Alert archive / Louisiana / Mesoscale Discussions in 2013

Mesoscale Discussion 21

January 12, 20137 counties in Arkansas; 20 parishes in Louisiana; 6 counties in Texas

The Storm Prediction Center issued this mesoscale discussion at Jan 12, 2013, 10:33 PM CST for 7 counties in Arkansas; 20 parishes in Louisiana; 6 counties in Texas. It was in force for 87 min.

The area

The outline as issued.

The facts

Issued: Jan 12, 2013, 10:33 PM CST (04:33 UTC)
Expired: Jan 13, 2013, 12:00 AM CST
In force: 87 min
Issued by: the Storm Prediction Center
Chance of a watch: 40%

All 33 counties

Ashley County, AR, Bradley County, AR, Calhoun County, AR, Columbia County, AR, Drew County, AR, Ouachita County, AR, Union County, AR, Bienville Parish, LA, Bossier Parish, LA, Caddo Parish, LA, Caldwell Parish, LA, Claiborne Parish, LA, De Soto Parish, LA, Grant Parish, LA, Jackson Parish, LA, Lincoln Parish, LA, Morehouse Parish, LA, Natchitoches Parish, LA, Ouachita Parish, LA, Red River Parish, LA, Richland Parish, LA, Sabine Parish, LA, Union Parish, LA, Vernon Parish, LA, Webster Parish, LA, West Carroll Parish, LA, Winn Parish, LA, Angelina County, TX, Nacogdoches County, TX, Panola County, TX, Sabine County, TX, San Augustine County, TX, Shelby County, TX

The radar

The Shreveport radar (KSHV), 62 km from the middle of the area, began a volume at 04:35 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.

The watches that followed

Watches SPC issued over any of the same counties within four hours of this discussion.

Tornado Watch 4Jan 12, 11:30 PM CST

The nearest sounding before it

Shreveport, LA (KSHV), launched 00:00Z, 4.6 hours before the alert went out and 57 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: 1010 J/kg
Mixed-layer CIN: -5 J/kg
Mixed-layer LCL: 620 m above ground
0-6 km bulk shear: 39 kt
0-1 km storm-relative helicity: 340 m²/s²
0-3 km storm-relative helicity: 398 m²/s²
Significant tornado parameter: 1.8
Supercell composite: 9.8

Open this sounding in Storm Lab

The text

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

602 
ACUS11 KWNS 130434
SWOMCD
SPC MCD 130433
ARZ000-LAZ000-TXZ000-130600-

MESOSCALE DISCUSSION 0021
NWS STORM PREDICTION CENTER NORMAN OK
1033 PM CST SAT JAN 12 2013

AREAS AFFECTED...PORTIONS E TX...WRN/NRN LA...EXTREME SRN AR.

CONCERNING...SEVERE POTENTIAL...WATCH POSSIBLE

VALID 130433Z - 130600Z

PROBABILITY OF WATCH ISSUANCE...40 PERCENT

SUMMARY...CONVECTION IS GROWING IN COVERAGE/INTENSITY OVER
PREFRONTAL WARM SECTOR IN E TX.  GIVEN STRENGTH OF AMBIENT WIND
FIELDS AND SHEAR PROFILES...ANY SUSTAINED WARM-SECTOR CONVECTION MAY
OFFER RISK OF DAMAGING GUSTS AND A FEW TORNADOES...WHETHER FROM
SUPERCELLS OR CIRCULATIONS EMBEDDED IN QUASI-LINEAR SEGMENTS.
ADDITIONAL WW MAY BE REQUIRED IF CURRENT INCREASING TRENDS CONTINUE.

DISCUSSION...SFC MESOANALYSIS SHOWS SOMEWHAT WAVY ARCTIC COLD FRONT
FROM NERN AR SWWD THROUGH LIT AREA TO CADDO PARISH LA...BASICALLY
BISECTING WW 3...THEN SWWD ACROSS PANOLA AND HOUSTON COUNTIES TX TO
MIDDLE TX COASTAL PLAIN.  FRONT WILL PROCEED SEWD ACROSS MOST OF
DISCUSSION AREA THROUGH 09Z.  THIS BOUNDARY IS SHARPLY DEFINED AND
STG...UNDERCUTTING CONVECTION DEVELOPING ALONG OR IMMEDIATELY AHEAD
OF IT.  THIS PROCESS SHOULD QUICKLY HALT SVR WIND AND TORNADO
POTENTIAL WITH SUCH ACTIVITY.  HOWEVER...MODIFIED SHV/LCH RAOBS
SUGGEST MINIMAL MLCINH OVER WARM SECTOR IN EXTREME E TX AND
LA...ALONG WITH 500-1000 J/KG MLCAPE.  LOW-LEVEL MOISTURE TRANSPORT
AND WAA SHOULD OFFSET/RESTRICT PACE OF DIABATIC SFC COOLING ENOUGH
TO KEEP EFFECTIVE LIFTED PARCELS ROOTED AT OR VERY NEAR SFC FOR
SEVERAL MORE HOURS.  LLJ MAY DIMINISH WITH TIME AFTER 06Z...BUT
STILL SHOULD CONTRIBUTE TO ENLARGED HODOGRAPHS THROUGH MOST OF
PRE-DAWN HOURS.  AS SUCH...SLY-SSELY SFC WINDS BENEATH LLJ SHOULD
RENDER EFFECTIVE SRH VALUES 250-500 J/KG OVER MUCH OF THIS
AREA...WITH ONLY GRADUAL SFC VEERING EXPECTED OVERNIGHT.
MEANWHILE...DEEP-LAYER SHEAR SHOULD REMAIN FAVORABLE FOR ORGANIZED
SVR...E.G. EFFECTIVE SHEAR MAGNITUDES 55-65 KT OVER MOST OF
PREFRONTAL AIR MASS.

..EDWARDS/COHEN.. 01/13/2013

...PLEASE SEE WWW.SPC.NOAA.GOV FOR GRAPHIC PRODUCT...

ATTN...WFO...JAN...LZK...LCH...SHV...HGX...

LAT...LON   30979522 32729408 33699285 33689161 33469126 32229161
            31009301 30879455 30979522

SPC that day

Everything else SPC issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
Jan 12, 6:18 PM CST	Mesoscale Discussion	SPC	Arkansas, Bradley, Calhoun, Clark, Clay, Cleburne and 48 more	117 min
Jan 12, 9:25 PM CST	Mesoscale Discussion	SPC	Arkansas, Bradley, Calhoun, Clark, Clay, Cleveland and 31 more	2 hr
Jan 12, 11:29 PM CST	Tornado Watch	SPC	Arkansas, Ashley, Bradley, Calhoun, Chicot, Cleveland and 9 more	5 hr
Jan 13, 1:18 AM CST	Mesoscale Discussion	SPC	Ashley, Chicot, Desha, Drew	87 min
Jan 13, 3:53 AM CST	Mesoscale Discussion	SPC	Arkansas, Ashley, Bradley, Chicot, Desha, Drew and 5 more	97 min
Jan 13, 2:30 PM CST	Mesoscale Discussion	SPC	Ascension, Assumption, Lafourche, Livingston, Orleans, Saint Charles and 5 more	90 min

Nearby

Mesoscale Discussions in Louisiana, 2013 Mesoscale Discussions in Arkansas, 2013 Mesoscale Discussions in Texas, 2013 Every mesoscale discussion in 2013

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.