Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0719 AM CDT Sat Jul 18 2026
Valid 181300Z - 191200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PORTIONS
OF THE NORTHEAST...MID ATLANTIC AND LOWER GREAT LAKES...
...SUMMARY...
Severe thunderstorms are expected across portions of the Ohio Valley
and Lower Great Lakes into the Mid-Atlantic and Northeast this
afternoon and evening. Damaging gusts will be the primary threat,
but a few tornadoes are possible, including perhaps a strong
tornado.
...OH Valley and Great Lakes to the Mid Atlantic and Northeast...
Water-vapor imagery this morning shows a potent mid-level shortwave
trough moving southeast over the Upper Great Lakes towards the
Northeast during the period. The primary upper vorticity max will
rotate through the base of the larger-scale trough and through the
St. Lawrence Valley, before an upstream disturbance west of James
Bay this morning pivots southeast and reaches Maine/eastern Quebec
by early Sunday. Surface analysis this morning places a 1004-mb low
north of Georgian Bay and a cold front extending west-southwestward
into northern IA. An attendant warm front arching southeastward
through the Lower Great Lakes and draped over the Mid-Atlantic
states will advance northward as the cyclone develops east towards
northern Maine by late evening. Concurrently, the cold front will
push southeast across much of Great Lakes into the OH Valley.
South of the boundary, a very moist air mass was sampled this
morning by 12 UTC raobs and surface observations (i.e., upper 60s to
lower 70s F dewpoints). A few thunderstorm clusters this morning
have developed near the warm frontal zone from the Lower Great Lakes
into the Mid-Atlantic states. The early day storm activity will
likely be limited in storm vigor, due in part to initially
widespread cloud cover inhibiting strong heating in the areas
downstream over parts of PA/NY. Gradual dissipation of clouds and
heating coupled with the northeastward advancement of the warm
sector will promote a broad area to become moderately to strongly
unstable later today (1000-3000 J/kg MLCAPE) from the OH Valley
eastward into the Mid-Atlantic and parts of the Northeast.
A belt of westerly 30-45 kt 700-mb flow from northern VA into the
Northeast will aid in storm organization and severe potential this
afternoon and evening. Models differ substantially regarding
evolution/timing of possible bands of storms and clusters with
embedded cells. Regardless of specific details, it seems plausible
several multicellular bands of storms will develop during peak
heating and organize as these bands move east. Some of the more
intense cells will potentially become supercellular posing an
all-hazards severe risk. Have largely unchanged the previous
outlook given the inherent thunderstorm evolution-based
uncertainties. Potentially widespread damaging gusts [50-60 mph,
locally stronger (55-70 mph) perhaps paralleling the I-95 corridor
from Chesapeake Bay into southern New England]. A few tornadoes are
possible both from linear and cellular storm structures given some
low-level hodograph enlargement and moist low levels.
Farther west, veered low-level flow ahead of the cold front will
promote upscale growth into a few bands of cells and line segments
across the southern Great Lakes into the OH Valley. Damaging gusts
appear to be the primary hazard but a localized risk for hail and
perhaps a couple of tornadoes may accompany this activity.
...Northern Rockies...
The mid/upper-level low that has lingered over the Pacific Northwest
this week is forecast to move northeastward and weaken to an open
wave as it lifts across far northern ID/MT into southern Canada.
While only glancing over the northern Rockies, modestly enhanced
southwesterly flow aloft will be maintained across the region
through the day/evening. Monsoon moisture and strong diurnal
heating will support widely scattered storm development during the
afternoon, with MLCAPE increasing to near 1000 J/kg and effective
shear of 30+ kt. This could support a threat of isolated hail and
localized strong to severe gusts with the more robust storms.
..Smith/Broyles.. 07/18/2026
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