Day 1 Convective Outlook
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Summary
Severe thunderstorms with the potential for damaging winds, hail, and tornadoes are expected across portions of the Ohio Valley, Great Lakes, Mid-Atlantic, and Northeast from this afternoon through tonight. Scattered severe storms may also occur across the Northern Rockies and northern Gulf Coast during the day.
Product Text
000 ACUS01 KWNS 180559 SWODY1 SPC AC 180557 Day 1 Convective Outlook NWS Storm Prediction Center Norman OK 1257 AM CDT Sat Jul 18 2026 Valid 181200Z - 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 into the evening. Some threat may linger into late tonight. ...OH valley and Great Lakes to the Mid Atlantic and Northeast... An unseasonably strong upper trough is forecast to move from southern ON into the northeast US through early Sunday. As the trough intensifies, a 60+ kt mid-level jet will overspread portions of eastern Canada and the Northeast, aiding in deepening a surface low across the eastern St Lawrence Valley. Strong surface mass response will lift a warm front northward across eastern NY and southern New England. South of the boundary, a very moist air mass will be in place ahead of a cold front over the OH Valley and Great Lakes. A rather broad area, is expected to be conducive for numerous strong to severe storms with the potential for widespread damaging winds over the Northeastern US today and tonight. One or more clusters of showers/thunderstorms appears likely to be ongoing at the start of the period from central PA and southern NY into eastern ON as isentropic ascent increases along the advancing warm front. The impact of this overnight/early morning convection is somewhat uncertain, but most guidance shows substantial air mass recovery in its wake by midday with dewpoints in the mid 70s across much of the Mid Atlantic. Given the degree of moisture and minimal inhibition present, even modest diurnal heating should support rapid destabilization with MLCAPE of 2000-3000 J/kg common by early afternoon. Several different forcing regimes are expected to be active which makes the convective evolution highly uncertain. The early morning storms near the advancing warm front in PA could re-intensify as the air mass to their south and east gradually destabilizes. This would favor an increasing risk for damaging gusts. 35-45 kt of deep-layer shear could also support some supercell structures. A couple of tornadoes (some possibly strong) may occur, given favorable backed low-level flow and ESRH of 200-300 m2/s2 near the front from southeastern PA, into NJ and southern NY. Additional storms are likely to develop along a pre-frontal confluence axis/lee trough east of the higher terrain from southern PA into eastern MD and northern VA to northern NC. Large buoyancy and deep-layer shear of 30-40 kt would favor a mixed mode of line segments and supercells. These storms should move eastward toward DelMarVa and the I-95 Corridor by early afternoon. RAP/HRRR soundings show 40-50 kt of flow aloft as the upper trough and jet streak move overhead. Regardless of storm mode, the high PWAT content (2-2.5 inches) should foster strong downdrafts and momentum transfer to the surface. Widespread damaging gusts are likely from northeast VA, eastern MD, into southeast PA and NJ. Higher wind probabilities (60%) may be needed if confidence in a more linear/cluster storm mode develops. To the west, ahead of the cold front, a broken band of cells and clusters is likely from NY/VT to the Lower Great Lakes. The strongest vertical shear should reside across upstate NY into northwest PA and northeast OH, where a few supercells are possible. While farther west, strongly veered surface flow should result in more linear storm modes. Multiple wind-damage swaths may accompany these storms, along with some potential for isolated hail. 70s F surface dewpoints and enhanced low-level SRH could also support a tornado threat with any discrete or embedded supercells, especially closer to the surface low and near the effective warm frontal zone where low-level shear is more favorable. ...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. ...Gulf Coast... A remnant MCV may serve as a focus for diurnal thunderstorm development along the northern Gulf Coast today. While vertical shear is limited, locally moderate to strong buoyancy and very high PWAT values could support sporadic downbursts with damaging gusts. However, confidence in how widespread or sustained the severe threat is too low for wind probabilities. ..Lyons/Weinman.. 07/18/2026 $$