Heat in Alabama, July 13, 2018

Area Forecast Discussion...UPDATED
National Weather Service Huntsville AL
959 AM CDT Fri Jul 13 2018

.NEAR TERM...(Rest of Today)
Issued at 959 AM CDT Fri Jul 13 2018

A strengthening upper-ridge continues to build south into the
Tennessee Valley this morning, promoting a clear and sunny sky. Thus,
it's been another warm and very humid start to the day as dewpoints
remain in the mid 70s across most of North Alabama this morning. Some
locations along the Tennessee River are experiencing dewpoints in
the upper 70s. As heating has kicked in temperatures have easily
warmed into the 85 to 90 degree range as of 15z, with heat indices
already in the mid to upper 90s -- to around 100 degrees. With
additional heating expected into the early afternoon (highs in the
low to mid 90s) -- and little air mass modification, think Heat Index
values will peak at or above 105 degrees across all but Northeast
Alabama and Southern Middle Tennessee. As such, have expanded the
Heat Advisory into Cullman, Limestone, Morgan, and Madison Counties
from 17-00z.

As mentioned above, sufficient moisture exists underneath this ridge
to support some isolated to widely scattered convection. 12z
soundings do indicate some subsidence from the ridge that will likely
suppress coverage somewhat. However, a weak dewpoint "front" was
noted across Middle and East Tennessee where values north of this
semi-diffuse boundary are in the upper 60s and south of it in the
mid/upper 70s. This feature may be able to kick off a few storms,
along with the terrain of Northeast Alabama. They'll likely be around
2000-25000 J/kg of CAPE to work with so a strong pulse storm or two
can't be ruled out. However, do think coverage will be more limited,
which has been suggested by some of the latest mesoscale models.

.SHORT TERM...(Saturday through Sunday night)
Issued at 302 AM CDT Fri Jul 13 2018

Little change in the environment is anticipated on Saturday as the
front meanders around TN/AL. There is also the potential for
remnant outflow boundaries to cause development as daytime heating
increases instability after erosion of the nocturnal inversion. Not
sure if we'll need a heat advisory on Saturday as more cloud cover
may exist, but should still be hot with heat index values around
100-103.

The synoptic ridge begins to dampen and shift south as a sheared
shortwave trough moves east across the northern flank of the ridge
over the Midwest. This dampening of the ridge will decrease
subsidence and increase the storm potential on Sunday as temperatures
rise into the low 90s. Have kept higher POPs on Sunday due to this
change in the pattern. These storms may linger into the evening as
outflow boundaries move across the region and the shortwave trough
moves east towards the TN Valley.

.LONG TERM...(Monday through Thursday)
Issued at 302 AM CDT Fri Jul 13 2018

The medium range models are both leaning more and more to a pattern
change next week. This equates to the upper ridge breaking down
across the southeast U.S. and allowing an active and progressive
train of troughs traversing the upper Midwest and Great Lakes. On
Monday, one more day of typical pulse convection is expected during
the afternoon into the evening. Then, the tail end of one these
troughs will drop into the TN valley and southern Appalachians late
Monday night into Tuesday. With a very moist and buoyant airmass in
place, the trough and associated cold front will likely trigger
multiple MCS clusters along the front. These should arrive in the TN
valley before sunrise Tuesday, and last through much of the morning
hours. The precipitation should end from northwest to southeast by
Tuesday night as drier and "cooler" air arrives behind the front.
After a welcome break from the very humid air on Wednesday, flow will
return to the south-southwest again by Wednesday night into Thursday.
Another incoming northwest flow shortwave arrives Thursday night,
which amplifies significantly according to the GFS, and to a lesser
degree by the ECMWF. This may bring one or more periods of
thunderstorms.

&&

.AVIATION...(For the 12Z TAFS through 12Z Saturday morning)
Issued at 631 AM CDT Fri Jul 13 2018

Fog over portions of northern Alabama and southern middle Tennessee
is causing drops in VIS to IFR/LIFR and at times VLIFR especially at
KHSV, K4A9, K8A0, and KMDQ. KHSV has been experiencing oscillating
VIS and have incorporated a tempo group through 14Z in that TAF.
Expect most of the fog to dissipate by 14-15Z across the rest of the
area. Then, isolated to scattered storms are expected to develop
later this afternoon/evening as a stalled front causes storm
development. Coverage of storms is uncertain and will amend TAFs if
storms approach KHSV/KMSL. Fog formation may occur again during the
14/06Z-14/12Z.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...Heat Advisory until 7 PM CDT this evening for ALZ001>007-016.

TN...NONE.
&&

NEAR TERM...AMP.24
SHORT TERM...SL.77
LONG TERM...17
AVIATION...SL.77

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

Hazardous Weather Outlook...UPDATED
National Weather Service Huntsville AL
941 AM CDT Fri Jul 13 2018

ALZ001>010-016-TNZ076-096-097-140545-
Lauderdale-Colbert-Franklin AL-Lawrence-Limestone-Madison-Morgan-
Marshall-Jackson-DeKalb-Cullman-Moore-Lincoln-Franklin TN-
941 AM CDT Fri Jul 13 2018

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

.DAY ONE...Today and Tonight...

A Heat Advisory is in effect for portions of North Alabama from Noon
until 7 pm. Heat index values will reach 105 to 108 degrees in these
areas. Please see the Heat Advisory product for more information.

Isolated to scattered thunderstorms will develop by this afternoon.
A couple of strong thunderstorms will be possible, capable of
producing wind gusts to around 50 mph.

.DAYS TWO THROUGH SEVEN...Saturday through Thursday...

From Saturday into Monday, isolated to scattered thunderstorms are
expected each day, mainly during the afternoon and early evening
hours.

A greater risk of more widespread thunderstorms will occur late
Monday night into Tuesday as a cold front moves through the area.
Some thunderstorms may be capable of producing strong winds.

.SPOTTER CALL TO ACTION STATEMENT...

Activation of storm spotters and emergency management personnel is
not anticipated at this time.