Alert archive / Alabama / Tornado Watches in 2011

Tornado Watch 245

April 27, 201125 counties in Alabama; Greene County, Perry County, Mississippi

The Storm Prediction Center issued this tornado watch at Apr 27, 2011, 9:42 PM CDT for 25 counties in Alabama; Greene County, Perry County, Mississippi. It was in force for 4 hr.

The area

The outline as issued.

The facts

Issued: Apr 27, 2011, 9:42 PM CDT (02:42 UTC)
Expired: Apr 28, 2011, 2:00 AM CDT
In force: 4 hr
Issued by: the Storm Prediction Center
Largest hail expected: 1.5 in
Strongest gust expected: 60 kt

All 27 counties

Autauga County, AL, Bibb County, AL, Butler County, AL, Calhoun County, AL, Cherokee County, AL, Chilton County, AL, Choctaw County, AL, Clarke County, AL, Clay County, AL, Cleburne County, AL, Conecuh County, AL, Coosa County, AL, Crenshaw County, AL, Dallas County, AL, Etowah County, AL, Hale County, AL, Lowndes County, AL, Marengo County, AL, Monroe County, AL, Perry County, AL, Shelby County, AL, St. Clair County, AL, Talladega County, AL, Washington County, AL, Wilcox County, AL, Greene County, MS, Perry County, MS

The radar

The Birmingham radar (KBMX), 44 km from the middle of the area, began a volume at 02:44 UTC, the scan nearest the moment this went out.

Open the radar archive at this moment

The archived warnings, watches and discussions in force at that minute are drawn over the scan, this one among them.

What SPC expected

The probabilities SPC published with the watch, for the watch area over its life.

Two or more tornadoes: 40%
One or more EF2 to EF5 tornadoes: 20%
Ten or more severe wind reports: 30%
One or more gusts of 65 knots or more: 30%
Ten or more severe hail reports: 40%
One or more hailstones of 2 inches or more: 30%
Six or more combined severe hail and wind reports: 70%

Warnings issued inside it

7 tornado, severe thunderstorm and flash flood warnings for its counties while it was in force.

Issued	Product	Office	Counties	Lasted
Apr 27, 9:42 PM CDT	Severe Thunderstorm Warning	MOB	Wilcox	33 min
Apr 27, 9:46 PM CDT	Tornado Warning	BMX	Chilton, Coosa, Tallapoosa	37 min
Apr 27, 9:51 PM CDT	Severe Thunderstorm Warning	BMX	Calhoun, Cherokee, Etowah	39 min
Apr 27, 10:39 PM CDT	Severe Thunderstorm Warning	BMX	Chambers, Clay, Coosa, Elmore, Randolph, Tallapoosa	49 min
Apr 27, 11:15 PM CDT	Severe Thunderstorm Warning	BMX	Dallas, Lowndes	45 min
Apr 27, 11:31 PM CDT	Severe Thunderstorm Warning	BMX	Autauga, Elmore, Lowndes, Montgomery	44 min
Apr 28, 12:01 AM CDT	Severe Thunderstorm Warning	BMX	Autauga, Bullock, Elmore, Lee, Lowndes, Macon and 3 more	49 min

Discussions around it

SPC mesoscale discussions over the same counties from four hours before the watch to its end.

The nearest sounding before it

Birmingham, AL (KBMX), launched 00:00Z, 2.7 hours before the alert went out and 44 km from the middle of its area. This is the routine balloon launch nearest before it, the air the forecaster had in hand, not a measurement taken for the alert.

Skew-T log-p

Temperature in red, dew point in green, the mixed-layer parcel dashed.

Hodograph

The wind turning with height, colored by layer: ground to 1 km, 1 to 3 km, 3 to 6 km, 6 to 10 km.

Mixed-layer CAPE: 3065 J/kg
Mixed-layer CIN: -4 J/kg
Mixed-layer LCL: 711 m above ground
0-6 km bulk shear: 73 kt
0-1 km storm-relative helicity: 448 m²/s²
0-3 km storm-relative helicity: 480 m²/s²
Significant tornado parameter: 9.6
Supercell composite: 33.6

Open this sounding in Storm Lab

The text

As transmitted, in the capitals the NWS used until 2016.

