Special Weather Statement, NWS Birmingham, AL

Area Forecast Discussion
National Weather Service Birmingham AL
335 PM CDT Sat May 20 2017

.SHORT TERM...
Through Tonight.

Showers and thunderstorms have formed into broken line segments
across Central Alabama this afternoon, with one line oriented
north to south near the MS state line and pushing slowly
eastward. Another is oriented from west to east near the US
Highway 80 corridor and pushing northward. In addition, seeing
scattered activity ahead of these two more focused features. This
activity is in response to an upper level shortwave moving
north from the Gulf of Mexico and an approaching trough and cold
front from the west. The HRRR has done a good job in handling the
progression of these features and convective trends today, and
the near term PoP forecast has been based on hi-res model guidance
and radar trends.

Through tonight, this activity will continue to become more
widespread as it moves both east and north. We have received a
couple small hail reports with stronger storms this afternoon
across the local area, and a couple wind gusts measured by ASOS of
20-25kts. These impacts will continue to be possible this
afternoon. Forecast soundings become saturated with PW values
nearing 2.0 inches as we move into the late afternoon and early
evening hours. Activity will train over the same area, and with
high rainfall rates, flash flooding primarily in urban and known
problem locations will become the main threat. Those with outside
activities through tonight should plan on heavy rainfall and
lightning.

Tomorrow's forecast comes with a great deal of uncertainty, as
rain chances will depend on how storms evolve tonight and how
quickly they push to the east. Moisture remains high across the
area as upper level flow remains out of the southwest and the
surface front back to our west tomorrow morning. Expect PoPs to
increase for at least a portion of the area tomorrow, but
confidence is not high enough to use definite wording for any
particular timeframe yet.

14

.LONG TERM...
Sunday through Friday.

The cold front will move into the area Sunday and Sunday night.
The main upper-level dynamics will pass well north of the area,
with just the southern edge of the trough passing over our area.
However, tropical moisture will remain in place along with weak
shortwaves in the southwest flow aloft. Therefore, rain chances
will remain high, but the exact convective details of what time it
will rain and what time there will be breaks remains uncertain.
Instability should be less than today given convective
overturning, but a few strong storms with gusty winds will remain
possible. Weak 0-6 km bulk shear values should prevent any
organized severe weather. As discussed in the short term section,
the threat for heavy rainfall will continue to be monitored.

The cold front should stall somewhere over the area by Monday as
it becomes parallel to the westerly flow aloft and separated from
the parent system. The deeper moisture should move east of the
area, but weak shortwaves in the westerly flow aloft interacting
with the front should result in continued chances of scattered
showers and storms, primarily across the southern half of Central
Alabama. A fast- moving, possibly convectively enhanced shortwave
will move through the Gulf Coast region through the base of an
large and anomalously deep trough amplifying over the central and
eastern US Monday night and Tuesday. This may be accompanied by a
surface low and LLJ. Rain chances will begin to increase Monday
night as isentropic lift develops ahead of this system, followed
by a potential MCS on Tuesday. There's still alot of uncertainty
regarding the timing and strength of this system. If it tracks far
enough inland, a threat for damaging winds and even brief
tornadoes could develop across parts of southern Central Alabama
south of the warm front. On the other hand, a track along the
coast would minimize surface-based instability further inland, a
scenario that often occurs. There's still too much uncertainty
due to the small size of the wave and mesoscale details to mention
anything in the HWO. Heavy rainfall will also be something to
monitor as well.

Wednesday a more potent shortwave will round the base of the
anomalous trough and a cold front will move into the area. 0-6 km
bulk shear, upper- level forcing, and cooling aloft will all be
impressive for late May with this system. However, moisture
return will be very questionable behind Tuesday's system due to
low- level winds turning more northwesterly Tuesday night without
much of a southerly LLJ by Wednesday, so instability and
associated severe threat will be uncertain with dewpoints only in
the low 60s. The upper- level forcing and front will be enough to
trigger a good chance of showers and embedded storms, also aided
by a -18C cold pool at 500 mb. Dry and refreshingly cooler
conditions will move in for Thursday.

32/Davis

&&

.AVIATION...
18Z TAF Discussion.

Showers and thunderstorms will become more widespread through the
afternoon hours as a storm system approaches from the west. Activity
is showing signs of organizing into clusters and lines, which
follows with hi-res near term guidance. Given both those trends,
have transitioned from SHRA VCTS to TSRA for several hours at
different times through the afternoon and evening for each terminal.
Any shower or storm will produce heavy rainfall and gusty winds to
20kts. Strongest storms could produce gusts to 40kt. Winds will
generally remain from the south to southeast through the period.
Additional development is possible Sunday morning, but will greatly
depend on how activity evolves overnight.

Timing for predominant TSRA will likely be adjusted through the
afternoon and evening as trends evolve.

14

&&

.FIRE WEATHER...

Widespread rain and storms will continue tonight, with high rain
chances tomorrow ahead of a cold front. The front is expected to
stall across the area, leaving elevated rain chances in the
forecast through mid week. Critical fire weather conditions are
not expected.

