NWS Storm Prediction Center Norman OK
0647 AM CST Tue Feb 14 2017
Valid 141300Z - 151200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS THIS MORNING
OVER A SMALL PART OF THE MIDDLE-UPPER TX COASTAL AREA...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS INTO
AFTERNOON...ELSEWHERE FROM THE TX COASTAL BEND TO EAST TX AND
WESTERN LA...
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS TONIGHT ACROSS
IMMEDIATE COASTAL AREAS FROM THE MS RIVER MOUTH TO THE WESTERN FL
PANHANDLE...
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE WESTERN
SLIGHT-RISK AREA ACROSS PORTIONS OF CENTRAL/SOUTHERN MS AND AL...
...SUMMARY...
Severe thunderstorms are expected to shift eastward over southeast
Texas toward the Sabine River region through early afternoon, with a
few tornadoes and damaging winds possible. After a relative lull
when the risk becomes marginal, the threat of severe weather will
increase conditionally tonight into Wednesday morning, from the
mouth of the Mississippi River to the coastal Florida Panhandle.
...Synopsis...
A high-amplitude large-scale pattern will persist over North
America, characterized by troughing from eastern Canada across the
Great Lakes to the central Appalachians, and a sharp ridge from
Yukon and the NW Territories across the northern Rockies and CA.
East of that ridge, a strong/southern-stream shortwave trough was
evident in moisture-channel imagery from western KS southward across
the southern High Plains to the TX Big Bend region and adjoining
parts of northern MX. As a basal vorticity max aloft, now near the
Big Bend, ejects northeastward, the shortwave trough as a whole will
deamplify gradually, but still remain strong through most of the
period. By 00z, the 500-mb trough should reach eastern OK and
central TX, with a transient closed low possibly developing over
eastern OK. Thereafter the perturbation will enter a broader-scale
confluent-flow regime, reaching the Mid-South and lower Mississippi
Valley near the end of the period.
A complex surface pattern was evident in an 11Z analysis in and near
the outlook region, with a weak low between CLL-CRS and a wavy cold
front from there southwestward across the western part of the lower
Rio Grande Valley region. A diffuse warm front was drawn from the
low southeastward across the HOU metro area and over shelf waters
just off the LA coast. The surface low pressure should consolidate
better through this afternoon over southeast and east TX, with a
fairly well-defined low crossing the Sabine River valley near Toledo
Bend Reservoir mid-late afternoon. By 00z, the surface low should
be located over central or northern LA, with the cold front
southwestward across southern LA and the northwestern Gulf. Through
the overnight hours, the surface low should weaken and accelerate
east-northeastward toward west-central AL. By 12Z, the cold front
should extend from there southwestward over southeastern MS,
southeastern LA and the west-central Gulf.
...South-central/southeast TX to western LA...
A gradually organizing complex of strong-severe thunderstorms is
forecast to continue shifting eastward to east-northeastward across
the outlook area into at least early-mid afternoon, noticeably
faster than progged by most convection-allowing model solutions from
the 00Z-09Z initialization cycles. This activity still will cross
an area of enhanced low-level SRH favorable for embedded supercells
and QLCS circulations supporting tornado/wind risk. By mid-late
afternoon, we currently anticipate a general decrease in both
large-scale upper support and inflow-layer instability for most of
the complex as it proceeds deeper into LA, though isolated severe
and a persistent supercell or two cannot be ruled out.
In the meantime, lateral expansion of the MCS is possible both
northward and southward; however, the extent of that is uncertain
due to:
1. Capping on the south end (evident in the 12Z CRP RAOB) versus
both frontal forcing and the glancing influence of large-scale
DCVA/cooling aloft on the inversion layer sampled around 700 mb.
2. Northern extent of favorable near-surface destabilization/
effective-inflow parcels, amidst the rather lax thermal/theta-e
gradient represented by the warm-frontal zone.
In between, the area around I-10 and southward to the middle/upper
TX coast, near the axis of SPC tornado watch 32, represents the most
favorable severe-weather corridor through early afternoon.
For additional information and nowcast updates, refer to watch-based
mesoscale discussions, to be issued as needed through the morning
hours.
...Southeast LA to coastal FL Panhandle...
A conditional slight risk for tornadoes and marginal
thunderstorm-wind threat remains apparent over this region
overnight, as the surface low tracks eastward across MS. Related
isallobaric forcing should help to maintain generally southerly
surface winds beneath an eastward-shifting, 45-55-kt southwesterly
LLJ. The net effect will be to strengthen low-level bulk shear and
hodograph sizes with time in any given spot, while deep shear also
increase as the shortwave trough crosses the MS River and height
gradients tighten to its south and southeast. Forecast hodographs
accordingly show an eastward spread of effective SRH in the 250-450
J/kg range and effective-shear magnitude near 50 kt along the coast
where moisture and instability will be most favorable.
Thermodynamically, lapse rates aloft will be rather weak for much of
the period. However, subtle large-scale ascent from both low-level
warm advection and weak DCVA should steepen lapse rates overnight
enough, in tandem with surface dew points mid 60s F near the
northern rim of the most substantial Gulf return flow, to remove
CINH and support 500-1000 J/kg MLCAPE. Buoyancy will diminish
substantially inland, making the potential even more isolated and
conditional northward. The main uncertainty at this time involves
low-level/pre-frontal forcing for convective-scale lift, and
resulting coverage/mode of storms in an environment otherwise
favorable for supercells. Any sustained cells will have the
potential to generate cyclic mesocyclones with attendant tornado
threat and potential for at least isolated nontornadic wind damage.
..Edwards/Broyles.. 02/14/2017
