re: Severe weather - DFW Metroplex (and now across the South)

quote:

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SPC AC 281226

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0726 AM CDT Tue Apr 28 2020

Valid 281300Z - 291200Z

...THERE IS A MODERATE RISK OF SEVERE THUNDERSTORMS ACROSS PARTS OF
EASTERN OKLAHOMA...FAR WESTERN ARKANSAS AND NORTHEAST TEXAS...

...SUMMARY...
Widespread severe thunderstorms are expected to develop this
afternoon and evening across parts of the Ozarks into the southern
Plains. Damaging winds with gusts possibly exceeding 65 mph, very
large hail and a few tornadoes can be expected.

...Southern Plains/Arklatex/Ozarks/Mid to Upper Mississippi
Valley...
An upper-level trough will dig southeastward from the northern
Plains into the central Plains today. An associated mid-level jet
will dive southeastward into the central Plains. The leading edge of
the stronger mid-level flow associated with the jet will overspread
a corridor of maximized low-level moisture located from northeast
Texas north-northeastward into the Ozarks. Moisture advection ahead
of the system, will increase surface dewpoints into the 60s F across
most of Oklahoma, Arkansas and Missouri by afternoon. In response to
surface heating, the moisture will contribute to moderate
destabilization with MLCAPE values reaching the 2000 to 4000 J/kg
range across parts of the moist sector. Thunderstorms are forecast
to first initiate to the south of a surface low on the northern edge
of the moist airmass in eastern Iowa around midday. Thunderstorms
will then likely develop south-southwestward along a cold front from
northern Missouri into southeast Kansas during the afternoon. This
convection is forecast to grow upscale as a squall line organizes
and moves south-southeastward across the southern Plains and Ozarks.
The moisture and instability combined with strong deep-layer shear
will be favorable for severe thunderstorms across a widespread area.

RAP forecast soundings along the most unstable parts of the front
during the late afternoon increase MLCAPE into the 3000 to 4000 J/kg
range and show 0-6 km shear generally from 30 to 40 kt. This
environment will support supercells with large hail, especially
early in the event before the squall line becomes organized. The
potential for supercells with hailstones greater than 2 inches in
diameter is expected to be greatest from far southeast Kansas into
central Oklahoma and far northern Texas where instabilty is forecast
the strongest. Supercells early in the event may also be accompanied
by a tornado threat. The tornado threat is expected to be greatest
across eastern Oklahoma and far western Arkansas where a low-level
jet will strengthen during the late afternoon and early evening.
Forecast soundings in this area increase 0-3 km storm relative
helicities into the 250 to 400 m2/s2 range suggesting a more
substantial tornado threat will be possible. For this reason, have
added a 10 percent tornado probability contour across parts of the
moderate risk area.

Increasing low-level convergence during the early evening along the
front should gradually result in the development of a nearly
continuous squall line. This squall line is forecast to become more
organized as it moves southward across southwest Missouri, western
Arkansas and east-central Oklahoma. Wind damage will be likely along
the leading edge of the squall line. Wind gusts over 65 kt will be
possible ahead of the faster moving parts of the squall line. A few
tornadoes may also occur with rotating cells embedded in the line. A
widespread wind damage threat should continue into parts of
north-central and northeast Texas during the mid to late evening
before a gradual weakening takes place due to overnight decreasing
instability.

..Broyles/Guyer.. 04/28/2020

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