SPC 1200Z Day 1 Outlook
Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1250 AM CDT Mon May 09 2022
Valid 091200Z - 101200Z
...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FOR PARTS OF THE
UPPER MISSISSIPPI VALLEY...
...SUMMARY...
A complex and conditional severe risk will accompany multiple rounds
of thunderstorms through the period across the Upper Mississippi
Valley this morning through tonight. Large hail, damaging gusts, and
a few tornadoes are all possible.
...Synopsis...
A broad cyclonic upper flow regime will be in place across the
western half of the CONUS today, with an embedded mid-level impulse
ejecting into the Upper Mississippi Valley this afternoon and
evening. A 990 mb surface low should be established across central
SD this morning, and is poised to deepen considerably through the
day while tracking north-northeast towards Manitoba. Near and ahead
of the surface low, rapid northward advection of deep low-level
moisture should foster increasing buoyancy across the warm sector
given the presence of steep mid-level lapse rates. Scattered
thunderstorms (some strong to severe) are expected to be in progress
across the northern Plains at the start of the period, which will
complicate later afternoon/evening severe potential ahead of an
approaching cold front across the Upper Mississippi Valley. However,
modest convergence along the cold front, preceded by some airmass
recovery during the day, suggests that a conditional risk for more
widespread strong to severe thunderstorms will exist later in the
period.
...Upper Mississippi Valley - Early to Mid Morning...
The leading edge of an 80+ kt southwesterly 500 mb jet streak will
be overspreading the nose of a 50-60 kt 850 mb jet over the eastern
Dakotas at the start of the period (12Z), where robust elevated
convection will already be in place. While most guidance indicates
that these storms will be rooted around 850 mb, 50-60 kts of
effective bulk shear and well over 2000 J/kg MUCAPE should foster
the continued eastward progression of these elevated storms. Given
8+ C/km mid-level lapse rates, some severe hail will be possible
with the strongest storms moving from northeast SD into central MN.
Modest damaging-gust potential may also accompany storms where
mid-morning boundary-layer mixing may foster more efficient downward
momentum transport.
...Minnesota into far northwest Wisconsin - Afternoon...
A conditional severe risk may accompany any storms that manage to
intensify ahead of the cold front later into the afternoon/early
evening hours if airmass recovery can occur behind initial elevated
storms along the nose of the low-level jet. Guidance consensus
depicts strong tropospheric wind fields across MN by afternoon as
the breadth of the mid-level jet streak overspreads the Upper
Mississippi Valley and contributes to further intensification of the
surface low (centered over east ND by 18Z) and associated low-level
flow fields. Several guidance members depict long, curved hodographs
ahead of the cold front during the afternoon, which would support
organized storm structures (including supercells) capable of all
hazards. Despite strong low-level warm/moist-air advection
anticipated through the day, significant airmass modification would
be required to generate appreciable instability to support a more
substantial severe threat. Given uncertainties in airmass recovery,
higher severe probabilities have been withheld for now, but may need
to be introduced in future outlooks if greater recovery becomes
apparent.
...Southern Minnesota into Iowa and western Wisconsin - Evening...
Greater upper support will glance the Midwest along with the core of
the mid-level jet streak. As such, the greatest source of lift for
convection will be from low-level convergence associated with the
cold front, which will sweep across the Mississippi Valley during
the afternoon and evening hours. Convective inhibition is expected
to erode within a narrow corridor of the warm sector ahead of the
cold front by 00Z, from central and eastern IA into southern
MN/western WI. Mid to upper 60s F surface dewpoints overspread by a
mixed boundary layer and 8 C/km mid-level lapse rates will support
2000-3000 J/kg MLCAPE. Some veering with height through the
tropopause will yield modestly curved/elongated hodographs. With
convection expected to be relatively isolated ahead of the cold
front, small linear segments/semi-discrete supercells are expected
by late afternoon and will progress eastward through the evening.
Large hail, damaging gusts, and a few tornadoes may accompany the
more dominant, longer-lived storms.
..Squitieri/Darrow.. 05/09/2022
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