SPC AC 260559

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1259 AM CDT Fri Jun 26 2020

   Valid 261200Z - 271200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   EASTERN IOWA...SOUTHERN WISCONSIN...NORTHERN ILLINOIS...NORTHERN
   INDIANA AND SOUTHERN LOWER MICHIGAN...

   ...SUMMARY...
   A line of severe thunderstorms is expected to move from northeast
   Iowa across southern Wisconsin and northern Illinois into southwest
   Lower Michigan this afternoon and evening. Additional strong to
   severe storms are possible from the Mid Missouri Valley to the
   central High Plains.

   ...Great Lakes/Upper Mississippi Valley/Mid Missouri Valley...
   A shortwave trough will move east-southeastward across the Upper
   Mississippi Valley today and into the western Great Lakes region. At
   the surface, a cold front will advance southeastward into the mid
   Missouri Valley and Upper Mississippi Valley. Ahead of the front, a
   moist airmass will be in place with surface dewpoints generally in
   the upper 60s to near 70 F. As surface heating takes place today,
   moderate instability is expected to develop ahead of the front
   across much of the moist sector. Although there is some spread in
   the model guidance, model consensus suggests that convection will
   first initiate from southern Minnesota southward into Iowa early
   this afternoon. This convection is forecast to move quickly
   east-southeastward into southern Wisconsin and northern Illinois by
   early afternoon reaching southern Lower Michigan by late afternoon.
   It appears that a linear MCS will develop as this convection
   organizes and moves quickly east-southeastward across the region.

   Although the scenario is still far from certain, the setup should
   become favorable for a fast-moving line segment this afternoon.
   MLCAPE is forecast to reach the 1500 to 2500 J/kg range along a
   corridor from eastern Iowa into northern Illinois. Convection should
   move just slightly to the south of due east, along the instability
   gradient from northeast of Des Moines to near Chicago and then
   eastward to near Detroit. The NAM is forecasting a 40 to 50 kt
   mid-level speed max to move eastward to near Chicago by 21Z. This
   feature, along with an associated speed max at lower levels, should
   act as a rear inflow jet to this convective complex. This will help
   any line segment to obtain bowing characteristics and enhance the
   wind potential. NAM forecast soundings along the expected track of
   the MCS show veered low-level winds to the south-southwest and very
   steep lapse rates in the boundary layer. This should be favorable
   for wind damage along the leading edge of the line. Some uncertainty
   exists concerning the north-to-south extent of the wind damage
   threat but have used model consensus to place an enhanced risk in
   the area with the greatest probability of severe. Although isolated
   large hail will also be possible, this should be the lesser of the
   two threats. The severe threat with this linear MCS should reach
   southeast Lower Michigan and northern Ohio by early evening.

   Further to the west across the mid Missouri Valley, a pocket of
   strong instability is expected to develop by afternoon. At the
   surface, a cold front will move south-southeastward in the mid
   Missouri Valley by mid to late afternoon. Although deep-layer shear
   should be relatively weak, steep low-level lapse rates should be
   sufficient for a wind damage threat with multicells along and south
   of the front.

   ...Central Rockies/Central Plains...
   A subtle shortwave trough is forecast to move eastward across the
   central Rockies this afternoon. At the surface, a moist airmass will
   be in place from the foothills in central Colorado eastward across
   the Colorado Plains and into western Kansas. As surface heating
   takes place today, moderate instability is expected to develop from
   the mountains eastward. Thunderstorms should first initiate in the
   higher terrain of central Colorado with this convection moving
   eastward into the lower elevations during the afternoon. Forecast
   soundings at Colorado Springs and Pueblo suggest that MLCAPE values
   could reach the 1500 to 2000 J/kg range by afternoon. In addition to
   very steep low to mid-level lapse rates, 0-6 km shear is forecast to
   be in the 30 to 35 kt range. This environment may be enough to
   support supercells with storms that become more dominant in areas
   with the strongest instability. Any supercell that can develop
   should have an isolated large hail and wind damage threat. The
   stronger multicells may also be capable of producing hail and
   isolated damaging wind gusts. The severe threat should affect areas
   in far eastern Colorado and western Kansas by evening as a
   convective cluster moves across the region from west to east.

   ..Broyles/Bentley.. 06/26/2020

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