Isolated Severe Weather and an MCS
 in the Rio Grande Valley and
 Deep South Texas May 18, 2010:
A Use of High Resolution
 Convection Allowing Model


Andrew Kennedy and Alex Tardy, NWS Corpus Christi

 Rainfall

Isolated Severe

Radar Loops

Mesoanalysis

Satellite Loops

Models

Summary

In the early afternoon on May 17th, showers and thunderstorms developed across the Hill Country of South
Central Texas to west Texas.  These storms were triggered from a stalled frontal boundary that was situated
across central Texas.  Isolated thunderstorms were visible from the NWS office across Victoria and Goliad
County at 1800 UTC 17 May.  These cells quickly became dominated by the outflow and pushed this cooler
boundary layer air mass away from the storms.  The
southern Coastal Bend was slightly capped (warm layer
aloft) at this time and influenced by the stable sea breeze, so the outflow boundary did not redevelop additional
convection.  However, instability increased across deep South Texas when temperatures rose into the 80s
and 90s and dewpoint temperatures held in the 70s.  Surface-based CAPE (SBCAPE) values depicted 2500
to 4500 Joules/kg with the only convective inhibition (CIN) located along the immediate coast due to the sea
breeze and subsidence.  Bulk shear and helicity values in the 0 to 1-km layer were low, so the tornado threat
appeared to be minimal.  The main threat anticipated was heavy rain due to the precipitable water values of 1.6
to 1.8 inches.  A secondary threat was straight-line wind and hail if an organized system were to evovle.

By early evening, a line of storms were moving across South Central Texas.  The line had a strong cold pool
with tempeartures falling 20F within 3 hours behind the thunderstorms and outflow boundary.  Other storms were
across western Texas and moved into northern Mexico.  These thunderstorms would later merge with the
decaying line of thunderstorms and form one mesoscale convective system (MCS).

The
MCS maintanence probability from the Storm Prediction Center (SPC) depicted a 50 to 70 percent
probability that an MCS would maintain itself if it moved into the Rio Grande Valley. 
The MCS maintenance
is calculated using the maximum bulk shear in the 0 to 1-km and 6 to 10-km layers, 3 to 8-km lapse rate,
most unstable CAPE (MUCAPE) and 3 to 12-km mean wind speed.  850-mb moisture transport and theta-E
advection were both focused over the Rio Grande Valley north to west Texas.  Therefore, conditions were
favorable for an MCS to enter southwest Texas.

By late evening, the convective line over the Hill Country began to weaken as it moved towards South Texas. 
There was a severe storm which affected Victoria County and a tornado warning was issued due to radar rotation
evident in the lowest elevation slices. The western portion of the line merged with the cluster of storms over
northern Mexico and near the Rio Grande Valley. These storms began to organize into an intense line around
Laredo at 5 am CDT.  Heavy rain was reported in the city of Laredo (1.5 inches) and parts
of Webb County 
received up to 4 inches.  The line intensified futher as it continued to the southeast across deep South Texas.
3.5 inches of rain was reported 21 miles west of Kingsville at 730 am CDT within a 45 minute period.  In addition,
2 large mesquite branches,10 to 12 inches in diamter, were downed by the severe wind. The MCS continue into
the coastal waters of South Texas through the morning.

The
WRF-NMM 4-km accurately forecasted the convective mode and evolution of the event 6 to 12 hours
before it occurred.


Rainfall Totals

 

Rainfall Totals COOP and CoCoRaHS
Rainfall Totals.

CoCoRaHS and COOP rainfall.

 


Radar Loops

 

May 17 radar loop May 18 radar loop
KCRP base reflectivity loop May 17. Outflow boundary left over South Texas. KCRP base reflectivity loop 0249 to 1649 UTC 18 May 2010.

Satellite Loop

GOES loop

GOES infrared and 1-h NLDN lightning loop May 18 2010.


Isolated Severe Storms in Goliad and Victoria Counties

FSI over Goliad

Goliad overshooting tops

4-D storm cell investigator over Goliad May 17.

Severe storms displayed overshooting tops as viewed from the WFO on May 17.

Mesoanalyses

850 mb at 0500 UTC

700 mb

500 mb

850-mb wind and temperature at 0500 UTC

700-mb wind, temperature and dewpoint at 0500 UTC

500-mb height, temperature and wind at 0500 UTC

MCS maintenance

MCS at 03 UTC

MCS at 06 UTC

MCS maintenance at 0000 UTC

MCS maintenance at 0300 UTC

MCS maintenance at 0600 UTC

MUCAPE and Shear

Precipitable water at 05 UTC

Moisture convergence

Most unstable CAPE and CIN with effective shear (knots) at 0500 UTC

Precipitable water (inches) at 0500 UTC

Surface moisture convergence and mixing ratio at 0500 UTC

Models

NMM model loop

4-km total precipitation

0000 UTC 18 May 2010 4-km NMM 1-km AGL simulated reflectivity loop 

0000 UTC 18 May 2010 4-km NMM 36-h total preciptation.

 

 

 

 


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