463
ACUS01 KWNS 211642
SWODY1
SPC AC 211640

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1140 AM CDT Tue May 21 2024

Valid 211630Z - 221200Z

...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS ACROSS
IOWA...NORTHERN MISSOURI...NORTHWEST ILLINOIS...SOUTHEAST
MINNESOTA...AND SOUTHERN WISCONSIN...

...SUMMARY...
An outbreak of severe thunderstorms, including the potential for
strong tornadoes, large hail, and widespread damaging winds, is
expected this afternoon through evening, especially across eastern
Nebraska, Iowa, and Missouri to northern Illinois, southeast
Minnesota, and southern Wisconsin.

...Middle/Lower Missouri Valley and Midwest...
As an outbreak precursor, a well-organized cluster of storms across
south-central/east-central Iowa mid-late this morning has had a
history of producing at least isolated/episodic severe weather since
the predawn hours. These storms are focused along/north of an
outflow-reinforced warm front that will lift northward through late
today into tonight. This will be heavily influenced by a highly
dynamic mass response to a northeast-moving mid-level shortwave
trough that is readily evident in water vapor imagery over the
central High Plains this morning. That said, some short-term
uncertainties exist as of 15z observational data, with gusty
post-convective northeasterly winds still present across
south-central Iowa, which relates to some uncertainty with the
northward extent of the higher-magnitude severe risk across
Minnesota/Wisconsin later today, although latest 16z data features a
recent abatement of those northerly winds.

Regardless, intense storm development is anticipated into the
afternoon across a broad north/south corridor. A synoptically
evident and regional outbreak-favorable setup exists, especially by
mid/late May standards, highlighted by a coupled upper jet structure
over the Upper Midwest, strengthening deep-layer wind fields, and
steady cyclogenesis from the Nebraska/Kansas border vicinity this
morning northeastward toward the Minnesota/Wisconsin border tonight.
Low-level winds will remain strong diurnally ahead of the primary
cyclone, but a noteworthy strengthening of mid-level winds 3-7 km
AGL (reference 700mb/500mb) will rapidly occur through the
afternoon/evening, which will likely have direct outbreak-relevant
influences on initial supercell intensity, QLCS evolution and
overall fast east-northeastward storm motions later today.

Steepening mid-level lapse rates, noted in upstream 12z observed
soundings such as Topeka/Dodge City KS and Springfield MO, will
overspread an increasingly moist airmass generally characterized by
mid/upper 60s F surface dewpoints, contributing to warm-sector
MLCAPE to upwards of 2500-3000 J/kg this afternoon. Scattered to
numerous thunderstorms, including scattered fast-moving supercells,
are expected to initially form over parts of eastern Nebraska by
late morning/midday, and into Iowa and northern/central Missouri
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.
Additional, initially semi-discrete, supercellular development is
also plausible farther east across Iowa this afternoon in vicinity
of modifying outflow/warm front.

Sufficient airmass recovery for a surface-based, all-hazards severe
threat is expected across most of Iowa and potentially into southern
Minnesota as well, spreading into parts of western/northern Illinois
and Wisconsin. Several tornadoes, including some strong with EF2+
potential, are expected, along with large, damaging hail and severe
downdrafts. This includes the possibility of long-track
supercells/tornadoes and swaths of locally intense wind damage.

Later tonight, storms 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 vicinity.

...Ozarks/Oklahoma to south Texas...
Isolated to scattered thunderstorms are expected to form from
mid afternoon 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. Renewed storm development,
potentially severe, could also occur in the predawn hours of
Wednesday across Oklahoma near the front.

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 mid-level 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 Texas 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 Arkansas/Missouri, 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 Ontario
and southernmost Quebec. 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 Kansas two days ago -- is apparent
in satellite imagery over Lake Huron. This feature will move
eastward toward northern New York/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
sunset.

..Guyer/Wendt.. 05/21/2024

$$