Area Forecast Discussion
Issued by NWS Birmingham, AL

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FXUS64 KBMX 200049

Area Forecast Discussion
National Weather Service Birmingham AL
749 PM CDT Mon Mar 19 2018





The environment remains favorable for severe thunderstorms with
wind shear continuing to increase across our northern and central
counties. Though storm development remains more isolated across
our southern and central counties, any activity that develops and
intensifies will have the opportunity to tap into favorable
instability and dynamics that will support all convective hazards
from very large hail to tornadoes to damaging winds. Currently,
the most favorable area for severe storms in where the best low
level convergence is from Double Springs to Oneonta to Helfin.
These storms will slowly move south and east the nest few hours.

An effective surface dryline extended from near Natural Bridge in
Winston County southwest to near Vernon in Lamar County. The
severe weather threat will end as the dryline progresses further

We have launched special weather balloon soundings every 3 hours
since 18Z this afternoon and the trend over time has been
increasing wind shear across central Alabama while robust
instability has been maintained due to the solar heating much of
the area experienced for much of the afternoon along with
generally southerly winds that have helped maintain dew points
generally in the 60`s across much of our forecast area.

A limiting factor in convective development has been the remnant
Elevated Mixed Layer (EML) that has persistently been present in
the observed soundings aloft this afternoon. This dry layer has
helped to promote the accumulation of convective available
potential energy while only permitting the strongest of updrafts
to break through the cap. This has resulted in the cap holding
strong across much of our south-central counties since mid
afternoon while earlier development across northeast Mississippi
and northwest Alabama have maintained their development and
further expanded in extent and intensity over the past few hours.



00Z TAF Discussion.

Trend with this set of TAFs will be in a bit more optimistic
direction, as the higher coverage of convection thus far has
remained well to the north of central Alabama terminals. Will
maintain a VCTS for a couple of hours at most locations, although
it did push this back an hour or two from the previous forecast
(based on radar trends). Still quite a bit of uncertainty on how
far south the convection will get overnight. If radar trends over
the next couple hours show the storms eventually expanding in
area, then we can update the TAFs where necessary.

Winds will shift around to a more westerly direction overnight,
and start to increase and be more gusty after sunrise on Tuesday.
The models were giving me mixed signals on the development of
lower clouds after the rain/storm threat ends overnight. Given the
lack of clouds back to the west, I am inclined to believe the more
optimistic of the model solutions. But once the winds kick up
Tuesday, the cumulus field should start to fill in as well.




After some storms tonight, light showers are possible Tuesday,
before dry conditions arrive by Wednesday. RH values will be
elevated the next couple days. There are no fire weather


/Issued 649 PM CDT Mon Mar 19 2018/

Confidence has increased in the potential for explosive supercell
thunderstorm development late this afternoon into tonight
especially along and north of a line from Tuscaloosa to Calera to
Wedowee. A dangerous environment will also be in place south of
this line, but initiation of storms and storm coverage is

Currently, surface winds have veered to the southwest across most
of the area following the passage of a warm front. Normally this
would yield lower dewpoints and overall moisture quality, but this
does not appear to be the case. Dewpoints are holding in the mid
to upper 60s upstream across MS. Moisture quality should not be an
issue as this event progresses.

This event will hinge on evolution of the surface to 850mb wind
fields in response to the approaching upper-level trough. We are
already seeing this response across the northern half of MS where
winds are south to southeast near Tupelo and Columbus. Hi-res
models appear to be getting a better handle on the mesoscale
features and are slowly trending more aggressive with surface
pressure troughing and backing of winds in the 23-03z time frame.
HRRR guidance suggests the potential for 0-1km shear to exceed
40kts. This amount of shear combined with SBCAPE of 2000-3000 J/kg,
supports a risk for a few long-track tornadoes. A violent tornado
cannot be ruled out. The best chance of these enhanced ingredients
coming together for long-track tornadoes appears to be from I-65
eastward and as far south as Shelby, Talladega, Clay, and Randolph

Balloon launches are planned for 330 PM, 6 PM, and 730 PM to sample
critical changes in the low-level wind environment.



Gadsden     51  55  34  51  33 /  90  40  30   0   0
Anniston    53  56  35  53  33 /  90  40  30   0   0
Birmingham  49  54  37  56  36 /  80  40  20   0   0
Tuscaloosa  48  56  37  60  37 /  70  30  10   0   0
Calera      50  55  37  59  36 /  80  40  20   0   0
Auburn      56  59  38  58  36 /  70  20  20   0   0
Montgomery  57  60  39  62  38 /  60  20  10   0   0
Troy        55  62  41  62  37 /  50  20  10   0   0




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