- Day Three
acus02 kwns 201727
Storm Prediction Center ac 201727
Day 2 convective outlook resent 1
National Weather Service Storm Prediction Center Norman OK
1127 am CST Tue Feb 20 2018
Valid 211200z - 221200z
..there is a marginal risk of severe thunderstorms from East Texas
Showers and embedded thunderstorms are expected from Texas through
the lower Mississippi Valley to the Ohio Valley Wednesday.
Marginally severe storms are possible from East Texas into
Louisiana, with small hail possible into central Texas.
An amplified large-scale pattern will persist into Wednesday with an
upper ridge centered over the western Atlantic, and a large upper
trough across the western Continental U.S.. in between, a broad region of
strong southwest flow will exist from the Southern Plains into
eastern Canada. Within this area of southwest flow, an intense
low-amplitude shortwave trough will move from the upper Great Lakes
into Quebec, with an impressive 200+ kt upper level jet Max, with
generally neutral height tendencies aloft to the southwest, although
a weak impulse is expected to move across Texas.
At the surface, a cold front will extend from a low in Quebec
southwestward across the Ohio/Tennessee valleys, and across the lower MS
valley across southern Texas. A moist air mass with 60+ dewpoints will
exist ahead of the front from Texas into the Ohio Valley, with dewpoints
near 70 f across coastal Texas to the Sabine River. Warm advection in
the lower levels coupled with lift along and north of the cold front
will result in thunderstorm activity from Texas to OH, the strongest of
which will exist from Texas into la.
..East Texas into Louisiana...
Moderately strong southwest flow aloft will persist over the area
along with warm, moist southerly low-level flow. This is expected to
counteract/stall the front, perhaps resulting in a weak surface wave
along it. Models are in good agreement depicting a zone of
thunderstorms developing by late morning through afternoon across
east TX, continuing into western la. Low-level shear appears
marginally supportive of rotating storms should storm Mode be
cellular. However, a large number of storms could have negative
impacts as far as supercell potential. Any threat is expected to end
after 00z as lift from differential divergence tied to the large jet
Max to the north moves away from the area.
..much of central Texas...
The cold front will continue to drift southward during the day,
undercutting a moist and marginally unstable air mass and resulting
in elevated instability. This will be maintained by a broad region
of warm advection with 20-30 kt southerly 850 mb flow. Lift will
occur mainly via warm advection, although a storm or two cannot be
ruled out along the actual front across southern TX, but 0-3 km
lapse rates will be poor there. Given cool temperatures aloft and
sufficient instability, a few storms may be capable of marginal
hail, but the threat does not appear high enough for a marginal risk
at this time.