SPC AC 111958

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0258 PM CDT Fri May 11 2018

   Valid 112000Z - 121200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS SOUTHEAST WY
   VICINITY...

   ...SUMMARY...
   A few severe storms with large hail as the primary hazard are most
   likely to occur across the central High Plains this afternoon into
   early evening.

   ...Eastern NE/IA/northern IL/far southern WI...
   Confidence has increased for the inclusion of a Marginal risk for
   severe hail being produced by elevated storms that develop later
   this evening into the overnight from far eastern NE through IA to
   northern IL and far southern WI.  The environment across the central
   Plains will likely remain capped into the early portion of this
   evening.  Meanwhile, a midlevel shortwave trough, currently moving
   east-northeast through WY and eastern CO per mid-upper level
   cloudiness detected by satellite imagery, is expected to reach
   eastern NE/western IA by late evening and proceed toward northern IL
   by 12Z Saturday.  Forcing for ascent attendant to this trough and
   within increasing warm advection on the nose of strengthening
   southwesterly low-mid level winds accompanying the trough should
   prove sufficient for elevated storms.  This late evening/overnight
   activity should be located mainly north of a quasi-stationary
   boundary residing from northern MO into central IL tonight.  Strong
   elevated instability, steep midlevel lapse rates, and effective bulk
   shear of 25-35 kt should prove favorable for organized storms
   capable of producing severe-caliber hail.  As indicated in the 1630Z
   Day 1 outlook, orientation of shear vectors should result in storms
   maturing into multicell clusters that may limit the overall coverage
   of severe hail.  

   ...Eastern OK to southern MO...
   Thunderstorms that affected this area earlier today have mostly
   diminished.  A strengthening cap will limit further development, and
   no longer warrants maintaining a general thunderstorm area across
   these areas through tonight.

   ..Peters.. 05/11/2018

   .PREV DISCUSSION... /ISSUED 1128 AM CDT Fri May 11 2018/

   ...Central High Plains...
   Well north of a surface cold front that has progressed into the
   southern High Plains, easterly upslope flow will strengthen this
   afternoon as the pressure gradient increases. A confined pocket of
   greater insolation is underway across the Front Range from
   north-central CO into southeast WY to the west of a nearly
   stationary stratus deck over much of the plains. A few storms will
   likely form off the Laramie Range during the early-mid afternoon.
   Within a belt of enhanced mid-level southwesterlies around 35-45 kt
   at 500 mb, atop the boundary-layer easterlies, effective shear will
   be quite favorable for a discrete supercell or two. Large hail will
   be the primary hazard. The spatiotemporal window for surface-based
   supercells will be limited as capping/inhibition remains very strong
   within the stratus deck at lower elevations. This should result in
   the severe risk decaying rather rapidly eastward during the early
   evening.

   ...Southern Great Plains...
   Isolated storms should develop for a few hours along the dryline in
   the late afternoon to early evening. Hot temperatures in the 96-102
   F range should result in very high-based storms rooted around
   600-500 mb. Capping/inhibition is increasingly large with eastern
   extent from the dryline, resulting in a limited spatiotemporal
   window for the severe risk. Despite weak shear within the
   cloud-bearing layer, the very steep lapse-rate environment will
   support dry microbursts producing strong to severe wind gusts. 

   ...IA to southern Lower MI...
   Guidance is fairly consistent in suggesting scattered overnight
   convection forming north of the quasi-stationary front that should
   lie near the IA/MO border through central IL/IN. While NAM soundings
   appear unrealistically overdone with MUCAPE, the RAP and other
   guidance suggest moderate elevated buoyancy may develop along the
   northern periphery of the expansive EML plume over the central
   states. Within a belt of confluent mid-level westerlies, a few
   updrafts might produce hail approaching severe size. But given the
   westerly flow aloft paralleling the zone of broad isentropic ascent,
   convection should rapidly form into multicell clusters, likely
   limiting the overall potential for severe hail. Will monitor this
   area for possible upgrade to MRGL risk for hail in later outlooks.

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