911 
WWUS20 KWNS 280243
SEL5
SPC WW 280243
ALZ000-MSZ000-280700-

URGENT - IMMEDIATE BROADCAST REQUESTED
TORNADO WATCH NUMBER 245
NWS STORM PREDICTION CENTER NORMAN OK
940 PM CDT WED APR 27 2011

THE NWS STORM PREDICTION CENTER HAS ISSUED A
TORNADO WATCH FOR PORTIONS OF

       ALABAMA
       FAR SOUTHEAST MISSISSIPPI

EFFECTIVE THIS WEDNESDAY NIGHT AND THURSDAY MORNING FROM 940 PM
UNTIL 200 AM CDT.

TORNADOES...HAIL TO 1.5 INCHES IN DIAMETER...THUNDERSTORM WIND
GUSTS TO 70 MPH...AND DANGEROUS LIGHTNING ARE POSSIBLE IN THESE
AREAS.

THE TORNADO WATCH AREA IS APPROXIMATELY ALONG AND 45 STATUTE
MILES EAST AND WEST OF A LINE FROM GADSDEN ALABAMA TO 105 MILES
SOUTHWEST OF SELMA ALABAMA.  FOR A COMPLETE DEPICTION OF THE
WATCH SEE THE ASSOCIATED WATCH OUTLINE UPDATE (WOUS64 KWNS WOU5).

REMEMBER...A TORNADO WATCH MEANS CONDITIONS ARE FAVORABLE FOR
TORNADOES AND SEVERE THUNDERSTORMS IN AND CLOSE TO THE WATCH
AREA. PERSONS IN THESE AREAS SHOULD BE ON THE LOOKOUT FOR
THREATENING WEATHER CONDITIONS AND LISTEN FOR LATER STATEMENTS
AND POSSIBLE WARNINGS.

OTHER WATCH INFORMATION...CONTINUE...WW 235...WW 238...WW
239...WW 240...WW 241...WW 242...WW 243...WW 244...

DISCUSSION...WHILE STRONG MIDLEVEL VORTICITY MAXIMUM IS NOW
BEGINNING TO LIFT NEWD AWAY FROM THE REGION...LOW-LEVEL CONVERGENCE
SHOULD SERVE TO MAINTAIN TSTMS ALONG COLD FRONT PROGRESSING EWD
ACROSS WW AREA.  AMBIENT AIR MASS REMAINS VERY MOIST AND MODERATELY
UNSTABLE WITH MLCAPE VALUES OF 1500-3000 J/KG.  WHEN COUPLED WITH
EFFECTIVE SRH VALUES OF 300-500 M2/S2...ENVIRONMENT REMAINS
SUPPORTIVE OF SUPERCELLS CAPABLE OF TORNADOES...LARGE HAIL AND
DAMAGING WINDS WINDS.

AVIATION...TORNADOES AND A FEW SEVERE THUNDERSTORMS WITH HAIL
SURFACE AND ALOFT TO 1.5 INCHES. EXTREME TURBULENCE AND SURFACE
WIND GUSTS TO 60 KNOTS. A FEW CUMULONIMBI WITH MAXIMUM TOPS TO
500. MEAN STORM MOTION VECTOR 24040.


...MEAD

SPC that day

Everything else SPC issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
Apr 27, 7:55 PM CDT	Mesoscale Discussion	SPC	Autauga, Bibb, Blount, Butler, Calhoun, Cherokee and 27 more	65 min
Apr 27, 9:20 PM CDT	Mesoscale Discussion	SPC	Autauga, Bibb, Blount, Butler, Calhoun, Cherokee and 29 more	25 min
Apr 27, 11:09 PM CDT	Mesoscale Discussion	SPC	Barbour, Bullock, Chambers, Clay, Cleburne, Elmore and 7 more	36 min
Apr 28, 1:22 AM CDT	Mesoscale Discussion	SPC	Dale, Geneva, Henry, Houston	83 min

Nearby

Tornado Watches in Alabama, 2011 Tornado Watches in Mississippi, 2011 Every tornado watch in 2011

Sources

The product and its follow-ups from the Iowa Environmental Mesonet's archive of National Weather Service and Storm Prediction Center text; the sounding from the Mesonet's radiosonde archive; radar from the NEXRAD Level II archive.