Convective Outlook
— Day 1
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
000 ACUS01 KWNS 181221 SWODY1 SPC AC 181219 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 $$