The fine folks at the Storm Prediction Center are indicating that a severe thunderstorm watch will likely be required soon for portions of Southwest Texas and the Concho Valley. Strong low-level winds out of the south continue to pump in elevated instability totals. While temperatures at the surface remain fairly cool, we’re seeing instability values increasing in the mid-levels of the atmosphere. As such, the potential for some hail and gusty winds will be increasing as storms move into the region. A brief tornado is not out of the question, although that potential is low at this time. It may increase a bit later further to the east. This will be the first of several watches we’ll likely see issued today.

Mesoscale Discussion 0052
NWS Storm Prediction Center Norman OK
1257 PM CST Sun Jan 15 2017

Areas affected…Portions of southwest TX

Concerning…Severe potential…Severe Thunderstorm Watch likely

Valid 151857Z – 152130Z

Probability of Watch Issuance…80 percent

SUMMARY…The risk for severe thunderstorms will increase and spread
across the region during the next several hours. While there is some
uncertainty regarding the timing of more substantial increase in
severe risk, the issuance of a Severe Thunderstorm Watch is likely
within the next few hours.

DISCUSSION…Arcs of convection are spreading eastward/northeastward
across portions of West TX, within a region of strong DCVA preceding
an upper cyclone emerging over the TX Trans-Pecos region, and at the
leading edge of an attendant midlevel speed maximum/dry slot.
Antecedent continental polar air ahead of this activity continues to
modify, with a warm front advancing northward across portions of the
Edwards Plateau and vicinity. Two-hour surface pressure falls around
2.0-3.5 mb highlight the ongoing warm advection accompanying a 30-40
kt low-level jet, indicated by area VAD wind profiles. This will
support additional northward motion of the boundary during the next
several hours, especially as the upper cyclone emerges over the
Plains. As a result, higher theta-e air south of the boundary —
presently characterized by dewpoints in the middle 60s — will
continue to build northward. With modest diurnal heating of this air
mass, inflow for convection along the southern end of the
aforementioned arcs of convection should gradually become
surface-based during the next few hours.

Present indications are that the severe risk — aided by 45-55 kt of
effective shear combining with 500-1250 J/kg of MLCAPE near and
south of the boundary — will increase in the 20Z-22Z time frame,
and develop eastward/northeastward. A quasi-linear mode may tend to
support primarily a severe-wind risk. However, sufficiently steep
midlevel lapse rates may promote an isolated severe-hail risk —
particularly with line-embedded/leading supercell structures. Ample
low-level shear could support a tornado or two, though the
undercutting nature of line-related outflow, combined with the
ongoing incipient stages of boundary-layer destabilization, cast
doubt on a more substantial tornado risk.