A MCS ( Mesoscale Convective System ) – thunderstorm cluster will be pushing into the mountain area by mid to late afternoon ( 4:00 to 6:00 PM ) with a threat of damaging winds, lightning, and torrential rainfall.
*Several thunderstorm clusters will be moving east toward the Appalachians through this afternoon-evening. Models currently predict the second and third clusters to become most severe ( the final cluster possibly impacting a region west of the mountains…but remain alert throughout the remainder of this afternoon-evening ).
Storm Prediction Center – Damaging Wind Risk Forecast
The greatest threats remain west of the mountains according to the Storm Prediction Center, but local risks should be respected and have the potential to become greater than currently indicated with partial clearing in advance of the developing thunderstorm clusters adding to instability in high dewpoint air.
These storms will be capable of generating torrential rainfall, such that strong water level rises will also be at least a local threat ( ultimately, perhaps, becoming the greatest threat given such a wet May 2018…any activity that trains or moves repeatedly across the same corridor will therefore be especially prone to trigger flash flooding ).
GOES-16 Visible Image
Stay Tuned to NOAA Weather Radio and your favorite media sources for possible watches and warnings that may be needed.
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.
ALERT For Torrential Rain In Hit-Miss Showers And Downpours In Thunderstorms During May 15-18
Increasing moisture from the Gulf of Mexico and Caribbean will combine with disturbances in the westerlies to trigger showers and downpours in thunderstorms beginning on Tuesday afternoon and continuing through the work week.
Overnight Into Tuesday Morning
High clouds & mild. Light valley winds. Winds S-SW at less than 10 mph on mountain ridges-plateaus below 2700 feet. SSW to SW winds 5-15 mph, with higher gusts, on mountain ridges above 2700 feet. Temperatures varying from 50s in cooler mountain valleys to the mid-upper 60s.
Tuesday Afternoon
Mostly cloudy with a chance of hit-miss showers and thunderstorms. Heavy rainfall possible, especially by late afternoon. Light & variable winds, except gusty in heavier storms. Temperatures varying from upper 60s to lower 70s at highest elevations to the lower 80s ( warmer south into the Great Valley ).
Tuesday Night Into Wednesday Morning
Mostly cloudy with a chance of showers & thunderstorms, especially during the evening. Areas of fog, dense in places receiving heavy rain. Light winds, except S-SW at 5-15 mph with higher gusts on mid-upper elevation mountain ridges. Temperatures in the 50s to lower-mid 60s.
Wednesday Afternoon Through Friday Afternoon
Partly-mostly cloudy with intervals of hit-miss showers and downpours in thunderstorms. Locally heavy rain amounts. Temperatures in 60s & 70s during the day and 50s to lower 60s at night ( generally coolest at upper elevations ).
Weather Discussion ( Summer Pattern )
A jump straight from spring into a summer-like pattern during the past week will kick into high gear during coming days as tropical moisture increases both horizontally and vertically through the atmosphere.
Wild Stonecrop ( Sedum ternatum )
Although the forest canopy has now closed at the lower elevations in the foothills of eastern Kentucky, and within the Tennessee Valley, it remains partially to fully open in higher elevations of the mountains.
NAM 12 KM Model Total Rainfall Forecast Through 8 AM Thursday – May 17
Humidification and relative mildness will enhance the progression of spring upward through higher elevations during the next week to 10 days, with a threat for tropical downpours to signal that this transition into summer is for real with staying power through the next couple of weeks.
European Model Ensemble Mean 500 MB Height Anomalies: Days 6-10
Bermuda High pressure off the east Coast and a general southwesterly flow aloft will continue the warm, humid pattern with intervals of showers & thunderstorms during the next week to 10 days ( while it will not rain, of course, all the time, a wetter and more humid pattern is coming to signal an early arrival of summer in the Mountain Empire ).
True summer will arrive as the forest canopy matures across all elevations and evapotranspiration increases to feed low-level moisture back into the overlying atmosphere ( * ).
*This process starts in lowlands, below 2000 feet, and progresses upward through higher terrain locations as spring progresses. It annually begins during the explosive April 15-May 15 period, at elevations below 3000 to 3500 feet, and advances to the highest summits through late May into early June.
