469
ACUS01 KWNS 181952
SWODY1
SPC AC 181951

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0251 PM CDT Sat Jul 18 2026

Valid 182000Z - 191200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS THE
LOWER GREAT LAKES AND MID-ATLANTIC...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS PORTIONS OF
WESTERN TO CENTRAL MONTANA...

...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.

...20z Update...
The previous forecast remains on track with only minor adjustments
required based on recent convective trends. Lingering clouds and
stratiform precipitation across northern NY and into VT/NH continues
to limit confidence in the degree of destabilization that will occur
through the remainder of the day. Latest high-res ensemble guidance
has captured recent trends and depicts limited potential for intense
convection through this evening. Severe wind/hail probabilities have
been reduced to reflect the decreasing potential. For additional
short-term forecast details across the lower Great Lakes see MCD
#1650.

Further west, the 5/15% wind probabilities have been expanded
slightly eastward to account for trends in recent guidance that
depict some threat for severe winds across central MT later tonight
as thunderstorms begin to cluster. Given building buoyancy, strong
shear across the region (per recent mesoanalysis estimates), and
building cumulus within the terrain, environmental trends appear to
support the eastward expansion. See MCD #1651 for additional
short-term details.

..Moore.. 07/18/2026

.PREV DISCUSSION... /ISSUED 1120 AM CDT Sat Jul 18 2026/

...Mid-Atlantic to the lower Great Lakes this afternoon/evening...
Ongoing convection across central PA/NY will continue to spread
eastward through early afternoon within a warm advection zone along
and north of a surface warm front.  The warm front will move
northward today in advance of a surface cyclone and associated
midlevel trough now in the vicinity of Lake Huron.  In the wake of
the ongoing convection, destabilization will occur as boundary-layer
dewpoints rise to near 70 F and surface temperatures into the  80s,
resulting in moderate buoyancy in the warm sector.  Destabilization
will occur earlier immediately south-through-west of the ongoing
storms across eastern PA/NJ, and a little later this afternoon into
central NY.

Additional thunderstorm development is beginning in southwest PA and
vicinity as of 16z and a continued increase in storms is expected
into early-mid afternoon along a cold front and along pre-frontal
confluence zones/lake-enhanced boundaries.  A mixed convective mode
(line segments and clusters) is expected, with embedded bowing
segments and a chance for pre-frontal supercells in PA/NY/NJ where
vertical shear and hodograph curvature will be larger along the warm
front.  Wind damage will be the primary severe threat with both the
frontal convection and pre-frontal storms, with a few tornadoes
possible with supercell structures and/or embedded mesovortices.

...Northern Rockies late this afternoon/evening...
Scattered thunderstorm development is expected later this
afternoon/evening across western MT and vicinity, within the
northwest fringe of the midlevel ridge and monsoonal moisture plume.
 Midlevel flow will be somewhat enhanced to the immediate southeast
of a midlevel trough moving over southeast BC, thus some organized
storm structures will be possible with faster storm motions compared
to yesterday.  Occasional severe outflow gusts and large hail up to
1.5 inches in diameter will be the main threats.

$$