Forecast Discussion


000
FXUS64 KMEG 201735
AFDMEG

Area Forecast Discussion
National Weather Service Memphis TN
1235 PM CDT Fri Jul 20 2018

.UPDATE...Aviation Discussion.


&&

.PREV DISCUSSION... /issued 1010 AM CDT Fri Jul 20 2018/

Skies are mostly cloudy across portions of east Arkansas with
mostly sunny skies across the remainder of the forecast area.
Temperatures this morning range from the mid 70s to mid 80s.
Some showers are moving through east Arkansas at this time with
strong to severe thunderstorms moving south through central
Arkansas.

The clouds and rain have kept temperatures from warming up across
portions of east Arkansas and these areas may not see Heat Index
values reach above 100 degrees this afternoon. However, since
the rain was not widespread, will leave Excessive Heat Warning
alone for now.

Will update forecast to remove morning wording.

ARS

&&

.PREV DISCUSSION... /issued 329 AM CDT Fri Jul 20 2018/

Dangerous Heat Expected Across the Mid South Today...
Severe Weather Outbreak from Late Afternoon into Tonight...

A very busy short-term forecast period is in store for the Mid
South. Initially, dangerous heat and humidity is expected today
across the region before severe thunderstorms develop from late
afternoon into tonight.

High temperatures this afternoon should top out in the middle to
upper 90s. These very hot temperatures will combine with middle to
upper 70s dewpoints to support heat index values between 105-109
degrees east of the Mississippi River, with 110-115 degree heat
index values along and west of the river. A heat advisory is now
in effect for all areas east of the Mississippi River, with an
excessive heat warning now in effect for areas along and west of
the river.

The very hot and humid airmass in place today will set the stage
for a potentially explosive severe thunderstorm event from late
this afternoon into tonight. The airmass will become very unstable
late this afternoon, with MUCAPE values expected between
5000-6000 j/kg. In addition, mid level lapse rates will begin to
steepen to between 7.5 and 8.0 c/km, with the approach of an
anomalously strong 60 kt mid level jet that will overspread a very
hot and humid surface airmass. As mid level temperatures cool
slightly, the weak capping in place should erode by late afternoon
and allow for explosive thunderstorm development. Strong deep
layer shear and low level helicity values between 100-200 m2/s2
strongly support supercell development with initial late afternoon
activity across northern portions of the forecast area. Any
supercell thunderstorm will be capable of very large hail
(potentially 2 inches or greater), damaging winds, and a few
tornadoes from late this afternoon into this evening. The greatest
threat appears across northwest Tennessee where a Moderate Risk
for Severe Storms resides. Eventually, supercells may cluster and
bring an increasing damaging wind threat to the entire area. Large
hail and isolated tornadoes cannot be ruled out even later
tonight further south, from I-40 southward. The remainder of the
Mid South, outside of the Moderate Risk area, is in an Enhanced
Risk for Severe Storms.

Storms should push into southern areas late tonight with the
severe threat gradually diminishing towards sunrise. A cold front
will drop south across the region with slightly cooler
temperatures and lower humidity values on Saturday across northern
sections. A few storms may still be possible across southern
sections into Saturday afternoon as the atmosphere still remains
very warm and humid ahead of the front.

A cutoff upper level low will drop southwestward from the Ohio
Valley across the Tennessee Valley, then across the central Gulf
Coast through the early and middle part of next week. This will
bring a return to near to slightly below normal temperatures to
the Mid South. In addition, deeper moisture associated with the
upper low will advect back into the area from the east, with
nearly daily chances for showers and thunderstorms through the
extended forecast period.

JLH

&&

.AVIATION...18Z TAFs

Very conditional forecast this afternoon and tonight dependent on
daytime heating, a weak midlevel disturbance and numerous outflow
boundaries lingering from prior convection. It looks less likely
that we will see thunderstorms this afternoon...but short term high
resolution guidance continues to bring a round of showers and
thunderstorms across the Midsouth tonight. Brief periods of MVFR
or lower visibility, along with locally gusty winds is likely in
association with thunderstorms. Included 3-4 hour TEMPO groups
based on model guidance timing. Storms should come to an end
by sunrise with the exception at perhaps TUP. Tomorrow looks
mostly dry at JBR, MKL and MEM. A few storms may linger near TUP.

Prevailing wind will be mainly from the South to Southwest
5-10kt this afternoon and overnight, shifting more from the West
to Northwest Tomorrow.

30

&&

.MEG WATCHES/WARNINGS/ADVISORIES...
AR...Excessive Heat Warning until 8 PM CDT this evening for Clay-
     Craighead-Crittenden-Cross-Greene-Lawrence-Lee AR-
     Mississippi-Phillips-Poinsett-Randolph-St. Francis.

MO...Excessive Heat Warning until 8 PM CDT this evening for Dunklin-
     Pemiscot.

MS...Excessive Heat Warning until 8 PM CDT this evening for Coahoma-
     DeSoto-Panola-Quitman-Tallahatchie-Tate-Tunica.

     Heat Advisory until 8 PM CDT this evening for Alcorn-Benton MS-
     Calhoun-Chickasaw-Itawamba-Lafayette-Lee MS-Marshall-Monroe-
     Pontotoc-Prentiss-Tippah-Tishomingo-Union-Yalobusha.

TN...Excessive Heat Warning until 8 PM CDT this evening for
     Lauderdale-Shelby-Tipton.

     Heat Advisory until 8 PM CDT this evening for Benton TN-Carroll-
     Chester-Crockett-Decatur-Dyer-Fayette-Gibson-Hardeman-
     Hardin-Haywood-Henderson-Henry-Lake-Madison-McNairy-Obion-
     Weakley.

&&

$$