020
ACUS01 KWNS 210544
SWODY1
SPC AC 210543

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1243 AM CDT Fri Jun 21 2024

Valid 211200Z - 221200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE NORTHERN
HIGH PLAINS TO THE UPPER MISSISSIPPI VALLEY AND OVER THE FOUR
CORNERS REGION...

...SUMMARY...
Scattered strong to severe storms are expected from parts of the
northern High Plains into the upper Mississippi Valley on Friday.
Scattered strong/severe thunderstorms are also possible in the Four
Corners vicinity. A few strong storms may also be noted into parts
of the lower Great Lakes to southern New England.

...Northern High Plains to the upper Mississippi Valley...

Latest water-vapor imagery depicts a low-amplitude short-wave trough
over eastern WA/OR. This feature is forecast to shift into the
northern Rockies later today. Surface high pressure over
MB/SK/Dakotas will ensure southeasterly low-level flow deep into MT
ahead of the short wave. 40kt 500mb flow will develop across central
MT as the ridge flattens and scattered robust convection should
easily develop within a flow regime favorable for supercells.
Forecast sounding for central MT exhibit favorable deep-layer shear
with strong buoyancy and relatively low cloud bases. Scattered
supercells should evolve by early afternoon then propagate southeast
along the instability axis toward south-central MT by early evening.
Very large hail, gusty winds, and even a few tornadoes can be
expected.

Downstream across the upper MS Valley, low-level warm advection will
focus along a frontal zone that is expected to be draped across
WI/southern MN into southeast SD. Latest model guidance suggests a
weak short-wave trough will approach eastern SD/MN border by 22/00z.
This feature should encourage some increase in the LLJ across IA
into southern MN. Strong diurnal heating south of the boundary will
ease CINH across the warm sector and scattered storms are expected
to develop. Updrafts may initially develop along the cool side of
the boundary but with time surface-based updrafts are expected.
Hail, wind, and perhaps a few tornadoes can be expected with
supercells near the front.

...Four Corners region...

Remnants of TS Alberta are progressing west across Mexico as upper
ridging builds west into the middle of the CONUS. This flow regime
is creating favorable trajectories across the southwestern US for
seasonally high PW plume to advance into the Four Corners region.
Weak low-latitude short-wave trough over southern CA will eject
across the Great Basin which will strengthen mid-level flow across
southern UT into southwestern CO. Scattered supercells should
develop along the southern fringe of this short wave where strong
shear will support organized longer-lived updrafts. Large hail is
the primary risk but some tornado risk does exist with this
activity.

...Lower Great Lakes to southern New England...

Strong boundary-layer heating will once again be noted across the
lower Great Lakes region into portions of southern New England. It
appears a zone of low-level confluence will extend across the upper
OH Valley into southern New England which should help focus
scattered convection by peak heating. Gusty winds will be the
primary concern with the most robust updrafts.

..Darrow/Wendt.. 06/21/2024

$$