SPC 0730Z Day 3 Outlook

Day 3 Outlook Image

Day 3 Convective Outlook  
NWS Storm Prediction Center Norman OK
0228 AM CDT Thu Apr 22 2021

Valid 241200Z - 251200Z


Isolated to scattered severe thunderstorms may occur Saturday across
parts of the Southeast. All severe hazards appear possible,
including large hail, damaging winds, and a few tornadoes.

An upper trough should continue moving eastward from the lower/mid
MS Valley across the Southeast and Mid-Atlantic on Saturday. A
strong south-southwesterly low-level jet is forecast to advance
quickly eastward from AL Saturday morning across GA and into SC by
the afternoon. At the surface, a weak low over the lower MS Valley
and Mid-South should develop northeastward towards the central
Appalachians during the day, with some uncertainty whether it
deepens substantially or not. Regardless, rich low-level moisture
should move northward across AL/GA/SC, supporting a threat for
surface-based thunderstorms.

Strong to severe convection may be ongoing Saturday morning from
portions of southern LA into MS and AL in association with the
low-level jet. As this activity spreads eastward through Saturday
afternoon, it may continue to pose at least an isolated severe
threat so long as it does not outpace the low-level moisture return.
A rather strong mid-level jet attendant to the upper trough will
support similarly strong deep-layer shear. Forecast soundings across
the warm sector in GA/SC from both the 00Z NAM and ECMWF show a
veering/strengthening wind profile with height through mid levels,
potentially supporting supercells. Some eastward advection of an EML
from the Plains also appears possible across the warm sector, which
combined with the increasing low-level moisture and diurnal heating
should foster moderate to locally strong instability along/south of
the warm front.

All severe hazards, including the potential for a few tornadoes,
appear possible given the favorable thermodynamic and kinematic
environment forecast as thunderstorms move eastward across the
Southeast through the afternoon and early evening. The main
uncertainties are the possible limiting effect of morning convection
on the development of instability, and predominant storm mode.
Finally, there also appears to be a conditional threat for severe
thunderstorms behind the initial morning activity across southern LA
into MS/AL, as strong instability should develop ahead of an
eastward-moving cold front. However, low-level flow is forecast to
quickly veer to westerly behind the passage of the previously
mentioned low-level jet. This should limit convergence along the
front, and the most pronounced large-scale ascent associated with
the upper trough should be mostly east of this region by Saturday
afternoon. Even so, if convection can develop in this regime, then
both isolated large hail and damaging winds would be possible.

..Gleason.. 04/22/2021

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