Damaging Winds and Lightning Strike in Tennessee, June 28, 2018

Area Forecast Discussion
National Weather Service Huntsville AL
1053 AM CDT Thu Jun 28 2018

.MESOSCALE DISCUSSION...
Issued at 1053 AM CDT Thu Jun 28 2018

Potent atmospheric instability in advance of QLCS in middle TN will
be strong fuel for severe weather during the next several hours.
Although deep layer bulk shear is only 20-25kt, storm relative inflow
will be sufficient to maintain updrafts as the cold pool outflow
arrives. In fact, 1km SRH of 150+ given the south movement along
with the strong deep layer CAPE may be sufficient for mesocyclones as
well. The tornado threat is relatively low however, but non-zero.
The main threat will be strong downburst winds of 50-60kt. The latest
HRRR run depicted a strong bow echo signal with this surging south
which may not be far from reality. Thus a Severe Thunderstorm Watch
is in effect until 00Z.

&&

.NEAR TERM...(Rest of Today)
Issued at 852 AM CDT Thu Jun 28 2018

Taking a look at the new model guidance this morning, it looks like
NW AL will see storms slightly later in the day. With heat index
values already forecast from 105-107 in the far western areas, went
ahead and upped temps a deg or two, into the lower 90s, and issued a
heat advisory for Lauderdale, Colbert, Franklin and Lawrence
counties. This is in effect from 11AM to 7PM. Hires guidance looks
consistent in holding back the precip until after 18-19z. A few may
sneak into S. Middle TN and NE AL earlier than that. If these storms
make it here faster than currently forecast, the heat advisory will
bust. Even then, it will still be very hot today!

Previous Near Term-

Latest water vapor imagery shows a fairly amplified upper ridge
encompassing much of the central CONUS, with a trough swinging
through the eastern Great Lakes region and a trough along the West
Coast. Area radars show an area of showers and thunderstorms
developing over Kentucky and northern Tennessee, moving eastward over
the Cumberland Plateau. The current trends would indicate that this
activity would generally move east of the area later this morning,
perhaps clipping the far eastern portion of the area.

Overall, forecast confidence is a bit low through at least the next
12 hours or so. The upper ridge will slowly build east through the
day, with the forecast area remaining between the ridge and the
trough along the east coast. Hires guidance suggests that an MCS will
develop to our north and slide to the south-southeast through the TN
Valley later this morning and afternoon. Given the current activity,
and potential for any lingering outflow boundaries, this is
certainly a plausible scenario. Dewpoints will remain in the lower
to mid 70s and with highs rising into the upper 80s to lower 90s,
instability will certainly be enough to help generate thunderstorms
if any lift is available. The overall severe potential is on the
lower side, given less shear for storm organization. However, given
the amount of instability and the PW values, we could see isolated
pockets of strong wind gusts. Storms will also be capable of
producing hail, given plenty of CAPE between -10 and -30 degrees,
though with forecast freezing levels near 15000 feet, not sure how
much severe hail we will see. If storms can get going sooner than
currently forecast (i.e. before the afternoon hours) there may be a
little bit of deep layer shear to give us a more organized threat for
strong storms. However based on current trends, am more confident on
storms developing/moving in during late morning and afternoon.

.SHORT TERM...(Tonight through Saturday night)
Issued at 502 AM CDT Thu Jun 28 2018

The MCS from the daytime hours will quickly push south by 00Z, with
the potential for few lingering storms across the east. Not expecting
much coverage with these, so have only kept a slight chance for
storms to account for this. Storms should completely diminish by
midnight. Temperatures will once again be mild and only fall into the
lower 70s across the area.

The upper ridge will continue to build eastward during the overnight
hours, though short term models are pretty consistent on a low
developing along the Gulf and lifting northward around the eastern
periphery of the ridge. This will bring an even greater influx of
moisture to the TN Valley, along with the potential for showers and
thunderstorms to develop sooner than what we would normally see
during the summertime. Given some timing uncertainties, will not
introduce pops in the forecast prior to 12Z, with only slight chance
in between 12 and 15Z. Rain chances then increase for the latter
part of the morning and into the afternoon hours, as the lift
associated with the upper low provides an enhancement to diurnally
driven thunderstorms. PoPs may need to be increased, however a lower
confidence will limit PoPs in the lower 40 percent range. The best
chance showers and thunderstorms will be west of I-65 where the
influence of the upper low is greatest during our peak heating hours.
The center of the upper level high will move into the Ohio Valley
late Friday night, with the TN Valley remaining on the southern
periphery of the high and the northern periphery of a low that short
term models keep around the area through the remainder of the period.
Thus, scattered thunderstorms will be possible once again on
Saturday. Instability will remain high both Friday and Saturday and
slightly more dry air in the mid levels may lead to a higher chance
for microbursts on both days. Though slightly lower PWs on Saturday
may limit the overall coverage of thunderstorms.

