SPC 2000Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0159 PM CST Tue Mar 05 2024
Valid 052000Z - 061200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS PARTS OF THE
SOUTH-CENTRAL STATES AND THE FL PENINSULA...
...SUMMARY...
Isolated severe thunderstorms remain possible across a portion of
the South-Central States into this evening, and across central to
south Florida this evening through late tonight.
...20Z Update...
Primary change has been to reorient the cat 1-MRGL risk area over
the South-Central States in the wake of extensive convection that
has largely weakened across LA and MS. Redevelopment should focus on
the remnant outflow boundary that arcs northwestward across east TX
and intersects a slow-moving front. Relatively greatest potential
for sustained development should be across northeast TX to southeast
OK, where a few storms may produce isolated large hail. In the wake
of extensive overturning, a dearth of downstream instability suggest
the severe threat should remain relatively short-lived and wane
after dusk. Elevated convection within a weaker sheared environment
may persist longer across the Mid-South with small hail being the
most likely hazard.
No adjustments have been made to the FL risk area, with some
convective potential this evening and more probable thunderstorm
occurrence overnight.
..Grams.. 03/05/2024
.PREV DISCUSSION... /ISSUED Issued by 15th OWS Scott Air Force Base IL/
...Synopsis...
In mid/upper levels, a low-amplitude, zonal to gently cyclonic flow
pattern is present over most of the CONUS, ahead of a synoptic
trough over the northeastern Pacific between 130W-140W. That trough
will approach coastal northern/central CA near the end of the period
with an embedded 500-mb low west of MRY, then move inland on day 2.
Downstream, a southern-stream shortwave trough -- with several
associated/small-scale vorticity maxima -- was apparent in
moisture-channel imagery over eastern parts of OK/TX to the
mid/upper TX Coast. This feature is expected to shift eastward
across the Gulf Coast States and northern Gulf through the period,
reaching parts of MS/AL and the FL Panhandle by 00Z, then moving
across GA/FL in piecemeal fashion around the end of the period and
early day 2. A weaker perturbation -- initially evident over parts
of the Mid-Atlantic region and adjoining ocean waters -- should
deamplify as it moves northeastward across New England through
tonight.
At the surface, a cold front was analyzed at 16Z from western QC
across Lake Huron to southern MO, southeastern OK, north-central TX,
and the TX South Plains region. A weak, attached low initially east
of DAL should drift southward with the boundary and weaken further
today, while another frontal-wave low forms ahead of the front over
northern AL overnight. The front itself should move slowly
southward across central/east TX, AR, and the lower Ohio Valley
through the period.
...Arklatex to lower Mississippi Valley/Delta regions...
The threat for marginally severe hail is ongoing with widely
scattered elevated thunderstorms ongoing over portions of AR into
the Arklatex region and nearby LA/east TX. Though predominantly
multicellular in character, due to lack of more intense vertical
shear, MUCAPE in the 1000-2000 J/kg range, supported by patches of
favorable low-level moisture away from convectively modified air
masses, should continue to support episodic development generally
shifting eastward to east-southeastward across the outlook area into
this afternoon.
Sporadic large hail on a pulse basis will remain a threat, though
isolated damaging to severe gusts cannot be ruled out as well.
Several clusters or small complexes of thunderstorms may develop as
the morning activity moves toward the lower Mississippi Valley, with
the messier mode lessening hail potential locally but maintaining
some pulse-wind threat. Strongest mid/upper winds also will be
displaced over the Great Lakes and northern Gulf in distinct
northern and subtropical streams -- north and south of the greatest
concentrations of convection. This should maintain modest deep
shear and a mostly multicell mode for inland activity.
Though the prefrontal boundary layer should destabilize diurnally,
especially over western parts of the outlook area ahead of the cold
front -- mid/upper forcing for ascent will be displaced farther east
into the Delta region with time -- and ultimately over parts of the
Southeast containing less-suitable low-level moisture and
instability. Though isolated strong-severe afternoon thunderstorms
may form closer to the front in east TX, AR and LA, large-scale
subsidence behind the trough will foster stable layers that may
inhibit sustained convection over the greatest low-level instability
today. With associated coverage uncertainties, overall
unconditional severe probabilities are kept at marginal levels for
the time being.
...FL...
As the large-scale ascent/destabilization passes over the north-
central/northeastern Gulf waters -- including the Loop Current --
low/middle-level lapse rates should steepen with weakening MLCINH.
This may support upscale organization of convection now near and off
the northern coastline into a quasi-linear MCS that would move
east-southeastward across the Gulf, reaching the West Coast of FL
late this evening or overnight, before shifting eastward over at
least parts of the peninsula. Additional convection may form late
during the diurnal heating cycle this afternoon, and/or in a zone of
increasing warm advection and low-level convergence/lift ahead of
the MCS this evening.
While both synoptic and convection-allowing guidance is inconsistent
on mesoscale timing of the complex, or greatest coverage areas of
pre-MCS activity, the general notion of increasing convective
potential from late afternoon onward over FL is supported by the
pattern and by ongoing offshore convective trends. As such, while
the severe threat still looks conditional (especially given some
nocturnal stabilization likely to occur inland over FL), at least a
marginal unconditional risk now is apparent, given increasing deep
shear progged across most of the area, and at least marginal
low-level instability with around 500-1500 J/kg MLCAPE away from
convective outflows. Progged hodographs generally are not
particularly large, though storm-scale and boundary interactions can
modify them locally to become more favorable, amidst 35-45-kt
effective-shear magnitudes.
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