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
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