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Source: US National Oceanic and Atmospheric Administration

SPC AC 100550

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1250 AM CDT Sat Oct 10 2020

Valid 111200Z – 121200Z

…THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE NORTHERN/CENTRAL PLAINS INTO THE UPPER MIDWEST…

…SUMMARY…
Isolated strong to severe thunderstorms may occur Sunday across
parts of the northern/central Plains into the Upper Midwest, with
marginally severe thunderstorms also possible across parts of the
Southeast.

…Northern/Central Plains into the Upper Midwest…
A large-scale upper trough over western Canada and the northern
Rockies will move eastward Sunday into central Canada, the
northern/central Plains, and Upper Midwest. This upper trough should
acquire a negative tilt by Sunday evening, while strong low to
mid-level south-southwesterly winds overspread these regions. At the
surface, a cold front will sweep eastward across the Dakotas and NE
Sunday afternoon/evening, and continue into the Upper Midwest Sunday
night. An EML will likely suppress convective development along the
front through much of the day until it encounters mid 50s to perhaps
lower 60s surface dewpoints late Sunday afternoon across far eastern
SD, southwestern MN, and vicinity. The steep mid-level lapse rates
present with the EML combined with modest low-level moisture return
ahead of the front should support a narrow corridor of around
500-1000 J/kg of MLCAPE by peak afternoon heating.

Strong forcing for ascent associated with the approaching upper
trough will likely encourage convective initiation by late Sunday
afternoon into the evening along the eastward-advancing cold front.
30-40 kt of effective bulk shear will foster organized updrafts,
with the steep mid-level lapse rates perhaps supporting some large
hail with initial convective development. The severe wind threat
should increase by Sunday evening into parts of MN and
western/central IA as storms congeal along the cold front and
quickly grow upscale into a fast moving line. This squall line
should gradually weaken with eastward extent across the Upper
Midwest Sunday night as it outpaces the already modest low-level
moisture return and weak instability. But, it may still pose an
isolated strong/gusty wind risk until the instability becomes too
meager to support surface-based storms. How far east this gusty wind
potential will last Sunday night remains uncertain, so have expanded
the Marginal Risk eastward into more of southeastern MN and
central/eastern IA. Latest guidance also suggests that there is less
potential for low-level moisture return into northern MN, so severe
probabilities have been reduced across this area.

…Southeast…
The remnants of Delta are forecast by the National Hurricane Center
to move northeastward from the TN Valley towards the upper OH Valley
on Sunday. The surface low is forecast to remain rather weak through
the period, although a warm front should also advance slowly
northward across the Carolinas through Sunday evening. Around 30-35
kt of mid-level southwesterly flow should be present across parts of
eastern GA into SC/NC in association with Delta’s remnant mid-level
circulation. NAM forecast soundings across these areas show enough
veering/strengthening of the wind profile in low/mid levels to
support modest storm organization. Around 500-1500 J/kg of MLCAPE
should also be present along/south of the warm front as low-level
moisture gradually increases and surface temperatures warm through
the day. Even so, mid-level lapse rates will remain poor, which
should temper updraft strength to some extent. Regardless, given the
marginal deep-layer shear and sufficient instability forecast,
isolated strong to damaging winds appear possible with any storms
that form either along an eastward-advancing surface trough or the
warm front. A brief tornado also cannot be ruled out, primarily near
the warm front Sunday afternoon/evening, where low-level flow is
forecast to be slightly stronger compared to locations farther
south.

..Gleason.. 10/10/2020

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