Convective Outlook

Day 4-8 Outlook
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Day 4-8 Convective Outlook  
NWS Storm Prediction Center Norman OK
0326 AM CDT Sat Sep 18 2021

Valid 211200Z - 261200Z

Medium-range model output is quite varied concerning mid/upper flow
developments from the middle of the coming work week into next
weekend.  It appears that large-scale ridging may generally prevail
across much of the West into the Great Plains, with troughing across
and east of the Mississippi Valley.  However, short wave
developments remain much more unclear, starting with the evolution
of one significant short wave trough forecast to dig into the
central Great Plains at the outset of the period.  It may be that
this contributes to the evolution of a broad mid-level low over the
lower Ohio Valley vicinity by mid to late week.  However, it is not
certain that this will be accompanied by anything more than weak
cyclogenesis, and potential for substantive destabilization with an
appreciable risk of severe weather appears relatively low.  However,
given the apparent low pattern predictability, it is possible that
this could still change in later outlooks for this period.

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Updated 18 September 2021 | 3:29 am

SPC 0730Z Day 3 Outlook
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Day 3 Convective Outlook  
NWS Storm Prediction Center Norman OK
0200 AM CDT Sat Sep 18 2021

Valid 201200Z - 211200Z


Strong thunderstorms may impact parts of the lower Missouri Valley
and adjacent portions of the central Great Plains Monday afternoon
and evening, accompanied by at least some risk for severe weather.

Within a belt of westerlies extending across the mid-latitude
Pacific into North America, there may be some further amplification
of large-scale mid-level ridging shifting into western North America
and downstream large-scale mid-level troughing shifting east of the
Rockies through the Great Plains.  It appears that the troughing
will be comprised of at least a couple of smaller-scale
perturbations, including one digging across parts of the eastern
Great Basin and Colorado/Wyoming Rockies.  This likely will trail a
lead impulse, which might undergo considerable deformation across
parts of the northern Great Plains into the upper Mississippi Valley
Monday through Monday night.  There is substantive spread among the
various model output concerning this development, but guidance has
generally indicated that surface wave development along a cold front
advancing out ahead of the larger-scale mid-level troughing will
remain modest to weak.

...Parts of lower Missouri Valley and adjacent Great Plains...
Of primary concern regarding convective potential for this period,
further boundary-layer moistening ahead of the
eastward/southeastward advancing cold front could contribute to
moderately large CAPE beneath a remnant plume of elevated
mixed-layer air.  By mid to late Monday afternoon, the axis of this
plume may be in the process of becoming suppressed southeastward
across parts of eastern Nebraska and Kansas into western Iowa and
Missouri.  While deep-layer shear along this corridor may be
sufficient for organized convective development, including
supercells, it appears that there will be a tendency for storms to
be undercut by the front, with stronger mid/upper support for
large-scale ascent lagging to the cool side of the front.  This is
expected to mitigate the overall severe weather potential, but a
window of opportunity for convection capable of producing strong
wind gusts and severe hail might not be out of the question.

..Kerr.. 09/18/2021

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Updated 18 September 2021 | 2:02 am