Valid 221300Z – 231200Z
…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM SOUTHEAST
TEXAS TO WESTERN MISSISSIPPI TODAY…
Strong to briefly severe thunderstorms are possible from southeast
Texas to western Mississippi today.
In mid/upper levels, an already split flow pattern will split again
during the period, as a nearly closed Pacific low west of the CA
coast closes and cuts off from the branch already diverting around
the large southwestern-CONUS cyclone. The bigger/inland cyclone has
one weakening circulation center apparent in moisture-channel
imagery over northern AZ near the Grand Canyon, with another
developing over eastern CO. The resulting/eastern 500-mb low should
shift eastward across KS through 12Z tomorrow, on the northern rim
of a positively tilted trough crossing NM and the southern High
Plains. Meanwhile, a strong, positively tilted trough initially
over the upper Great Lakes will move eastward across the Northeast,
becoming neutrally to negatively tilted late overnight across New
England and the western Maritimes of Canada.
At the surface, 11Z analysis showed a cold front from central NY to
central KY, across southern AR, to a weak frontal-wave low over
northeast TX, then southwestward across south-central TX. By 00Z,
the front should extend from the VA/NC line across northern AL,
through the low shifted to northern LA, to near the middle TX coast
and central Coahuila. By 12Z, the cold front should be located
across SC, becoming a warm front just east of the low over the
northwestern AL/middle TN region, then a cold front across southern
LA and the northwestern Gulf.
…West Gulf Coastal Plains/Delta region…
Widely scattered to scattered thunderstorms are expected to form on
either side of the front as the boundary crosses east TX and LA
through the period. Activity developing along and just ahead of the
front may be undercut rather quickly. However, convection farther
into the warm sector should mature/organize gradually, yet enough to
pose a marginal wind/tornado threat, before weakening in relatively
stable, more poorly modified air east of the Delta region.
Subjective and objective analyses show gradually weakening surface
moisture/theta-e with northeastward extent from southeast TX across
the lower Mississippi Valley, into progressively less-completely
modified return flow. This loose gradient will be maintained
through the day in the warm sector, but also will shift eastward.
The net effect will be an area of marginally favorable/surface-based
effective inflow expanding across central LA toward west-central/
southwest MS, before being overtaken by the faster-moving cold front
this afternoon and evening. Although lapse rates will be modest
throughout the troposphere, with prefrontal lift remaining subtle,
modified model soundings suggest small MLCINH values as well, as an
inversion found near 700 mb in the 12Z LCH sounding slowly weakens.
Cloud cover will render diurnal heating patchy and slow, but
sufficient to support surface-based thunderstorms from around midday
onward prior to frontal passage. Hodographs are expected to
decrease slowly through the day in step with LLJ trends, but
sufficient low-level and deep shear are possible to support at least
transient supercell structures, along with a few organized
multicells. The entire parameter space will become less favorable
this evening and overnight.