Severe Weather Threat Continues Through Monday (June 24)

An Enhanced Threat Of Severe Thunderstorm Development Has Been Issued Along With A Severe Thunderstorm Watch

Valid 241630Z – 251200Z

…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS
OF THE CENTRAL APPALACHIANS…

…SUMMARY…
Scattered strong to severe storms capable of damaging winds will
affect the central/southern Appalachians and Ohio Valley today.
Other more isolated severe activity is possible today over northern
Illinois/southern Wisconsin and vicinity, and over
west-central/southwest Texas.

…Central/Southern Appalachians…
Morning satellite and radar imagery show a well-defined remnant MCV
over southern KY. This trough is embedded with a larger upper
trough rotating across the OH/TN valleys. The air mass ahead of the
feature is heating rapidly and will become very unstable by
mid-afternoon. Strengthening midlevel winds, coupled with strong
instability and affects of the MCV will pose a more substantial risk
of bowing thunderstorm structures capable of damaging wind gusts. A few supercells cannot be ruled out, along with a risk of isolated
tornadoes. Therefore have upgraded much of WV and vicinity to an
ENH risk and 30% severe wind probabilities.

1249 AM CDT Mon Jun 24 2019

Valid 241200Z – 251200Z

…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE GREAT LAKES SOUTHWARD INTO THE SOUTHERN APPALACHIANS AND IN PARTS OF THE SOUTHERN PLAINS…

…SUMMARY…
Scattered storms, some severe with hail or wind, are expected today the southern Great Lakes southward across the Ohio Valley into the Southern Appalachian Mountains. Other severe storms may occur across parts of the southern Plains.

…Ohio Valley/Southern and Central Appalachians…
An upper-level trough will move eastward across the Mississippi
Valley today as southwest mid-level flow remains over east-central
U.S. At the surface, a cold front will advance eastward into the
Ohio and Tennessee Valleys as a corridor of maximized low-level
moisture sets up ahead of the front. Surface dewpoints in the upper 60s F should result in moderate instability by midday across much of the moist sector. As surface temperatures warm and low-level convergence increases along the front, scattered convection is
forecast to initiate along and ahead of the front. Linear MCS
development will be possible as storms increase in coverage and move northeastward across the moist sector during the afternoon. RAP forecast soundings ahead of the front by 21Z from northern Kentucky southward into the southern Appalachian Mountains show MLCAPE values from 1000 to 2000 J/kg with steep low-level lapse rates. This along about 30 kt of 0-6 km shear should be favorable for isolated damaging wind gusts. Hail will also be possible with the stronger cells embedded in the any line that becomes established.

SPC authored by Broyles/Cook…06/24/2019

An enhanced threat for W-NW flow Mesoscale Convective Systems (MCS) is being monitored, with the potential for widespread wind damage generating Derecho formation. In addition, supercells and an enhanced flash flood risk will be a significant concern. While the exact track is yet to be determined for these systems, confidence for their development is high, and locations across the Ohio and Tennessee valleys into the Appalachians will be at risk.

While the above graphic does not look ominous yet, I expect significant upgrades to be made by the SPC during the next 24 to 48 hours as this setting gets closer in time.

An enhanced flash flood risk will remain a concern given so much rainfall already observed during June 1-20 and in 2019 which, like 2018, has been another very wet year.

Superintendent Andrew Greear, of the City of Norton Water Plant, reported 7.45″ of June rainfall up to 9:00 AM Thursday, June 20, which brought the 2019 tally to 40.28″ for the city (official NWS rain gauge total).

June rainfall totals of 8.00″ to 9.00″, with locally higher amounts, have been common in upper elevations of the High Knob Massif where 2019 precipitation tallies have reached 45.00″ to 50.00″ .

Locally higher amounts of rainfall are exemplified by Wayne & Genevie Riner who had measured 9.58″ through the morning of June 20 (34.10 in 2019), on Long Ridge of Sandy Ridge in southern Dickenson County, Va.

Development of an air mass with extreme instability is the big concern, with simply sick or outrageous convective energy being predicted by multiple forecast models along and just behind a surface warm frontal boundary in developing W-NW flow aloft. Past climatology of similar settings dictates that this is an ominous signal. Storms will fire within and along the gradient of extremely unstable air.

A very dry, cool air mass over the Upper Great Lakes and across the northeastern USA will be in dramatic contrast to a very humid, moisture laden air mass overspreading the Ohio-Tennessee valleys and southern Appalachians.

