SPC AC 300557

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1257 AM CDT Sat Sep 30 2023

   Valid 301200Z - 011200Z

   ...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FOR PORTIONS OF
   THE UPPER MS VALLEY...CENTRAL AND SOUTHERN HIGH PLAINS...AND WESTERN
   INTO CENTRAL UTAH...

   ...SUMMARY...
   Isolated severe gusts may occur across portions of the
   southern/central High Plains and western into central Utah this
   afternoon. An instance or two of severe hail also cannot be ruled
   out across portions of the Upper Mississippi Valley.

   ...Synopsis...
   A broad, pronounced mid-level trough will continue to amplify over
   the Interior West today as upper ridging builds over the CONUS east
   of the Rockies. Beneath the mid-level trough, a large surface
   cyclone should become established over the Great Basin, with the
   surface cyclonic flow extending as far northeast as the northern
   Plains, where the northward advection of low-level moisture will be
   taking place. Strong tropospheric wind fields overspreading a deep,
   mixed, but marginally buoyant boundary layer will support scattered
   strong to isolated severe storms across portions of the Great Basin,
   eastward to the central and southern High Plains. At the terminus of
   a Great Plains low-level jet, warm-air advection may support
   isolated strong to severe thunderstorms given the presence of
   elevated buoyancy.

   ...Central and Southern High Plains...
   A mid-level impulse embedded within the broader upper trough will
   traverse the southern Rockies and approach the central High Plains
   through the afternoon, providing locally stronger forcing for ascent
   to support scattered thunderstorms. These storms will develop atop a
   well-mixed, dry boundary layer extending up to 600 mb in spots.
   Though hodographs are not overly long, modestly stronger flow aloft,
   combined with downward momentum transport driven by rainwater
   evaporation in the deep and dry sub-cloud layer, should support at
   least isolated severe gusts with the stronger storms during the late
   afternoon and evening hours.

   ...Western into central Utah...
   A belt of 50-70 kt 500 mb south-southwest to north-northeasterly
   flow will accompany the leading edge of the broad upper trough
   during the afternoon, and is poised to overspread a dry, well-mixed
   boundary layer. Through the afternoon, strong southerly flow
   associated with the aforementioned surface cyclone may encourage
   dewpoints to approach the 35-40 F range, resulting in marginal (i.e.
   less than 500 J/kg) SBCAPE. Though buoyancy should be quite scant,
   strong forcing along a surface cold front will foster a line of
   low-topped thunderstorms immediately preceded by strong (albeit
   meridional, unidirectional) speed shear and accompanying elongated
   hodographs. When also considering the impacts of evaporative cooling
   in a dry boundary layer with steep low-level lapse rates, organized
   cells or small bows within the parent convective line may produce a
   couple of severe gusts by late afternoon.

   ...Upper Mississippi Valley...
   By mid to late afternoon, the gradual approach of an upper trough
   will encourage the strengthening of a low-level jet over the Plains
   states. A plume of low-level moisture (including low to mid 60s F
   surface dewpoints) will continue to advect northward to a diffuse
   surface-850 mb baroclinic zone draped across the northern Plains
   into the Upper MS Valley. Isentropic ascent along this boundary will
   be sufficient for scattered thunderstorm development by late
   afternoon. By this point, 7-8 C/km mid-level lapse rates will
   overspread the low-level moist axis, boosting MLCAPE to over 2000
   J/kg. Forecast soundings depict short hodographs with appreciable
   low-level curvature, suggesting that any storms that form will be
   multicellular in nature, capable of producing a couple instances of
   severe hail by late afternoon or early evening.

   ..Squitieri/Lyons.. 09/30/2023

   NOTE: THE NEXT DAY 1 OUTLOOK IS SCHEDULED BY 1300Z