Alert archive / Louisiana / Mesoscale Discussions in 2012

Mesoscale Discussion 1830

August 29, 20124 states

The Storm Prediction Center issued this mesoscale discussion at Aug 29, 2012, 1:55 AM CDT for 4 states. It was in force for 2 hr.

The area

The outline as issued.

The facts

Issued: Aug 29, 2012, 1:55 AM CDT (06:55 UTC)
Expired: Aug 29, 2012, 4:00 AM CDT
In force: 2 hr
Issued by: the Storm Prediction Center

All 44 counties

Baldwin County, AL, Mobile County, AL, Washington County, AL, Escambia County, FL, Ascension Parish, LA, Assumption Parish, LA, East Baton Rouge Parish, LA, East Feliciana Parish, LA, Iberville Parish, LA, Jefferson Parish, LA, Lafourche Parish, LA, Livingston Parish, LA, Orleans Parish, LA, Plaquemines Parish, LA, Saint Bernard Parish, LA, Saint Charles Parish, LA, Saint Helena Parish, LA, Saint James Parish, LA, Saint John the Baptist Parish, LA, Saint Tammany Parish, LA, Tangipahoa Parish, LA, Terrebonne Parish, LA, Washington Parish, LA, West Baton Rouge Parish, LA, Amite County, MS, Covington County, MS, Forrest County, MS, George County, MS, Greene County, MS, Hancock County, MS, Harrison County, MS, Jackson County, MS, Jefferson Davis County, MS, Jones County, MS, Lamar County, MS, Lawrence County, MS, Lincoln County, MS, Marion County, MS, Pearl River County, MS, Perry County, MS, Pike County, MS, Stone County, MS, Walthall County, MS, Wayne County, MS

The radar

The Mobile radar (KMOB), 125 km from the middle of the area, began a volume at 06:55 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 nearest sounding before it

Slidell Radar S, LA (KSIL), launched 00:00Z, 6.9 hours before the alert went out and 79 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: 275 J/kg
Mixed-layer CIN: 0 J/kg
Mixed-layer LCL: 509 m above ground
0-6 km bulk shear: 41 kt
0-1 km storm-relative helicity: 392 m²/s²
0-3 km storm-relative helicity: 473 m²/s²
Significant tornado parameter: 0.5
Supercell composite: 8.0

Open this sounding in Storm Lab

The text

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

850 
ACUS11 KWNS 290656
SWOMCD
SPC MCD 290655
ALZ000-FLZ000-MSZ000-LAZ000-290900-

MESOSCALE DISCUSSION 1830
NWS STORM PREDICTION CENTER NORMAN OK
0155 AM CDT WED AUG 29 2012

AREAS AFFECTED...PORTIONS SERN LA...SRN MS...SWRN AL...WRN PANHANDLE
OF FL.

CONCERNING...TORNADO WATCH 593...

VALID 290655Z - 290900Z

THE SEVERE WEATHER THREAT FOR TORNADO WATCH 593 CONTINUES.

SUMMARY...GREATEST UNCONDITIONAL TORNADO POTENTIAL WILL CONTINUE TO
BE WITH MINI-SUPERCELLS EMBEDDED WITHIN OUTER BAND CURRENTLY
EXTENDING SEWD FROM MOB AREA ACROSS COASTAL/OFFSHORE WATERS OF AL.
INNER BANDING OFFSHORE MS COAST ALSO MAY OFFER BRIEF SPINUPS OVER
COASTAL MS AND ADJACENT ISLANDS. THESE FEATURES SHOULD EXPERIENCE
NET WWD SHIFT IN STEP WITH THAT COMPONENT OF ISAAC'S TRANSLATION.
GIVEN CONVECTIVE STRUCTURE OF THIS SYSTEM...AND DECREASE IN
LOW-LEVEL SHEAR EWD...ISOLATED SUPERCELLS E OF OUTERMOST BAND APPEAR
IMPROBABLE...AND WW MAY BE CLEARED FROM E-W ACCORDINGLY.

DISCUSSION...GREATEST JUXTAPOSITION OF BUOYANCY AND LOW-LEVEL
VERTICAL SHEAR WILL REMAIN WITH OUTER-BANDING FEATURES AND
INDIVIDUAL/EMBEDDED SUPERCELL ELEMENTS MOVING OFF GULF.  IN
NOCTURNAL ABSENCE OF DIABATIC SFC HEATING...LOW-LEVEL BUOYANCY
ARISES FROM SUFFICIENT MARINE-LAYER THETAE...ADVECTING NWWD FROM
GULF ADJACENT TO OUTER BANDING AND BECOMING PART OF INFLOW LAYER FOR
EMBEDDED CELLS.  MODIFIED SOUNDINGS ACCORDINGLY SUGGEST HIGHEST
OVERLAND 0-3 KM CAPE VALUES ALONG COAST FROM AL EWD...CONTRIBUTING
TO TOTAL MLCAPE OF AROUND 500-1000 J/KG.  CAPE WILL CONTINUE TO
DIMINISH WITH NWWD/INLAND EXTENT DUE TO PRECIP EFFECTS...AND DROPS
OFF SHARPLY NW OF NEARLY COLLOCATED SFC TROUGH AND WEAK BAROCLINIC
BOUNDARY.  AT 6Z THIS BOUNDARY WAS DRAWN FROM NEAR GPT NEWD TO NEAR
MGM...AND SHOULD MOVE SLOWLY NWWD APACE WITH TC TRANSLATION.
VWP...RAOBS AND MODIFIED MODEL SOUNDINGS SUGGEST GREATEST EFFECTIVE
SRH OVER COASTAL AL AND EXTREME SERN MS...GRADUALLY DIMINISHING WITH
EWD EXTENT AWAY FROM STRONGEST 925-850 MB WINDS THAT CONTRIBUTE TO
LARGE HODOGRAPHS.  AS SMALL CAPE/SRH OVERLAP NOW OVER COASTAL AL AND
SERN MS SHIFTS WWD...SO WILL RELATIVE MAX IN TORNADO THREAT.

..EDWARDS.. 08/29/2012

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

ATTN...WFO...MOB...JAN...LIX...LCH...

LAT...LON   30288728 30198808 30238865 30188902 30008881 29698885
            29538903 29398931 29358915 29258911 29218895 28948911
            29058925 28818947 29098934 29218950 29278978 29079019
            29019065 30239145 31429116 31878864 31138733 30428711
            30288728

SPC that day

Everything else SPC issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
Aug 29, 5:46 AM CDT	Mesoscale Discussion	SPC	Baldwin, Mobile, Washington	89 min
Aug 29, 6:36 AM CDT	Tornado Watch	SPC	Baldwin, Mobile, Washington	9 hr
Aug 29, 11:48 AM CDT	Mesoscale Discussion	SPC	Baldwin, Mobile, Washington	117 min
Aug 29, 3:17 PM CDT	Mesoscale Discussion	SPC	Baldwin, Mobile, Washington	28 min
Aug 29, 3:53 PM CDT	Tornado Watch	SPC	Baldwin, Mobile, Washington	8 hr
Aug 29, 5:45 PM CDT	Mesoscale Discussion	SPC	Barbour, Bullock, Butler, Coffee, Covington, Crenshaw and 6 more	90 min

Nearby

Mesoscale Discussions in Louisiana, 2012 Mesoscale Discussions in Mississippi, 2012 Mesoscale Discussions in Alabama, 2012 Every mesoscale discussion in 2012

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.