Tornadoes in Alabama, April 27, 2017

Area Forecast Discussion
National Weather Service Birmingham AL
845 AM CDT Thu Apr 27 2017

.UPDATE...
MESOSCALE UPDATE.

&&

.DISCUSSION...

The line of storms can't get out of AL soon enough.  Very warm,
moist, and unstable low level air along and southeast of I-85 may
warm sufficiently for some reintensification of convection in the
marginal risk area. Cells have been able to organize briefly with
broad rotation, but can't seem to sustain a single updraft for a
prolonged period of time. Just about the time the cells are
organized - they dissipate. With additional heating the concern
is that cells may be able to tap into the rich air. At the same
time however, with the upper low and best dynamics pulling out to
the north, shear profiles are on the decline through the morning.
Bottom line is that while there may be sufficient fuel for perhaps
significant thunderstorms, organization of sustained updrafts may
be difficult to achieve. We will continue to keep a sharp eye on
the radar and monitor storm scale features.

02/JD

&&

.AVIATION...
12Z TAF Discussion.

Main line of convection now exiting MGM and will impact TOI after
13Z. Most of the convection will be across the southern sites
through 17Z. Low clouds will persist through the morning in the
west and early afternoon in the east/southeast. Skies will clear
afterwards. Some stratus/fg will develop across the south and move
northward after 6Z. This has now been persistent for two runs and
confidence is growing. Included IFR ceilings and vis for MGM and
TOI after 6Z and hinted at some lower cigs making it into ASN
before 12Z. Lower clouds may invade much of the area just outside
of this TAF cycle.

16

&&

.FIRE WEATHER...

A cold front will continue to bring showers and thunderstorm to the
area through this morning. Storms will be strong to severe with
heavy rainfall. Drier conditions are expected by late afternoon,
with another good chance of storms late Sunday afternoon into Sunday
night. There could also be some isolated showers and storms on
Friday but those chances remain small. Otherwise, there are no fire
weather concerns.

&&

.PREV DISCUSSION...
/Issued 647 AM CDT Thu Apr 27 2017/
SHORT TERM...
This morning.

As of 08:30Z/3:30AM, a line of thunderstorms continues to move
eastward through central Alabama. The environment is characteristic
of weak surface-based instability/few swaths of modest mixed-layer
instability (across southern counties), alongside deep-layer bulk
shear values around 50-knots + high helicity values, and steep lapse
rates above the top perimeter of a low-level inversion.

We launched a special 07Z balloon, which unfortunately popped very
early as it went inside downstream convective elements. We gathered
just enough data to see that a low-level inversion remains strong,
centered just above 850mb at 15.6C/60F. We also had a thermal
inversion around 500mb on the 00z sounding and it appears that the
temperature reading may have been reflecting that once again before
transmission stopped. Steep lapse rates were observed above the
inversion, but the inversion is a big detriment tonight. Speed and
directional wind shear remain well-off, with quite a bit of
curvature on the hodograph. A bit of veer-back-veer profiling was
evident, though. Dew points have been rising several degrees just
ahead of the line of storms, which is where capping is most offset.
Given background wind shear and influxes in surface/low-level
moisture, we will have to remain vigilant for any instance of a
storm becoming surface-based. The highest potential (which is low
overall) for severe storms is primarily south of I-20 for the
remainder of the overnight and into the morning hours post-sunrise.
Refer to the Severe Thunderstorm Watch area for our focus area.
That'll be the best zone for accessed instability to overlap shear
values + the more cellular nature of the storms. We could still
have an instance(s) of damaging straight-line winds, hail, and
mayhaps a brief tornado. Farther north, rain with some boomers is
expected.

89^GSatterwhite

LONG TERM...
Tonight through Wednesday.

All of the rain should be east of the area by tonight and skies
will continue to clear. However, the southeast will be in close
enough proximity to the front that some post frontal stratus may
redevelop. Numerous short term models concur on this idea and made
sure to include in the Sky grids through 10 am. On Friday the
boundary lifts back northward during the day. This added boundary
may be enough to trigger an isolated thunderstorm into the area
during the peak heating of the day, especially with readings in
the mid to upper 80s.

Once this boundary lifts northward by Friday night we should stay
dry for much of Saturday and Saturday night. Models are hinting at
some orographic lifting resulting in isolated thunderstorms
Saturday afternoon in the northeast but will stay dry for now,
given the overall environment.

As we get into Sunday, models have slowed down a little bit with
the overall timing of the front. Still looking for the potential
for some strong to severe storms with it, but it now may not work
into the area until late Sunday afternoon and push through Sunday
night. Much more to be discussed with regards to the severe
potential but with active weather on the radar this morning, will
leave that to the next update.

After this system moves out during the overnight period looks like
we will be drier and a touch cooler as we work into the first week
of May, with another system just on the outside skirts of this
forecast.

16

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     77  56  86  65  89 /  50   0  20  10  10
Anniston    77  59  85  67  89 /  70   0  20  10  10
Birmingham  78  62  87  69  89 /  30   0  20  10  10
Tuscaloosa  78  61  87  70  90 /  20   0  20  10  10
Calera      78  62  86  68  89 /  30   0  20  10  10
Auburn      78  65  84  68  88 /  70  10  20  10  10
Montgomery  82  66  88  68  91 /  60  10  20  10  10
Troy        82  68  87  68  89 /  70  10  20  10  10

&&

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

&&

Hazardous Weather Outlook
National Weather Service Birmingham AL
458 AM CDT Thu Apr 27 2017

ALZ011>015-017>050-281300-
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-
458 AM CDT Thu Apr 27 2017

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

.DAY ONE...Today and Tonight.

A line of strong to marginally severe storms will continue to push
through Central Alabama until Noon today. The main threat will be
damaging winds south and east of the interstate 85 corridor.

.DAYS TWO THROUGH SEVEN...Friday through Wednesday.

There is also a threat of severe thunderstorms across all of central
Alabama late Sunday afternoon and into Sunday night. The threats
include damaging winds, large hail, and tornadoes.

.SPOTTER INFORMATION STATEMENT...

Activation of storm spotters and emergency management may be needed
late tonight and into Thursday morning, and again Sunday into Sunday
night.

Special Weather Statement
National Weather Service Birmingham AL
841 AM CDT THU APR 27 2017

ALZ037-271415-
Tallapoosa-
841 AM CDT THU APR 27 2017

...A STRONG THUNDERSTORM WILL AFFECT NORTHERN TALLAPOOSA COUNTY...

At 841 AM CDT, a strong thunderstorm was located near Jacksons Gap,
or near Dadeville, moving northeast at 55 mph.

Half inch hail is possible with this storm.

Locations impacted include...
Alexander City, Dadeville, Jacksons' Gap, New Site, Goldville,
Jacksons Gap, Hillabee Creek, Daviston, Dare Park, Eagle Creek,
Horseshoe Bend and Horseshoe Bend National Military Park.

If you are on or near Lake Martin, get out of the water and move
indoors or inside a vehicle. Remember, lightning can strike out to
10 miles from the parent thunderstorm. If you can hear thunder, you
are close enough to be struck by lightning. Move to safe shelter
now! Do not be caught on the water in a thunderstorm.

LAT...LON 3276 8587 3275 8588 3282 8596 3311 8581
      3311 8559 3292 8559
TIME...MOT...LOC 1341Z 225DEG 48KT 3284 8586

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