SPC AC 161730

   Day 2 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1230 PM CDT Mon Sep 16 2024

   Valid 171200Z - 181200Z

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE CENTRAL
   HIGH PLAINS INTO NORTHEAST MONTANA...

   ...SUMMARY...
   Isolated to scattered severe gusts and hail are possible Tuesday
   across a large swath of the High Plains region, mainly from late
   afternoon into the evening. The greater concentrations of severe
   storms may occur over the central High Plains into northeast
   Montana.

   ...Synopsis...

   A mid/upper cyclone is forecast over the Great Basin Tuesday
   morning. This system will pivot northeast across the central Rockies
   and into the northern High Plains through the forecast period. This
   will bring a belt of strong south/southwesterly deep-layer flow
   across CO/WY and adjacent portions of the High Plains. A surface low
   will deepen over MT through the period, with a surface trough
   extending southward through the central/southern High Plains. A warm
   front is forecast to extend west to east from southeast MT into
   north-central SD during the morning. With time, this boundary will
   lift northward toward the International border during the nighttime
   hours. As the system deepens and the low-level baroclinic zone over
   the Plains strengthens, 850-700 mb flow is forecast to markedly
   increase during the afternoon/evening to 30-50 kt. While deeper
   boundary-layer moisture is forecast to remain east of the surface
   trough axis and north of the warm front, steep midlevel lapse rates
   will support pockets of weak to moderate instability. An arc of
   thunderstorm activity, with a mix of line segments and discrete
   cells, are expected to develop in the vicinity of the surface
   low/warm front south/southeast along the surface trough during the
   afternoon and evening hours.

   ...Eastern MT...

   Strong ascent will overspread the region as the upper trough pivots
   northeast during the late afternoon/evening. Boundary-layer moisture
   will be greatest to the north of the warm front across
   north-central/northeast MT, but capping will likely remain in place.
   Storm motion will likely result in convection remaining elevated to
   the cool side of the warm front. Nevertheless, steep midlevel lapse
   rates and favorable vertical shear (elongated/straight hodographs)
   will support some risk for large hail. As stronger forcing and
   sufficient erosion of capping can overlap during the evening, strong
   to severe wind gusts also will be possible. 

   ...Northern/Central High Plains Vicinity...

   Convection is expected to develop over the higher terrain of CO/WY
   during the afternoon. Given strengthening mid/upper flow and weak
   instability, some locally strong gusts could occur with this initial
   activity near the core of the upper low over western/central CO. As
   convection shifts east/northeast through the afternoon and into the
   evening, strong heating is expected to result in a steep low-level
   lapse rate environment across the High Plains. While deep
   boundary-layer moisture will remain further east across the Plains,
   steep midlevel lapse rates and surface dewpoints in the 50s F will
   result in a corridor of around 1000-1500 J/kg MLCAPE from far
   eastern CO into parts of western NE/SD. Strengthening midlevel flow
   atop a deeply mixed boundary-layer will likely support severe gusts
   (isolated gusts to 75 mph possible) with linear segments spreading
   northeast across the central High Plains during the late
   afternoon/evening. 

   ...Southern High Plains Vicinity...

   Further south, large-scale ascent will be weaker, with stronger
   height falls focused north of the region. Nevertheless, low-level
   convergence along the surface trough and modest instability (up to
   1000 J/kg MLCAPE) will support initial thunderstorm development over
   the higher terrain of south-central CO into north-central NM.
   Forecast soundings show supercell wind profiles across the adjacent
   High Plains by afternoon. Midlevel lapse rates will be somewhat
   modest (around 6.5-7 C/km), but elongated/straight hodographs and
   plenty of instability through the 700-300 mb layer suggest isolated
   hail is possible. Steep low-level lapse rates also will support
   isolated strong to severe gusts.

   ..Leitman.. 09/16/2024

   NOTE: THE NEXT DAY 2 OUTLOOK IS SCHEDULED BY 0600Z