re: Tornado! High Risk Day w/ 3 Active PDS Watches

Tornado watch coming soon for S. Louisiana



Mesoscale Discussion 0093
NWS Storm Prediction Center Norman OK
0234 PM CST Sat Jan 21 2017

Areas affected...Portions of southern MS...LA

Concerning...Severe potential...Tornado Watch likely

Valid 212034Z - 212300Z

Probability of Watch Issuance...80 percent

SUMMARY...The risk for severe thunderstorms capable of large hail,
damaging winds, and tornadoes will be increasing through the late
afternoon and into the evening. The issuance of a Tornado Watch is
likely within the next few hours.

DISCUSSION...The isallobaric response to 2-hour pressure falls on
the order of 2-4 mb across the lower MS Valley will be the continued
northward advance of a warm front analyzed approximately west-east
across southern LA. The air mass south of this boundary is
characterized by surface dewpoints in the middle 60s to around 70F,
supporting moderate to locally strong buoyancy -- aided by steep
midlevel lapse rates sampled by the 18Z Slidell sounding (around 8
C/km in the 700-500-mb layer). As this air mass continues to spread
northward within a modest warm-advection regime well in advance of
stronger deep ascent farther west, diurnally reduced capping and
modest ascent will likely support an increase in surface-based
thunderstorm development -- especially during the late afternoon and
evening as more vigorous ascent approaches. Initial shallow
convective towers are already evolving along the warm frontal zone
across parts of southern LA, in proximity to pockets of stronger
insolation, per visible imagery.

While there is some uncertainty regarding the timing of more robust
convective development given the aforementioned capping, effective
shear around 45-60 kt will likely support supercell structures.
Initially, large hail will be likely, though elongating/increasingly
curved low-level hodographs into the evening hours suggest that the
tornado risk will likely increase. Damaging winds will also be
possible in conjunction with small-scale, congealing convective
clusters.

..Cohen/Hart.. 01/21/2017