SPC AC 270559

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1259 AM CDT Sun Apr 27 2025

   Valid 281200Z - 291200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS FROM CENTRAL
   IOWA INTO SOUTHEAST MINNESOTA AND WESTERN WISCONSIN...

   ...SUMMARY...
   An outbreak of severe weather remains likely across portions of the
   Upper Midwest, centered on Monday afternoon and evening. Very large
   hail, strong tornadoes, and damaging winds are expected.

   ...Synopsis...
   A strong (~75 knot) mid-level jet streak will advance from the
   central Plains to the Upper Midwest on Monday. As this occurs, a
   surface low will deepen across the Upper Midwest with an eastward
   advancing dryline to its south and a cold front to the
   west-southwest. A warm front will extend east of this surface low
   and move rapidly north through the day, spreading mid 60s dewpoints
   across much of the Upper Midwest. 

   ...MN/IA/WI...
   Thunderstorms will likely be ongoing at the beginning of the period
   north of a warm front across Minnesota and Wisconsin amid moderate
   isentropic ascent. Isolated large hail will be possible from these
   storms. A surface low will be located somewhere near eastern South
   Dakota and will start to advance east during the morning. This
   surface low is expected to deepen somewhat through the day (perhaps
   to the low 990s mb range) as it moves toward the Twin Cities. There
   is a consistent signal for thunderstorms to develop along the cold
   front as it intersects the surface low/dryline in west-central
   Minnesota Monday afternoon. After some initial hail threat, these
   storms will likely become quickly linear along the front with an
   increasing severe wind threat. Strong low-level shear will also
   favor some line-embedded tornadoes, particularly if there area any
   areas within the line which become more favorably oriented to the
   low-level shear vector. 

   Additional thunderstorms are forecast to develop along the dryline
   from southern Minnesota to west-central Iowa during the afternoon on
   Monday. 00Z CAM guidance is not overly bullish with convective
   development south of the MN/IA border, likely due to relatively weak
   convergence along the dryline in western Iowa from most high
   resolution guidance. However, the lack of convection from much of
   the guidance does not seem realistic considering the overall
   pattern. ECMWF/GFS/NAM are fairly consistent with a deepening
   surface low across Minnesota and moderate to strong height falls
   across the dryline. The southern extent of the previous Day 3
   moderate risk corresponds well to the southern extent of the more
   favorable height falls, and convective precipitation signal from the
   ECMWF/GFS. Therefore, no adjustments to the southern extent of the
   moderate risk were deemed necessary. However the moderate risk was
   expanded slightly westward to account for the expectation for
   earlier convective initiation (19-21Z) than shown by most guidance.
   Given very strong effective shear and over 3000 J/kg MLCAPE, expect
   any supercells which develop to become quickly severe with a threat
   for all severe weather hazards. 

   As the surface low across Minnesota deepens the low-level jet is
   also forecast to strengthen across Iowa, Minnesota and Wisconsin.
   This will elongate low-level hodographs and support an increasing
   tornado threat during the evening. STP values of 3 to 5 are common
   across much of the warm sector ahead of anticipated storms with some
   guidance indicating values of 7 to 10. Therefore, any mature,
   discrete supercells which can form and maintain within the open warm
   sector on Monday late afternoon/evening will pose a threat for
   strong to potentially intense tornadoes. 

   ...Eastern Kansas and northern Missouri to West Texas...
   Overall forcing will be weaker farther south along the dryline, but
   subtle height falls across a mostly uncapped warm sector could
   result in isolated to scattered supercell development Monday
   afternoon. A secondary area of higher severe weather probabilities
   may exist from portions of West Texas into southwest Oklahoma where
   the nocturnally strengthening low-level jet could support scattered
   supercell development Monday evening. However, weak height rises
   cast some doubt on storm coverage during the evening, precluding
   higher probabilities at this time.

   ..Bentley.. 04/27/2025

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