SPC AC 171957

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0257 PM CDT Thu Apr 17 2025

   Valid 172000Z - 181200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM EASTERN
   NEBRASKA INTO CENTRAL IOWA...

   ...SUMMARY...
   Isolated to scattered severe thunderstorms are expected, mainly from
   late afternoon into early tonight from eastern Nebraska into western
   and central Iowa.  Very large hail to 3 inches in diameter or
   greater, isolated severe gusts of 60-70 mph, and a couple of
   tornadoes will all be possible.

   ...20Z Update...
   Very minor changes were made to the outlook with this update. A
   cluster of thunderstorms is evolving immediately ahead of the
   surface low over southern MN, where low-level convergence is
   maximized amid boundary-layer heating and decreasing inhibition.
   Given around 40 kt of deep-layer shear (per MPX VWP), this activity
   should gradually intensify through the afternoon, with a risk of
   large hail and locally damaging wind gusts. Reference MCD #458 for
   more information.

   Farther south, the latest surface observations and radar imagery
   depict gradually increasing boundary-layer convergence ahead of a
   well-defined triple point over south-central/southeast NE. While
   satellite imagery still suggests strong capping at the base of the
   EML, expectations are still for isolated/discrete storm development
   near the triple point in southeast NE later this afternoon into the
   evening. The primary concern will be large to very large hail with
   initially discrete supercells, with an increasing risk of damaging
   winds and a couple of tornadoes into this evening. For additional
   details, reference MCD #459.

   ..Weinman.. 04/17/2025

   .PREV DISCUSSION... /ISSUED 1129 AM CDT Thu Apr 17 2025/

   ...NE/IA/MN/MO through early tonight...
   A belt of strong southwesterly mid-upper flow will extend from the
   southern High Plains to the central Great Plains and upper MS
   Valley, downstream of a large-scale trough over the West. 
   Visible-satellite imagery late this morning shows a capped
   stratocumulus shield from eastern OK northward into eastern KS and
   the MO Valley.  This low-level moisture depicted on satellite
   corresponds with a 10.0-11.5 g/kg lowest 100-mb mean mixing ratio
   and dewpoints in the upper 50s to lower 60s.  Moisture will continue
   northward on southerly flow to the east of a weak surface wave which
   is expected to move along an associated baroclinic from the mid MO
   Valley to the upper MS Valley, while a lee cyclone develops
   south-southeastward across the central High Plains.  

   Strong heating and continued moisture advection will result in
   moderate to strong destabilization from eastern KS to eastern
   NE/western IA and southern MN.  Models continue to show initial
   thunderstorm development around 20z in southern MN and around 22z in
   eastern NE (near and just east of the cold front/dryline triple
   point).

   Model forecast soundings over southern MN will favor a mix of
   supercells and storm clusters capable of producing large hail
   (generally 1-1.75 inch diameter) and isolated strong-severe gusts
   (50-60 mph), and perhaps a brief tornado.  Farther south, greater
   buoyancy and elongated hodographs will be more favorable for
   supercells.  A long-track supercell or two is forecast from eastern
   NE this evening into western/central IA through early tonight. 
   Large to giant hail (peak diameter 2.75 to 3.5 inches) will be
   possible with the more intense phase of the supercell activity.  A
   narrow window of opportunity may exist for a couple of tornadoes
   this evening with sufficient moisture (boundary-layer dewpoints near
   60 F) as low-level shear increases.  Severe gusts will be possible
   with the stronger storms before this activity wanes late this
   evening into the overnight.

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