SPC 2000Z Day 1 Outlook

Day 1 Outlook Image

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0246 PM CDT Sun Oct 11 2020

Valid 112000Z - 121200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS
AFTERNOON INTO EARLY TONIGHT FROM SOUTHWEST MN TO EXTREME NORTHEAST
KS...

...SUMMARY...
Severe thunderstorms are possible late this afternoon and evening
across the Upper Midwest and Middle Missouri Valley. A brief/weak
tornado or two and isolated damaging gusts will be possible this
afternoon across parts of the Carolinas.

...20Z Update..

...MN/IA/eastern NE this evening through early tonight...
Overall expectations outlined in the previous forecast (appended
below) remain unchanged. A narrow corridor of upper 50s/low 60s
dewpoints currently extends thorough eastern SD and into far
southeast ND and southwest MN. Cold front currently extends from
south-central Manitoba south-southwestward through central SD and
into northeast CO. Convective inhibition is beginning to erode
within the corridor of higher low-level moisture, with further
decrease in this inhibition anticipated over the next few hours.
Primary initiation zone still appears to be near the MN/SD border
south-southwestward into central NE around 22-00Z. Large hail is
possible initially, before storms evolve quickly into a line capable
of damaging outflow gusts.

...Carolinas...
Severe threat associated with the remnants of Delta is expected to
continue for the next several hours amid modest instability and
vertical shear. Primary threat is damaging wind gusts and a tornado
or two. Greatest threat potential exists in the vicinity of the warm
front, which currently extends from near LUX (in western SC)
northeastward to BBP (near the central NC/SC border) and then back
southeastward off the far southeast NC coast.

..Mosier.. 10/11/2020

.PREV DISCUSSION... /ISSUED 1122 AM CDT Sun Oct 11 2020/

...MN/IA/eastern NE this evening through early tonight...
In association with a midlevel shortwave trough moving eastward from
WY/MT, a surface cold front will progress east-southeastward across
the northern/central Plains today and reach the upper MS Valley
overnight.  A narrow corridor of low-level moistening is underway
ahead of the cold front (boundary-layer dewpoints of 56-60 F),
though the breadth and quality of the moisture return has been
limited by the influence of tropical cyclone Delta across the
Southeast.  The modest low-level moistening is occurring beneath a
plume of steep midlevel lapse rates near 9 C/km, which is also
contributing to a strong cap.  It appears that surface
heating/mixing, continued moisture advection, and focused ascent
along the front will all be necessary for thunderstorm initiation
this evening, starting around 23z near the MN/ND border and
developing southward toward eastern NE/western IA by late evening.

Strong/linear forcing for ascent along the front and storm motions
undercut by the front support rather rapid upscale growth into a
squall line, while the narrow corridor of buoyancy and cap will
limit the potential for pre-frontal storm development.  The steep
midlevel lapse rates and MLCAPE of 1000-1500 J/kg will support some
potential for large hail, especially with any embedded supercells
given effective bulk shear in excess of 40 kt.  Otherwise,
convection evolving quickly into a line suggests that damaging
outflow gusts will become the main threat this evening.  The severe
threat will diminish gradually overnight as buoyancy diminishes and
convective inhibition increases with eastward extent.

...Carolinas this afternoon...
The remnants of tropical cyclone Delta will continue to move
east-northeastward from the southern Appalachians and weaken. 
Vertical shear will remain at least marginally favorable for
supercells given lingering 30-40 kt southwesterly midlevel flow and
backed low-level winds/stronger low-level shear along a warm front
approaching the SC/NC border.  The threat for a brief tornado or two
will be focused where the convection in central SC (along the
primary confluence band) moves northeastward and interacts with the
zone of somewhat stronger low-level shear along the warm front this
afternoon.

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