SPC AC 172000

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0300 PM CDT Tue Sep 17 2024

   Valid 172000Z - 181200Z

   ...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
   THE CENTRAL HIGH PLAINS...

   ...SUMMARY...
   Thunderstorms capable of producing scattered to numerous severe wind
   gusts and isolated large hail should occur this afternoon and
   evening across much of the High Plains. Some of the wind gusts may
   be significant (75+ mph).

   ...20Z Update...
   Relatively minor adjustments were made to the wind probabilities
   across parts of the southern Rockies/High Plains where convection
   has increased and shown some signs of linear organization. A
   significant wind gust or two is possible as this activity tracks
   north-northeastward into better moisture. The remainder of the
   forecast remains on track and is unchanged.

   ..Wendt.. 09/17/2024

   .PREV DISCUSSION... /ISSUED 1126 AM CDT Tue Sep 17 2024/

   ...High Plains and Central/Southern Rockies...
   Expansions have been made to the Slight and Enhanced Risks across
   the northern/central High Plains to account for latest trends in
   guidance and observations. The Marginal Risk has also been expanded
   westward to include more of western CO and western/central WY, where
   isolated strong to severe winds are already ongoing with high-based
   convection.

   A strong upper trough/low over the Great Basin will pivot
   east-northeastward across the northern/central Rockies through this
   evening. An associated 50-60 kt mid-level jet will move eastward
   across the central/southern Rockies this afternoon, and over parts
   of the adjacent High Plains through this evening. As large-scale
   ascent with the upper trough and mid-level jet overspreads these
   regions, a surface low is forecast to deepen over eastern MT through
   this evening, while another low also develops over the central High
   Plains. A surface trough will extend southward from the central High
   Plains low, and it should sharpen as it develops eastward across the
   central/southern High Plains through the day.

   Greater low-level moisture, characterized by generally mid 50s to
   low 60s surface dewpoints, will gradually increase today along/east
   of the surface trough, while remaining more limited across the
   higher terrain of the central/southern Rockies. Moderate instability
   should develop to the east of the surface trough across much of the
   High Plains through peak afternoon heating. Strengthening deep-layer
   shear with the approach of the mid-level jet will support organized
   convection, including some potential for initial supercells across
   the northern High Plains (eastern MT and vicinity). Related threat
   for large hail will exist with any of this more discrete convection.
   Otherwise, thunderstorms will continue to develop this afternoon
   over the central/southern Rockies while producing isolated strong to
   severe wind gusts in a weakly unstable but strongly sheared
   environment. This activity will eventually spread off the higher
   terrain and into the central/southern High Plains.

   Steepened low-level lapse rates with robust daytime heating, along
   with more limited/shallow low-level moisture along/west of the
   surface trough, will likely support a gradually increasing severe
   wind threat with the high-based convection spreading
   east-northeastward across the southern/central Rockies. As these
   clusters and linear segments encounter greater low-level moisture
   and related instability along/east of the surface trough across the
   High Plains late this afternoon into early evening, the threat for
   scattered to numerous severe wind gusts should increase. This may
   particularly be the case across parts of the central High Plains,
   where the best combination of steepened low-level lapse rates,
   moderate instability, moderate to strong deep-layer shear, and
   greatest linear convective signal exists in 12Z guidance. Some
   chance for isolated gusts of 75+ mph may also be realized across
   this region with the more intense clusters spreading
   east-northeastward in tandem with the mid-level jet. The threat for
   severe winds is expected to gradually decrease with eastward extent
   across the Plains this evening as convection encounters increasing
   MLCIN with nocturnal cooling of the boundary layer.

   ...Florida...
   A weak mid/upper-level low will remain centered over the Southeast
   today. Modest mid-level westerly flow will persist over FL on the
   southern periphery of the upper low. Scattered to numerous
   thunderstorms should develop later this afternoon along/south of a
   remnant front, and along the Atlantic Coast sea breeze. Even with
   daytime heating of a rather moist low-level airmass, poor mid-level
   lapse rates should temper updraft intensities to some extent, even
   as moderate instability develops. That said, the stronger cores may
   be capable of producing isolated strong to damaging winds, with
   20-25 kt of deep-layer shear aiding in modest thunderstorm
   organization. Some adjustments have been made to the Marginal Risk
   across parts of the FL Peninsula, based on latest observational
   trends and 12Z guidance.

   ...Southeast Virginia/Northeast North Carolina...
   A weak convective band is ongoing across far northeast NC into
   southeast VA, on the northeast side of a mid/upper-level low over
   the Southeast. Occasional strong/gusty winds may occur with the more
   robust cores this afternoon as the convective band pivots further
   into southeast VA. Even so, weak instability, modest low/mid-level
   flow, and related weak shear should temper overall thunderstorm
   organization. Accordingly, severe wind potential should remain low.

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