Special Weather Statement, NWS Birmingham, AL

Area Forecast Discussion...UPDATED
National Weather Service Birmingham AL
606 AM CST Mon Jan 2 2017

.UPDATE...
For 12Z Aviation.

&&

.DISCUSSION...

An active severe weather day is shaping up across the Gulf States,
and confidence in the local severe weather threat has increased
somewhat this morning. However, the threat across our forecast
area remains very conditional on mesoscale features and quite
complicated. Overnight convection extending from the southeast tip
of Louisiana into Southeast AL and southern Georgia has created an
effective cold front which is now beginning to stall. This
boundary should begin to return northward later this morning and
become the effective warm front. To the north of this boundary,
the airmass is rather stable and characterized by temperatures and
dewpoints in the lower 60s. Some modification of the airmass is
possible north of the effective warm front as SSW 850 mb flow
continues to feed moisture into the region, but the strongest CAPE
should remain south of the boundary. For this reason and in the
scenarios discussed below, we will continue to show the northern
one-third of the forecast area as having no discernible severe
weather risk.

Part 1 (Noon to 8 PM): Elevated thunderstorm activity has formed
this morning in Southwest Mississippi and may reach our southwest
counties late this morning into the early afternoon. At the same
time, airmass modification may lead to surface-based instability
as these storms arrive into the CWA. There is some suggestion by
recent model information that another effective warm front will
develop in the vicinity of the Highway 80 corridor, enhancing the
threat for tornadoes and damaging winds. To account for this, our
severe weather timeframe will be shown starting at noon with an
elevated risk area extending from Demopolis to Phenix City
southward and a lower threat to the north.

Part 2 (8 PM to 3 AM): We are now also concerned about the
potential for a more substantial nocturnal severe weather
episode mainly along and south of the Interstate 85 corridor.
Short range ensemble guidance and other individual models are
coming to a consensus regarding the development of a 45-55 kt LLJ
tonight across southern Alabama along with large clockwise-curved
hodographs. This LLJ could promote the advancement of much
stronger CAPE into our S/SE counties after 8 PM. Mid-level lapse
rates have also been trending stronger with time, and forecast
soundings show an increasingly dangerous environment. What
initially may form into a QLCS could break up into supercells as
the 500 mb trough deamplifies and and 0-6 km shear vectors become
westerly or even west-northwesterly. The threat appears to
continue much longer than previously thought, and an elevated risk
for tornadoes and damaging winds will be maintained through 3 AM.
A strong tornado or two may also be possible, and that possibility
will need to be evaluated in later updates.

There is still not a good model consensus beyond Wednesday,
leading to continued low forecast confidence. Ensemble members
continue to show a wide range of possibilities from wintry
precipitation to all rain to totally dry conditions for Thursday
through Saturday. A slight chance for a wintry mix was kept in for
Friday into Friday for consistency.

87/Grantham

&&

.AVIATION...
12Z TAF Discussion.

Band of showers across the U.S. 80 corridor and southward this
morning. A few lightning strikes near TOI otherwise expecting TS
activity to increase during the day from the south/southwest
spreading across/into all areas of TAFs by afternoon as a warm
front progresses northward and ahead of a cold front to our
northwest. Until then IFR cigs and MVFR fog will be common along
I-20 corridor behind earlier rain shield. Some storms could be
strong this afternoon and tonight especially in the MGM/TOI area.

08

&&

.FIRE WEATHER...

Wet conditions will continue through tonight with drier conditions
to follow. There are no fire weather concerns at this time.

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     67  58  65  47  51 /  90 100  20  10  10
Anniston    68  59  66  48  54 /  90 100  20  10  10
Birmingham  70  59  66  49  53 /  90  90  20  10  10
Tuscaloosa  71  58  68  49  54 / 100  80  10  10  10
Calera      70  59  67  49  54 /  90  90  20  10  10
Auburn      69  60  68  50  58 /  90 100  20  10  10
Montgomery  73  60  72  52  61 / 100  90  10  10  10
Troy        72  59  72  51  63 /  90  90  10  10  10

&&

.BMX WATCHES/WARNINGS/ADVISORIES/...
Dense Fog Advisory until 9 AM CST this morning for the following
counties: Bibb...Blount...Calhoun...Cherokee...Cleburne...
Etowah...Fayette...Greene...Hale...Jefferson...Lamar...Marion...
Pickens...Shelby...St. Clair...Sumter...Talladega...Tuscaloosa...
Walker...Winston.

Flash Flood Watch through 6 AM Tuesday for the following
counties: Barbour...Bullock...Chambers...Lee...Lowndes...Macon...
Montgomery...Pike...Russell.

&&

87/08

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