Mesoscale Discussion 138

Issued

Feb 28, 2021, 4:40 PM CST (22:40 UTC)

Expired

Feb 28, 2021, 6:15 PM CST

In force

95 min

Issued by

the Storm Prediction Center

Concerning

Tornado watch 13

The Memphis radar (KNQA), 20 km from the middle of the area, began a volume at 22:42 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.

• Severe Thunderstorm Watch 14Feb 28, 5:50 PM CST
• Severe Thunderstorm Watch 15Feb 28, 7:10 PM CST

215 
ACUS11 KWNS 282241
SWOMCD
SPC MCD 282240 
TNZ000-KYZ000-ALZ000-MSZ000-MOZ000-ARZ000-010015-

Mesoscale Discussion 0138
NWS Storm Prediction Center Norman OK
0440 PM CST Sun Feb 28 2021

Areas affected...Eastern Arkansas into northern
Mississippi...western Tennessee and far southern Kentucky

Concerning...Tornado Watch 13...

Valid 282240Z - 010015Z

The severe weather threat for Tornado Watch 13 continues.

SUMMARY...Storms are expected to intensify over the next 1 to 2
hours along and ahead of a cold front. Damaging wind will be the
primary threat with potential for a couple tornadoes.

DISCUSSION...Storm mode has been messy thus far across Arkansas with
occasional stronger cells. The 18Z LZK RAOB showed MLCAPE around
1000 J/kg with minimal inhibition. Temperatures in the warm sector
have warmed further with SPC mesoanalysis suggesting around 1500
J/kg MLCAPE ahead of ongoing convection. The 18Z BNA RAOB still
showed a significant warm nose around 600 mb which limits the
overall instability of the profile and likely explains the weaker
storm intensity with northern extent. This warm nose has likely
eroded or will soon as height falls overspread the region. This,
combined with assistance from the strengthening low-level jet,
should allow storms to intensify and organize into a squall line
over the next 1 to 2 hours. This strengthening is supported by the
latest HRRR which suggests a maximum in storm intensity between 23Z
and 02Z. The current convection in Woodruff County Arkansas at 2230Z
is likely the beginning stages of a more organized squall line. 

Once storms organize, there will likely be a several hour window
this evening which appears favorable for damaging winds and even a
couple tornadoes. The KNQA VWP shows 50 knots of southwesterly flow
at less than 1km and 0-1 SRH in excess of 300 m2/s2. This favorable
low-level shear, combined with 1000 to 1500 J/kg MLCAPE (per SPC
mesoanalysis) yields STP values around 1 to 2 across the region. A
linear storm mode, mostly parallel to the shear vector will likely
temper a greater tornado threat given the environment, but a couple
tornadoes are possible, especially within more organized bowing
segments within the squall line.

..Bentley.. 02/28/2021

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

ATTN...WFO...LMK...OHX...HUN...PAH...MEG...JAN...LZK...SHV...

LAT...LON   34329332 35789098 36468945 37048723 37058657 36688614
            35528646 34968775 34168956 33449209 33499328 34329332

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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.