SPC Aug 28, 2020 0600 UTC Day 1 Convective Outlook

SPC 1200Z Day 1 Outlook

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
1237 AM CDT Fri Aug 28 2020

Valid 281200Z - 291200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE UPPER MISSISSIPPI VALLEY...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
THE LOWER GREAT LAKES AND TENNESSEE VALLEY...

...SUMMARY...
Severe storms with large hail, damaging winds and a couple tornadoes
are possible today over parts of the upper Mississippi Valley and
southern Great Lakes. Other severe storms may occur across the
Tennessee and lower Mississippi Valleys with a few tornadoes and
damaging winds the primary threats.

...Upper Mississippi Valley/Southern Great Lakes...
An upper-level trough will move southeastward into the upper
Mississippi Valley today. At the surface, a cold front will advance
southeastward across the upper Mississippi and mid Missouri Valleys.
Ahead of the front, dewpoints in the lower to mid 70s F will
contribute to moderate to strong instability by afternoon. By 21Z,
the front is forecast to be located from near Omaha, Nebraska
northeastward to Madison, Wisconsin. Thunderstorms are expected to
form along the front during the mid afternoon with convection moving
southeastward across the region. Multiple line segments appear
likely to organize along the front during the late afternoon and
early evening.

Forecast soundings along the instability axis for 00Z/Saturday show
MLCAPE in the 2000 to 3000 J/kg range. Wind profiles are forecast to
have strong speed shear in the mid-levels, resulting in 0-6 km shear
of 40 to 50 kt. In addition, 700-500 mb lapse rates are forecast to
be near 7.0 C/km. This environment should support supercells with
large hail. However, supercell development may remain confined to
areas that destabilize the most. The favored storm mode could
transition to linear as low-level convergence increases along the
boundary in the early evening. Wind damage would be possible with
the faster moving line segments. The severe threat could continue
into the mid to late evening with the more persistent line segments.

Further to the east across the southern Great Lakes region, the
airmass will be very moist with surface dewpoints in the 70s F. In
response, a corridor of moderate to strong instability is forecast
to develop from northern Indiana into central Ohio. Thunderstorms
that can form along this corridor of instability during the late
afternoon will have potential to produce damaging wind gusts and
hail.

...Tennessee and Lower Mississippi Valleys...
Tropical Depression Laura will move east-northeastward across the
mid Mississippi Valley today. Multiple bands of convection
associated with Laura will move across northern sections of the
central Gulf Coast states and the Tennessee Valley. Pockets of
moderate instability are forecast to develop in some areas as
surface temperatures warm. This combined with strong low-level shear
profiles associated with Tropical Depression Laura, will create
conditions favorable for rotating storms and possibly a couple
tornadoes. The area with the greatest tornado potential should be
from north-central Mississippi northeastward into western and middle
Tennessee where low-level shear is forecast to be maximized this
afternoon.

...Central Plains...
West-northwest mid-level flow will be in place today across the
central Plains as a cold front advances south-southeastward across
the region. By 21Z, the front should be positioned from western
Kansas northeastward into far southeast Nebraska. Along and south of
the front, surface dewpoints in the lower to mid 60s F will allow
moderate instability to develop by afternoon. Although large-scale
ascent across the central Plains will be weak, increasing low-level
convergence along the front should be enough for isolated
thunderstorm development. The moderate instability, combined with
steep low to mid-level lapse rates and 0-6 km shear in the 20 to 30
kt range, will support an isolated severe threat. Hail and strong
gusty winds will be the primary threats near and after peak heating.

..Broyles/Lyons.. 08/28/2020

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from SPC Forecast Products http://www.spc.noaa.gov/products/outlook/day1otlk_1200.html