Mesoscale Discussion 657

Issued

May 17, 2021, 6:25 PM CDT (23:25 UTC)

Expired

May 17, 2021, 7:30 PM CDT

In force

65 min

Issued by

the Storm Prediction Center

Concerning

Severe potential...watch unlikely

Chance of a watch

20%

Counties Acadia Parish, LA, Ascension Parish, LA, Assumption Parish, LA, Iberia Parish, LA, Iberville Parish, LA, Lafayette Parish, LA, Saint James 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

The Lake Charles radar (KLCH), 146 km from the middle of the area, began a volume at 23:24 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.

383 
ACUS11 KWNS 172326
SWOMCD
SPC MCD 172325 
LAZ000-180030-

Mesoscale Discussion 0657
NWS Storm Prediction Center Norman OK
0625 PM CDT Mon May 17 2021

Areas affected...Southern Louisiana

Concerning...Severe potential...Watch unlikely 

Valid 172325Z - 180030Z

Probability of Watch Issuance...20 percent

SUMMARY...Brief, weak tornadoes remain possible across southern LA
this evening, though the duration of the threat remains uncertain.

DISCUSSION...A broken QLCS associated with a gradually decaying MCV
continues to move east/southeast across southern LA. Convective
cells continue to develop along and ahead of this line, but have not
acquired rotation as rapidly or for a long as prior convection
earlier this afternoon. This may be related to gradually diminishing
MLCAPE across the region (per RAP mesoanalysis trends) as well as a
less favorable shear in the lowest 1-2 km across south-central LA as
compared to southwest LA earlier today. Nonetheless, velocity data
from KLCH continues to show transient rotation with cells developing
along the line, and more persistent rotation associated with one or
two cells just offshore. Curvature in the 0-2 km layer noted in KLIX
VWP observations, along with nearly 1000 J/kg MLCAPE, suggests that
the environment remains supportive of at least a low-end tornado
threat for the next few hours. One or two additional brief, weak
tornadoes are possible, but the potential for a more robust tornado
threat appears low given gradual stabilization and the weakening
trend of the MCV.

..Moore/Grams.. 05/17/2021

...Please see www.spc.noaa.gov for graphic product...

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

LAT...LON   29609311 29969287 30309252 30649229 30529149 30419102
            30089059 29419048 29009063 28999089 29189126 29359260
            29609311

Mesoscale Discussions in Louisiana, 2021Every mesoscale discussion in 2021

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.