SPC 2000Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0232 PM CDT Fri Oct 23 2020

Valid 232000Z - 241200Z


Threat for a few severe storms capable of damaging wind, hail and
perhaps a tornado will continue this afternoon into the early
evening over a portion of the Great Lakes and Ohio Valley.

Forecast reasoning as laid out in prior outlooks remains reflective
of current convective/severe weather evolution across the outlook
area.  Some minor line adjustments have been made to account for the
western edge of the outlook areas, to reflect progression of
advancing convection.  Tweaks have also been made to the slight risk
area to account for counties currently within WW #501.  Otherwise
however, Local severe risk will continue across the outlook area
through early evening.

..Goss.. 10/23/2020

.PREV DISCUSSION... /ISSUED 1120 AM CDT Fri Oct 23 2020/

...Great Lakes/OH Valley this afternoon/evening...
Within the northern stream, a shortwave trough will progress from
the upper MS Valley to the upper Great Lakes by this evening.  An
associated surface cyclone in lower MI will gradually deepen and
move northeastward toward Lake Huron, as a trailing cold front
translates southeastward across Lower MI, the mid MS Valley, and the
southern Plains.

12z regional soundings support the possibility of both low-level
moisture advection from southwest-to-northeast toward Lower MI in
advance of the cold front, and downward mixing of moisture from
aloft with daytime heating.  The net result should be boundary-layer
dewpoints increasing into the low-mid 60s in southeast Lower MI and
the upper 60s across the lower OH Valley.  Some weak elevated
convection is ongoing in the warm sector across the lower OH Valley,
along the corridor of the low-level jet and where convective
inhibition is weak for parcels originating near 850 mb.  Additional
warm sector convection will be possible through the day as the low
levels destabilize with heating/mixing.  

The primary focus for thunderstorm development will be the zone of
ascent along and just ahead of the cold front through the afternoon.
MLCAPE is expected to range from 1000-2000 J/kg (greatest across the
lower OH Valley) in advance of the cold front, while low-midlevel
flow/shear will be stronger toward lower MI.  The net result will be
an environment favoring bands of storms capable of producing
damaging winds with sufficiently strong downdrafts and momentum
transfer.  A tornado cannot be ruled out with embedded rotating
storms given effective SRH close to 150 m2/s2 and effective bulk
shear near 35 kt, and marginally severe hail may also occur with
midlevel lapse rates of 7-7.5 C/km.  The severe threat will diminish
by late evening across OH as the boundary layer stabilizes and the
primary shortwave trough passes farther to the north. 

...East TX area through this evening...
Convection has organized as an MCS across north TX this morning with
a leading convective line and weak MCV.  The downstream environment
across southeast TX appears conducive to maintaining some form of
this convection through the afternoon, with the southwest extent of
the convection limited by more substantial midlevel capping.

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