Heat in Alabama, July 2, 2023

Area Forecast Discussion
National Weather Service Huntsville AL
947 AM CDT Sun Jul 2 2023

...New NEAR TERM...

.NEAR TERM...
(Rest of Today)
Issued at 946 AM CDT Sun Jul 2 2023

A cluster of showers and thunderstorms are already developing over
west and north central Mississippi at this hour. This is all
developing along a pre-frontal boundary that stretches from
extreme northeastern Arkansas into southern Arkansas and northeast
Texas. Very moist and unstable air is in place ahead of this
boundary already this morning, with temperatures in the lower to
mid 80s and dewpoints in the 75 to 80 degree range.

A decent 700 mb shortwave is also seen aloft associated with this
boundary. This makes sense based on meso-analysis picking up on 30
to 40 knots of shear near that feature aloft. RAP13 seems to be
the only model picking up on this convection and shows this pre-
frontal trough axis and shear well. This model develops the
convection northeastward into northern Alabama and southern
middle Tennessee as those areas become more unstable with the
morning heating and as the pre-frontal trough axis moves east
into the area late this morning into the afternoon hours.

Already, we have MUCAPE between 2000 and 3000 J/KG over much of
northern Alabama and mid level lapse rates 6.5 to 7.0 degrees/km.
In addition, DCAPE parameters are between 1000 J/KG and 1300 J/KG.
This will provide a good deal of dry air aloft to help accelerate
downdrafts once convection gets going. The instability and lapse
rates will only increase as the morning goes on into the early
after hours. Especially given higher shear today, expecting
another round of strong to severe thunderstorms to develop very
late this morning into the afternoon hours. This activity should
dissipate or become very widely scattered toward the early evening
hours as the forcing with this pre-frontal trough axis moves east
of the area.

Despite the convection late this morning into the afternoon hours,
temperatures should still be able to climb into the lower to mid
90s either later this morning or into the mid afternoon hours.
With dewpoints expected to climb into the mid to upper 70s during
that period. The combination of the two should produce heat index
values between 105 and 109 degrees today. Cloud cover and
precipitation should keep excessive heat criteria from being met.

In addition, a cold front remains in place northwest of the
pre-frontal trough axis that stretches from southeastern Missouri
into eastern Oklahoma. This is slow to move south, but will
eventually push southeast into southern middle Tennessee overnight
tonight. This may produce another round of strong to more
marginally severe thunderstorm activity. However, with loss of
daytime heating and some working over of the atmosphere with
expected early convection this afternoon, more uncertainty exists
with this round of storms.

&&

.SHORT TERM...
(Tonight through Tuesday)
Issued at 340 AM CDT Sun Jul 2 2023

Convection should gradually wane in coverage and intensity this
evening with a loss of daytime heating and a more stable
environment returns. Muggy conditions are expected as lows fall
into the lower 70s with near 100 percent relative humidity.
Slightly lower heights thanks to a pattern change as synoptic
troughing will make for a slightly cooler start to an abbreviated
work week with highs in the lower 90s. This will help reduce heat
index values mostly below 105 degrees or Heat Advisory criteria.

The daytime heating of course will destabilize the atmosphere and
bring more afternoon showers and thunderstorms. Some of the
storms could produce marginally strong to severe outflow wind
gusts. Locally heavy rainfall, small hail, and frequent lightning
are all possible in the stronger storms. A near replay is expected
for Independence Day with highs rising into the upper 80s to
lower 90s and heat index values in the mid/upper 90s. More shower
and thunderstorm chances are expected, especially in the afternoon
and early evening. Rain chances however will be higher July 4th,
given forcing from a frontal boundary nearing the region from the
north, which will help to enhance rain production.

&&

.LONG TERM...
(Tuesday night through Saturday)
Issued at 340 AM CDT Sun Jul 2 2023

Large scale atmospheric features for the initial part of the long
term will largely resemble those early in the week. The region will
continue to be in a quasi-zonal upr flow pattern with diffuse,
transient short waves instigating shower/thunderstorm development on
Wednesday. Moisture will tend to remain fairly steady-state on
Wednesday, but may tend to increase regionally on Thursday with
pooling/convergence ahead of a cold front pushing southward. Overall,
lapse rates aloft will tend to be lower Wed and Thurs, but pockets of
dry air in the mid-levels combined with fairly steep low-lvl lapse
rates and sufficiently high PWs could yield strong outflow winds with
thunderstorm activity. The longer term guidance suite has indicated
the possibility of a sfc cold front crossing the region perhaps on
Thursday, adding a further dynamic forcing mechanism for convection
and resulting in higher POPs. Slightly drier low-lvl air may filter
into the region on Friday into Saturday, but detailed characteristics
of the boundary layer air mass are too uncertain to resolve at this
time. Recent model runs have wavered a bit regarding whether or not
the drier continental air is able to make much inroads this far
south, unsurprisingly.

Progressing late into the week and the weekend, an invigorated sub-
tropical jet appears increasingly likely to develop across the mid-
section of the CONUS, including the OH/TN Valley regions. Assuming
relatively high moisture availability overall in the TN, and with
increasing upr flow and prospects for ample shear, better
thunderstorm organization would appear to be feasible late in the
week into next weekend. This might entail MCS activity in the region
perhaps once again given the upr flow regime and orientation of
moisture/thermal gradients upstream.

&&

.AVIATION...
(06Z TAFS)
Issued at 1140 PM CDT Sat Jul 1 2023

No changes to previous aviation forecast reasoning, as it still
appears as if HZ/BR will develop overnight given moist boundary
layer conditions and light SSW winds. For this reason, we have
introduced MVFR vsby beginning arnd 7Z at both HSV/MSL. Although
an isolated SHRA or TSRA will be possible invof residual outflow
boundaries during the early morning hours Sunday, the greatest
spatial coverage of storms tomorrow appears as if it will occur
during the aftn/evening, and we have included PROB30 groups for
convection btwn 19-01Z, when AWWs may be required for
lightning/strong wind gusts/hail. Outside of storm impacts,
conditions will remain VFR, with a SW wind of 10G18 knots that
will gradually subside during the evening.

&&

.HUN WATCHES/WARNINGS/ADVISORIES...
AL...Heat Advisory until 8 PM CDT this evening for ALZ001>010-016.

TN...Heat Advisory until 8 PM CDT this evening for TNZ076-096-097.

&&

NEAR TERM...KTW
SHORT TERM....RSB
LONG TERM....KDW
AVIATION...70