Alert archive / Louisiana / Mesoscale Discussions in 2009

Mesoscale Discussion 521

April 18, 200926 parishes in Louisiana

The Storm Prediction Center issued this mesoscale discussion at Apr 18, 2009, 7:24 PM CDT for 26 parishes in Louisiana. It was in force for 36 min.

The area

The outline as issued.

The facts

Issued: Apr 18, 2009, 7:24 PM CDT (00:24 UTC)
Expired: Apr 18, 2009, 8:00 PM CDT
In force: 36 min
Issued by: the Storm Prediction Center

All 26 counties

Acadia Parish, LA, Allen Parish, LA, Ascension Parish, LA, Assumption Parish, LA, Calcasieu Parish, LA, Cameron Parish, LA, East Baton Rouge Parish, LA, East Feliciana Parish, LA, Evangeline Parish, LA, Iberia Parish, LA, Iberville Parish, LA, Jefferson Davis Parish, LA, Lafayette Parish, LA, Livingston Parish, LA, Pointe Coupee Parish, LA, Saint Charles Parish, LA, Saint Helena Parish, LA, Saint James Parish, LA, Saint John the Baptist Parish, LA, Saint Landry Parish, LA, Saint Martin Parish, LA, St. Mary Parish, LA, Terrebonne Parish, LA, Vermilion Parish, LA, West Baton Rouge Parish, LA, West Feliciana Parish, LA

The radar

The Lake Charles radar (KLCH), 116 km from the middle of the area, began a volume at 00:25 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

Lake Charles Mun, LA (KLCH), launched 00:00Z, 0.4 hours before the alert went out and 115 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: 1481 J/kg
Mixed-layer CIN: -11 J/kg
Mixed-layer LCL: 448 m above ground
0-6 km bulk shear: 41 kt
0-1 km storm-relative helicity: 260 m²/s²
0-3 km storm-relative helicity: 345 m²/s²
Significant tornado parameter: 2.3
Supercell composite: 16.7

Open this sounding in Storm Lab

The text

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

000 
ACUS11 KWNS 190025
SWOMCD
SPC MCD 190024 
LAZ000-MSZ000-TXZ000-190100-

MESOSCALE DISCUSSION 0521
NWS STORM PREDICTION CENTER NORMAN OK
0724 PM CDT SAT APR 18 2009

AREAS AFFECTED...SRN LA

CONCERNING...TORNADO WATCH 166...

VALID 190024Z - 190100Z

THE SEVERE WEATHER THREAT FOR TORNADO WATCH 166 CONTINUES.

TORNADO WATCH WILL BE ALLOWED TO EXPIRE AT 01Z.  

EXPANSIVE MCS OVER THE NWRN GULF OF MEXICO AND SWRN LA WILL LIKELY
CONTINUE EWD OVER S-CNTRL AND SERN LA IN CONJUNCTION WITH MESO-LOW
/INVOF CAMERON PARISH AT 0020Z/ DURING THE EVENING HOURS.  LAST HOUR
TRENDS IN RADAR MOSAIC SHOW A WEAKENING TREND WITH CONVECTION ALONG
THE LA COAST /CAMERON PARISH/ BUT BRIEF/WEAK CYCLONIC CIRCULATIONS
HAVE BEEN OBSERVED IN SWRN LA.  AS SUCH...TORNADO POTENTIAL...ALBEIT
LIMITED...REMAINS IN THE NEAR TERM ACROSS PARTS OF SWRN LA.

00Z SURFACE ANALYSIS DEPICTS LOWER QUALITY MOISTURE OVER SERN LA
WITH DEWPOINTS ONLY IN THE MIDDLE 60S F COMPARED TO OBSERVATIONS
EARLY TODAY OVER SWRN LA WITH DEWPOINTS IN THE UPPER 60S.  WITH ONLY
MODEST MID-LEVEL LAPSE RATES AROUND 7 C/KM IN THE H7-H5 LAYER PER
00Z LIX RAOB AND SURFACE TEMPS EXPECTED TO REMAIN IN THE LOWER
70S...WILL MOST LIKELY LEAD TO A GENERAL WANING IN THE OVERALL
SEVERE THREAT AS MCS PUSHES E INTO MORE MARGINAL THERMODYNAMIC
ENVIRONMENT.  AS A RESULT...A WW IS NOT EXPECTED DOWNSTREAM OF WW
166 OVER SERN LA.

..SMITH.. 04/19/2009

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

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

LAT...LON   29189216 29359379 29739391 30109376 30449311 30949246
            31039161 30989078 30409030 29859035 29279059 29119131
            29189216

SPC that day

Everything else SPC issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
Apr 18, 9:18 PM CDT	Mesoscale Discussion	SPC	Acadia, East Baton Rouge, East Feliciana, Iberville, Lafayette, Pointe Coupee and 5 more	57 min
Apr 19, 12:43 PM CDT	Mesoscale Discussion	SPC	Livingston, Saint Helena, Saint Tammany, Tangipahoa, Washington	3 hr

Nearby

Mesoscale Discussions in Louisiana, 2009 Every mesoscale discussion in 2009

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.