- Day Three
acus03 kwns 180818
Storm Prediction Center ac 180817
Day 3 convective outlook
National Weather Service Storm Prediction Center Norman OK
0217 am CST Mon Feb 18 2019
Valid 201200z - 211200z
..there is a marginal risk of severe thunderstorms into the day
Wednesday across parts of southeastern Louisiana and Mississippi
into adjacent portions of southwestern Alabama...
Strong thunderstorms may continue to impact parts of the lower
Mississippi Valley into the day Wednesday, accompanied by at least
some risk for severe weather.
Models indicate little change to the general large-scale pattern
from Tuesday into Wednesday. A belt of mid-latitude westerlies
likely will remain amplified across the eastern Pacific into western
North America, while flow remains broadly confluent downstream,
across eastern North America, to the north of prominent subtropical
ridging centered to the east of the Bahamas.
While another strong short wave impulse continues to dig along the
U.S. Pacific coast, toward the base of the larger-scale western U.S.
Troughing, a downstream impulse emerging from the troughing is
forecast to progress around the crest of the larger-scale eastern
U.S. Ridging. At the same time, a gradual erosion of cold surface
ridging appears likely to proceed to the east of the Mississippi
Valley, and it appears that a modest cyclone may evolve within
deepening surface troughing accompanying the lead short wave
impulse, across the upper Midwest and Great Lakes region Wednesday
through Wednesday night.
Considerable convective development, including embedded
thunderstorms, may be ongoing early Wednesday across the lower
Mississippi and Ohio valleys, largely driven by warm advection and
moisture return above the residual cold/stable surface-based air.
Forcing for this elevated destabilization and convection is expected
to spread east-northeastward across the Appalachians and northern
mid Atlantic region through the period, while additional convective
development persists along a trailing quasi-stationary baroclinic
zone across the Tennessee and lower Mississippi valleys into the
Texas Gulf Coast vicinity.
Much of this convection may remain rooted above a cold/stable layer.
However, a corridor of weak to modest boundary-layer instability
developing across parts of the lower Mississippi Valley by 12z
Wednesday may linger into at least midday, while slowly spreading
eastward. It appears that vertical shear will remain strong enough
across this region to maintain an environment at least marginally
conducive to organized severe thunderstorm development, perhaps
including supercells with the risk for a tornado or two.