SPC AC 300730
Day 3 Convective Outlook
NWS Storm Prediction Center Norman OK
0230 AM CDT Fri Apr 30 2021
Valid 021200Z - 031200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SUNDAY INTO SUNDAY
EVENING ACROSS PARTS OF SOUTHEAST TEXAS...SOUTHERN AND EASTERN
LOUISIANA...MUCH OF MISSISSIPPI AND ADJACENT PORTIONS OF WESTERN
ALABAMA AND SOUTHWESTERN TENNESSEE...
...SUMMARY...
Severe thunderstorms are possible Sunday into Sunday evening across
much of the lower Mississippi Valley.
...Synopsis...
A couple of significant short wave perturbations are forecast to
remain progressive within a southern branch of split mid-latitude
westerlies across the central and southern tier of the U.S. Models
indicate that the lead impulse will lose amplitude as it accelerates
northeastward in negatively tilted fashion, into a more broadly
confluent regime across and northeast of the lower Mississippi
Valley. This will coincide with the increasing amplification of the
evolving upstream mid-level troughing, which is forecast to dig
southeastward through much of the Four Corners states by late Sunday
night.
Preceding the trailing perturbation, a broadening plume of warm and
capping elevated mixed-layer air is forecast to overspread much of
the southern Great Plains. Cold surface ridging appears likely to
nose southward to the lee of the northern Rockies in the wake of the
lead southern branch impulse, and mid-level troughing digging within
the northern branch. Within broad surface troughing shifting
southeastward ahead of the cold air, the more prominent surface low
is expected to migrate southeastward across the Texas Panhandle
vicinity into western portions of northern Texas by 12Z Monday. It
appears that this will be accompanied by an increasing influx of
low-level moisture beneath the warming mid-levels, across the
southern Great Plains into lower Mississippi Valley.
...Upper Texas coast into lower Mississippi Valley...
Models continue to indicate a window of opportunity for vigorous
convective development in advance of the eastward advecting plume of
elevated mixed-layer air. Ahead of the remnant mid-level cold core
of the lead perturbation, a cluster of thunderstorms may be
spreading across the upper Texas coast vicinity into Louisiana early
in the period. This may be in the process of weakening, with
activity tending to progress into an initially cooler/drier
low-level environment associated with surface ridging across and
east of the lower Mississippi Valley. However, most model output
does suggest that a modest (30-40 kt around 850 mb) southerly
low-level jet preceding this convection will supported elevated
moisture return across much of southern/eastern Louisiana and
Mississippi. And guidance suggests that daytime heating and
downward mixing of this moisture will contribute to a destabilizing
boundary layer through Sunday afternoon.
A zone of enhanced lower/mid tropospheric warm advection in the
immediate wake of the lead perturbation may provide one focus for
vigorous new thunderstorm development across southern Louisiana
during the day Sunday. This forcing could support an upscale
growing and organizing cluster of storms, aided by strong deep-layer
supported by an associated 50 kt southwesterly 500 mb jet. If this
does not negatively impact areas to the north, other thunderstorm
development may increase by Sunday afternoon along the axis of
destabilization through Mississippi. This may be initially
discrete, but convection may tend to grow upscale along outflow
spreading northeastward from the storm cluster to the south.
Aided by the development of moderate CAPE, strong shear and
lingering relatively cool mid-levels, supercell structures capable
of producing severe hail, damaging surface gusts and perhaps a few
tornadoes appear possible.
..Kerr.. 04/30/2021
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