Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0258 PM CDT Sat Jun 21 2025
Valid 212000Z - 221200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
NORTHERN NEW YORK...VERMONT AND NEW HAMPSHIRE...
...SUMMARY...
Thunderstorms associated with severe wind gusts and hail will be
possible tonight across New York and New England. Isolated severe
storms will also be possible across portions of the Great Lakes, the
northern High Plains and Middle Missouri Valley.
...20z Update...
Minimal changes were made to the prior valid outlook. Thus far,
strong convection across the Great Lakes region has remained on the
Canadian side of the border. Continued heating and weak mid-level
ascent overspreading a trailing composite front/lake breeze boundary
may eventually support scattered storm development over the UP of
Michigan this afternoon/evening. Isolated damaging gusts and some
hail are possible with moderate CAPE and vertical shear. These
storms should grow upscale across the lakes and through southern
Ontario, reaching western New England late tonight as an MCS. This
MCS may pose a risk for damaging gusts late tonight. Have adjusted
the eastern edge of the Slight and Marginal Risk areas for the
latest hi-res guidance.
Otherwise the outlook remains unchanged, see the previous
discussion.
..Lyons.. 06/21/2025
.PREV DISCUSSION... /ISSUED 1149 AM CDT Sat Jun 21 2025/
...Great Lakes...
Strong/intermittently severe thunderstorms continue across the
northern Lower Peninsula of Michigan this morning, associated with
the long-lived severe MCS that moved across the northern
Plains/upper Midwest Friday afternoon and night. Although the
immediate downstream environment is only slowly warming due to the
presence of mid/high-level clouds, some slow destabilization is
occurring. This will maintain at least some continued risk for a
strong/severe storm through early afternoon across northern Lower MI
and adjacent waters of Lake Huron.
Across the MI Upper Peninsula, a warm front is expected to sharpen
across this area, along with rising heights in the wake of the
convectively-augmented vorticity maximum now moving through southern
Ontario. Heating of a very moist boundary layer will result in
strong to extreme instability (MLCAPE in excess of 3000 J/kg),
however the presence of a pronounced cap should suppress storms
during the afternoon. As a modest low-level jet strengthens this
evening, isolated elevated severe storms will be possible on the
immediate cool side of the front, within an environment supportive
of organized storms including supercells. Large hail and damaging
winds will be possible.
...NY into Western New England...
Thunderstorms moving across Lake Huron/southwestern Ontario at 16z
will continue east, and most morning CAM guidance suggests storms
will remain west of western NY. Of greater concern will be the
potential for storms to move into northern NY/western New England
late tonight ahead of a southeast-moving cold front, where modest
destabilization and strong deep-layer shear will support storm
organization. Morning hi-res guidance supports a loosely organized
linear structure, and perhaps supercells with an attendant risk for
strong winds and severe hail late tonight. The Level 2/Slight Risk
has been expanded east into northern VT based on this expectation.
...Eastern MT into Western ND...
Isolated thunderstorms have weakened over eastern MT this morning,
with additional isolated redevelopment possible this afternoon and
tonight as ascent associated with minor perturbations within
southwest mid-level flow moves across the area. Modest instability
and strong deep-layer shear will support updraft organization and a
risk for severe hail and/or damaging gusts.
...Central NE/SD/ND into the Upper Midwest...
Surface low currently over central SD is expected to move little
this afternoon, with low-level moisture advection throughout the
warm sector east of this low. Air mass recovery/modification
following the overnight MCS is expected, and this will contribute to
a sharpening warm-frontal zone. Strong to extreme buoyancy (i.e.
MLCAPE greater from 3000 to 5000 J/kg) is expected by this evening
along this warm front from the ND/SD border vicinity eastward
through central MN into northern WI. Some low-level convergence is
anticipated in this region, but height rises will occur within the
large-scale environment. Additionally, warm mid-level temperatures
(16 to 17 deg C at 700 mb) are expected, capping the airmass to deep
convection this afternoon. Isolated thunderstorm development will be
possible mainly this evening as a low-level jet strengthens, with a
risk for severe wind and isolated hail.
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