846
ACUS01 KWNS 181630
SWODY1
SPC AC 181628

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1128 AM CDT Tue Jun 18 2024

Valid 181630Z - 191200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF THE
UPPER MISSISSIPPI VALLEY INTO THE MID MISSOURI VALLEY/CENTRAL PLAINS
AND SOUTHERN HIGH PLAINS...

...SUMMARY...
Scattered strong to severe storms are expected this afternoon and
evening over parts of the Upper Mississippi Valley southwestward
into the central/southern High Plains. Large hail and
severe/damaging winds should be the main threats, but a few
tornadoes may also occur.

...Upper Mississippi Valley into the Mid Missouri Valley/Central
Plains and Southern High Plains...
A pronounced upper trough will advance eastward across the northern
Plains and Canadian Prairie provinces today, with multiple embedded
mid-level perturbations forecast to move northeastward across the
northern/central Plains and upper MS Valley through the period. At
the surface, a 997 mb low currently over eastern SD will develop
northeastward towards the MN Arrowhead through this evening. A
secondary surface low should remain located over the
southern/central High Plains. Between these two lows, a surface cold
front will continue to progress east-southeastward this afternoon
and evening. This boundary will be the primary focus for severe
thunderstorms today. Based on latest guidance, the broad Slight Risk
extending from the upper MS Valley into the mid MO Valley/central
Plains and southern High Plains has generally been expanded to
account for severe/damaging wind potential continuing into at least
mid evening.

Current expectations are for robust thunderstorms to develop by
early afternoon across the upper MS Valley and mid MO Valley
along/near the cold front. With a rather moist airmass in place
across these regions based on latest surface observations and area
12Z soundings, moderate instability is expected to develop this
afternoon, even though cloud cover may temper diurnal heating to
some extent, especially in MN. The stronger mid-level southwesterly
flow should tend to remain displaced to the cool side of the surface
front. Still, sufficient deep-layer shear should exist to support a
mix of multicells and supercells with this initial development.
Given the linear forcing of the front, upscale growth into multiple
bowing clusters is anticipated later this afternoon and continuing
into the evening. Both large hail and severe/damaging winds appear
possible, with a transition to a mainly damaging wind threat as
clusters become dominant. Enough low-level shear may also support
some threat for a few tornadoes, especially in MN with any supercell
that can be maintained.

Additional intense convective development is expected with southward
extent into the central Plains along the front, perhaps delayed
until a little later in the afternoon as stronger daytime heating
will be needed to erode a cap/lingering MLCIN. Strong instability
and deep-layer shear should be present over parts of western/central
KS, where multiple supercells may develop near/east of the secondary
surface low. Isolated very large hail may occur with this activity
given the favorable environment and presence of steep mid-level
lapse rates. A few tornadoes may also occur in a narrow corridor
across southwest KS this evening if supercells can be maintained as
low-level shear increases. Otherwise, severe/damaging winds (with
perhaps some reaching 75-80 mph on an isolated basis) appear
probable as convection likely grows upscale into a bowing cluster
across central KS late this afternoon/early evening. Scattered
thunderstorms may also develop across the southern High Plains
(OK/TX Panhandle regions) along/east of a dryline. Severe wind gusts
should be the main threat if this activity occurs, as the boundary
layer is forecast to become very well mixed, with steep low-level
lapse rates present.

...Lower Great Lakes/Upper Ohio Valley into the Northeast...
Recent water vapor satellite and radar imagery show multiple widely
dispersed thunderstorm clusters and remnant MCVs present across the
lower Great Lakes into the Northeast. This activity is occurring on
the western/northern periphery of a prominent upper ridge centered
over the Mid-Atlantic States and Carolinas. Surface heating will aid
ample destabilization this afternoon across a broad portion of the
upper OH River Valley into upstate New York and northern New
England. Isolated to widely scattered thunderstorms will once again
develop and increase into the afternoon, influenced in some areas by
the aforementioned MCVs. Localized downbursts with strong/damaging
wind gusts will be possible through the diurnal heating cycle. Have
maintained a broad Marginal Risk to account for this potential, as
confidence in a more focused corridor of damaging wind potential
remains low.

...Lower/Middle Texas Coast...
Low-level easterly flow will gradually increase late tonight into
early Wednesday morning as Potential Tropical Cyclone 1 develops and
moves westward over the southwest Gulf, per latest NHC forecast.
While instability should remain weak, with poor lapse rates aloft,
strengthening low-level shear may support a low chance for a tornado
with any low-topped cells that can approach/reach parts of the
lower/middle TX Coast near the end of the Day 1 period. This threat
should continue into the Day 2/Wednesday time frame.

..Gleason/Halbert/Lyons.. 06/18/2024

$$