Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1258 AM CDT Fri Jul 17 2026
Valid 171200Z - 181200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
THE NORTHERN ROCKIES...GREAT LAKES AND OHIO VALLEY TO THE SOUTHERN
APPALACHIANS....
...SUMMARY...
Strong to potentially severe storms are possible today across parts
of western Montana and northern Idaho, and also across parts of the
Upper Midwest and northern Great Lakes. More isolated strong to
severe storms with damaging gusts are possible over parts of the
Ohio Valley and southern Appalachians.
...Northern Rockies and Adjacent Plains...
An upper low over the Pacific Northwest is forecast to lift slowly
northward as an expansive upper ridge continues to build across the
western US. East of the low, a broad area of monsoon moisture will
persist over much of the Rocky Mountains. Weak ascent on the
periphery of the upper low and diurnal heating across the higher
terrain should allow for scattered thunderstorm development through
the afternoon. While displaced from the core of the stronger flow
aloft as the upper low drifts to the north, sufficient effective
shear of 25-35 kt will exist for occasional storm organization.
Isolated hail and severe gusts could accompany the strongest storms
into the evening across ID and western MT.
...Upper Midwest and northern Great Lakes...
An upper trough initially over AB/SK will move southward into the
northern Great Lakes by early Saturday. A belt of stronger mid-level
flow accompanying the upper trough will aid in deepening a surface
low as it moves into western ON. A warm front will lift north with
the deepening low over the Upper Midwest. A cold front trailing the
surface low will provide a focus for thunderstorm development over
northern MN, WI and the UP of MI this afternoon and evening.
A cluster of elevated thunderstorms is likely to be ongoing early in
the period near or just across the international border. These
storms could reinvigorate with a risk for some damaging gusts and
hail, though confidence in the convective evolution is low. Later in
the day ahead of the cold front, moderate buoyancy and sufficient
deep-layer shear will conditionally support a threat for isolated
supercells or organized multicell clusters, with a threat of hail
and damaging winds. Any persistent supercell may also pose at least
a brief tornado threat, especially in closer proximity to the
surface low and warm front where low-level shear will be maximized.
As the main upper trough continues to southeast one or more clusters
of strong to severe storms may persist into the overnight along the
warm front. However, confidence in this scenario is lower owing to
the prior convection and uncertainties of diurnal development along
the cold front. Still, a strengthening wind field and ample moisture
could support a risk for damaging winds into northern Lower MI
overnight.
...OH Valley to the southern Appalachians...
A warm and seasonably moist air mass will support moderate to
locally strong buoyancy beneath the upper ridge from the OH valley
toward the Southeast and southern Appalachians. A weak, convectively
enhanced sub-tropical disturbance moving northward and the higher
terrain should focus more widespread diurnal thunderstorm activity
from portions of IN/OH, into eastern KY/TN, the western Carolinas
and northern GA. Despite weak deep-layer shear below 20 kt, moderate
to strong MLCAPE (2000-3000 J/kg) and PWATS exceeding 2 inches may
allow for sporadic damaging gusts with any stronger multicell
clusters.
...FL...
CAM guidance shows vigorous storm development just offshore of the
FL Gulf Coast Friday morning, in association with a weak midlevel
trough/low. Additional storm development is expected by afternoon
across much of the peninsula. Large buoyancy and high PWATs suggest
localized strong to severe gusts cannot be ruled out near the coast
and inland this afternoon.
..Lyons/Weinman.. 07/17/2026
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