299
ACUS01 KWNS 171959
SWODY1
SPC AC 171957

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0257 PM CDT Thu Apr 17 2025

Valid 172000Z - 181200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM EASTERN
NEBRASKA INTO CENTRAL IOWA...

...SUMMARY...
Isolated to scattered severe thunderstorms are expected, mainly from
late afternoon into early tonight from eastern Nebraska into western
and central Iowa.  Very large hail to 3 inches in diameter or
greater, isolated severe gusts of 60-70 mph, and a couple of
tornadoes will all be possible.

...20Z Update...
Very minor changes were made to the outlook with this update. A
cluster of thunderstorms is evolving immediately ahead of the
surface low over southern MN, where low-level convergence is
maximized amid boundary-layer heating and decreasing inhibition.
Given around 40 kt of deep-layer shear (per MPX VWP), this activity
should gradually intensify through the afternoon, with a risk of
large hail and locally damaging wind gusts. Reference MCD #458 for
more information.

Farther south, the latest surface observations and radar imagery
depict gradually increasing boundary-layer convergence ahead of a
well-defined triple point over south-central/southeast NE. While
satellite imagery still suggests strong capping at the base of the
EML, expectations are still for isolated/discrete storm development
near the triple point in southeast NE later this afternoon into the
evening. The primary concern will be large to very large hail with
initially discrete supercells, with an increasing risk of damaging
winds and a couple of tornadoes into this evening. For additional
details, reference MCD #459.

..Weinman.. 04/17/2025

.PREV DISCUSSION... /ISSUED 1129 AM CDT Thu Apr 17 2025/

...NE/IA/MN/MO through early tonight...
A belt of strong southwesterly mid-upper flow will extend from the
southern High Plains to the central Great Plains and upper MS
Valley, downstream of a large-scale trough over the West.
Visible-satellite imagery late this morning shows a capped
stratocumulus shield from eastern OK northward into eastern KS and
the MO Valley.  This low-level moisture depicted on satellite
corresponds with a 10.0-11.5 g/kg lowest 100-mb mean mixing ratio
and dewpoints in the upper 50s to lower 60s.  Moisture will continue
northward on southerly flow to the east of a weak surface wave which
is expected to move along an associated baroclinic from the mid MO
Valley to the upper MS Valley, while a lee cyclone develops
south-southeastward across the central High Plains.

Strong heating and continued moisture advection will result in
moderate to strong destabilization from eastern KS to eastern
NE/western IA and southern MN.  Models continue to show initial
thunderstorm development around 20z in southern MN and around 22z in
eastern NE (near and just east of the cold front/dryline triple
point).

Model forecast soundings over southern MN will favor a mix of
supercells and storm clusters capable of producing large hail
(generally 1-1.75 inch diameter) and isolated strong-severe gusts
(50-60 mph), and perhaps a brief tornado.  Farther south, greater
buoyancy and elongated hodographs will be more favorable for
supercells.  A long-track supercell or two is forecast from eastern
NE this evening into western/central IA through early tonight.
Large to giant hail (peak diameter 2.75 to 3.5 inches) will be
possible with the more intense phase of the supercell activity.  A
narrow window of opportunity may exist for a couple of tornadoes
this evening with sufficient moisture (boundary-layer dewpoints near
60 F) as low-level shear increases.  Severe gusts will be possible
with the stronger storms before this activity wanes late this
evening into the overnight.

$$