SPC 0600Z Day 2 Outlook

Day 2 Outlook Image

Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1246 AM CDT Tue Jul 27 2021

Valid 281200Z - 291200Z


Thunderstorms associated with severe wind gusts, isolated large hail
and a couple tornadoes will be possible from Wednesday into
Wednesday night across parts of the upper Mississippi Valley and
Great Lakes region. Marginally severe storms could also occur in the
central Appalachians and Mid-Atlantic.

...Upper Mississippi Valley/Great Lakes...
A large area of high pressure will remain over the western and
central United States on Wednesday, with northwest mid-level flow
located from the upper Mississippi Valley to the eastern Seaboard.
At the surface, a low will move across the upper Mississippi Valley
as a cold front advances southward across the northern Plains. A
warm front should be located across the western and southern Great
Lakes region. Along and to the south of the warm front, surface
dewpoints will likely range from 70 to 75 F, with moderate to strong
instability developing by afternoon. Model forecasts move a subtle
shortwave trough from southern Canada into the Great Lakes region as
a 40 to 50 kt low-level jet moves across the upper Mississippi
Valley. Convective initiation is expected to take place at the
northwestern end of the instability corridor and to the north of the
low-level jet during the afternoon. This convection should organize
quickly with a severe complex of storms moving southeastward across
Wisconsin during the late afternoon and early evening. It appears
that MCS development will take place in the evening.

NAM forecast soundings along the track of the MCS near Madison,
Wisconsin show an impressive thermodynamic and kinematic
environment. MLCAPE is forecast to peak near 4000 J/kg across
central Wisconsin late Wednesday afternoon with 0-6 km shear in the
60 to 70 kt range. This combination of instability and shear will
likely be favorable for a linear MCS associated with damaging wind
gusts. A northwest-to-southeast swath of wind damage may occur
across parts of northern, central and eastern Wisconsin. As the MCS
moves southeastward into lower Michigan during the late evening, a
wind damage threat should continue. A couple of tornadoes and some
hail will also be possible along the track of the MCS, mainly with
supercells that develop within the MCS itself. The wind-damage
threat could persist into the early overnight period across the
lower Great Lakes region.

...Central Appalachians/Mid-Atlantic...
An upper-level trough is forecast to move southeastward across the
Northeast on Wednesday. A moist airmass will likely be in place
across parts of the central Appalachians, where moderate instability
is forecast to develop by afternoon. Although large-scale ascent
will not be that strong, isolated convective initiation should take
place in the higher terrain as surface temperatures warm up. In
addition to the instability, deep-layer shear will be strong enough
to contribute to a marginal severe threat. A few marginally severe
gusts will be possible in the mid to late afternoon as low-level
lapse rates become steep.

..Broyles.. 07/27/2021

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