SPC AC 230554

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1254 AM CDT Sat May 23 2020

   Valid 231200Z - 241200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS NORTHERN
   ILLINOIS AND OVER PORTIONS OF THE HIGH PLAINS...

   ...SUMMARY...
   Scattered severe thunderstorms, including a tornado threat, will
   develop across northern Illinois later this afternoon. Isolated
   severe thunderstorms, some with very large hail, will be noted
   across the Plains.

   ...Northern IL...

   Upper low over southwestern IA is forecast to eject into eastern IA
   by 18z, then weaken some as it ejects into southern WI by early
   evening. 50kt 500mb speed max should translate across northern MO
   into northern IL during the afternoon, which will enhance shear and
   the likelihood for supercells immediately ahead of the upper low.
   Latest short-range model guidance suggests mid 60s surface dew
   points will advance north across IL into a region of favorable
   large-scale forcing for ascent. Forecast soundings suggest
   thunderstorms should develop by late morning across eastern IA
   beneath the upper low, while downstream, portions of northern IL
   will experience a bit more destabilization as partial sunshine
   should allow the boundary layer to warm with surface temperatures
   rising into the mid-upper 70s. As a result, MLCAPE values should
   exceed 1500 J/kg within a sheared regime favorable for supercells.
   Latest thinking is scattered supercells will evolve by early
   afternoon near the IA/northwestern IL border then spread east toward
   the Chicago metro by early evening. Hail, wind, and a few tornadoes
   can be expected with this activity.

   ...Northern/Central Plains...

   Weak large-scale height falls will spread across the northern High
   Plains during the day1 period as upper troughing shifts across the
   northern Rockies. Downstream, high-level diffluent flow should
   encourage cloud-top venting and scattered thunderstorms are expected
   to develop along a surface boundary draped across the western
   Dakotas into eastern WY. Strongest boundary-layer heating will
   likely focus near the wind shift, thus greater buoyancy, and likely
   storm intensity, should focus along this corridor. Even so,
   progressive short-wave trough should nudge deep convection off the
   High Plains into portions of southeastern SD/northeastern NE late as
   LLJ is dislodged into this region during the overnight hours. Very
   large hail may accompany late-afternoon supercells.

   ...Southern High Plains...

   Broad troughing with weak southwesterly high-level flow will persist
   across the southwestern US into the southern High Plains Saturday as
   the mean trough remains located near 110W longitude. Relatively cool
   mid-level temperatures (-11C at 500mb) and very steep mid-level
   lapse rates will lead to extreme instability just east of the dry
   line over west TX. Strong boundary-layer heating across far west TX
   into the western TX Panhandle will allow surface parcels to reach
   their convective temperatures around 20-21z. As a result, scattered
   supercells should develop by mid afternoon from the TX South Plains,
   south into northern Mexico. Very large hail (possibly > 3 inches)
   could be noted with these slow-moving storms, especially early in
   the convective cycle.

   ..Darrow/Lyons.. 05/23/2020

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