ACUS01 KWNS 211302
SWODY1
SPC AC 211300

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0800 AM CDT Tue May 21 2024

Valid 211300Z - 221200Z

...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS OVER MUCH OF IA
AND NEARBY PARTS OF SOUTHEASTERN
MINNESOTA...WISCONSIN...ILLINOIS...AND MISSOURI...

...SUMMARY...
An outbreak of severe thunderstorms, including the potential for
strong tornadoes, is expected mainly this afternoon to early
evening. The greatest threat is over Iowa and parts of adjacent
states.

...Synopsis...
In mid/upper levels, a persistent, slow-moving cyclone will meander
over southern SK through the period, while a separate closed low
digs southeastward from northern BC to WA. These will anchor a mean
trough extending from the northern Rockies across southern CA and
offshore from northern Baja. In the downstream southwesterlies, a
shortwave trough was evident in moisture-channel imagery over the
High Plains from western NE to northeastern NM. This perturbation
should pivot northeastward to southern MN, central IA and
northwestern MO by 00Z, then northeastward over western and northern
parts of the upper Great Lakes by 12Z tomorrow.

At the surface, 11Z analysis showed lows near SUX and CNK, along a
cold front arching to southwestern KS and the Raton Mesa. The lows
should consolidate today into a more-coherent cyclone center and
move to southern MN by 00Z, with cold front to southeastern IA,
central MO, and central OK, becoming quasistationary to warm across
western OK and the northern TX Panhandle, to a low in southeastern
CO. A dryline -- initially analyzed from a frontal intersection
over west-central KS south-southwestward to between MAF-FST, should
shift eastward today over southern/central OK, northwest/west-
central TX, and the Edwards Plateau. By 12Z, the low should become
occluded over the Boundary Waters region, with cold front across
Lower MI, southern IL, southeastern MO, southern OK, northwest TX,
and southeastern NM.

...Upper Mississippi to lower Missouri Valleys...
An ongoing, organized band of thunderstorms, with a history of
producing several measured severe gusts in central/eastern NE --
will sweep across parts of IA through the remainder of the morning.
This activity will overtake isolated strong-severe convection near
the outflow boundary, where relatively maximized low-level vorticity
and instability will be most strongly juxtaposed. Severe gusts,
sporadic large hail, and a tornado or two all are possible. See SPC
Severe Thunderstorm Watch 275 and related mesoscale discussions for
near-term guidance on this threat.

That activity is a precursor to the main event, which is expected to
sweep across much of the same areas (and portions of adjoining
states) from midday through the evening. Scattered to numerous
thunderstorms (including multiple fast-moving supercells) are
expected to form over parts of eastern NE from midday into early
afternoon, and over IA and northern/central MO this afternoon. This
will occur as the cold front impinges on a retreating outflow
boundary from the morning storms, and an airmass destabilizing
rapidly on both sides of the boundary from a combination of diurnal
heating and intense warm advection. Sufficient airmass recovery for
a surface-based, all-hazards severe threat is expected across most
of IA and probably into southern MN as well, spreading into parts of western/northern IL and WI ahead of the main convective arc.
Several tornadoes -- some strong (EF2+ damage potential) are
possible, along with large, damaging hail and severe downdrafts.
Given potential for cell motions of 45-50 kt, potential exists for a
few long-track mesocyclones/tornadoes.

The approach of the shortwave trough will increase large-scale lift
-- now evident with post-frontal convection over southwestern NE and northwestern KS that may be trackable across southern NE to the
earliest substantial convection in the midday hours. The ascent
should spread over the region, atop that from the front and surface
heating, supporting the development of steep low/middle-level lapse
rates. With surface dewpoints recovering into the upper 60s to
around 70 F, a corridor of 2000-3000 J/kg MLCAPE should develop over
much of IA, with slightly weaker (but still favorable) values
clipping parts of southern MN as well. Activity should begin to
gradually outrun the corridor of favorable surface-based buoyancy in
the evening after crossing the Mississippi River, but may remain
well-organized in terms of severe-wind threat across Lake Michigan.
The largest low-level hodographs should be near and north of the
remnant outflow boundary, with effective SRH exceeding 300 J/kg --
but will be favorable for potentially tornadic supercells farther
south into southern IA and parts of MO as well.

...Ozarks to south Texas...
Isolated to scattered thunderstorms are expected to form from
midafternoon into evening near the front and dryline, offering large
to very large hail and locally severe gusts. Hail at least 3 inches
in diameter is possible. A tornado or two is not out of the
question -- especially in northern areas (Ozarks vicinity) under the
fringes of the gradient-flow/deep-shear influence from the ejecting
shortwave trough -- and with high-CAPE storm-scale processes amid
favorable deep shear elsewhere.

Though capping from an EML will limit convective potential for much
of the day, a combination of strong surface heating, rich boundary-
layer moisture, and frontal/dryline lift will weaken MLCINH enough
by mid/late afternoon to support deep convective development. Upper
60s to low 70s F surface dewpoints should become common beneath a
high equilibrium level (deep troposphere) and steep midlevel lapse
rates, supporting a corridor of 3000-4000 J/kg MLCAPE (locally
higher). Sufficient veering of winds with height, and effective-
shear magnitudes in the 45-60 kt range, will support supercell
potential. Activity over TX my weaken within a few hours after
sunset as nocturnal boundary-layer cooling re-establishes the strong
capping beneath the EML. However, farther north across parts of
AR/MO, a weaker EML influence may permit activity to persist later
and farther east this evening, before weakening.

...Adirondacks region to northern New England...
Widely scattered to scattered thunderstorms should develop over
portions of the Adirondacks and White Mountains today, and perhaps
also move into the area from nearby parts of extreme eastern ON and southernmost QC. Damaging gusts and isolated severe hail will be
possible.

A mid/upper-level shortwave trough and embedded MCV -- related to
the significantly severe MCS over KS two days ago -- is apparent in
satellite imagery over Lake Huron. This feature will move eastward
toward northern NY/New England and times well with the diurnal-
heating cycle there. Activity should form as large-scale DCVA/lift
and enhancement of midlevel flow -- preceding the MCV/shortwave
trough -- spread atop a destabilized, suitably moist boundary layer,
heated to convective temperature over elevated terrain. These
processes will erode MLCINH to negligible levels, fostering MLCAPE
commonly in the 1000-1500 J/kg range, locally/briefly near 2000
J/kg. Deep shear should remain modest, with effective-shear
magnitudes generally 25-35 kt, though enough veering with height in
low levels may develop to shape somewhat enlarged hodographs.
Multicell and transient supercell modes should predominate, with the
severe threat diminishing markedly after dark.

..Edwards/Goss.. 05/21/2024

$$
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