SPC Moderate Risk for July 19, 2019

SPC AC 191236

   Day 1 Convective Outlook
   NWS Storm Prediction Center Norman OK
   0736 AM CDT Fri Jul 19 2019

   Valid 191300Z - 201200Z

   ...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS EAST-CENTRAL MN
   AND NORTHERN WI...

   ...SUMMARY...
   A regional outbreak of severe thunderstorms with strong tornadoes
   and potentially widespread significant wind damage may occur from
   central Minnesota across northern Wisconsin during the mid-afternoon
   to evening.

   ...Upper Midwest/Great Lakes...
   Upgrading to Moderate Risk this outlook for the potential threats of
   multiple strong tornadoes and a derecho later this afternoon and
   evening. CAPE/shear/SRH parameter space in this region is forecast
   to be exceptionally rare for mid July amid a predominately zonal
   flow regime across the northern states.

   A convective cluster is ongoing across north-central SD aided by
   warm/moist advection around 700 mb along a pronounced baroclinic
   zone. 00Z CAM guidance had a decent handle on this activity earlier
   this morning and are consistent with a decaying trend through late
   morning as advection shifts east amid pronounced MLCIN as sampled by
   the 12Z Aberdeen sounding. There is a low confidence scenario that a
   strong enough cold pool is becoming established with this early-day
   cluster such that total decay does not occur. If this occurs, robust
   boundary-layer heating downstream may be sufficient for an
   increasing threat during the afternoon of strong to severe wind
   gusts. This could still yield a scattered to widespread damaging
   wind scenario, albeit in an earlier time frame than progged by
   guidance.

   The more probable scenario is for an extremely unstable air mass
   (MLCAPE > 4000 J/kg) to become established to from southeast SD
   through central/southern WI as the elevated mixed-layer overspreads
   surface dew points rising into the mid to upper 70s. A surface
   cyclone over central SD should track towards the Twin Cities through
   early evening. Strong low-level convergence near/northeast of this
   cyclone along the warm front should sustain surface-based storm
   development across central into east-central MN. Once initiation
   occurs, intense supercells will develop rapidly, with an attendant
   threat for all severe hazards including strong tornadoes amid 50-60
   kt effective shear and 0-3 km SRH > 300 m2/s2. The very warm/moist
   thermodynamic profiles will also support generation of strong cold
   pools which should eventually yield a bowing, forward-propagating
   MCS across northern WI into parts of northern Lower and Upper MI.
   Strength of the vertical shear suggests the potential for intense
   bowing segments capable of yielding a derecho with significant
   severe wind gusts.

   ...Lower Great Lakes...
   An MCV progressing east over the Lower Great Lakes this afternoon
   should provide enough lift for scattered thunderstorms development.
   Air mass over the region will be moderately to strongly unstable but
   modestly sheared. Expectation is for predominantly multicellular,
   outflow-dominant storms capable of isolated damaging wind gusts.

   ...SD tonight...
   Another round of 700-mb warm/moist advection will occur tonight
   within a tight baroclinic zone well north of a surface front arcing
   across NE into northeast WY. Large elevated buoyancy and strong
   effective shear will support a risk for isolated large hail. The
   risk for severe wind will likely be tempered by a more stable
   sub-cloud layer in the second convective round.

   ..Grams/Gleason.. 07/19/2019

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