423
ACUS01 KWNS 281248
SWODY1
SPC AC 281247

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0747 AM CDT Fri Jun 28 2024

Valid 281300Z - 291200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF THE
CENTRAL PLAINS INTO PARTS OF THE MIDDLE AND UPPER MISSISSIPPI
VALLEY...

...SUMMARY...
Large hail, damaging wind and a few tornadoes will be possible from
parts of the central Plains into parts of the Middle and Upper
Mississippi Valley today and tonight.

...Synopsis...
The main mid/upper-level perturbation influencing convective
potential this period is a strong shortwave trough accompanying a
small cyclone now centered over southwestern SK.  The cyclone is
expected to open up today, with a 00Z trough position from
southwestern MB southwestward across northern/western ND to
northeastern WY.  The trough should accelerate eastward across the
remainder of the Dakotas and MN overnight, reaching WI and Lake
Superior by 12Z tomorrow.  This will occur as a closely following
shortwave digs southeastward across southern MB toward northwestern
MN.  The subtropical ridge will build across the Southeast, southern
Plains and into parts of the Desert Southwest.  Despite rising
absolute height values, this will yield a tightening height gradient
across the central Great Plains between the ridge and the
northern-stream trough.

The 11Z surface analysis showed a low over eastern ND, with cold
front extending across eastern SD, central NE, northwestern KS, and
east-central CO.  A surface trough was drawn from a cold-frontal
intersection over central NE south-southwestward to the southern
High Plains.  The low should cross the Boundary Waters region of the
MN/ON border this afternoon through 00Z, when the cold front should
extend to southern MN, southeastern NE, through a weak frontal-wave
low over central/north-central KS, to southwestern KS, becoming a
diffuse warm front over eastern CO.  By 12Z, the low should reach
that portion of ON northeast of Lake Superior, with cold front to
eastern Upper MI, Lake Michigan, southeastern WI, then near a line
from DVN-MKC-ICT-DHT.

...Central Plains to Mississippi Valley...
This severe-risk area is a composite of two separate potential
convective episodes, with the western (second) potentially lasting
long enough and far enough east to overlap into the hours-earlier
starting region of the first.

1.  Lower Missouri Valley, eastern NE/northern KS eastward:
An extensive area of clouds and precip, with isolated to widely
scattered, embedded, non-severe thunderstorms, was apparent in radar
composites and satellite imagery from astern MN and WI across parts
of IA, northern/western MO and eastern KS.  Although this will delay
substantial diurnal heating across much of the mid/upper Mississippi
and lower Missouri Valley regions, sufficient destabilization (from
diabatic heating and warm advection) is expected by midafternoon to
support surface-based development near the cold front.  Favorable
low-level moisture (dewpoints in the upper 60s to low 70s F) will
support a prefrontal plume of MLCAPE in the 2000-3500 J/kg range.

Modest low-level flow will limit ambient hodograph size, but
sufficient deep shear will exist (effective-shear magnitudes 35-45
kt) to support both supercells and organized multicells.  Backing of
winds along and just north of the front also will enlarge hodographs
in a narrow corridor, with vorticity from the boundary potentially
ingested into favorably positioned updrafts for locally boosted
tornado potential.  Development should be most likely and dense near
and northeast of the frontal-wave low, south of which low-level lift
will weaken amid rising heights aloft, and buoyancy will decrease
toward the OK border.  As such, probabilities for this lobe of the
outlook area have been tightened northward somewhat.  Still, upscale
coalescence into one or more clusters/lines should occur, with
severe wind the greatest threat this evening into tonight, and a
tornado or two possible.

2.  Central High Plains and eastward:
Widely scattered to scattered thunderstorms are expected this
afternoon over higher terrain of western/central CO, with additional
initiation possible on the foothills, Cheyenne Ridge and Palmer
Divide.  This convection will be supported by sustained diurnal
heating and related weakening of MLCINH, along with moist advection
and upslope lift by the easterly component of post-frontal flow.
That moist advection also should offset moisture loss from vertical
mixing enough to maintain buoyancy, with MLCAPE in the 250-800 J/kg
range.  A deep subcloud mixed layer with steep lapse rates will
encourage strong-severe downdrafts.

Though weak, low-level flow will veer with height, leading to long
hodographs and contributing to effective-shear magnitudes ranging
from around 30 kt in the southwestern part of the outlook (where
coverage also is more uncertain but conditional severe potential
still apparent) to 45-50 kt over and downshear from the Cheyenne
Ridge.  As such, a high-based mix of multicells and supercells is
possible, offering mainly damaging gusts and large hail (each with
some significant severe possible).  One or more clusters may evolve
and move eastward to southeastward over KS this evening and
overnight, with severe gusts the main concern.

..Edwards/Kerr.. 06/28/2024

$$