SPC Jun 21, 2025 0600 UTC Day 1 Convective Outlook

SPC 1200Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1228 AM CDT Sat Jun 21 2025

Valid 211200Z - 221200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE GREAT LAKES REGION...

...SUMMARY...
Thunderstorms associated with marginally severe wind gusts and hail
will be possible today from the northern Plains into the Upper
Mississippi Valley and Great Lakes.

...Great Lakes...
A mid-level ridge will move quickly eastward into the Northeast
today, as trailing shortwave trough moves through the Great Lakes.
An MCS with some severe potential will likely be ongoing at the
start of the period in the vicinity of Lake Superior.
Warm-advection-related convection is expected to develop along the
southern edge of the decaying MCS from the Upper Peninsula of
Michigan eastward into northern sections of the Lower Peninsula of
Michigan. As low-level lapse rates steepen during the morning,
isolated damaging wind gusts will be possible with short line
segments. During the afternoon, the brunt of the convection is
forecast to move across southwestern Ontario. The southern edge of
this cluster may move back into the lower 48, affecting parts of New
York during the late afternoon. In central New York, moderate
instability is forecast along with northwest mid-level flow around
50 knots. This should be enough for an isolated wind-damage threat
with the faster-moving short line segments. Hail will also be
possible with the stronger cores.

Further south into parts of northern Pennsylvania, moderate
instability is forecast to develop by afternoon. Due to a lack of
large-scale ascent, convective coverage is expected to be very
isolated. Any cell that can initiate in the higher terrain could
have potential for marginally severe wind gusts.

...Northern Plains/Upper Mississippi Valley...
Anticyclonic southwesterly mid-level flow will be in place today
across much of the north-central U.S. At the surface, a post-frontal
airmass will be in place over much of the northern Plains.  Behind
the front, a moist airmass will remain over parts of the Dakotas,
where surface heating should yield moderate instability by
afternoon. Although large-scale ascent will remain weak across most
of the region, pockets of low-level convergence are forecast to
develop within this airmass. This should be enough for isolated
convective initiation during the mid to late afternoon. Marginally
severe wind gusts and hail will be the primary threats.

Further west into the northern High Plains, scattered thunderstorms
are expected to develop during the overnight period, ahead of a
mid-level trough in the northwestern U.S. Although a relatively dry
airmass will be in place, low to mid-level lapse rates will be
steep, and could be enough for a marginal wind-damage threat.

..Broyles/Moore.. 06/21/2025

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