&&

.PRELIMINARY POINT TEMPS/POPS...
Gadsden     70  78  63  78  59 /  80  70  70  20  10
Anniston    70  78  66  79  60 /  80  70  70  30  10
Birmingham  72  78  65  80  61 /  80  70  70  20  20
Tuscaloosa  72  80  65  82  62 /  80  70  70  30  40
Calera      71  78  65  80  62 /  80  70  70  30  30
Auburn      71  79  67  80  63 /  80  70  80  50  30
Montgomery  73  82  68  83  64 /  80  70  70  50  40
Troy        72  81  67  82  64 /  80  70  80  50  40

&&

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

&&

Hazardous Weather Outlook
National Weather Service Birmingham AL
348 AM CDT Sat May 20 2017

ALZ011>015-017>050-211200-
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-
348 AM CDT Sat May 20 2017

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

.DAY ONE...Today and Tonight.

Numerous showers and thunderstorms will develop this afternoon. A
couple isolated storms could produce damaging wind gusts up to 60
mph between noon and 8 PM west of a Montgomery to Heflin line.

.DAYS TWO THROUGH SEVEN...Sunday through Friday.

No hazardous weather is expected at this time.

.SPOTTER INFORMATION STATEMENT...

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

Special Weather Statement
National Weather Service Birmingham AL
353 PM CDT SAT MAY 20 2017

ALZ011-014-202115-
Winston AL-Marion AL-
353 PM CDT SAT MAY 20 2017

...SIGNIFICANT WEATHER ADVISORY FOR NORTHEASTERN MARION AND
NORTHWESTERN WINSTON COUNTIES UNTIL 415 PM CDT...

At 353 PM CDT, Doppler radar was tracking strong thunderstorms along
a line extending from Hackleburg to near Haleyville. Movement was
northeast at 40 mph.

Winds in excess of 40 mph will be possible with these storms.

Locations impacted include...
Haleyville, Hackleburg, Needmore, Ashridge, Bear Creek, Weiss, Posey
Field Airport, Tessner, Delmar, Sipsey River Rec Area and Pebble.

LAT...LON 3430 8731 3414 8747 3421 8795 3431 8785
TIME...MOT...LOC 2053Z 211DEG 33KT 3426 8785 3418 8760

JH/05

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Special Weather Statement
National Weather Service Birmingham AL
455 PM CDT SAT MAY 20 2017

ALZ017-019-021-024>028-202245-
Calhoun AL-Clay AL-Jefferson AL-Shelby AL-Talladega AL-St. Clair AL-
Cleburne AL-Blount AL-
455 PM CDT SAT MAY 20 2017

...SIGNIFICANT WEATHER ADVISORY FOR TALLADEGA...BLOUNT...SOUTHWESTERN
CALHOUN...NORTHEASTERN SHELBY...NORTHEASTERN JEFFERSON...ST. CLAIR...
SOUTHWESTERN CLEBURNE AND NORTHEASTERN CLAY COUNTIES UNTIL 545 PM
CDT...

At 455 PM CDT, Doppler radar was tracking strong thunderstorms along
a line extending from near Blount Springs to near Greystone to near
Talladega. Movement was northeast at 25 mph.

Pea size hail and winds in excess of 40 mph will be possible with
these storms.

Locations impacted include...
Birmingham, Hoover, Vestavia Hills, Anniston, Oxford, Mountain Brook,
Trussville, Talladega, Gardendale, Pell City, Irondale, Leeds, Moody,
Chelsea, Fultondale, Oneonta, Childersburg, Springville, Odenville
and Blountsville.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

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

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

&&

LAT...LON 3397 8634 3394 8637 3395 8632 3371 8609
      3352 8558 3326 8623 3330 8655 3379 8701
      3380 8695 3387 8696 3387 8692 3391 8692
      3397 8677 3406 8671 3405 8668 3409 8669
      3409 8668 3412 8660 3418 8656
TIME...MOT...LOC 2155Z 241DEG 23KT 3389 8685 3345 8658 3336 8612

JH/05

Special Weather Statement
National Weather Service Birmingham AL
459 PM CDT SAT MAY 20 2017

ALZ019-021-029-202245-
Calhoun AL-Randolph AL-Cleburne AL-
459 PM CDT SAT MAY 20 2017

...SIGNIFICANT WEATHER ADVISORY FOR NORTHERN RANDOLPH...SOUTHEASTERN
CALHOUN AND CLEBURNE COUNTIES UNTIL 545 PM CDT...

At 458 PM CDT, Doppler radar was tracking strong thunderstorms along
a line extending from 7 miles north of Delta to near Morrison Cross
Roads to 8 miles south of Ephesus. Movement was northeast at 20 mph.

Pea size hail and winds in excess of 40 mph will be possible with
these storms.

Locations impacted include...
Anniston, Oxford, Jacksonville, Heflin, Wedowee, Ranburne,
Fruithurst, Edwardsville, Woodland, Morrison Cross Roads, Abernathy,
Fosters Cross Roads, Graham, Newell, Choccolocco, Trickem, Lake
Cleburne, Micaville, Big Springs and Hollis Crossroads.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

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

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

&&

LAT...LON 3320 8534 3333 8551 3341 8565 3349 8565
      3350 8576 3348 8577 3350 8580 3385 8574
      3370 8535 3325 8526
TIME...MOT...LOC 2158Z 211DEG 19KT 3354 8565 3345 8543 3328 8528

JH/05