Mesoscale Discussion 1789

Issued

Sep 28, 2021, 7:01 PM CDT (00:01 UTC)

Expired

Sep 28, 2021, 8:30 PM CDT

In force

89 min

Issued by

the Storm Prediction Center

Concerning

Severe potential...watch unlikely

Chance of a watch

5%

Counties Atascosa County, TX, Bandera County, TX, Bee County, TX, Bexar County, TX, DeWitt County, TX, Dimmit County, TX, Frio County, TX, Goliad County, TX, Guadalupe County, TX, Karnes County, TX, Kinney County, TX, La Salle County, TX, Live Oak County, TX, Maverick County, TX, McMullen County, TX, Medina County, TX, San Patricio County, TX, Uvalde County, TX, Wilson County, TX, Zavala County, TX

The Austin/San Antonio radar (KEWX), 134 km from the middle of the area, began a volume at 00:02 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.

653 
ACUS11 KWNS 290001
SWOMCD
SPC MCD 290001 
TXZ000-290130-

Mesoscale Discussion 1789
NWS Storm Prediction Center Norman OK
0701 PM CDT Tue Sep 28 2021

Areas affected...Portions of south Texas

Concerning...Severe potential...Watch unlikely 

Valid 290001Z - 290130Z

Probability of Watch Issuance...5 percent

SUMMARY...Isolated strong/severe storms ongoing near the Mexican
border and far south Texas may produce a few severe wind or hail
reports over the next several hours. Storm coverage is expected to
gradually decrease with the loss of diurnal heating suggesting a
watch is not needed.

DISCUSSION...On the southern fringes of a weak upper low observed
from regional WV imagery across the southern Rockies, isolated to
widely scattered thunderstorms are ongoing across portions of
northern Mexico and southern Texas. Surface obs and SPC mesoanalysis
show these storms are ongoing in a warm, moist and unstable
environment with 2000-2500 J/kg of MLCAPE and 2-6km lapse rates of
7-8 C/km. Vertical shear remains weak with only 25-30 kts of
deep-layer shear in place owing to relatively weak mid-level flow.
The favorable buoyancy/shear profiles should be sufficient for a few
organized multicells and perhaps a weak supercell.

As ongoing storms move to the east northeast with time, occasional
strengthening is possible, with a risk for damaging downburst winds
or hail. Recent radar trends have shown this with two lead storms
over Zavala and Frio counties over the last half hour. The
downstream environment should remain favorable for a lower-end
severe risk for at least a couple more hours. After sunset,
increasing nocturnal inhibition and the movement of the main
synoptic ascent from the upper low away from convection should
result in a gradual downtrend in coverage/intensity. Storms may
linger for a few hours after dark across the southern parts of the
discussion area, closer to the coast, where a greater reservoir of
warm/unstable air exists. However, weak vertical shear suggests
little organization potential and overall confidence in convective
evolution is low. Given the limited spatial/temporal threat window
for wind and hail, a weather watch is unlikely.

..Lyons/Grams.. 09/29/2021

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

ATTN...WFO...CRP...EWX...

LAT...LON   29080079 29460052 29710014 29899944 29899854 29269720
            29039714 28729709 28359717 27989743 27769818 27769828
            27829910 27879942 27889997 29080079

Mesoscale Discussions in Texas, 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.