Severe Thunderstorm Warning 121, NWS Morristown, TN

AREA FORECAST DISCUSSION
National Weather Service Morristown TN
315 PM EDT Thu Jul 23 2020

.SHORT TERM...(Tonight and Friday)

Key Messages:

1. Widespread thunderstorms through this evening capable of
producing locally heavy rain and localized flooding and gusty winds.

2. Scattered thunderstorms again Friday, but coverage will be less.

Discussion:

Rest of This afternoon/evening...

A line of convection was located over the Plateau as of 1820Z with
scattered multicell clusters over SE TN and across NE TN and SW VA.
This activity will continue moving ENE through the evening as a
shortwave progresses toward the eastern Great Lakes with the surface
cold front and prefrontal trough being pulled gradually eastward.
The main hazard the rest of the day is torrential rainfall producing
localized flooding. MRMS estimated 2-3 inches of rain in an hour
over northern Hamilton County early this afternoon, and this goes to
show how moist the atmosphere is today. Latest mesoanalysis depicts
PWATs of 1.8 to 2+ inches over the entire region, and soundings show
a moist and deep warm cloud layer. This will allow any convection to
continue to produce efficient rainfall rates. The severe threat is
extremely low. MLCAPE has increased to just over 1000 J/Kg (highest
south of I-40) with up to 2000 Joules at the surface due to the
sunshine early this afternoon, but the moist profiles are keeping
DCAPE and low-level lapse rates quite low at only 500 J/Kg and 5-7
C/Km respectively. Shear is also very weak at only 10-15 kts in the
0-6 Km layer. Any lines or bowing multicells could produce gusty
winds to 30-40 kts, but severe gusts are generally not expected.

Tonight and Friday...

As the aforementioned shortwave moves through the eastern Great
Lakes, a longwave trough will temporarily set up over the eastern
CONUS for the end of the week with surface high pressure building
across the central Great Lakes. This will push the cold front from
the lower OH Valley slowly south through the southern Appalachians
tonight and Fri. We will find ourselves at the base of the longwave
trough, so the cold front will have a hard time clearing south of
the region. Guidance depicts the boundary getting hung up near the I-
40 corridor much of Fri which will keep the chance for
showers/storms elevated. Kept slight chance to chance PoPs tonight
with the boundary in the area, but confined the highest PoPs Fri to
areas S of I-40. Afternoon MLCAPE will build to just over 1000 J/Kg
again with LI's of -5 to -8, but believe the most widespread
convection will develop near the frontal convergence, so have likely
PoPs in the S Plateau and S Valley with chance farther N. Not
expecting severe weather since DCAPE and shear remain weak, but
continued PWATs of 1.8 to 2 inches will support locally heavy
rainfall again. Lows tonight will remain mild with upper 60's/low
70's. Highs Friday have bust potential depending on cloud cover with
the boundary in the vicinity. Stayed with NBM highs giving low/upper
80's from the N Plateau through NE TN and SW VA and upper 80's/low
90's farther S.

Garuckas

.LONG TERM...(Friday Night through Thursday)

Hot, above normal temperatures and diurnal convection will continue
to be the primary pattern through the extended. There are some hints
that relief from the heat could be coming late next week.

A subtle upper shortwave and surface boundary will be east and south
of the region by Friday night and Saturday. This should bring some
drier conditions Friday night with a clearing sky from north to
south. Upper heights remain near 591 to 593 dam on Saturday and
Sunday with above normal temperatures and diurnal convection having
greatest coverage across the higher elevations and southern half of
the forecast area. With weak shear, pulse convection will continue
to be the primary convective mode with the potential for locally
heavy rain and isolated damaging microburst winds. Temperatures will
remain above normal with ridging in place.

On Monday, an approaching upper shortwave will be moving east across
the Upper Mississippi Valley and Great Lakes with height falls
across the Appalachians. This is forecast to result in additional
scattered convection, and greater convective coverage. As the
shortwave and its associated cold front approach the area on
Tuesday, likely PoPs are anticipated across the forecast area with
widespread scattered showers and thunderstorms during the afternoon.
With PW near 2.0 inches, localized heavy rain and flash flooding
will be possible, especially if the GFS solution occurs with a
slower, and stalling, cold front across the Tennessee Valley.

