Alert archive / Louisiana / Mesoscale Discussions in 2021

Mesoscale Discussion 669

May 18, 20217 parishes in Louisiana

The Storm Prediction Center issued this mesoscale discussion at May 18, 2021, 3:10 AM CDT for 7 parishes in Louisiana. It was in force for 95 min.

The area

The outline as issued.

The facts

Issued: May 18, 2021, 3:10 AM CDT (08:10 UTC)
Expired: May 18, 2021, 4:45 AM CDT
In force: 95 min
Issued by: the Storm Prediction Center
Concerning: Severe potential...watch unlikely
Chance of a watch: 20%

Counties Jefferson Parish, LA, Lafourche Parish, LA, Orleans Parish, LA, Saint Charles Parish, LA, Saint John the Baptist Parish, LA, Saint Tammany Parish, LA, Terrebonne Parish, LA

The nearest sounding before it

Slidell Radar S, LA (KSIL), launched 00:00Z, 8.2 hours before the alert went out and 88 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: 386 J/kg
Mixed-layer CIN: -6 J/kg
Mixed-layer LCL: 682 m above ground
0-6 km bulk shear: 24 kt
0-1 km storm-relative helicity: 93 m²/s²
0-3 km storm-relative helicity: 155 m²/s²
Significant tornado parameter: 0.0
Supercell composite: 0.0

Open this sounding in Storm Lab

The text

As transmitted.

033 
ACUS11 KWNS 180811
SWOMCD
SPC MCD 180810 
LAZ000-MSZ000-180945-

Mesoscale Discussion 0669
NWS Storm Prediction Center Norman OK
0310 AM CDT Tue May 18 2021

Areas affected...portions of southeast LA

Concerning...Severe potential...Watch unlikely 

Valid 180810Z - 180945Z

Probability of Watch Issuance...20 percent

SUMMARY...A brief, weak tornado is possible across parts of
southeast Louisiana early this morning.

DISCUSSION...Showers and thunderstorms across southeast LA have
shown transient, weak rotation the past hour or so. Overall, this
activity is occurring in weak instability and modest shear. However,
low level southeasterly winds around 20-30 kt are resulting in
modest low level hodograph curvature. Furthermore, surface dewpoints
near 70 F are aiding in moderate low level instability with a
corridor of 0-3 km MLCAPE around 100-125 J/kg evident in latest
mesoanalysis. Poor low level lapse rates and overall marginal
larger-scale environment will limit longevity/intensity of organized
cells. However, the moist/unstable low levels amid somewhat
favorable low level shear could result in a brief/weak tornado. The
overall threat is expected to remain limited, and a watch is not
expected at this time.

..Leitman/Hart.. 05/18/2021

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

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

LAT...LON   29649079 30299049 30569011 30608983 30438948 29898952
            29258972 28909014 28789071 28869106 29119114 29649079

SPC that day

Everything else SPC issued on the same UTC day.

Issued	Product	Office	Counties	Lasted
May 18, 2:30 PM CDT	Mesoscale Discussion	SPC	Bossier, Caddo	2 hr

Nearby

Mesoscale Discussions in Louisiana, 2021 Every mesoscale discussion in 2021

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.