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.
CoCoRaHS and COOP rainfall.
|KCRP base reflectivity loop May 17. Outflow boundary left over South Texas.||KCRP base reflectivity loop 0249 to 1649 UTC 18 May 2010.|
GOES infrared and 1-h NLDN lightning loop May 18 2010.
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 at 0000 UTC MCS maintenance at 0300 UTC MCS maintenance at 0600 UTC 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
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 at 0000 UTC
MCS maintenance at 0300 UTC
MCS maintenance at 0600 UTC
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