SPC 1730Z Day 2 Outlook

Day 2 Outlook Image

Day 2 Convective Outlook  
NWS Storm Prediction Center Norman OK
1228 PM CDT Tue Jul 27 2021

Valid 281200Z - 291200Z


Thunderstorms associated with severe wind gusts, large hail to very
large hail, and a couple tornadoes will be possible Wednesday
afternoon into Wednesday night across parts of the Upper Midwest and
extending into the southern Great Lakes region.

...Upper Midwest into the southern Great Lakes...
A cluster or two of showers/thunderstorms may be ongoing Wednesday
morning from northern MN southward into perhaps the IA/WI border
vicinity within a warm advection regime.  This activity will likely
dissipate by the late morning.  A subtle mid-level disturbance,
cresting the mid-level ridge centered over the central Great Plains,
will move eastward into western MN by late afternoon.  A warm
frontal zone will become draped from the southwest Great Lakes
northwestward through WI into northern MN.  Strong heating and a
very moist boundary layer will lead to a very unstable airmass
developing by mid afternoon over the Upper Midwest (2500-4000 J/kg
MLCAPE, locally higher).  Suite of convection-allowing models show
an initial cluster of supercells will probably develop during the
late afternoon into the early evening.  Large to very large hail,
severe gusts, and a couple of tornadoes are possible.  Enlarged
hodographs near the warm front will favor strong mesocyclones but it
is unclear how much of a tornado risk will ultimately develop due to
storm-scale interactions.  Nonetheless, an initial cluster of storms
will likely grow upscale into a forward-propagating severe MCS over
the western Great Lakes during the evening with localized intense
gusts (75+ mph) possible with HP supercell to QLCS transitions or
with the more pronounced bowing segments.  It seems plausible this
MCS will move across southern Lake Michigan into the southern Great
Lakes during the overnight.  The coverage/intensity of gusts will
probably lessen with time as the squall line encounters weaker
buoyancy during the pre-dawn hours.  

...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 forcing
will be weak, isolated storms are possible.  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

..Smith.. 07/27/2021

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