Valid 101300Z – 111200Z
…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
MISSISSIPPI…LOUISIANA AND EXTREME SOUTHEASTERN ARKANSAS…
Severe thunderstorm winds, isolated hail and a few tornadoes will be
possible today from south Texas to northern Alabama, with the
greatest threat over parts of Mississippi, Louisiana and extreme
In mid/upper levels, a split-flow pattern persists across the
western CONUS, with a closed cyclone centered just offshore from the
SAN area and northern Baja. Through the remainder of this period,
the cyclone should pivot eastward across northern Baja/Sonora, with
the 500-mb low moving to near TUS by 12Z tomorrow. A broad fetch of
west-southwesterly flow aloft will extend from the eastern rim of
the cyclone across TX and much of the South. This flow likely will
contain subtle vorticity maxima racing along a height gradient that
will tighten gradually as the southern-stream cyclone shifts
eastward, and a northern-stream perturbation crosses the northern
Plains and upper Mississippi Valley.
At the surface, 11Z analysis showed a cold front from northeastern
OH to extreme western KY, through a weak frontal-wave low near DEQ,
southwestward between ACT-AUS to the Big Bend area of southwest
Texas. By 00Z, the front should extend across portions of PA,
eastern KY, middle TN, northern LA, east TX, to central Coahuila.
By 12Z, the slowly moving front should reach the southern
Appalachians, northern AL, and parts of southeast/deep south TX.
…South TX to Tennessee Valley…
Scattered thunderstorms are expected to develop episodically along
and ahead of the front through the period, with warm-sector
convection most dense and intense this afternoon into evening.
Damaging gusts, sporadic hail and a few tornadoes all are possible
in aggregate, with the “slight risk” area representing the most
persistent zone of juxtaposition between convective maturation and
favorable environmental parameter space.
Frontal thunderstorms may access surface-based buoyancy long enough
to mature to severe levels in terms of damaging-wind and tornado
potential, though the front may undercut some such activity.
Front-parallel flow aloft and likely messy storm modes cast some
uncertainty on the organization of those threats. Additional
convection, forming in warm-sector/low-level convergence zones, will
have greater access to favorable thermodynamic conditions for more
time, before merging into the precip/convective swath more directly
associated with the front. Isolated severe hail also is possible
either in the warm sector, or in the post-frontal zone of elevated
low-level warm advection.
Patchy/sporadic insolation and low-level theta-e advection will
destabilize the warm sector from southwest-northeast throughout the
day, amidst rich source-region moisture as observed in upstream RAOB
and GPS PW data. Accordingly, MLCAPE away from precip/convection
should increase into the 1000-1500 J/kg range this afternoon
(locally/briefly higher). Vertical shear will be favorable for
supercells, with effective-shear magnitudes reaching 45-55 kt. A
strong/45-60-kt LLJ should help effective SRH to commonly reach
200-300 J/kg, despite a slight westerly component to surface winds.
Heavy to excessive rainfall also is possible in parts of this area,
mainly over MS/AL, with WPC “high risk” and “critical weather day”
declared for that threat. Please refer to WPC rainfall discussions
and local forecasts and flash-flood products for more information.
Isolated to widely scattered thunderstorms are expected to develop
today in a zone of strongly difluent mid/upper flow, marginal but
adequate low-level moisture, and low-level warm advection/
destabilization east of the mid/upper-level cyclone. Along the
eastern rim of the most-difluent area aloft, somewhat stronger
mid/upper winds will enable enough deep shear (effective values
35-45 kt) to support a few organized thunderstorms. Low/middle-
level lapse rates should be steep enough to support 300-600 J/kg
MLCAPE as well. The most intense cells may briefly offer small hail
and strong gusts; however, severe potential appears too isolated and
conditional for a categorical risk area at this time.