Day 1 Severe Weather Outlook

Valid 261630Z – 271200Z


The greatest severe-weather threats today will involve thunderstorm
winds and hail, in a corridor from western/central Missouri to
northeastern Oklahoma.

…MO/KS/OK through tonight…
A midlevel trough over the Dakotas is in the process of evolving
into a closed low near the international border. A series of
embedded speed maxima will rotate through the southern periphery of
the trough from NE/KS to the middle MS Valley. At the surface, a
cold front will continue to move southeastward across KS toward OK.
A weak low is expected to form near the intersection of this cold
front and a separate baroclinic zone near the KS/MO border by late
afternoon. This baroclinic zone will be reinforced and likely
shifted south by outflow and differential heating associated with
the ongoing storms near Kansas City, and the evolution of these
ongoing storms pose the greatest uncertainty in the forecast for
this afternoon/evening across MO.

Large buoyancy (MLCAPE of 3000-4500 J/kg) is expected in the warm
sector this afternoon as surface heating occurs within a moist
boundary layer (mid 70s dewpoints) beneath steep midlevel lapse
rates. The cap is weaker with eastward extent across MO, and the
ongoing storms could become rooted at the surface by early
afternoon, which would suggest the potential for large hail and
damaging winds this afternoon into central MO. In the wake of the
ongoing convection, renewed storm development is expected 21-23z
near the triple point and along the cold front near the KS/MO
border, though the northward extent of the threat is uncertain given
the influence of the ongoing storms. Deep-layer vertical shear will
be sufficient for supercells, while low-level hodograph
curvature/SRH will be maximized near the outflow-reinforced
baroclinic zone this afternoon/evening in MO. Cross-frontal shear
vectors with storm motions to the east-southeast 25-30 kt should
allow semi-discrete storms for a couple of hours late this
afternoon/evening, with an attendant threat for a couple of
tornadoes and isolated very large hail. Otherwise, cell mergers
should lead to upscale growth into clusters/line segments. Damaging
winds will become a primary threat with the clusters/line segments
in an environment favoring strong downdrafts, and the threat for
wind damage will persist into the early overnight hours.

Weaker forcing for ascent and a warmer elevated mixed layer will
tend to delay storm initiation until this evening farther southwest
along the cold front into northern OK. Wind profiles will be
sufficient for marginal supercells and/or organized clusters capable
of producing isolated large hail. Damaging winds should be the
primary threat given steep low-level lapse rates, large DCAPE, and
precipitation loading with updrafts in a large CAPE environment.

…IL/IN this afternoon…
A lead shortwave trough will eject northeastward from IL toward
northern IN and Lower MI and weaken this afternoon, along with an
accompanying surface cyclone in northern IL. Boundary-layer
dewpoints near 70 F are present in the warm sector of the cyclone,
though widespread clouds will tend to slow surface heating. There
is some potential for renewed storm development this afternoon in a
broken band across eastern IL into northwestern IN, where low-level
shear will be marginally favorable for supercells and perhaps a
brief tornado.

..Thompson/Cook.. 08/26/2019


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