SPC AC 121638

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   1138 AM CDT Thu Sep 12 2024

   Valid 121630Z - 131200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS EASTERN
   MONTANA AND WESTERN NORTH DAKOTA...

   ...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS THE
   SOUTHEAST...

   ...SUMMARY...
   A few tornadoes are possible today from northern Florida to
   north-central Alabama. Scattered large hail and severe wind gusts
   (some significant) are expected across eastern Montana into western
   North Dakota this afternoon through early/mid-evening.

   ...Northern Great Plains including eastern MT/western ND...
   Have upgraded portions of eastern Montana and western North Dakota
   for a heightened/focused potential for severe storms capable of
   large hail and potentially significant-caliber severe wind gusts
   later this afternoon through around mid-evening.

   Strong-severe thunderstorms are expected, along and behind the
   front, as early as early/mid-afternoon, with the most probable
   severe potential arising from activity moving north-northeastward
   from northern Wyoming and far southern Montana. High-based
   supercells and multicells are possible, offering large hail and
   severe wind gusts. These storms may aggregate and grow upscale into
   one or two bowing clusters, racing north-northeastward accompanied
   by a swath of better-organized, potentially significant (65+ kt)
   gust potential. 

   This scenario will be supported by trough-related cooling mid-level
   temperatures and steep low/mid-level rates atop a modestly moist
   boundary layer, as the exit region of a strong upper-level jet
   overspreads the region. MLCAPE of 500-1200 J/kg and 45-55 kt
   effective-shear magnitudes will support the severe potential
   including fast-moving and relatively long-lived storms into western
   North Dakota. The near-surface airmass over the central Dakotas
   should stabilize with time and eastward extent tonight, limiting
   duration and eastward shift of the threat.

   ...Southeast...
   Current Tropical Depression Francine is forecast to continue moving
   generally northward over Mississippi, and to lose definition as it
   interacts more closely with the mid/upper-level low. Associated
   deep-layer winds will continue to weaken, but low-level shear will
   remain favorable through most of the day, well to the east-southeast
   of its center. A destabilizing airmass along and south of the
   warm/marine front will support a continued, slow inland shift of
   supercell potential amid enlarged hodographs. The favorable
   environment will be bounded on the west by a dry slot and veering
   low-level flow, on the northeast by more stable air in heavy precip
   north of the front, and on the south by a gradual weakening of
   low-level winds and shear as the cyclone center pulls away and
   fills. In the favorable corridor, where inflow parcels are
   surface-based, expect effective SRH in the 200-500 m2/s2 range, very
   rich moisture, low LCL, and weak MLCINH. In the shorter term, a
   tornado threat may persist this afternoon across northern Florida,
   while potentially increasing later this afternoon through early
   evening near the warm front across southern into central Alabama.

   ..Guyer/Jirak.. 09/12/2024

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