Mesoscale Discussion 2045

Issued

Dec 18, 2021, 9:02 AM CST (15:02 UTC)

Expired

Dec 18, 2021, 11:00 AM CST

In force

118 min

Issued by

the Storm Prediction Center

Concerning

Severe potential...watch possible

Chance of a watch

40%

The Houston radar (KHGX), 23 km from the middle of the area, began a volume at 15:04 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.

317 
ACUS11 KWNS 181503
SWOMCD
SPC MCD 181502 
LAZ000-TXZ000-181700-

Mesoscale Discussion 2045
NWS Storm Prediction Center Norman OK
0902 AM CST Sat Dec 18 2021

Areas affected...Portions of southeast/coastal TX into southwestern
LA

Concerning...Severe potential...Watch possible 

Valid 181502Z - 181700Z

Probability of Watch Issuance...40 percent

SUMMARY...Storms may pose an increasing threat for damaging winds
and severe hail this morning. A tornado or two may also occur. Watch
issuance is possible.

DISCUSSION...Linear convection is ongoing this morning along a cold
front extending across southeast TX and into northwestern LA. A
large degree of front-parallel flow aloft is present over this
region, and tendency so far this morning has been for updrafts to be
undercut by the front. Still, there appears to be some potential for
pre-frontal storms to develop over the next couple of hours along
one or more low-level confluence zones. One of these zones is
already evident in radar and surface observations, extending
southward from the front across the middle TX Coast. A rather moist
low-level airmass exits ahead of the front, with surface dewpoints
generally in the upper 60s to low 70s. Even with widespread
cloudiness, modest diurnal heating and steepened mid-level lapse
rates are already supporting MLCAPE in the 1000-1500 J/kg range
ahead of the front. Although low-level winds are not overly strong,
around 30-40 kt of deep-layer shear will support storm organization.

Any convection that can develop ahead of the front will have the
potential to become supercellular while posing both a large hail and
damaging wind threat. It also appears possible that one or more
small bowing line segments may accelerate ahead of the front later
this morning and continue into the early afternoon along the TX
Coast and perhaps into parts of southwestern LA. Damaging winds
would be the primary threat with this activity if it occurs. A brief
tornado or two also cannot be ruled out this morning with around
100-150 m2/s2 of effective SRH present. However, low-level winds are
expected to gradually diminish later this morning and into the
afternoon as the main upper trough and stronger low-level forcing
shifts eastward from the OH Valley across the Mid-Atlantic and
Northeast today. Given expectations for a gradual increase in severe
potential through the rest of the morning, watch issuance may
eventually need to be considered for some portion of the
middle/upper TX Coast into southwestern LA.

..Gleason/Kerr.. 12/18/2021

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

ATTN...WFO...LCH...SHV...HGX...CRP...EWX...

LAT...LON   27999678 28229773 28589794 29399785 29879765 30319690
            31199546 31179460 30909283 30499228 29939213 29449200
            29679326 29449457 28829536 28449617 27999678

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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.