ACUS01 KWNS 172003
SWODY1
SPC AC 172002
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0302 PM CDT Sun May 17 2026
Valid 172000Z - 181200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE CENTRAL PLAINS AND MID MISSOURI VALLEY...
...SUMMARY...
Severe thunderstorms are expected this afternoon into tonight across
parts of the southern/central Plains to the Upper Midwest.
Supercells with very large hail and strong tornadoes are possible
initially this afternoon, before one or more bowing clusters leads
to a greater threat for severe/damaging winds this evening.
...20z Update portions of the northern MO Valley and Midwest...
Earlier elevated convection across southern SD has gradually
strengthened into a broader convective complex ahead of the cold
front this afternoon. Current expectations are for this cluster, and
additional supercells to evolve near the synoptic warm front near
the SD/NE/IA and MN borders this afternoon/evening. Strongly backed
low-level flow will support a tornado risk with a mixed convective
mode before upscale growth into an MCS/bowing cluster this evening.
Thereafter, a significant damaging wind threat with a few embedded
tornadoes may evolve along the warm frontal corridor across northern
IA and southern MN tonight. Damaging wind probabilities have been
increased ahead of this cluster.
Farther south, a CIG2 tornado area was considered across eastern NE
and west-central IA given the presence of STP 4-7 possibly
overlapping with discrete supercells. However, numerous storms are
expected ahead of the cold front with expected interactions casting
uncertainty on the longevity of the more discrete storms.
Regardless, a volatile thermodynamic and kinematic environment (RAP
ESRH 300-400 m2/s2) could support the threat for a higher-end
tornado this evening across southeastern NE and southwestern IA
should supercells remain more discrete.
...Southern OK and North TX...
Modest ascent from a subtropical jet over portions of TX could
support isolated thunderstorm development in the Red River Vicinity
to central TX this evening. A broadly unstable environment with 70s
F surface dewpoints of sufficient bulk shear for supercells could
support some hail or damaging wind risk with any convection able to
develop. Confidence in this scenario remains very low.
...Central High Plains...
Air mass destabilization is ongoing in the post-frontal upslope flow
regime across the central High Plains. Supercells remain likely with
large hail, damaging gusts and a tornado or two. Eventually one or
more clusters may evolve and spread eastward with an increased risk
for damaging gusts tonight. Higher probabilities were shifted
southward slightly to better align with the observed cold frontal
position.
Else where the outlook remains unchanged, see the previous
discussion.
..Lyons.. 05/17/2026
.PREV DISCUSSION... /ISSUED 1129 AM CDT Sun May 17 2026/
...Synopsis...
12Z soundings across the central and southern Plains reveal steep
mid-level lapse rates (8.8 deg C per km at OUN, SGF, and TOP) atop
returning low-level moisture (100 mb mean mixing ratios of 15.4 and
13.1 g/kg at OUN and FWD, respectively). An MCV, from overnight
storms across the Plains, is progressing into northern IL/southern
MN. 12Z DVN sounding sampled a corridor of 50 kt at the base of this
MCV.
Morning surface analysis reveals an intricate surface pattern across
the High Plains/Plains, with multiple lows within the lee troughing
that extends from eastern MT through the central High Plains, a pair
of warm fronts, and a residual outflow boundary across NE. The
deepest surface low is over the far southern CO/KS border vicinity,
with a warm front extending east-northeastward, roughly along the 70
deg F isotherm. A dryline also extends southward from this low into
far west TX. Another low is in place farther north over western NE.
Another warm front, this one along a sharp gradient in low-level
moisture, extends northeastward from this low into far southeast SD
before arcing more eastward across far southern MN and continuing
across from southern WI into central Lower MI. The aforementioned
outflow also extends eastward from this low across central NE.
All of these boundaries will likely influence the severe potential
today, combining with an emerging upper trough and moderate to
strong instability, to support an expansive and complex scenario
today.
...Central Plains into the Upper Midwest...
