SPC AC 241248
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0748 AM CDT Fri Sep 24 2021
Valid 241300Z - 251200Z
...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS PARTS OF NEW
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.
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.
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