The GFS and ECMWF differ with respect to timing on Wednesday through
the end of the week as the ECMWF amplifies the longwave trough and
brings the cold front south of the Tennessee Valley with drier and
cooler conditions. The GFS stalls the front with zonal flow midweek
prior to a second shortwave amplifying longwave troughing late week.
With these significant differences in global model guidance have low
confidence in the forecast mid to late next week. However, global
model consensus is aligning in better agreement that troughing and
cooler temperatures are possible by late next week.

JB

&&

.AVIATION...
18Z TAF DISCUSSION.

Scattered TSRA will peak in coverage this afternoon through mid
evening bringing occasional reductions in flight categories. Tough
to time individual storms, so carried VCTS into the evening. CHA
will dry out the fastest with VFR prevailing tonight, but a few
SHRA as well as areas of fog could lead to fluctuations between
VFR and MVFR at TYS and TRI late tonight. Periods of IFR fog are
possible, but not confident enough to include in the TAF.
Scattered TSRA will begin to redevelop Friday morning leading to
temporary reductions while VFR otherwise prevails.

Garuckas

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             74  91  73  92  72 /  20  60  30  50  20
Knoxville McGhee Tyson Airport, TN  72  90  70  91  71 /  40  60  30  40  20
Oak Ridge, TN                       72  91  70  91  70 /  50  50  20  40  10
Tri Cities Airport, TN              68  87  67  89  67 /  40  50  20  40  10

&&

.MRX WATCHES/WARNINGS/ADVISORIES...
NC...NONE.
TN...NONE.
VA...NONE.
&&

Hazardous Weather Outlook
National Weather Service Morristown TN
320 PM EDT Thu Jul 23 2020

NCZ060-061-TNZ012-013-035-036-067-068-081>087-098>102-241930-
Cherokee-Clay-Scott-Campbell-Morgan-Anderson-Roane-Loudon-
Sequatchie-Bledsoe-Rhea-Meigs-McMinn-Northwest Monroe-
Southeast Monroe-Marion-Hamilton-Bradley-West Polk-East Polk-
320 PM EDT Thu Jul 23 2020 /220 PM CDT Thu Jul 23 2020/

This Hazardous Weather Outlook is for portions of southwest North
Carolina and east Tennessee.

.DAY ONE...This Afternoon and Tonight

Scattered to numerous showers and thunderstorms will continue this
evening. Moderate to heavy rainfall is possible at times which
could lead to localized flash flooding.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not needed at this time.

TNZ014>018-037>047-069>074-VAZ001-002-005-006-008-241930-
Claiborne-Hancock-Hawkins-Sullivan-Johnson-Union-Grainger-Hamblen-
Northwest Cocke-Cocke Smoky Mountains-Northwest Greene-
Southeast Greene-Washington-Unicoi-Northwest Carter-
Southeast Carter-Knox-Jefferson-Northwest Blount-
Blount Smoky Mountains-North Sevier-Sevier Smoky Mountains-Lee-
Wise-Scott-Russell-
320 PM EDT Thu Jul 23 2020

This Hazardous Weather Outlook is for portions of east Tennessee
and southwest Virginia.

.DAY ONE...This Afternoon and Tonight

Scattered to numerous showers and thunderstorms will continue this
evening. Moderate to heavy rainfall is possible at times which
could lead to localized flash flooding.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

Spotter activation is not needed at this time.

Special Weather Statement
National Weather Service Morristown TN
420 PM EDT Thu Jul 23 2020

TNZ039-040-042-043-232115-
Hamblen TN-Northwest Greene TN-Northwest Cocke TN-
Southeast Greene TN-
420 PM EDT Thu Jul 23 2020

...SIGNIFICANT WEATHER ADVISORY FOR NORTHERN COCKE...SOUTHWESTERN
GREENE AND SOUTHEASTERN HAMBLEN COUNTIES UNTIL 515 PM EDT...

At 419 PM EDT, Doppler radar was tracking a strong thunderstorm near
Parrottsville, or 8 miles north of Newport, moving east at 10 mph.

Winds in excess of 45 mph will be possible with this storm.

Locations impacted include...
Morristown, Greeneville, Newport, Parrottsville, Mosheim, Caney
Branch, Bybee, Cedar Creek, Lowland and Mohawk.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Torrential rainfall is also occurring with this storm, and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 3627 8294 3602 8284 3597 8321 3605 8324
      3606 8324 3608 8325 3609 8324 3611 8325
      3613 8328 3614 8328
TIME...MOT...LOC 2019Z 252DEG 10KT 3608 8316

DH

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