SPC AC 171957

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0257 PM CDT Thu Jun 17 2021

   Valid 172000Z - 181200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS SOUTHEAST
   MN...SOUTHERN WI...NORTHERN PORTIONS OF IA/IL...

   ...SUMMARY...
   Severe storms are expected late this afternoon into tonight across 
   parts of the Upper Mississippi Valley, from central/southern
   Wisconsin and southern Minnesota into northern portions of Iowa and
   Illinois. A few tornadoes, damaging wind gusts exceeding 75 mph, and
   hail up to the size of baseballs will be possible.

   ...Discussion...
   The only changes this forecast update are the following:
   1) Added a 5-percent wind probability over parts of the central High
   Plains for late afternoon/early evening for the risk of isolated
   microbursts.  Extremely steep lapse rates from the surface up to 450
   mb will favor evaporation potential with any locally stronger cores
   that can develop.  Slightly stronger mid- to high-level flow over
   northeast CO compared to areas farther south will aid in some
   eastward storm motion.  
   2) Lowered severe probabilities on the western edge of the
   previously issued SLGT and ENH risk areas over northern IA to
   reflect model guidance showing a lower probability for storm
   development over this region.  
   3) Minor adjustment to the northern part of the MRGL Risk over the
   Ozarks based on mid afternoon satellite trends.  

   Elsewhere, the forecast remains on track and no other changes were
   made.

   ..Smith.. 06/17/2021

   .PREV DISCUSSION... /ISSUED 1130 AM CDT Thu Jun 17 2021/

   ...Upper MS Valley...
   Primary change from prior outlook is to expand greater probabilities
   farther north/east across parts of WI and IL, with the expectation
   of the wind threat persisting longer in this region tonight. 

   A mid-level impulse/speed max over SD, responsible for morning ACCAS
   across the eastern portion of the state, will aid in ascent along a
   southeast-moving cold front. Thunderstorm initiation is expected
   across west-central WI into southeast MN as MLCIN wanes near the
   front during the late afternoon. Activity should eventually
   backbuild southwestward into northeast/north-central IA this
   evening, but uncertainty is greater with how far into central IA
   convection will ultimately become sustained owing to greater
   influence of the EML and persistence of a relatively hot/dry
   boundary-layer across KS/southern NE into southern IA. 

   While guidance will probably be somewhat overdone with the degree of
   evapo-transpiration-induced moistening ahead of the front based on
   12Z morning soundings and ongoing drought, large buoyancy with
   MLCAPE of 2500-3500 J/kg should develop within a confined corridor
   given steep mid-level lapse rates. The mid-level speed max (around
   60 kts at 500 mb) will yield strong effective shear, especially with
   northern extent. This suggests initial supercells will have the
   potential to produce very large hail despite the relatively high
   freezing levels. Enlarging low-level hodographs as a southwesterly
   low-level jet strengthens this evening should maximize the supercell
   tornado threat between 22-02Z. Storms are expected to predominately
   coalesce and grow upscale into one or more bowing clusters with the
   potential to produce significant severe wind gusts (greater than 75
   mph) and brief tornadoes this evening. The damaging wind threat may
   linger into the early overnight as strong low to deep-layer shear
   attempts to compensate for increasingly pronounced MLCIN.

   ...Ozarks to Lower Red River Valley..
   A remnant MCV over southwest MO will drift southward across western
   AR this afternoon. Strong surface heating where dew points remain
   from 70-75 F will support large MLCAPE over 3000 J/kg. Isolated to
   perhaps scattered thunderstorms should develop southwest from
   southern MO towards the AR/OK/TX border area. A belt of 20-30 kt
   mid-level northeasterlies may provide adequate shear to support a
   couple loosely organized multicell clusters. Isolated damaging
   downbursts along with severe hail are possible during the late
   afternoon and early evening.

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