Special Weather Statement, NWS Birmingham, AL

Area Forecast Discussion
National Weather Service Birmingham AL
224 PM CDT Wed Jun 27 2018

.SHORT TERM...
Through Tonight.

A weak upper level trough is moving through the area this
afternoon. This added lift combined with daytime heating and an
unstable airmass will lead to scattered showers and storms through
the rest of the afternoon and early evening. Some storms could be
strong to severe, particularly across the northeast.

Main area of activity is currently along and north of the I-20
corridor, with storms moving in a general easterly direction.
Also watching an area of strong storms moving northward from the
Gulf Coast, into south Alabama. These might impact locations south
of the US 80 corridor around 3-330pm.

Activity will diminish this evening as the trough moves eastward
and we lose daytime heating. Models have backed off on the
potential for an overnight/early Thursday morning MCS affecting
the area. Trends are for the activity to be later in the day
tomorrow. Will trend rain chances back overnight tonight.

14

.LONG TERM...
Thursday through Wednesday.

The ridging at 850mb continues to strengthen Thursday as an upper
level shortwave kicks off through Central TN early Thursday morning.
The GFS, ECMWF, and NAM all have this shortwave sliding almost due
southward through AL/GA through the day on Thursday, but they differ
in exact timings. Meanwhile, another shortwave is sliding through
the Mid MS Valley, developing an MCS. Models are all over the place
with how this pattern plays out. Some show the shortwave in TN
pushing an MCS southward through Central AL and/or Georgia on
Thursday and some show the MCS to our North and West sliding all the
way into our area by late Thursday. The ARW actually plays both
sides and moves an MCS through in the morning from the shortwave in
TN, then a second shortwave Thursday evening with the remnants of
storm to our northwest. It's difficult to forecast how this will
play out 2 days out due to the mesoscale interactions, but with at
least some consistency across models, I can't ignore the possible
damaging wind threat from thunderstorms on Thursday.

While all this is taking place, the low level ridging strengthens,
especially over MS and the western portions of Central AL. 1000-
850mb thickness values are forecast to be at around 1440m, which
would easily support temps in the upper 90s, assuming rain doesn't
move through at or before peak heating. This complicates the
hazardous weather forecast because it creates the possibility for
3 scenarios: First scenario would be that western portions of our
area could see heat indices 105-109, warranting a heat advisory,
while areas in the east and northeast see an MCS move through mid-
morning, with the threat of damaging winds. The second scenario
would bring an MCS in earlier in the day, limiting daytime heating
and making a heat advisory unnecessary, while still presenting a
damaging wind threat. The third scenario would have the ridge
building in stronger and pushing any shortwave and resulting MCS
just east of our area, creating a situation where heat advisories
are needed for a good portion of Central AL. At this point in
time, based on what models are currently showing, I'm going with
the first scenario in my forecast and will continue mentioning
high confidence for heat impacts for portions of Central AL, and
will also introduce a lower confidence threat for damaging winds.
The reason for low confidence in the severe portion is due to the
complexity of MCS propagation and overall inability for models to
resolve it very well this far in advance.

As we go into Friday and into the weekend, models have some energy
from the shortwave lingering back over the Southeastern States, but
keep it generally weak as upper level ridging builds in aloft and
the low level ridging remains in place. Scattered afternoon
thunderstorms look to be possible each day as heat indices remain in
the 100-108 range area-wide. Therefore, will continue mentioning
heat impacts in the HWO through Saturday.

By Sunday, the center of the high pressure and the upper level ridge
has shifted northeastward, allowing 1000-850mb thickness values to
drop to around 1420-1430m across Central AL Monday through
Wednesday. This will mark a return to more seasonable temperatures
with highs in the upper 80s to near 90, which will limit heat index
values from reaching heat advisory criteria. Therefore, will not
carry anything in the HWO beyond Saturday. Typical diurnal
thunderstorm activity will be likely, so have carried 30-50% PoPs in
the afternoons with 15-25% in the overnight hours each day for the
first half of next week.

25/Owen

&&

.AVIATION...
18Z TAF Discussion.

Isolated showers and thunderstorms are developing across the area,
and will use SHRA in prevailing conditions for several terminals. As
activity increases in strength, will transition to VCTS at 19-20Z
given uncertainty in coverage. Stronger storms will produce gusty
winds, frequent lightning, and torrential rainfall.

Confidence has decreased with the potential for an overnight MCS,
and high res guidances suggests any thunderstorm potential would
come later in the forecast period tomorrow. Removed the PROB30
groups.

14

&&

.FIRE WEATHER...

Isolated to scattered afternoon storms are possible with daytime
heating for the next few days. Very warm temperatures with triple
digit heat indices will be the biggest issue for crews. Humidity
values remain high, and there are no fire weather concerns at this
time.

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     72  93  71  93  71 /  30  50  40  50  20
Anniston    73  93  72  93  72 /  30  50  40  50  20
Birmingham  75  94  74  93  74 /  20  50  40  50  20
Tuscaloosa  76  96  75  95  74 /  20  50  30  50  20
Calera      75  93  73  92  72 /  20  50  40  50  20
Auburn      73  91  74  92  74 /  20  50  40  50  30
Montgomery  75  94  75  93  75 /  20  40  40  50  30
Troy        74  93  74  93  74 /  20  40  40  50  30

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
Heat Advisory until 7 PM CDT this evening for the following
counties: Autauga...Barbour...Bibb...Bullock...Chilton...
Dallas...Elmore...Fayette...Greene...Hale...Jefferson...Lamar...
Lowndes...Macon...Marengo...Marion...Montgomery...Perry...
Pickens...Pike...Russell...Shelby...Sumter...Tuscaloosa...Walker.

&&

Hazardous Weather Outlook
National Weather Service Birmingham AL
424 AM CDT Wed Jun 27 2018

ALZ011>015-017>050-281230-
Marion-Lamar-Fayette-Winston-Walker-Blount-Etowah-Calhoun-Cherokee-
Cleburne-Pickens-Tuscaloosa-Jefferson-Shelby-St. Clair-Talladega-
Clay-Randolph-Sumter-Greene-Hale-Perry-Bibb-Chilton-Coosa-Tallapoosa-
Chambers-Marengo-Dallas-Autauga-Lowndes-Elmore-Montgomery-Macon-
Bullock-Lee-Russell-Pike-Barbour-
424 AM CDT Wed Jun 27 2018

This Hazardous Weather Outlook is for the counties served by the
National Weather Service office in Birmingham.

.DAY ONE...Today and Tonight.

Heat indices may reach 105 between noon and 7pm today, especially in
the southwestern portions of Central Alabama. Strong to severe
thunderstorms are possible between 1pm and 8pm in northeastern
portions of Central Alabama. The main threat will be damaging winds
up to 60 miles per hour

.DAYS TWO THROUGH SEVEN...Thursday through Tuesday.

Heat indices will may reach 105 degrees for portions of Central
Alabama each afternoon Thursday through Saturday.

Strong to severe thunderstorms are possible Thursday afternoon for
portions of Central Alabama. The main concern will be damaging winds
up to 60 miles per hour.

.SPOTTER INFORMATION STATEMENT...

Activation of storm spotters and emergency management may be needed
this afternoon and Thursday afternoon.

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