LIVE MAP (ABOVE) ... SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1247 AM CDT Fri Jul 28 2023
Valid 281200Z - 291200Z
...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS PRIMARILY OVER
IOWA...NORTHERN ILLINOIS...SOUTHERN WISCONSIN AND PARTS OF
SURROUNDING STATES...
...SUMMARY...
Severe storms are most probable from southeast South Dakota across
Iowa, northern Illinois and southern Wisconsin this afternoon and
evening, with corridors of damaging wind and large hail. Other
scattered strong to severe storms are expected from Ohio into
Virginia, and across the northern High Plains.
...Synopsis...
Moderate northwest flow aloft will exist over the northern Plains
into the upper Great Lakes and across much of the Midwest, with an
upper high across the Four Corners region. A stalled front will
stretch roughly from the SD/NE border eastward toward WI/IL, with a
very moist and unstable air mass along and south of it. Midlevel
height falls are forecast across the upper MS Valley and Great Lakes
late in the day, with a substantial area of thunderstorms moving
east/southeast across much of the Midwest.
Elsewhere, the base of an upper trough will skirt northern New
England, enhancing deep-layer shear and providing cooling aloft to
support scattered storms across ME, perhaps with marginally severe
hail.
...Midwest...
A few showers or storms may remain over parts of WI and lower MI
during the morning, as unstable air interacts with an outflow
boundary from the overnight convection. To the south, an expansive
area of unstable air with 70s F dewpoints will remain from the
middle MO Valley eastward across much of the Midwest. Heating will
lead to strong instability from eastern SD and NE to the OH River,
with a belt of stronger flow aloft near the elongated stationary
front. Heating will be strongest across NE, IA and IL, and this
warming air mass will shift northeastward through the afternoon and
interact with the boundary. While model differences do exist, the
general idea is for storms to form anywhere from central SD into
southern WI by mid/late afternoon, eventually merging into one or
more MCSs. The high degree of convective feedback in the forecast
wind fields suggest a fairly significant swath of damaging winds may
develop from IA into northern IL and surrounding areas. In addition,
isolated very large hail will be possible as deep-layer shear will
be favorable along the stationary front.
...Southern MT into the northern High Plains...
Upper ridging will occur over the northern Rockies today, with
veering surface winds from northeast to easterly, aiding westward
transport of moisture. Meanwhile, strong heating beneath cool
midlevel temperatures will lead to steep lapse rates, and effective
shear will be near 50 kt. As such, storms forming over the high
terrain of southwest MT and extending southeastward across WY may
produce marginal hail during the late afternoon and early evening.
...VA into MD...
Strong heating will occur today, with a weak lee trough developing.
Convergence and an uncapped air mass will lead to scattered storms
over western into central VA, with inverted-v profiles and MLCAPE
over 2500 J/kg favoring locally strong downbursts.
...Northern ME southwestward toward far northern NY...
Scattered storms are likely to develop over southern Quebec during
the afternoon along a front and in association with a progressive
shortwave trough. This wave will lengthen hodographs with 40-50 kt
effective shear extending southward into New England, and provide
cooling aloft which will steepen midlevel lapse rates. Sufficient
MLCAPE around 1000 J/kg may favor a supercell or two producing
severe hail.
..Jewell/Bentley.. 07/28/2023
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