Severe Thunderstorm Warning 187, NWS Birmingham, AL

Area Forecast Discussion
National Weather Service Birmingham AL
117 PM CDT Tue Aug 11 2020

.UPDATE...
Midday Update and 18Z Aviation.

&&

.SHORT TERM...
/Updated at 1238 PM CDT Tue Aug 11 2020/

Scattered thunderstorms will continue through the afternoon and
into the evening. Instability is 2000-3000 J/kg with DCAPE values
around 1100 to 1200 J/kg. PW values are higher in the southern
half of the area around 1.95...and west of I65...so would expect
the concentration of potentially severe storms to be in these
areas. With the cloud cover experienced in the northeast through
the morning...storms may not have enough recovery time to develop
high enough cores...so including a lower threat for severe
thunderstorms in these areas. Activity will continue through the
evening...weakening after sunset. Patchy fog development is
possible in the early morning in areas that received storms later
in the afternoon and early evening. Temperatures today will be in
the mid to upper 90's with heat indices in the triple digits.

Wednesday...diurnally driven thunderstorms will be possible
through the afternoon and evening. Similar instabilities are
expected in the 2000-3000 J/kg range...with PW values around 1.9.
DCAPE and lapse rates are less impressive over most of the
area...though south of I85...there seems to be enough dry air for
another risk for severe thunderstorms. Cloud cover looks to be
higher as well. Edged the afternoon high temperatures a degree or
two lower...though heat indices will once again be in the triple
digits in the afternoon.

Previous short-term discussion:
/Updated at 0356 AM CDT Tue Aug 11 2020/

Today and Tonight.

A convective outflow boundary is moving south through the TN
Valley early this morning which is acting to initiate showers and
thunderstorms across northern AL. This feature will be a big
player in today's overall convective activity as the airmass to
the north should remain more stable. I expect this boundary will
eventually slow down and become more diffuse with time as the MCS
responsible for it has significantly decayed overnight, so
pinpointing it's position later this morning will certainly help
identify areas of best rain chances. For now I've focused PoPs
west of I-65 and southeastward toward the MGM area where best
theta-e and instability values are progged this afternoon,
generally along and south of the boundary. This area is also west
of the 'worked-over' airmass from yesterday's convection as
depicted by a deficit in buoyancy/theta-e on recent mesoanalysis.
In addition to the best overall rain chances, temperatures in the
mid 90s and dewpoints in the low 70s will not only support SBCAPE
of 3,000+ J/kg for convection here, but heat indices of 100-105
degrees as well. This will be maintained in the HWO, but there's
not enough confidence in spatial or temporal coverage to issue a
Heat Advisory at this time. Furthermore, the combination of
residual dry air in the 700-500 mb layer and favorable 0-3 km
lapse rates this afternoon also suggests the risk for damaging
downburst winds and perhaps small to quarter size hail in
strongest cores/updrafts. Isolated severe threats will be
maintained in the HWO as a result.

Wednesday morning and afternoon.

It's a bit of a repeat forecast for Wednesday as summer heat and
humidity is going nowhere fast. Another afternoon of low to mid
90s is expected with scattered to numerous showers and
thunderstorms, as well as heat indices in the 100-105 range. I
kept PoPs a little more generalized and focused on areas of best
moisture quality during this time. Guidance is in normal
disagreement on this notion, but areas south seem to be a common
theme for PWs approaching or exceeding 1.8-2.0". Thus, I placed
likely PoPs across these areas with chances PoPs elsewhere as most
of Central AL remains wedged between two perturbations (one to
our northwest and a weak/diffuse disturbance drifting to our
southeast). The convectively enhanced shortwave to our northwest
will be watched as this could promote additional activity during
the late afternoon but I kept PoPs truncated in expectation that
it remains too far away during the best overall diurnal
environment.

40/Sizemore

.LONG TERM...
/Updated at 0356 AM CDT Tue Aug 11 2020/

Thursday through Monday.

A gap or weakness in the subtropical ridge will persist over the
Deep South on Thursday, with a weak 500 mb low and trough axis
moving toward Northwest Alabama. Deep-layer cyclonic flow around
this feature will be associated with lift and enhanced moisture
across the state. Showers and storms are likely with POPs ranging
from 50 percent in the west to 70 percent in the east. Nearly the
same setup is expected for Friday.

Due to different handling of upstream disturbances driven by
convection, models disagree on the evolution of the upper-level
pattern after Friday. There is however, general agreement for
falling rain chances, but the timing and magnitude is in question.
For now, our forecast will show a gradual drop in POPs over the
Saturday through Monday period.

87/Grantham

&&

.AVIATION...
18Z TAF Discussion.

Scattered thunderstorms are developing across the state...with
activity remaining through the evening. The best coverage of
convection will be around KTCL...KMGM...and KTOI...so included
VCTS through the afternoon. Storms could impact the other TAF
sites...but since coverage and timing were uncertain...left
mention out of TAF for now and will update if needed. Any
activity will weaken after sunset...with fog development possible
overnight into the early morning.

24

&&

.FIRE WEATHER...

Moisture content will remain high through the end of the week, and
scattered showers and storms can be expected through Wednesday. The
pattern should become even wetter for Thursday and Friday with rain
chances in the 50-70 percent range.

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     71  94  72  92  71 /  20  40  30  60  40
Anniston    73  94  72  93  72 /  20  50  30  60  40
Birmingham  74  94  74  92  74 /  20  40  30  60  40
Tuscaloosa  75  95  74  93  74 /  20  40  30  60  40
Calera      73  94  72  91  72 /  20  40  30  60  40
Auburn      72  93  72  89  72 /  30  50  40  70  40
Montgomery  74  96  74  94  74 /  30  60  40  60  40
Troy        73  95  73  92  73 /  30  60  40  70  40

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
None.

&&

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Special Weather Statement
National Weather Service Birmingham AL
130 PM CDT Tue Aug 11 2020

ALZ042-111900-
Lowndes AL-
130 PM CDT Tue Aug 11 2020

...SIGNIFICANT WEATHER ADVISORY FOR WEST CENTRAL LOWNDES COUNTY UNTIL
200 PM CDT...

At 130 PM CDT, Doppler radar was tracking a strong thunderstorm near
Gordonville, or near Mosses. This storm was nearly stationary.

Pea size hail and winds in excess of 40 mph will be possible with
this storm.

Locations impacted include...
Mosses, White Hall and Gordonville.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

Torrential rainfall is also occurring with this storm, and may lead
to localized flooding. Do not drive your vehicle through flooded
roadways.

Frequent cloud to ground lightning is occurring with this storm.
Lightning can strike 10 miles away from a thunderstorm. Seek a safe
shelter inside a building or vehicle.

&&

LAT...LON 3218 8681 3222 8682 3223 8681 3224 8681
      3224 8682 3225 8681 3226 8678 3227 8669
      3225 8665 3216 8666 3214 8670 3214 8675
TIME...MOT...LOC 1830Z 154DEG 1KT 3222 8675

89^GSatterwhite