Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1130 AM CDT Thu Aug 21 2025
Valid 211630Z - 221200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
EASTERN SOUTH DAKOTA INTO NORTHWEST MINNESOTA...
...SUMMARY...
Scattered severe thunderstorms are possible across portions of the
northern Plains and Upper Midwest this afternoon into tonight, with
severe wind gusts and large hail the primary concerns. A few strong
to severe storms may also occur across parts of southern Arizona.
...Northern Plains into the Upper Midwest...
Enhanced mid/upper-level westerly flow will persist today across the
northern Plains/Upper Midwest, sandwiched between upper troughing
over the Canadian Prairie provinces and a stout mid/upper-level
anticyclone centered over the Four Corners/Southwest. Multiple
low-amplitude perturbations, some associated with ongoing convection
across eastern MT into ND and northern MN, will move eastward today
and potentially aid in additional thunderstorm development this
afternoon and evening. At the surface, the primary low associated
with the upper trough will remain well north in Canada, with a
trailing cold front expected to continue moving east-southeastward
across the northern Plains and Upper Midwest through the period.
There is still considerable uncertainty regarding potential robust
convective development and subsequent evolution along/near the cold
front later today. This is mainly due to the stronger large-scale
ascent aloft forecast to remain in central Canada, and capping
concerns with southward extent along the front in the northern
Plains due to fairly warm low/mid-level temperatures. Multiple
potential scenarios remain plausible, ranging from little to no
convection developing this afternoon/evening, as some recent
convection-allowing model guidance suggests, to multiple
supercells/clusters forming in a narrow zone of favorable
instability and shear.
Given these considerations, confidence is lower than usual regarding
severe potential across the northern Plains/Upper Midwest. Still, a
narrow corridor of moderate to strong instability will likely exist
late this afternoon and into the evening along/near the front from
northwest MN into eastern SD, aided by steep mid-level lapse rates
and diurnal heating. Modest low-level flow veering to westerly at
mid/upper levels will foster sufficient deep-layer shear (around
30-40 kt) to support organized convection, including the potential
for a few supercells and clusters. Scattered large hail and
severe/damaging winds may occur with this activity if it develops
and can be sustained. So, a Slight Risk has been included with this
update to account for this potential.
Farther west, a post-frontal regime will exist this
afternoon/evening across the northern High Plains of northeast WY
and vicinity. Low-level upslope flow against the higher terrain may
promote a supercell or two to develop and spread east-southeastward
through the late afternoon and evening, with mainly an isolated hail
threat. Some potential for clustering is also apparent this
evening/overnight across parts of SD along/near the surface front.
If this occurs, then a continued threat for strong to locally severe
gusts may persist. Have therefore connected the Marginal Risks
across SD for this possibility.
...Arizona...
Modest mid/upper-level easterly flow will persist today over
southern AZ, to the south of an anticyclone aloft centered over the
Four Corners. Convection that initially develops over the higher
terrain of southeast/east-central AZ may gradually develop westward
into lower elevations through the late afternoon and continuing into
the evening. Ample daytime heating of a seasonably moist low-level
airmass already in place (reference observed 12Z PHX sounding) will
act to further steepen low/mid-level lapse rates. At least isolated
strong to severe gusts may occur with these thunderstorms given the
favorable thermodynamic/lapse rate environment. The Marginal Risk
has been expanded some to account for latest short-term guidance
trends.
..Gleason/Supinie.. 08/21/2025
Read more CHECK UPDATE ZOOM GRAPHIC
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