Day 1 Severe Weather Outlook

Valid 191300Z – 201200Z

…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
THE LOWER MISSOURI VALLEY REGION AND IOWA…AS WELL AS FROM THE
OZARKS TO THE LOWER OHIO VALLEY…

…SUMMARY…
Severe thunderstorms appear possible through tonight over a broad
part of the central U.S., with the greatest threats over parts of
southeastern Nebraska/southern Iowa/northern Missouri, and from
parts of the eastern Ozarks to the lower Ohio Valley.

…Synopsis…
In mid/upper levels, the closed cyclone near RNO should meander
erratically over western NV and the adjoining Sierra through the
period. A strong shortwave trough — initially apparent in
moisture-channel imagery over the Four Corners region — will pass
from the peripheral cyclonic-flow field of the NV low, through a
broader-scale ridge over the south-central Rockies, then across the
central Plains late this afternoon into evening. Increasing
cyclonic flow over the upper Mississippi Valley — around a
northern-stream perturbation digging southeastward over the Dakotas
and MN — then will absorb and deamplify the central Plains feature.
To its south, a belt of strong deep-tropospheric southwesterlies
will cover much of the central/southern Plains and middle
Mississippi Valley.

At the surface, a cyclone was analyzed at 11Z over
east-central/northeastern CO, with a synoptic warm front eastward
across northern KS to central MO and western KY. The low is
expected to move along the synoptic frontal zone and across the
northern KS/southern NE area today, before redeveloping
northeastward over Lakes Michigan and Huron overnight, in response
to the blending of the shortwaves to its west. A trailing cold
front will sweep southward over the central/southern Plains and
mid/upper Mississippi Valley through tomorrow morning. The cold
front should reach southern WI, northern MO, southeastern KS, and
the TX Panhandle by 12Z. The warm front will cross portions of
southeastern NE, northwestern MO, IA, and northern IL ahead of the
surface low(s) this afternoon and evening.

The threats should be relatively maximized in and near the
“Enhanced” areas, with a relative minimum (but still at least
isolated severe thunderstorms possible) in between, and trailing
southwestward across portions of TX.

…Northern MO/NE/IA and vicinity…
Scattered thunderstorms, including several supercells, are expected
to develop in one or two arcs today, near the southern NE/IA warm
front and across parts of northeastern KS/northwestern MO, near the
nose of a low/middle-level dry slot forced by the ejecting shortwave
trough. Deep-layer forcing for ascent will strengthen through the
day as the mid/upper perturbation approaches. The related mass
response will lead to increasing low-level lift and backing of
surface flow along and very near the warm front, though veered
surface winds are probable southward into the warm sector. The
frontal zone will act as a localized low-level SRH/lift maximum,
supporting tornado potential with any storms that can maintain
enough residence time in it before encountering too much poleward
stable air.

Thermodynamic support will arise from steepening midlevel lapse
rates, diurnal surface heating behind early cloud cover/precip, and
a ribbon of favorable low-level moisture advection into the
corridor. Forecast soundings suggest dew points ranging from low
50s west to low/mid 60s east can support a ribbon of 1500-2500 J/kg
peak MLCAPE over parts of southeastern NE, northwestern MO,
northeastern KS and southwestern IA. Buoyancy should decrease
eastward amidst weaker lapse rates, and westward amidst less
moisture. Deep shear will be highly variable under strongly
difluent mid/upper-level flow, with contribution from near-frontal
backing, but generally stronger near the Missouri Valley and over
IA. Though overlapping somewhat, the greatest thunderstorm-wind
threat appears somewhat eastward-displaced with respect to large
hail. The convective regime and associated severe potential should
spread eastward across the outlook area into early evening, before
encountering gradually stabilizing boundary-layer air.

…Ozarks to Ohio Valley, southwestward to north TX…
This area is somewhat more uncertain in terms of severe coverage,
and the greatest potential likely will be delayed until late
afternoon into evening. Still, sufficient destabilization is
anticipated behind the morning convective/precip plume to support
severe thunderstorms in an environment of strong deep shear. In
aggregate, scattered thunderstorms should form in a southwest/
northeast-oriented low-level convergence zone ahead of the surface
cold front, in an environment characterized by weakening MLCINH,
boundary-layer heating and theta-e advection, rich low-level
moisture, and deeply buoyant profiles despite modest lapse rates
aloft. This combination should support a corridor of 1000-2000 J/kg
peak MLCAPE.

Hodograph enlargement by a strong southwesterly LLJ will support
effective SRH 300-400 J/kg, at least from northern AR to
southwestern IN. Any sustained supercells or bow/LEWP features in
this setting will be capable of tornadoes, as well as damaging wind
and sporadic hail. Greater instability but weaker shear is expected
southwestward across the Arklatex to northeast TX, and vice versa
into IN.

…South TX overnight…
Widely scattered to scattered thunderstorms may develop overnight
over parts of Mexico adjacent to the LRD-DRT corridor, and/or in the
trailing segment of the convergence zone discussed above, and move
northeastward into south TX. Any such activity may produce severe
gusts/hail, and a tornado cannot be ruled out. The air mass across
the region will remain richly moist, with surface dew points mid 60s
to lower 70s F under favorable lapse rates aloft, supporting
1500-2500 J/kg MLCAPE and slightly larger MUCAPE. Deep shear will
be weaker than farther northeast, but with effective magnitudes
still around 40-45 kt. Numerical guidance supports the threat but
differs on timing/coverage. At this time, too many uncertainties
remain regarding the most dominant area(s) for potential severe-
thunderstorm development, as well as storm coverage, to boost
unconditional probabilities. Some part of south TX may need an
upgrade in later outlooks.

..Edwards/Goss.. 03/19/2020

$$

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