Expectation is for the northern warm front mentioned in the synopsis
to continue northward throughout the day while as cold front sweeps southeastward across SD and NE. Thunderstorm development is
anticipated as the cold front interacts with the returning low-level
moisture. Moderate to strong buoyancy (i.e. MLCAPE from 2000 to 3000
J/kg) will support robust thunderstorms from southeast SD through
south-central NE, potentially as far south as central KS. This
activity along the front will become linear quickly with damaging
gusts as the primary risk. Some cell-in-line structures could
support large to very large hail, particularity early in the
convective cycle.
Within this larger area of severe wind and hail potential, two areas
appear to have a greater tornado threat. One is from
northeast/east-central NE into far southeast SD. Here, the influence
of a residual outflow boundary could support more discrete
development ahead of the front. Any discrete development in the warm
sector would likely be supercellular and capable of all hazards,
including very large hail (over 2.5") and a strong tornado. The
other area is where the convective line interacts with the warm
front amid a strengthening low-level jet, likely far northern IA/far
southern MN. Here, increased low-level shear coupled with
augmentation from interaction with the front could result in
significant severe gusts as well as lined-embedded QLCS tornadoes.
...Central High Plains...
Thunderstorm development still appears likely across northeastern CO
this afternoon in a low-level upslope flow regime. This activity
will likely become supercellular and capable of producing large to
very large hail given the presence of steep mid-level lapse rates,
strong deep-layer shear, and moderate instability. Just how far
east/northeast (into central NE) thunderstorms remains strong is a
question of timing. Earlier storms could interact with the front as
it moves through NE, potential leading to persistence or
reintensification. However, later timing would take these storms
into central NE behind the cold front. Steep mid-level lapse rates
in place could still support these storms for a few hours, but the
overall intensity will likely gradually decrease.
...Southern WI into Lower MI...
An MCV is current progressing into northern IL/southern MN. Modest
buoyancy and moderate deep-layer shear will support isolated
elevated thunderstorms capable of small hail and locally strong wind
gusts through early afternoon. This MCV is expected to continue
northeastward into Lower MI, potentially interacting with a warm
front moving north across the region this afternoon. 12Z DVN
sounding sampled enhanced mid-level flow with this MCV, which could
result in enough shear to support organized convection along and
near the warm front later this afternoon/evening. Strong multicells
and an isolated supercell or two could occur, with an attendant risk
for damaging winds, hail, and perhaps a tornado.
...South-Central KS into the eastern TX Panhandle and western OK...
Some guidance continues to show at least isolated high-based
convection developing by late afternoon across parts of
north-central KS southward into the eastern TX Panhandle/western OK.
Any thunderstorms which can be sustained would likely be
supercellular and capable of all severe hazards, including large to
very large hail and tornadoes. Increasing low-level shear after 00Z
supports a conditional risk for a strong tornado.
-- No changes to these areas from the earlier 13Z Outlook --
...Southeast...
On the western periphery of a surface high centered over the western
Atlantic, diurnally maximized thunderstorm development should focus
from the southern Appalachians across GA and the interior/western FL
Peninsula. Even though deep-layer shear will remain weak, a
favorable thermodynamic environment with steepened low-level lapse
rates should support strong to pulse-severe type convection capable
of producing isolated wind damage/downbursts and perhaps marginally
severe hail.
...Central and North Texas/ArkLaTex/Lower Mississippi Valley...
A fairly robust cap is expected to remain across the southern extent
of the warm sector today, with minimal/nebulous large-scale ascent
over the southern Plains. Still, some guidance suggests elevated
thunderstorms may develop through peak afternoon heating across
parts of northeast TX into the ArkLaTex/lower MS Valley in
persistent low-level warm advection regime. If this occurs, then
isolated severe hail and damaging winds would be possible given the
strong MUCAPE forecast.
...Mid-Atlantic...
Mid-level ridging should build over the Mid-Atlantic through the
afternoon, which should tend to suppress most convection. However, a
weak mid-level perturbation noted in water vapor imagery this
morning over OH may support isolated convective development this
afternoon across parts of MD/PA and vicinity. The overall
environment does not appear overly favorable for organized severe thunderstorms, but gusty winds could occur with any of the stronger
cores that may develop.
$$
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