SPC AC 270604

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0104 AM CDT Thu Jun 27 2024

   Valid 271200Z - 281200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM EASTERN
   MONTANA TO NORTHERN AND CENTRAL HIGH PLAINS....

   ...SUMMARY...
   Severe thunderstorms producing gusts to 80 mph, large hail greater
   than 2.5 inches, and a few tornadoes are possible across parts of
   eastern Montana into western portions of the Great Plains this
   afternoon and evening. Additional severe storms are possible over
   parts of the western US and the Southeast.

   ...MT and the Dakotas...
   An upper ridge centered over the Rockies will weaken under the
   influence of a deepening mid-level low over the Pacific Northwest.
   Ascent from the deepening low and strengthening flow aloft will aid
   in the development of a surface lee cyclone over eastern MT.
   Surface-pressure falls will increase low-level advection of rich
   boundary-layer moisture over the central and northern Plains
   northwest into eastern MT and the western Dakotas by midday.
   Steepening mid-level lapse rates, increasing large-scale ascent,
   moist advection, and diurnal heating will rapidly destabilize the
   air mass, allowing for scattered thunderstorm development.

   Convective development appears likely over several areas of the
   northern Rockies/High Plains by early to mid afternoon. A cold front
   trailing the low in central MT will likely initiate first, followed
   by the surface low in eastern MT and western ND, and along a
   trailing dryline into portions of western SD and far northeastern
   WY. With moderate buoyancy and increasingly strong deep-layer shear
   of 40-50 kt, convection should rapidly organize into supercells.
   Steep mid-level lapse rates and the initial discrete mode will favor
   large to very large hail with these supercells. Backed southeasterly
   upslope flow and large low-level CAPE near the surface low also
   suggest the potential for a couple tornadoes, especially with
   supercells that remain discrete into the early evening. Eventual
   upscale growth into a cluster or MCS with a threat for damaging
   gusts is likely later this evening, and could persist overnight with
   ample buoyancy and a 40-50 kt southerly low-level jet over the
   northern Plains.

   ...Central and southern High Plains...
   Widely scattered storm development is expected late this afternoon
   along the dryline from western SD into western NE, as temperatures
   warm and inhibition is removed from upslope flow and the approach of
   the upper low. Rich moisture (dewpoints in the upper 60s to low 70s
   F) and steep mid-level lapse rates will support large buoyancy by
   early afternoon. Albeit weaker, on the southern fringes of the
   better flow aloft, adequate deep-layer shear will support supercells
   capable of all hazards, with eventual upscale growth occurring this
   evening.

   Farther south, more isolated storm coverage is expected into parts
   of eastern CO and western KS. Likely higher based, owing to very
   warm temperatures in the upper 90s to low 100s F, damaging winds and
   hail will be possible with a mix of mutlicells and transient
   supercells. Some clustering is expected into the evening hours with
   the risk for damaging gusts shifting into parts of central KS and
   northern OK. However, the exact eastward extent remains unclear,
   given stronger capping and weakening upper-level support to the
   south and east.

   ...Eastern UT into Western CO and southern WY...
   Modest ascent from the approaching trough combined with diurnal
   orographic circulations will support scattered thunderstorm
   development across parts of the Intermountain west from mid morning
   through the afternoon. Relatively high dewpoints in the low 50s F
   and steep lapse rates will aid in afternoon destabilization, with
   MLCAPE of 1000-1500 J/kg forecast. Scattered organized storms,
   including supercells, should form over the higher terrain and along
   the trailing cold front/wind shift eastern UT into southwest WY and
   northwest CO. Damaging winds and hail are likely with the more
   robust storms. While low-level shear is not overly strong, favorable
   terrain interactions and the supercell mode could also support a
   tornado or two.

   ...Southeast...
   An upper trough over the eastern COUS will phase with a modest
   subtropical vort max rounding the eastern periphery of the rapidly
   flattening ridge over the southeastern CONUS. Subtle ascent from the
   phasing of the two troughs will support scattered afternoon
   thunderstorms over portions of AL/GA and into the Carolinas along a
   diffuse surface boundary. A very warm and moist air mass will allow
   for moderate to large buoyancy supporting strong updrafts despite
   poor mid-level lapse rates. While effective shear appears to be only
   modest (generally less than 15-20 kt), heavily water-loaded
   downdrafts may produce isolated pulse-type damaging gusts. A remnant
   MCV/weak upper-level support may also allow for the development of a
   loosely organized cluster of storms with time over portions of
   eastern GA and southern SC. A localized corridor of more sustained
   severe potential may develop, though confidence in exactly where
   remains low.

   ..Lyons/Wendt.. 06/27/2024

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