Current 500 MB (18,000 Feet) Flow Field

A strong (for June) upper air trough digging into the Pacific Northwest will help develop upper ridging downstream, with the Mountain Empire becoming located on the eastern side of a developing ridge axis in W-NW flow aloft into this weekend. Very moist and extremely unstable air will be developing in low-levels of the atmosphere with off-the-charts values of lifted indices, CAPE, supercell potential, etc being forecast (some of the highest values I have seen over this part of the world on forecast models).

Interactive Full-Screen Doppler Radar

Ideally, the worst storm clusters will fire southwest and northeast of the Mountain Empire (bad for those locations), but that can simply not yet be known and it will be surprising if the mountains are not impacted given the gradient of unstable air predicted by models.

Stay tuned to NOAA Weather Radio and your favorite media sources for updates on this potentially dangerous weather setting expected to develop this weekend.

ALERT For Localized Flash Flooding And Strong Thunderstorms Through Thursday (June 18-20) With Heavy-Excessive Rainfall

*Saturated ground and a slow moving weather system will combine to enhance the risk of flash flooding through Thursday (June 20). Local flash flooding rains were observed during Monday, June 17, across portions of Wise and Dickenson counties.

Weather Headlines

*Strong to severe thunderstorm development will be possible Wednesday (June 5).

Shower and thunderstorm development, with a threat of heavy rain and locally damaging winds and hail, is being monitored for Wednesday.

The threat of heavy to locally excessive rainfall is also being monitored for late week into early next week during the June 7-11 period.

Previous Headlines And Discussion

A gusty wind shift to the north will bring much cooler air into the mountains late Sunday Night into early Monday AM, with temperatures at high elevations falling into middle to upper 40s versus mid-upper 50s at lower-middle elevations. Wind chills will drop into the 30s at highest elevations.

*Update – Minimums in the 30s to lower 40s were observed within colder mountain valleys into the morning hours of June 4 (as expected).

The Mountain Empire will be on the southern fringe of unseasonably cool air, with values 10-20+ degrees below average for early June, that will be centered over the upper Great Lakes and New England during Monday into Tuesday of June 3-4.

Temperatures on Monday will be autumn-like, especially in the high country, with max readings varying from 50s to lower 60s above 3000 feet in the High Knob Massif to the upper 60s to lower 70s at lower-middle elevations (warmer to the south in the Great Valley of eastern Tennessee).

This will set the stage for chilly temperatures into Tuesday AM, with only the potential for high clouds acting to prevent readings from reaching their full cooling potential. MIN temperatures in the 30s are expected in colder mountain valleys from the High Knob Massif northeast along the Appalachians into Pennsylvania and New York where such cold temperature will become more widespread in nature. MINS in the 40s to low 50s will be common across much of the area, outside of colder mountain valleys.

Microclimate links the atmosphere, biosphere, hydrosphere-cryosphere and lithosphere together (they collectively dictate it) with a few indicator species in the High Knob Massif being Betula alleghaniensis, Viburnum lantanoides, Trillium undulatum, Maianthemum canadense, Osmundastrum cinnamomeum, Catharus fuscescens, Catharus guttatus, Cardellena canadensis, Setophaga caerulescens and Setophaga magnolia to note a few.

Having cold temperatures, and even frost, in high valleys from the High Knob Massif to Burkes Garden northeast in selected locations, known as frost pockets, through the eastern West Virginia highlands into western Maryland is nothing atypical during summer.

Dewpoints around 875-850 MB are a key factor in locations where high terrain reaches upward into this elevation zone. July 17-August 14 is the only period not to feature a night to freezing or below in Burkes Garden, which has the longest data period of any station in southwestern Virginia.

Research at the University of Virginia’s College At Wise is showing that high valleys in the High Knob Massif tend to run colder than Burkes Garden on average, but any given night may be different and Burkes Garden may be colder depending upon the cloud cover, surface conditions related to decoupling and dewpoints at upper elevations between these two Appalachian frost pocket locations.

The current terrain model forecast heading into Monday Night-Tuesday AM will be favorable for decoupling of boundary layer winds with unseasonably dry, low dewpoint air within the 900-800 MB zone. The only negative factor could be high clouds at altitudes above 25,000 to 30,000 feet in the atmosphere.

A stormy pattern with a notable above average precipitation trend, especially along the front ranges of the mountains, will develop in wake of this cool spell. This argues against unseasonable June heat and instead for muggy conditions with a positive feedback developing between surface moisture, cloudiness, and general wetness.