ACUS01 KWNS 130536
SWODY1
SPC AC 130534

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1234 AM CDT Mon Apr 13 2026

Valid 131200Z - 141200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE UPPER MISSISSIPPI RIVER VALLEY...

...SUMMARY...
Severe thunderstorms are likely across the Upper Mississippi Valley
and Great Lakes region with potential for large to very large hail,
damaging wind, and tornadoes. A more conditional and isolated risk
for severe storms is possible across portions of the southern Plains
late this afternoon and evening.

...Synopsis...
Early-morning water-vapor imagery depicts an upper trough with
multiple embedded vorticity maxima over the Great Basin. While the
primary upper wave will remain over the West for today, a leading
impulse is forecast to eject towards the northern Plains and into
the Great Lakes through tonight. At the surface, a weak lee cyclone
over the central High Plains will migrate northeastward in tandem
with the upper impulse, likely reaching the southern MN vicinity by
early evening. As this occurs, a plume of mid-60s dewpoints will
spread north into the upper MS Valley along and south of a warm
front. Scattered thunderstorm development along and north of the
warm front is anticipated by late afternoon as forcing for ascent
gradually increases. Elsewhere, more isolated strong to severe
thunderstorms are possible across portions of TX and OK where deep
mixing along a dryline may result in sustained deep convection. 

...Upper MS Valley...
A combination of shortwave ridging and capping at the base of an EML
will limit thunderstorm development through much of the day across
southern MN into central WI. This will allow for continued moisture
return as a warm front lifts into the region with MLCAPE values
forecast to reach 2000-2500 J/kg by around 21 UTC. Thunderstorm
development will become increasingly probable through the 21-00 UTC
period as isentropic ascent strengthens ahead of the approaching
upper disturbance. Lingering inhibition coupled with 45-50 knot
effective bulk shear should promote initially discrete supercells
along or just north of the surface boundary. Hodograph elongation
appears favorable for a large hail threat, possibly as large as 2-3
inches with the more robust cells. Additionally, enhanced low-level
SRH along the frontal zone may result in a corridor of relatively
higher tornado potential if convection can remain rooted along the
boundary. 

While the overall environment appears very favorable for intense
convection, east/northeasterly storm motions along or just north of
the warm front will likely promote storm advection towards the cool
side of the boundary and clustering/upscale growth through the
evening. This, combined with capping over the remainder of the warm
sector, limits confidence in a more widespread tornado threat, and
could modulate the potential for significant (2+ inch) hail within a
few hours after convective initiation. Nonetheless, a corridor of
higher hail potential remains evident in latest guidance along and
just north of the warm front where discrete supercells are most
probable. 30% hail probabilities have been shifted to reflect a
southward trend in frontal placement noted in most recent guidance.

...Southern Plains...
A moist and very unstable air mass will remain in place across TX
and OK today with MLCAPE values expected to increase to around 2500
J/kg by late afternoon. Ample diurnal heating/mixing on the western
periphery of the moisture plume will likely result in weak,
high-based convection by mid to late afternoon. Easterly storm
motions may promote downstream intensification as convection
migrates towards richer low-level moisture, but this will be
conditional on convection remaining sustained long enough to realize
the favorable thermodynamic environment. If this can occur,
deep-layer shear values on the order of 40 knots should promote
organized convection - likely in the form of supercells - with an
attendant threat for all hazards, including very large hail. While
both global and CAM ensemble guidance hint at this potential, the
lack of appreciable lifting mechanisms (aside from the dryline
circulation/boundary-layer mixing) casts considerable uncertainty on
overall storm coverage.

..Moore/Wendt.. 04/13/2026

$$