Given that the center of the high remains to our north and east,
temperatures during the short term period will be similar to those
felt the last couple of days. Highs will rise into the upper 80s to
lower 90s, with lows in the lower 70s. Heat indices will generally
remain below advisory criteria, though a few isolated areas across
the west may peak at 105 on Friday afternoon. Given the relatively
low coverage, no advisory is needed at this time.

.LONG TERM...(Sunday through Wednesday)
Issued at 502 AM CDT Thu Jun 28 2018

The extended portion of the forecast will begin with the forecast
area located within a region of weak steering currents between a
strong 500-mb high over the northern mid-Atlantic region, a weaker
ridge over the southern Plains, and an inverted mid-level trough in
the vicinity of the northeastern Gulf coast. Although global
deterministic and ensemble model guidance exhibits temporal
variability normal for this time range and season, a consensus of
the solutions suggests that the inverted trough axis will retrograde
across the region on Monday-Tuesday, around the base of a
strengthening high over the eastern Plains.

Large scale ascent related directly to the existence of the trough
will be rather weak, but the relative minimal in subsidence should
support scattered-locally numerous showers and thunderstorms in the
very moist airmass (PWAT values in the 1.8-1.9 inch range) that will
be in place over the TN Valley. This activity will be strongly tied
to the diurnal warming cycle, with highest POPs during the daylight
hours and only a slight chance of lingering convection during the
00-03Z periods. With forecast soundings depicting weak lower-middle
tropospheric wind fields and seasonably high values of CAPE, the main
threats with any storms will be brief gusty winds, lightning and
locally heavy rainfall.

During the period from Tuesday-Wednesday, the inverted trough will
advance further westward and away from the region and we will
experience an increasing influence from a ridge extending from the
mid-MS Valley into the western North Atlantic. The coverage of
diurnal showers and thunderstorms will be reduced this period, and
mainly confined to far western portions of the CWFA. With the ridge
displaced to our north and stronger deep-layer easterly flow over the
TN Valley, highs will remain in the u80s-l90s, with lows in the
u60s-l70s.

&&

.AVIATION...(For the 12Z TAFS through 12Z Friday morning)
Issued at 615 AM CDT Thu Jun 28 2018

VFR conditions will prevail through at least the first 6 hours of
the forecast period at the KMSL and KHSV terminals. Winds will
increase from the west by mid morning and become gusty during the
afternoon hours. A cluster of thunderstorms is expected to drop
southward later this morning and reach the KMSL terminal around 18Z
and the KHSV terminal around 19Z. Although confidence is not overly
high with regard to exact timing, there is at least enough confidence
to include thunder in a TEMPO group for each terminal. Any
thunderstorms that develop during this time would likely cause a drop
in visibility and increase in wind gusts. Thunderstorm chances
diminish by late afternoon, however there may be additional
development. Given the even lower confidence on a second cluster of
storms, will leave thunder out of the forecasts after 22Z. Winds will
lessen after sunset and be light overnight.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...Heat Advisory until 7 PM CDT this evening for ALZ001>004.

TN...NONE.
&&

MESOSCALE...17
NEAR TERM...LN
SHORT TERM...73
LONG TERM...70/DD
AVIATION...73

For more information please visit our website
at weather.gov/huntsville.

Hazardous Weather Outlook...UPDATED
National Weather Service Huntsville AL
844 AM CDT Thu Jun 28 2018

ALZ001>010-016-TNZ076-096-097-290445-
Lauderdale-Colbert-Franklin AL-Lawrence-Limestone-Madison-Morgan-
Marshall-Jackson-DeKalb-Cullman-Moore-Lincoln-Franklin TN-
844 AM CDT Thu Jun 28 2018

This Hazardous Weather Outlook is for north Alabama and portions of
southern middle Tennessee.

.DAY ONE...This Afternoon and Tonight...

Thunderstorm coverage and intensity will increase this afternoon as
a complex of storms pushes southward from middle Tennessee. Damaging
wind gusts will be the highest threat. Storms will end from
northwest to southeast this evening.

Prior to the onset of precipitation, locations in northwest Alabama
may see heat index values briefly reach the 105 to 107 degree range.
A Heat Advisory has now been issued for Lauderdale, Colbert,
Franklin and Lawrence counties through this evening.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday...

Scattered thunderstorms will be possible each day, with perhaps a
slightly higher coverage on Sunday and Monday as a weak upper level
disturbance moves westward across the outlook area.

Dangerous heat index values in the 105 to 110 degree range will be
possible across much of northwest and north central Alabama
tomorrow,
with these values confined to only portions of northwest Alabama on
Saturday and Sunday.

.SPOTTER CALL TO ACTION STATEMENT...

Formal activation of storm spotters and emergency management
personnel is not anticipated at this time. However...timely reports
of strong winds and hail will be greatly appreciated.