SPC 1300Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0753 AM CDT Sun Apr 17 2022
Valid 171300Z - 181200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
LOWER MISSISSIPPI VALLEY/CENTRAL GULF COAST REGION AND
SOUTH-CENTRAL TEXAS...
...SUMMARY...
Large hail, damaging wind gusts, and a few tornadoes, are possible
over parts of the lower Mississippi Valley/central Gulf Coast region
today into this evening. Severe/damaging hail will be the main
threat with thunderstorms over parts of south Texas.
...Synopsis...
In mid/upper levels, the previously zonal flow regime across most of
the CONUS will become much more amplified through the period --
mainly related to a couple of nearly phased, northern- and
southern-stream perturbations:
1. A trough located over the SK/AB border southward through central
MT, which is forecast to strengthen and move to the mid/upper
Mississippi Valley by 12Z tomorrow.
2. A somewhat lower-amplitude, but more directly influential,
shortwave trough apparent in moisture-channel imagery from western
KS to the TX Panhandle. This feature should move eastward to MO and
AR by 00Z, then the southern Appalachians and GA by the end of the
period.
At the surface, 11Z analysis showed a cold front from the western
Carolinas across northern AL, becoming quasistationary
west-southwestward to a low near GGG. The wavy boundary then
extended southwestward through parts of south-central TX. Another
baroclinic zone, which has been reinforced by prior convective
outflow and denotes the northern rim of best-modified
maritime/tropical conditions from the Gulf, was drawn near the
coastal FL Panhandle, west-northwestward across southwestern MS and
northern LA.
By 00Z, the low is expected to ripple eastward along the frontal
zone and across the ArkLaMiss region to central/northeastern MS,
with cold front southwestward across northern LA and south TX. To
its east, the marine warm front will shift inland and merge with the
synoptic front (where this already is occurring) across central MS
to southeastern AL. Overnight, the low should eject northeastward
away from the favorable low-level air mass, toward a 12Z position
over eastern KY. The cold front should extend from that low
southwestward across southeastern LA and the northwestern Gulf.
...Lower Mississippi Valley/central Gulf Coast...
Isolated severe hail may be noted from elevated convection this
morning across the Mid-South across northern MS/AL, north of both
fronts, in an environment characterized by somewhat favorable MUCAPE
and deep shear. Additional thunderstorms may develop this afternoon
across parts of the Southeast from southern AL eastward, offering a
marginal, mainly multicellular hail/gust potential amidst weaker
lapse rates and deep shear than will exist closer to the surface
low. However, the greatest severe potential should develop this
afternoon across portions of central/northern LA and
central/southern MS, perhaps spreading into parts of AL this
evening. The environment will favor supercells, with large to very
large hail, damaging to severe gusts, and a few tornadoes possible.
Well-organized multicell clusters and bowing segments also possible.
The approaching southern-stream shortwave trough will provide
favorable mid/upper support in the form of DCVA, as well as shifting
the outlook area under the left-exit region of a subtropical-jet
branch -- with difluent 500-250-mb layer winds. Tightening height
gradients aloft also will foster strengthening deep shear, though
near-surface winds may be modest in the warm sector and along the
boundaries. Surface flow also should veer to southwesterly with
time in the warm sector -- reducing but not removing favorable
low-level shear as hodographs shrink somewhat and get displaced
eastward away from the origin. Still, hodograph enhancements near
the boundaries, as well as 45-55-kt effective-shear magnitudes, will
support supercells. Surface dewpoints in the upper 60s to near 70
F, beneath layers of steep lapse rates related to a peripheral EML,
will combine with diurnal heating for very favorable buoyancy.
Preconvective MLCAPE in the 1500-2500 J/kg range should be common
along and south of the blended frontal zone this afternoon.
Severe potential should diminish tonight and with eastward extent as
the surface low and parent mid/upper-level trough depart the region,
veering the boundary-layer flow in the warm sector, in turn reducing
both convective lift and vertical shear. Meanwhile, nocturnal
low-level stabilization also will contribute to decreasing
convective coverage/intensity overnight.
...South TX...
Isolated to widely scattered thunderstorms are possible along the
front this afternoon, where lift the boundary impinges upon a
strongly heated boundary layer, and their combined lift may
reduce/overcome EML-related MLCINH. The most intense cell(s) may
offer large to very large hail and strong-severe gusts.
Peak/preconvective MLCAPE should reach the 2000-3000 J/kg range (a
substantial fraction being in layers favorable for hail growth) with
dew points in the 60s F, abundant inflow-layer moisture, and
suitable diurnal heating. A preponderance of guidance now
reasonably indicates afternoon development -- making potential less
conditional, within an environment common to large-hail events in
that area. Cloud-layer shear under the subtropical jet will favor
vigorous venting aloft. Nearly linear low/middle-level hodographs
with supercell-favorable deep shear (effective-shear magnitudes near
50 kt) are expected on and just east of the front, suggesting the
potential for splitting storms and left movers. To allow some room
for such activity, the outlook area has been enlarged somewhat on
the north end. Activity should diminish rapidly after dark, as the
boundary layer cools on both sides of the front.
..Edwards/Smith.. 04/17/2022
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