Day 1 Severe Weather Outlook

Valid 121200Z – 131200Z


Severe thunderstorms with tornadoes, large hail and wind damage are
likely across parts of the mid Mississippi, Ohio and Tennessee
Valleys this afternoon and evening. Isolated severe storms will also
be possible in parts of the Arklatex and Ozarks. A few marginally
severe storms could occur in the Desert Southwest.

…Arklatex/Mid Mississippi Valley/Ohio and Tennessee Valleys…
An upper-level trough will move eastward across the Great Plains
today as a 75 to 100 kt mid-level jet moves across the central
Plains and upper Mississippi Valley. A secondary 60 to 80 kt
mid-level jet will move into the lower Ohio Valley. Increasing
low-level moisture ahead of a cold front will result in surface
dewpoints in the 60 to 65 F range from southeast Missouri and
northeast Arkansas eastward across western Kentucky and Tennessee.
This will lead to the development of an east to west corridor of
instability by afternoon. Thunderstorms are forecast to first
develop across south-central Missouri this morning and spread
eastward into western Kentucky and northwest Tennessee by early to
mid afternoon. The instability combined with strong deep-layer shear
will be favorable for severe storms.

Concerning the finer-scale details, scattered thunderstorms should
develop near a surface low in south-central Missouri around midday
with this convection moving eastward across southern Illinois nad
into western Kentucky early this afternoon. This activity should
stay along and to the south of a warm front extending eastward from
the surface low. Cells that can move into the air south of the warm
front will likely be surface-based, and will have the greatest
chance to become severe. Forecast soundings near Paducah this
afternoon show MLCAPE near 750 J/kg and 0-6 km shear of 60-65 kt.
Hodographs show strong speed shear in the lowest 1 km AGL with
strong directional shear below 850 mb. This will be favorable for
supercells and tornadoes. The tornado threat will likely be greatest
in association with the 850 mb and 500 mb jet couplet. Storm
relative helicities of 450 to 550 m2/s2 will support a threat for
significant tornadoes as well. Supercells that move along and
parallel to the warm front should have the greatest potential for
tornadoes. A threat for large hail and wind damage will also be
associated with supercells. A wind-damage threat will also be
possible with bowing line segments.

Further southwest across the Arklatex and Ozarks, a cold front will
advance southeastward this afternoon. Model forecast suggest that a
pocket of instability will develop ahead of the front. Convection
that initiates along and ahead of the front will move
east-southeastward across the region. Surface dewpoints in the mid
60s F, MLCAPE of 500 to 750 J/kg and 0-6 km shear near 50 kt will
likely support the development of isolated severe storms with
potential for wind damage and hail.

…Desert Southwest…
An upper-level low will move slowly eastward off the coast of
southern California. A 75 to 100 kt mid-level jet will move through
the southeastern part of the system with the exit region affecting
the Desert Southwest today. A surface low will deepen beneath the
mid-level jet as a moist airmass with surface dewpoints in the 55 to
60 F range remains in place across southeast California and
southwest Arizona. Surface heating will allow a pocket of
instability to develop by afternoon with MLCAPE reaching 500 J/kg in
many locations. As scattered thunderstorms develop this afternoon,
the instability combined with moderate deep-layer shear should be
sufficient for a marginal severe threat. Wind damage and hail will
be possible with the stronger multicell clusters and short

..Broyles/Wendt.. 03/12/2020


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