Day 1 Severe Weather Outlook

Valid 121300Z – 131200Z

…THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM
SOUTHEASTERN MISSOURI AND NORTHEASTERN ARKANSAS TO PARTS OF THE
LOWER OHIO VALLEY…NORTHERN TENNESSEE AND CENTRAL KENTUCKY…

…SUMMARY…
The greatest severe-weather threat will be during the day from the
eastern Ozarks region to the lower Ohio Valley and central Kentucky,
featuring tornadoes, damaging wind and isolated large hail.

…Synopsis…
In mid/upper levels, a strong shortwave trough now over the Gulf of
Alaska will move southeastward and amplify, approaching the coastal
Pacific Northwest by 12Z tomorrow. As this occurs, a persistent/
cut-off cyclone now offshore from southern CA and northern Baja will
pivot northeastward across southern CA and the lower Colorado River
Valley tonight. In the northern stream, a series of closely
associated vorticity maxima and shortwaves will cause an eastward
shift of a synoptic trough now extending from north-central Canada
to the central Rockies. By 12Z, this should evolve into a
strong/closed cyclone over northern ON north or northeast of Lake
Superior, with cyclonic flow southward to the Ohio Valley.

Surface analysis at 11Z depicted a low near FSD with cold front
arching southwestward over northeastern CO, then into southern WY.
Another low was located over central KS near GBD, with warm front
east-southeastward over the Ozarks western/mid TN, and the
Carolinas. The northern low should move northeastward to near Lake
Superior and deepen considerably by 00Z in response to mid/upper-
level cyclogenesis. Meanwhile, the southern low should become a
triple-point feature as the cold front catches up, and is expected
to shift east-northeastward across central/southern MO to
southeastern IL by 00Z. The warm front then should extend across
parts of IN/OH and northern WV. Convective boundaries may exist
south of the warm front, with more baroclinicity. The cold front
should extend across southeastern MO, southeastern OK and northwest
TX. By 12Z, the cold front should reach southern PA, middle TN the
Arklatex region, and the southern TX Panhandle, with little or no
movement over the southern High Plains.

…Ozarks to central KY/northern TN and north TX…
Multiple episodes of severe thunderstorms are possible today, most
concentrated in and near the “enhanced risk” area, from mid/late
morning through afternoon. Supercells and organized multicells are
expected, with the potential for a few supercells to mature/persist
long enough for a tornado threat. Significant/EF2 rated tornadoes
may occur. Large hail is possible, especially from supercells, and
with modes potentially becoming clustered or messy with time, the
wind-damage potential will extend farther eastward. The severe
threat will extend down the front across AR to parts of north TX,
but in briefer, less-organized fashion with southwestward extent.

Activity is expected to develop initially in a zone of strong
low-level theta-e advection, south of the warm front and southeast
of the cold front, with MUCINH weakening as the warm sector
destabilizes from both that effect and weak diabatic surface
heating. Sustained surface-based parcels are uncertain with this
activity, especially in middle/northern parts, given its position on
the northern rim of the warm sector in a low-CAPE, moderate to
strong-shear setting. A second round of storms will form along the
surface cold front with access to greater CAPE and perhaps stronger
shear as well. Buoyancy generally will decrease northward through
the warm sector. By contrast, hodograph sizes increase, in
concordance with:
1. More-backed surface winds nearer to the warm front, and
2. A LLJ strengthening to 50-60 kt through the day, as part of the
mass response to the strong trough passing north of the area.
That trough also will tighten height gradients and boost deep shear,
rendering 45-50-kt effective-shear magnitudes in forecast soundings
in the “enhanced” area, weakening southward as pre-cold-frontal flow
becomes weaker and more-veered in the boundary layer.

A narrow corridor of theta-e advection ahead of the cold front,
combined with diurnal heating and relatively steep midlevel lapse
rates, will support a ribbon of 1000-2000 J/kg MLCAPE, transitioning
to pockets of 500-1000 J/kg MLCAPE south of the warm front as far
east as the southern Appalachians, away from areas of persistent
convection.

…West-central/southwest TX late tonight…
Isolated, perhaps widely scattered thunderstorms may develop
overnight — especially after 06Z. The main concerns would be large
hail and damaging gusts. Large-scale ascent will increase across
this area the last several hours of the period in two ways:
1. Early-stage mass response to the approaching Pacific
perturbation — a corridor of elevated low-level warm/moist
advection from the Rio Grande Valley.
2. Shots of DCVA related to small-scale perturbations (some
convectively induced/enhanced back across northwestern MX) ejecting
out of the cyclone within southwest flow.

Air-mass recovery above a relatively shallow/stable surface layer
may support MUCAPE in the 300-700 J/kg range, amidst 40-55-kt
effective-shear magnitudes. Enough residual subcloud drying may
persist to support gusts intense enough to penetrate to the surface.
Hail is possible, though buoyancy and lapse rates do not appear to
support more than marginally severe hail. Concentration of
convection in this regime is uncertain, with the potential very
conditional. Still, a consistency of synoptic and convection-
allowing guidance indicates non-negligible potential for a sustained
thunderstorm or two developing in a favorable environment,
compelling extension of marginal unconditional probabilities into
the region at this cycle. Activity may get as far as north-central
TX by the end of the period.

…Portions of AZ/southeastern CA…
Widely scattered to scattered thunderstorms should develop from
midday through this afternoon and evening, in an arc of strong
large-scale lift across northern Baja, southeastern CA and
northwestern Sonora. Isolated severe hail/gusts are possible. This
activity should move northwestward over parts of southern CA,
northward to northeastward over parts of AZ, beneath
correspondingly, strongly difluent mid/upper-level flow east of the
cyclone. Antecedent moist advection from subtropical latitudes has
resulted in anomalously rich low-level moisture, manifest as 50s to
60s F surface dew points across this region. In concert with
continued warm advection and muted diabatic heating beneath the
cloud cover, this should yield surface-based effective-inflow
parcels across much of the region. Peak MLCAPE will be highly
variable (300-1000 J/kg, locally higher) amidst favorably strong
mid/upper winds and cloud-layer shear. Mostly multicellular mode is
expected, though an isolated supercell or two may occur given the
presence of marginal effective shear and low-level SRH in a few
forecast soundings.

..Edwards/Goss.. 03/12/2020

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