SPC 0600Z Day 2 Outlook

Day 2 Outlook Image

Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1258 AM CDT Thu Apr 22 2021

Valid 231200Z - 241200Z


Isolated to scattered severe thunderstorms should occur Friday into
Friday night from parts of the southern Plains into the lower
Mississippi Valley. Large to very large hail, damaging winds, and
isolated tornadoes all appear possible.

...Southern Plains to the Lower Mississippi Valley...
An upper trough will eject eastward Friday from the Southwest and
northern Mexico across the southern Plains, eventually reaching the
lower/mid MS Valley by the end of the period. At the surface, a weak
low initially centered over the TX Panhandle should develop
southeastward through the day, likely reaching the southwestern OK
and western north TX vicinity by Friday evening. A dryline extending
southward from this low should mix eastward across west/central TX
before stalling near the I-35 corridor by the early evening. From
Friday evening into early Saturday morning the surface low should
develop eastward towards AR, while an attendant cold front sweeps
southeastward across OK/TX. A marine warm front should also lift
northward across parts of east TX, LA, and southern MS/AL from
Friday afternoon and continuing into the overnight as a strong
(40-50 kt) south-southwesterly low-level jet moves quickly eastward
across these areas.

There is still a fair amount of uncertainty how much convection will
develop along the dryline in central TX/southwestern OK through
Friday afternoon. Most guidance suggests the possibility of a
bimodal distribution of storms across the southern Plains, with the
triple point in southwestern OK late Friday afternoon and a
low-level warm advection regime in east/coastal TX through the
daytime hours being favored zones of initiation. Still, there
appears to be at least some chance for additional robust
thunderstorm development posing mainly an isolated large to very
large hail threat farther west along the dryline by peak afternoon
heating, as modest large-scale ascent with the upper trough
overspreads the southern Plains. Based on latest guidance being in
better agreement on the placement/eastward extent of the dryline,
severe probabilities have been adjusted slightly eastward in central

Any thunderstorms that form near the triple point in southwestern OK
will likely become severe supercells quickly. The main threat
initially should be large to perhaps very large (2+ inch diameter)
hail given steep mid-level lapse rates around 8 C/km, sufficient
low-level moisture and diurnal heating to support 1500-2500 J/kg of
MLCAPE, and effective bulk shear of 40-50 kt. By early Friday
evening, a modest strengthening of the low-level flow and gradually
increasing low-level moisture may support an isolated tornado threat
with any storms that can remain discrete. Although details remain
unclear, there also appears to be some potential for upscale growth
into a line or bow Friday evening/night as the cold front advances
southeastward. Damaging winds would become a greater risk across
parts of central OK into north TX if this upscale growth occurrs.

Farther east, convection may develop by early Friday afternoon from
parts of coastal/east TX into the ArkLaTex in association with a
30-40 kt low-level jet. This activity would occur in tandem with the
return of rich Gulf moisture across these regions. Thunderstorms may
have the potential to become elevated north of the marine warm front
as they move northeastward. Even so, boundary-layer instability is
expected to gradually increase through the day along/south of the
warm front, and deep-layer shear will remain strong. The potential
for large hail will exist with any supercells that can develop
initially, and isolated very large hail may occur as steep mid-level
lapse rates overspread this region. With time and continued
low-level warm advection occurring, storms will probably form into
one or more clusters as they move eastward into LA and southern MS
Friday evening/night. The damaging wind threat should increase as
this mode transition occurs, but isolated tornadoes also appear
possible with any convection that can remain at least semi-discrete,
as 0-1 km shear associated with the low-level jet is forecast to
remain quite strong.

Some consideration was given to greater hail probabilities
along/east of the dryline in OK/TX, but confidence in more than
isolated coverage of thunderstorms remains low. The best potential
for initiation appears to be near the triple point in southwestern
OK, but it may be that only one or two supercells form over this
area. Across east TX into the lower MS Valley, there may be a
greater and somewhat conditional threat for tornadoes given the
forecast low-level shear, but the probable mode transition from
supercells to a messy line may limit this potential somewhat. There
may need to be a higher categorical outlook with greater hail and/or
tornado probabilities introduced when these mesoscale uncertainties
become a bit clearer.

..Gleason.. 04/22/2021

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