ACUS01 KWNS 282002
SWODY1
SPC AC 282000

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0300 PM CDT Mon Jul 28 2025

Valid 282000Z - 291200Z

...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS THIS AFTERNOON
INTO TONIGHT FROM EASTERN SOUTH DAKOTA ACROSS SOUTHERN MINNESOTA AND
NORTHERN IOWA...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS OF
CENTRAL MONTANA...

...SUMMARY...
Severe storm clusters are expected to form this afternoon in South
Dakota and then spread southeastward into Minnesota and Iowa through
tonight.  Swaths of damaging winds, potentially 75-90 mph, a few
tornadoes and isolated large hail will all be possible.

...20Z Update...
The primary change with this update was removing the Slight risk
area across parts of the northern High Plains -- where the severe
risk has decreased behind an earlier cluster of storms. Elsewhere,
the previous forecast thinking remains the same. Thunderstorms are
evolving across the higher terrain in western MT, and will track
eastward into an increasingly moist/unstable air mass this
afternoon. Reference Severe Thunderstorm Watch 551 and MCD 1823 for details.

Farther east, thunderstorms are developing along a baroclinic zone
extending across SD. These storms will pose a risk of severe
downbursts as they develop into a hot/deeply mixed air mass in
south-central SD and north-central NE this afternoon (see MCD 1824).
This activity, and potentially new development along the baroclinic
zone, should eventually spread eastward into eastern SD -- where
intensification into one or more organized/intense clusters is still
expected into tonight.

..Weinman.. 07/28/2025

.PREV DISCUSSION... /ISSUED 1130 AM CDT Mon Jul 28 2025/

...Dakotas to MN/IA through tonight...
Around the northern periphery of a midlevel high covering the
southern Plains and Southeast, a series of embedded speed maxima
will progress from MT across the Dakotas to MN through early
Tuesday.  Elevated convection is ongoing in association with two
such speed maxima - one over ND and another moving eastward over
eastern MT.  At the surface, a lee cyclone is present near the SD/NE
border, and a baroclinic zone arcs into northeast SD and then
southeastward across southwest MN into northern IA.  A prior outflow
boundary (from dissipated overnight convection) will become more
diffuse with time as a result of strong surface heating across
IA/NE/SD within the old cold pool.

The elevated convection across southern ND, and moving from MT into
southwest ND, could persist in the post-frontal environment with
weak warm advection atop the frontal surface.  Steep midlevel lapse
rates and 40-50 kt midlevel flow suggests a continued threat for
occasional severe outflow gusts and isolated large hail.  The more
substantial severe-storm threat is expected to evolve farther
southeast by mid-late afternoon along the baroclinic zone in SD, to
the northeast of the lee cyclone.  Strong surface heating, continued
moisture advection and evapotranspiration will lead to the
development of strong-extreme buoyancy (3000-4500+ J/kg MLCAPE)
beneath an elevated mixed layer with 8.5-9 C/km midlevel lapse
rates.  Storms that form in SD are then expected to congeal into a
cluster or two that will subsequently spread east-southeastward
along the more northwest-southeast oriented baroclinic zone now
noted in visible imagery and surface observations from southwest MN
into northern IA.  The thermodynamic environment will favor both
intense updrafts and downdrafts, while deep-layer vertical shear
will be sufficient for an organized/bowing MCS.  Thus, there will be
the potential for swaths of significant-severe outflow (75-90 mph),
as well as isolated large hail and a few tornadoes (either with
initially discrete supercells or with embedded circulations with
bowing segments).

...Central MT this afternoon through early tonight...
An embedded speed max will crest the ridge and move from ID to
southwest MT this afternoon/evening.  Residual boundary-layer
dewpoints in the mid-upper 50s in the post-frontal environment,
along with surface heating and the north edge of the steeper
midlevel lapse rates will favor scattered thunderstorm development
by early-mid afternoon (near and just northeast of the higher
terrain southwest into central MT).  MLCAPE of 1500-2000 J/kg,
effective bulk shear in excess of 40 kt and seasonably cool midlevel
temperatures suggest the potential for initial supercells with large
hail (1-2.5 inches in diameter), and also the potential for upscale
growth into a cluster with the attendant threat for wind damage.

$$
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