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Sep 24, 2021 1300 UTC Day 1 Convective Outlook
Click to see valid 1Z - 12Z Day 1 Convective Outlook
Updated: Fri Sep 24 12:48:21 UTC 2021 (Print Version | 20210924 1300Z Day 1 shapefile | 20210924 1300Z Day 1 KML)
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 Forecast Discussion
   SPC AC 241248

   Day 1 Convective Outlook  
   NWS Storm Prediction Center Norman OK
   0748 AM CDT Fri Sep 24 2021

   Valid 241300Z - 251200Z


   Thunderstorms associated with a marginal wind-damage threat may
   affect parts of New England today.

   Broad cyclonic flow in mid/upper levels will be maintained across
   most of the north-central and eastern CONUS through the period,
   related to two primary cyclones:
   1.  A compact circulation initially over Georgian Bay of Lake Huron,
   and adjoining parts of ON, with trough southward over portions of
   OH/WV.  The low is forecast to eject northeastward to western QC and
   fill, with the basal shortwave perturbation over PA ejecting
   northeastward over central NY and weakening considerably.
   2.  An initially elongated cyclone from southern Nunavut across
   northeastern MB to westernmost parts of northern ON.  The southern
   portion of this circulation will split eastward slowly into
   northwestern ON through the period, while basal troughing amplifies
   to portions of WI, eastern IA and northern MO by 12Z tomorrow.

   At the surface, the slow-moving cold front related to the lead
   mid/upper cyclone was analyzed at 11Z from an occlusion triple point
   over the St. Lawrence River area north of VT, across western MA,
   central CT, eastern Long Island, then over Atlantic waters to the
   southern part of central FL and the south-central Gulf.  Over New
   England, slow eastward shift of near-surface baroclinicity will be
   encouraged at least as much by precip on its cool side as governing
   synoptic processes, given the near-meridional motion and weakening
   of the associated mid/upper perturbation.  The frontal zone may not
   exit New England completely before the end of the period, and
   farther south, should stall across FL.

   Elsewhere, a surface cold front related to the central Canadian
   mid/upper cyclone was drawn from northwestern ON southwestward
   across central MN, southeastern SD, west-central NE, and eastern CO.
   By 00Z, this front should reach Lake Superior, eastern WI, northern
   MO, the OK Panhandle or northern TX Panhandle, to near RTN.  By 12Z,
   the front should reach Lake Huron, northwestern OH, the lowest part
   of the Ohio Valley, the Ozarks and northern OK, stalling farther
   west and perhaps beginning to return northward near the CO/NM line. 
   Isolated to widely scattered thunderstorms are possible near the
   front this afternoon and evening across parts of the upper Great
   Lakes region, with locally strong gusts approaching severe limits. 
   However, available moisture and instability will be weak, and
   unconditional severe potential appears too low for a categorical
   area at this time.

   ...New England...
   A persistent band of scattered showers and isolated to widely
   scattered embedded thunderstorms is expected to shift slowly
   eastward across the outlook area today into this evening, as
   embedded elements move predominantly northward.  Isolated damaging
   to severe gusts are possible, and a tornado is possible.

   As slight height falls spread over the frontal/prefrontal corridor
   today, low-level convergence will be maintained in the frontal zone,
   given the expected continuation of relatively backed near-surface
   flow in the warm-sector mass response.  This also will help to
   maintain favorable low-level shear, with forecast soundings
   reasonably depicting 150-250 J/kg effective SRH and 25-30-kt
   magnitudes of 0-1-km shear.  Rich moisture also will continue
   spreading over the region, with surface dew points commonly in the
   mid 60s to low 70s F.  This, along with modest diurnal heating in
   cloud breaks, will offset weak mid/upper-level lapse rates enough to
   support MLCAPE in the 500-800 J/kg range, locally/briefly higher,
   with minimal MLCINH.  Counterbalancing factors will include messy/
   training convective mode, and lack of more-substantial deep shear. 
   Effective-shear magnitudes should remain only around 25-35 kt, as
   the strongest winds aloft will remain behind the surface front.

   ..Edwards/Jewell.. 09/24/2021



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