Day 1 Severe Weather Outlook

Valid 091300Z – 101200Z


Sporadic strong-severe thunderstorms are possible this afternoon
through tonight over the area from central and east Texas to
southern Oklahoma and parts of the Arklatex region.

Examination of available RAOB data and moisture-channel imagery
reveals a split-flow pattern developing in mid/upper levels, across
the western CONUS. This process will continue as a strong shortwave
trough — initially extending across northern portions of UT/NV to
the Pacific off north-central CA — digs southward. By 00Z, a
closed 500-mb low should develop just inland from SBA, which is
forecast to move roughly south-southeastward down the coast
overnight, crossing over the SAN area. By 12Z, the center of the
mid/upper cyclone should reach coastal northwestern Baja. To its
northeast, a northern-stream synoptic trough should take shape from
MB-WY. The associated cyclonic flow, from the Canadian High Plains
to the upper Mississippi Valley, will be traversed by several
shortwaves, including one vorticity banner splitting from the
current northern Great Basin perturbation and crossing the upper
Great Lakes tonight.

At the surface, a low was analyzed at 11Z over north-central KS,
with cold front across the eastern OK/northern TX Panhandles to
northeastern/north-central NM. By 00Z, the low should reach
southern WI, with cold front southwestward across eastern to
southwestern MO, northeastern/south-central OK, northwest TX, and
southeastern NM. By 12Z, the cold front should extend near a line
from CLE-EVV-LIT-AUS-DRT. As return flow continues through the
prefrontal warm sector, moisture increases over central/east TX, and
heating/mixing strengthens this afternoon over west/southwest TX
ahead of the front, a dryline will sharpen near a DRT-SPS line and
move slowly eastward. The dryline will be overtaken by the cold
front from north-south, through the remainder of the period.

…Central/east TX to southern OK and Arklatex…
Multiple episodes of thunderstorms are possible over the outlook
area from this afternoon through the late-overnight hours, each
including supercell potential. Large hail appears increasingly
probable, along with isolated damaging gusts, and a tornado cannot
be ruled out.

The western part of the moist sector — beneath the steepest
mid/upper-level lapse rates — also will gain the strongest
boundary-layer instability with a combination of afternoon
insolation/heating and continued theta-e advection. Surface dew
points in the 60s F should become common from southern OK southward
across central TX, with midlevel lapse rates greater than 7 deg
C/km, contributing to peak preconvective MLCAPE in the 1500-2000
J/kg range. Due to less-mature moisture return and low-cloud cover
today, MLCAPE will decrease with northeastward extent from the Red
River across eastern OK, though surface-based effective-inflow
parcels are likely as far east as the MLC area and may develop to
near the OK/AR border. 35-45-kt effective-shear magnitudes and
effective SRH of 150-250 J/kg (increasing eastward amidst
more-backed surface flow) will support supercells as well as
organized multicells. Strengthening MLCINH over central/south TX,
in proximity to a stronger EML, will reduce convective/severe
potential with southward extent.

This will enable afternoon/evening thunderstorm development along
the cold front over southern OK from the dryline intersection
northeastward. Such convection may become organized enough to
produce severe hail and strong/damaging gusts before being undercut
by the front or weakening in a slightly capped warm sector.
Isolated thunderstorms may form along the dryline itself over
northwest TX, but a combination of dry-entrainment potential on the
convective scale, and capping influence from advection of a Mexican
EML layer over the area, cast considerable uncertainty on
development/coverage of convection in that regime.

Weaker MLCINH is forecast in the warm sector across central/
north-central TX into southeastern OK, near the astern rim of the
EML, but still in a zone of associated steep mid/upper-level lapse
rates. Gradual building of strong-severe convection also is
possible in a zone of low-level lift associated with a northeast/
southwest-aligned, boundary-layer confluence/convergence axis, from
north TX to southeast OK. Finally, a secondary/later zone of
strengthening low-level lift (warm advection and convergence) may
develop late tonight across parts of east TX and northern LA,
possibly representing the early stages of the process described in
the SPC day-2 outlook.

The aggregate of all these potential scenarios compels considerable
expansion of the “marginal” severe-outlook area, with large-hail
potential this afternoon and evening becoming relatively
concentrated in and near the 15%/categorical “slight” upgrade area.

..Edwards/Mosier.. 02/09/2020


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