SPC AC 061739

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1239 PM CDT Mon Jul 06 2020

   Valid 071200Z - 081200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS CENTRAL
   AND EASTERN MT INTO WESTERN ND AND NORTHWEST SD...

   ...SUMMARY...
   Severe storms capable of intense damaging winds, large hail and a
   few tornadoes are expected from central Montana eastward across the
   northern Plains and into Minnesota Tuesday afternoon into the
   nighttime hours.

   ...Synopsis...

   An upper ridge over the northern/central Plains will shift eastward
   and break down on Tuesday/Tuesday night as a strong upper trough
   develops eastward across the northern/central Rockies. An associated
   band of enhanced southwesterly flow will overspread MT/WY eastward
   into the Dakotas and MN. Meanwhile, a surface low will deepen over
   MT and SK and develop eastward overnight, bringing a cold front
   across the northern High Plains into the western Dakotas by 12z
   Wednesday. Widespread thunderstorm development is expected ahead of
   the cold front from central MT into northern WY during the
   afternoon, spreading eastward across the Dakotas and into MN during
   the evening and overnight hours. 

   ...MT/WY into the western Dakotas...

   Strengthening southeasterly low level winds will transport low to
   mid 60s F dewpoints westward across eastern and central MT. Strong
   heating may result in some mixing of low level moisture, but will
   remain more than adequate to support strong destabilization by early
   afternoon across central/eastern MT into north-central/northeastern
   WY. Midlevel lapse rates will steepen as temperatures aloft cool
   with the approaching of the mid/upper trough, with forecast
   soundings showing values from 8-8.5 C/km common over the region.
   Storms are expected to initially develop by early afternoon over the
   higher terrain of western/west-central MT and spread eastward with
   time. Southeasterly low level winds through around 2.5-3 km veering
   to southwesterly above 700 mb will result in strong vertical shear
   supportive of supercells. Initial convection will be capable of
   strong downburst winds and hail over higher terrain. 

   Additional storm development is possible along the Big Horn
   mountains by late afternoon, spreading northeast across northeast MT
   and southeast MT. As storms develop eastward and encounter better
   moisture, very large hail (some greater than 2 inches in diameter)
   will be possible in addition to intense wind gusts. A low level jet
   is forecast to increase in this 00z-03z time frame, and a window of
   opportunity will exist for discrete cells producing a couple of
   tornadoes before upscale growth into one or more bowing segments
   occurs. Bowing convection should track northeast across eastern MT
   into the western Dakotas, producing potentially significant gusts
   (65+ kt) across parts of central into eastern MT, with widespread
   severe wind gusts expected to continue into northwest SD/western ND.


   ...WY/NE eastward across central/eastern Dakotas and MN...

   High-based convection is expected to develop along a surface trough
   extending southward across eastern WY into CO. This convection will
   mainly pose a threat for damaging gusts given a deeply-mixed and
   drier boundary layer compared to further north. Nevertheless, this
   convection will spread east/northeast through the evening and could
   develop into another bowing segment tracking across SD into
   central/eastern ND and MN during the nighttime hours. Damaging winds
   and hail will be possible with this activity. 

   A more conditional severe threat will exist across parts of
   central/northeast SD during the afternoon. Guidance suggests an MCS
   will be decaying across the region during the morning, leaving
   outflow boundaries and sharpening a warm front draped across central
   SD toward the ND/SD/MN border. This should suppress convection for
   much of the day also, as the airmass recovers. Some guidance
   develops convection along the warm front around 00z as the low level
   jet increases and strong warm advection overspreads the region ahead
   of the approaching surface low. If this occurs, a localized threat
   for supercells capable of large hail, damaging winds and a couple of
   tornadoes will exist across parts of north-central into northeast SD
   during the late afternoon/evening hours.

   ..Leitman.. 07/06/2020

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