SPC AC 051257

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0757 AM CDT Sun Aug 05 2018

   Valid 051300Z - 061200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE CENTRAL
   HIGH PLAINS TO THE UPPER GREAT LAKES...

   ...SUMMARY...
   Isolated to scattered severe storms, capable of large hail, damaging
   winds, and a couple tornadoes, will be possible from parts of the
   central High Plains to northern Michigan today.

   ...Central High Plains to the Mid-Missouri River Valley...
   A somewhat complex convective scenario is expected today.
   Regardless, this region will be susceptible to multiple rounds of
   potentially severe convection this afternoon and tonight with all
   convectively related hazards possible during the period, especially
   across Nebraska into Iowa where strong consideration of a
   categorical Enhanced Risk upgrade was given.

   Outflow continues to stall/gradually wash-out across northeast
   Nebraska/western Iowa with some regenerative convective development
   atop a surface cold pool in the wake an overnight
   severe-wind-producing MCS. Some sub-regional spatial uncertainty
   exists regarding the severe risk later today due to this, although
   gradual air mass recovery can be expected in most of these
   MCS/outflow-impacted areas.

   The region will be influenced by multiple eastward-spreading
   mid-level disturbances within a belt of strengthening mid/high-level
   westerlies (50+ kt in some cases), which will be relatively strong
   by August standards. One such disturbance is across southeast
   Wyoming/northern Colorado this morning. Ascent/mass response related
   to this feature could influence increasing severe storm development
   by mid/late afternoon in and around east-central Nebraska and points
   east through evening. A secondary semi-focused belt of ascent,
   related to a shortwave trough over the north-central Great Basin,
   will also be a factor of severe storms from interior Wyoming this
   afternoon eastward into Nebraska, potentially in the form of an MCS
   tonight.

   Coincident with cloud-filtered insolation and a strong elevated
   mixed layer, pockets of strong heating through the day will combine
   with surface dew points in the upper 60s/lower 70s and steep
   mid-level lapse rates to yield MLCAPE upwards of 2500-3000 J/kg
   across parts of central Nebraska. A south/southwesterly low-level
   jet is expected to strengthen over central/eastern Nebraska by
   evening, with scattered storms possibly developing from
   central/eastern Nebraska into western Iowa near a
   southwest-northeast oriented surface trough/effective warm front. In
   addition to a large hail and damaging wind threat, a localized
   enhancement in tornado potential may exist, owing to greater
   storm-relative helicity in association with the amplifying low-level
   jet. Over time, though, storm mergers should support damaging winds
   becoming the main threat.

   Farther west towards Wyoming, MLCAPE values should be closer to
   500-1500 J/kg. As mentioned, one focused area of convective
   initiation is expected here related to the secondary mid-level
   impulse influencing increasing storm development over the higher
   terrain. Aided by favorable easterly/upslope flow and ample
   effective shear, these cells (likely in the form of supercells)
   should intensify as they move towards southeast Wyoming and the
   Nebraska Panhandle, with a potential for damaging winds, large hail,
   and perhaps a tornado. Over time, the steep lapse-rate environment
   and layers of dry air aloft could foster a transition to small-scale
   bows, with a primary threat of severe wind gusts into the overnight
   as storms move eastward across Nebraska potentially as a
   well-organized/eastward-accelerating MCS.

   ...Portions of Minnesota/Wisconsin/Michigan...
   To the north of the overnight/early morning MCS, a corridor of
   relatively unperturbed unstable air remains especially across areas
   such as south-central/eastern Minnesota. This is confirmed by the
   12Z observed sounding from Minneapolis which featured a relatively
   steep mid-level lapse rate with 3000+ J/kg MLCAPE, and to a lesser
   extent at International Falls MN where a 7.3 C/km lapse rate was
   noted per the 12Z sounding. While the decaying early-day MCS to the
   south and any related MCV complicates the scenario, a modestly
   amplifying shortwave trough along the international border vicinity
   of northern Minnesota may influence diurnally increasing severe
   potential coincident with the previously highlighted plume of
   relatively unstable air as it advects east/northeastward coincident
   with a warming boundary layer.

   Accordingly, at least some severe risk appears probable from
   east-central/northeast Minnesota into northern Wisconsin and Upper
   Michigan this afternoon into evening given that 40-50 kt mid-level
   flow will overlie a warm/moist boundary layer. In tandem with fairly
   steep 700-500mb lapse rates, this thermodynamic environment will
   offer a stout, but probable severe threat where storms
   develop/mature. Relatively unidirectional flow suggests damaging
   winds will be the predominant severe threat, but supercells are
   possible and favorable low-level and deep-layer speed shear may
   support a few instances of large hail and perhaps a tornado.

   ..Guyer/Dial.. 08/05/2018

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