Valid 161300Z – 171200Z
…NO SEVERE THUNDERSTORM AREAS FORECAST…
Severe thunderstorms are not expected today and tonight.
The synoptic upper-air pattern will become more amplified through
the period, while remaining progressive. The main perturbation will
be a strong trough — now neutrally tilted and extending southward
over Pacific waters from a cyclone near the BC coast. The cyclone
is part of a Rex block with the anticyclonic portion over AK.
Through the period, the cyclone is forecast to retrograde across the
Gulf of Alaska, while the synoptic trough moves inland across
western North America and assumes negative tilt. By 12Z, this
trough should be located along an axis from west-central BC across
WA, southwestern ID to northern AZ. A strong/basal shortwave trough
and attendant vorticity lobe will be over southwestern UT and the
Kaibab Plateau/Grand Canyon region of AZ by then. As the trough
more closely approaches the West Coast, cooling air aloft and
steepening lapse rates, atop at least marginal low-level theta-e,
will support isolated thunder potential.
At the surface, 11Z analysis showed a low between GLF-RUT, and cold
front across northern NJ, northeastern MD, central VA, northwestern
SC, central MS, and northern LA, becoming quasistationary over east
through southwest TX. A weak surface trough intersected the front
near TYR and should ripple slowly westward gradually along the
boundary today, reaching the AUS area tonight. Gradual low-level
frontolysis is expected through the period across TX; however, some
boundary-layer baroclinicity should persist into day 2, amidst
strengthening return flow above the surface.
Widely scattered thunderstorms are underway, in an area of low-level
warm advection and frontal forcing from southwest through north-
central and northeast TX. Isolated to widely scattered, episodic
thunderstorms are possible through the remainder of the period over
much of west and north TX, the Red River region and eastward from
the Arklatex to part of the lower Mississippi Valley. This activity
will be enabled by continued moisture transport, frontal lift, warm
advection, and isentropic ascent of parcels to LFC. Low-level
convergence may remain regionally maximized near the front/trough
intersection, further contributing to convective potential, though
surface winds should remain rather modest. CAPE will be
increasingly elevated and weaker in magnitude with northward extent.
Mass response to the approaching synoptic trough will include the
formation of an extensive LLJ from TX to the Dakotas, the southern
part of which will contain favorably moist and at least marginally
unstable inflow-layer parcels in support of convective potential.
Areas of 35-45 kt effective-shear magnitudes are evident in some
forecast soundings, suggesting briefly well-organized multicells or
marginal supercell characteristics may occur on a localized,
short-lived basis. Lack of more robust lapse rates and buoyancy
should keep severe potential too low and conditional to assign
probabilities at this time.