Alert archive / Louisiana / Mesoscale Discussions in 2021
Mesoscale Discussion 411
The Storm Prediction Center issued this mesoscale discussion at Apr 23, 2021, 5:36 PM CDT for 18 parishes in Louisiana; 6 counties in Mississippi; 13 counties in Texas. It was in force for 54 min.
The area
The outline as issued.
The facts
- Issued
- Apr 23, 2021, 5:36 PM CDT (22:36 UTC)
- Expired
- Apr 23, 2021, 6:30 PM CDT
- In force
- 54 min
- Issued by
- the Storm Prediction Center
- Concerning
- Tornado watch 99
All 37 counties
Allen Parish, LA, Avoyelles Parish, LA, Catahoula Parish, LA, Concordia Parish, LA, De Soto Parish, LA, East Feliciana Parish, LA, Evangeline Parish, LA, Grant Parish, LA, LaSalle Parish, LA, Natchitoches Parish, LA, Pointe Coupee Parish, LA, Rapides Parish, LA, Red River Parish, LA, Sabine Parish, LA, Saint Landry Parish, LA, Vernon Parish, LA, West Feliciana Parish, LA, Winn Parish, LA, Adams County, MS, Amite County, MS, Franklin County, MS, Jefferson County, MS, Lincoln County, MS, Wilkinson County, MS, Angelina County, TX, Houston County, TX, Jasper County, TX, Nacogdoches County, TX, Newton County, TX, Polk County, TX, Sabine County, TX, San Augustine County, TX, San Jacinto County, TX, Shelby County, TX, Trinity County, TX, Tyler County, TX, Walker County, TX
The radar
The Ft Polk radar (KPOE), 64 km from the middle of the area, began a volume at 22:37 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.
The nearest sounding before it
Shreveport, LA (KSHV), launched 12:00Z, 10.6 hours before the alert went out and 134 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.
- Mixed-layer CAPE
- 0 J/kg
- Mixed-layer CIN
- No buoyant parcel
- Mixed-layer LCL
- 912 m above ground
- 0-6 km bulk shear
- 57 kt
- 0-1 km storm-relative helicity
- 232 m²/s²
- 0-3 km storm-relative helicity
- 252 m²/s²
- Significant tornado parameter
- 0.0
- Supercell composite
- 0.0
The text
As transmitted.
248
ACUS11 KWNS 232236
SWOMCD
SPC MCD 232236
MSZ000-LAZ000-TXZ000-232330-
Mesoscale Discussion 0411
NWS Storm Prediction Center Norman OK
0536 PM CDT Fri Apr 23 2021
Areas affected...Parts of east Texas...central Louisiana into
southwestern Mississippi
Concerning...Tornado Watch 99...
Valid 232236Z - 232330Z
The severe weather threat for Tornado Watch 99 continues.
SUMMARY...It still appears that the large, evolving cluster of
thunderstorms will continue to gradually become better organized as
it spreads into the lower Mississippi Valley through early evening,
accompanied by at least increasing potential for damaging wind
gusts. Trends are being monitored for an additional severe weather
watch downstream.
DISCUSSION...A large and sustained cluster of storms continues to
evolve, with strongest convection along a corridor arcing from
southeast of College Station, TX into the Fort Polk, LA vicinity.
This appears to be near the northern periphery of warmer and more
strongly capping mid-level air associated with subtropical ridging.
It is being supported by lift associated with lower/mid tropospheric
warm advection, beneath a strongly divergent upper flow field
associated with coupled speed maxima along the subtropical jet.
The forcing, and associated convection are forecast to continue
developing fairly rapidly east-northeastward into the lower
Mississippi Valley through 00-02Z.
Southerly 850 mb flow on the order of 40+ kt appears to be
supporting moistening inflow, with lifted parcels characterized by
at least modest CAPE on the order of 500-1000 J/kg. This likely
will continue, but the the motion of convection may still be
outpacing the advection of more substantive boundary-layer
instability, with leading stratiform precipitation tending to
maintain stable to neutral near-surface lapse rates northeast of
activity.
Still, there has been a strong signal within model output that an
evolving mesoscale convective vortex will be accompanied by
intensifying wind fields to 50-70+ kt within the 850-700 mb layer
across and east of the lower Mississippi Valley this evening. As
this occurs, the risk for potentially damaging wind gusts seems
likely to increase somewhere across central Louisiana into
southwestern Mississippi. A risk for tornadoes may also develop,
but this will largely hinge on further boundary-layer moistening and
destabilization of near-surface lapse rates.
..Kerr.. 04/23/2021
...Please see www.spc.noaa.gov for graphic product...
ATTN...WFO...JAN...LIX...LCH...SHV...HGX...
LAT...LON 31899450 32149338 32039205 31549027 30289163 30689321
30769383 30449548 30689597 31789509 31899450SPC that day
Everything else SPC issued on the same UTC day.
| Issued | Product | Office | Counties | Lasted |
|---|---|---|---|---|
| Apr 23, 12:43 PM CDT | Mesoscale Discussion | SPC | Allen, Beauregard, Calcasieu, Cameron, De Soto, Jefferson Davis and 3 more | 2 hr |
| Apr 23, 2:00 PM CDT | Tornado Watch | SPC | Acadia, Allen, Avoyelles, Beauregard, Calcasieu, Caldwell and 13 more | 7 hr |
| Apr 23, 11:29 AM CDT | Mesoscale Discussion | SPC | Archer, Bastrop, Bell, Bexar, Blanco, Bosque and 52 more | 91 min |
| Apr 23, 12:37 PM CDT | Severe Thunderstorm Watch | SPC | Anderson, Archer, Bastrop, Bell, Blanco, Bosque and 53 more | 7 hr |
| Apr 23, 12:43 PM CDT | Mesoscale Discussion | SPC | Baylor, Childress, Collingsworth, Cottle, Foard, Hall and 2 more | 3 hr |
| Apr 23, 3:14 PM CDT | Tornado Watch | SPC | Archer, Baylor, Childress, Clay, Collingsworth, Cottle and 7 more | 5 hr |
| Apr 23, 3:27 PM CDT | Mesoscale Discussion | SPC | Baylor, Foard, Hardeman, Wilbarger | 2 hr |
| Apr 23, 3:28 PM CDT | Mesoscale Discussion | SPC | Bosque, Brown, Comanche, Eastland, Erath, Hamilton and 11 more | 2 hr |
| Apr 23, 5:26 PM CDT | Mesoscale Discussion | SPC | Archer, Baylor, Clay, Foard, Hardeman, Knox and 2 more | 2 hr |
| Apr 23, 6:46 PM CDT | Mesoscale Discussion | SPC | Atascosa, Bell, Bexar, Blanco, Bosque, Burnet and 19 more | 89 min |
Nearby
Mesoscale Discussions in Louisiana, 2021Mesoscale Discussions in Texas, 2021Mesoscale Discussions in Mississippi, 2021Every 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; radar from the NEXRAD Level II archive.