Severe Thunderstorm Warning 109, NWS Morristown, TN

AREA FORECAST DISCUSSION
National Weather Service Morristown TN
334 PM EDT Sun Jul 5 2020

.SHORT TERM (Tonight and Monday)...

A seasonally active short term period is anticipated with ongoing
pulse convection into this evening and more chances tomorrow.
Localized flooding and isolated strong wind gusts continue to be the
largest concern within any convection that achieves notable vertical
extent. Otherwise, seasonally mild nighttime and seasonally hot
daytime temperatures are expected.

This Evening/Tonight

Currently, a mostly weak synoptic pattern is in place with much of
the synoptic-scale upper-level amplification north of the County
Warning Area (CWA). This amplification includes a trough centered
just north of the Montana/Canadian border with a ridge extending
from the upper Great Lakes into Ontario/Manitoba. Another upper
trough is located over New England. Locally, embedded shortwave
energy, weak 5-15 kt 850mb upslope flow, and differential heating
have initiated convection across much of the area with increased
concentration over higher terrain locations. The local environment
consists of notably high surface-based instability of 2,500 J/kg to
3,000+ J/kg and very minimal shear values of less than 10 kts. WBZ
heights of 12,000-14,000 ft AGL have outlined hail to be of lesser
concern compared to strong downburst winds and localized flooding.
DCAPE values of 800 to 1,300 J/kg are present area-wide, raising the
concern for localized downburst wind gusts within vertically defined
cells. Additionally, PWAT values of 1.2 to 1.7 inches and mid/lower
level winds of 15 kts or less have both raised awareness for
localized flooding, resulting from efficient rainfall potential and
limited storm movement. With considerable instability in place and
additional development anticipated, convection looks to continue
into the evening with slight chance PoPs lingering over TN/NC/VA
mountains until midnight. Tonight will be seasonally mild with
patchy fog expected from antecedent rainfall, calm winds, and
potential clearing skies late.

Heading into tomorrow, focus turns towards enhanced upper-level
shortwave energy over the Mid-South ahead of a surface low in the
Lower Mississippi Valley. With this surface low, a frontal boundary
will keep the highest chances for rain south of the CWA. However,
the slow northward progression and increasing lower to mid-level
southerly flow/moisture advection will cause a south to north
increase in PoPs during the day and into the later afternoon with
continued focus over the higher terrain. Current indications are for
relatively lower instability over the area with higher values
further west. However, the overall environment looks to be similar
to today with instability being more prevalent than shear values.

BW

.LONG TERM (Monday Night through Sunday)...

Weak troughing will be present across the Eastern CONUS to begin the
period with a surface low across the Gulf Coast states. This will
result in increased convective coverage across the Southeast CONUS.

Models are in better agreement with bringing the surface low to our
southeast and off the East Coast midweek. This will result in the
potential for some increased precip chances across our area,
especially the southern portions and higher elevations of the
forecast area. Temperatures remain near normal midweek with the
potential for warmest temperatures across northern sections of the
forecast area where precip coverage is forecast to be lower.

As the low strengthening off the East Coast, troughing amplifies
across the Eastern CONUS placing our forecast area within NW flow
from the end of the week through next weekend. Mostly summer diurnal
convection is anticipated, but a few MCV/vort maxima moving
through the NW upper flow may result in enhanced areas of shower and
thunderstorm activity at times. Temperatures are expected to be on
the increase again late week and next weekend as ridging increases
across the Central CONUS.

JB

&&

.AVIATION...
18Z TAF DISCUSSION.

The current expectation is for a seasonally eventful TAF period
with VCTS this afternoon into the early evening, along with
reductions to MVFR and potentially IFR tonight. Currently,
convection has initiated near and along higher terrain in east
Tennessee and southwest Virginia with VCTS already being reported
at TRI. The expectation is for further development by 19Z this
afternoon with diurnal diminishing during the evening. With
limited synoptic-scale features, winds largely remain light and
variable into tonight. Based on occurrences from this morning, a
similar meteorological environment, and more widespread
convection expected today, patchy fog is anticipated with
reduction to at least MVFR included in all 3 TAFs. Reduction to
IFR was included at TRI based on stronger model guidance
indication for calm winds and proximity with Boone Lake. The 11Z
to 13Z timeframe is the most likely window for fog development at
the sites, but future trends could indicate earlier development
and possibly longer presence of fog. However, confidence is still
not too high for extent and duration of impact. By midday
tomorrow, diurnal cumulus development is expected ahead of more
anticipated convection just after the TAF period.

BW

&&

.PRELIMINARY POINT TEMPS/POPS...
Chattanooga Airport, TN             71  90  71  86  71 /  20  30  30  80  30
Knoxville McGhee Tyson Airport, TN  71  91  70  88  71 /  20  30  20  60  30
Oak Ridge, TN                       71  92  70  88  71 /  20  30  20  60  20
Tri Cities Airport, TN              68  90  67  88  67 /  20  40  10  50  20

&&

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

Hazardous Weather Outlook...DELAYED
National Weather Service Morristown TN
336 PM EDT Sun Jul 5 2020

NCZ060-061-TNZ012>018-035>047-067>074-081>087-098>102-VAZ001-002-
005-006-008-061945-
Cherokee-Clay-Scott-Campbell-Claiborne-Hancock-Hawkins-Sullivan-
Johnson-Morgan-Anderson-Union-Grainger-Hamblen-Northwest Cocke-
Cocke Smoky Mountains-Northwest Greene-Southeast Greene-
Washington-Unicoi-Northwest Carter-Southeast Carter-Roane-Loudon-
Knox-Jefferson-Northwest Blount-Blount Smoky Mountains-
North Sevier-Sevier Smoky Mountains-Sequatchie-Bledsoe-Rhea-Meigs-
McMinn-Northwest Monroe-Southeast Monroe-Marion-Hamilton-Bradley-
West Polk-East Polk-Lee-Wise-Russell-
336 PM EDT Sun Jul 5 2020 /236 PM CDT Sun Jul 5 2020/

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

.DAY ONE...This Afternoon and Tonight

A few strong to possibly severe storms are possible this afternoon
and evening. Main concern will be strong gusty winds and heavy
rainfall with localized flash flooding possible.

.DAYS TWO THROUGH SEVEN...Monday through Saturday

Isolated strong to possibly severe storms are possible each
afternoon and early evening from Tuesday through Saturday. Main
concern will be strong gusty winds and heavy rainfall with
localized flash flooding possible.

.SPOTTER INFORMATION STATEMENT...

Spotter reports may be needed this afternoon. Please relay any
information about severe weather to the NWS while following local,
state, and CDC guidelines.

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