastern side of the chain with 8.92″ reported during May 1-20 ). Totals, as noted previously, have also varied greatly over more localized distances. Reported 1981-2010 precipitation totals are nearly the same for both peaks, to illustrate the anomalous nature of this short-term pattern ( * ).
*Mount LeConte typically reports much more than Grandfather Mountain, but during this pattern it has received around a FOOT less rainfall to again illustrate the anomaly associated with recent easterly flow of tropical air into the eastern flank of the Blue Ridge ( long-term MEAN annual flow has a westerly component to favor Mount LeConte for the most precipitation ).
While the regional trend of heaviest rains along and east of the Appalachians has been captured by NWS stations and a composite of Doppler Radar rain estimates, the local scale hit-miss nature must be looked at using individual Doppler radar estimation graphics ( like the MRX example below ).
MRX Doppler Rainfall Estimate For Monday ( May 21, 2018 )
Rain amounts during Monday afternoon varied by 2.00″ or more in Dickenson County over just a few air miles. Ditto for numerous other locations across the region.
