Day 1 Severe Weather Outlook

Valid 101300Z – 111200Z

…THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
SOUTHERN CALIFORNIA AND FROM PARTS OF KANSAS TO THE WESTERN
OZARKS…

…SUMMARY…
Isolated severe thunderstorms are possible this afternoon over parts
of southern California, and tonight from parts of Kansas to the
Ozarks. Strong winds with showers and isolated thunderstorms also
may affect portions of the Mid-Atlantic.

…Synopsis…
In mid/upper levels, split flow will exist over the western CONUS,
related to a well-defined cyclone. The elongated vortex is evident
in moisture-channel imagery west of southern CA and Baja, centered
near 31N127W. The associated 500-mb low is forecast to move
east-northeastward to very near the northern Channel Islands or
Point Conception by the end of the period, preceded by a compact
vorticity lobe crossing coastal southern CA around 00Z.

Downstream from the split, a zonal to gently cyclonically curved
height pattern will prevail across most of the CONUS. A shortwave
trough now over the northern Rockies will move east-southeastward
across the north-central Plains this evening and tonight, reaching
portions of IA/eastern NE by 06Z, before moving eastward and
weakening. A downstream shortwave trough — containing multiple
vorticity maxima — was apparent initially across portions of IN, OH
and Lower MI. This trough is expected to deamplify gradually as it
moves eastward through the period, while also phasing the vorticity
maxima in a more coherent, linear, positively tilted fashion. By
00Z the perturbation should reach southern QC, VT, eastern NY and
eastern PA, exiting New England around 06Z.

At the surface, 11Z analysis showed a low over the ON/QC border
region northeast of Georgian Bay, with cold front southwestward
across northwestern OH, southern IN, northeastern AR, and the
Arklatex region, becoming quasistationary or a slow-moving warm
front near another low located near ROW. The Canadian low should
move across northern ME around 00Z, with cold front across southern
New England, NJ, and WV, to a weak frontal-wave low over the
northern AL/middle TN area, then mostly a warm front to a low over
the southwestern KS/northwestern OK region. By 12Z, the latter low
should move to central or east-central OK, with cold front to the
southern TX Panhandle and warm front across AR.

…Southern CA…
Widely scattered thunderstorms are expected to move northward to
northeastward across this region during the afternoon into early
evening, as the aforementioned mid/upper-level cyclone approaches.
The most sustained/intense cells may produce damaging to near-severe
gusts. Small hail also is possible, but lack of more-robust
buoyancy should limit potential for severe hail.

A combination of diurnal heating and theta-e advection will
destabilize the region, with heating and mixing factoring into
instability potential more in inland areas, and MLCINH being rather
weak outlook-wide. MLCAPE of 300-500 J/kg is expected. Though
low-level hodographs generally will remain small, veering and
strengthening of winds with height are expected through the buoyant
layer, with 80-90-kt winds ventilating convection at and just below
anvil level, between 450-500 mb. This should support some
convective organization, with multicell and transient supercell
characteristics possible, along with small bowing structures.

…Central Plains to Ozarks…
Isolated to widely scattered thunderstorms should develop across
portions of central/eastern NE this afternoon, near an inverted
trough and weak frontal zone maximizing low-level lift. Though
moisture will be weak, strong/gusty surface winds may occur as
modest downdrafts from convective plumes supported by 50-200 J/kg
MUCAPE encounter pockets of favorably mixed boundary layer.

A separate, better-organized convective episode — rooted above a
low-level stable layer and north of the surface front — is expected
to start this evening over central KS and move southeastward toward
the Ozarks, while expanding and becoming better-organized as it
encounters somewhat greater moisture toward the southeastern
KS/northeastern OK/western Ozarks area prior to 12Z. This activity
also should be supported by steep midlevel lapse rates related to
the large-scale ascent from the approaching shortwave trough, rooted
in high RH. Forecast soundings suggest a dry subcloud layer, then
in turn, a shallow but very stable near-surface profile with
diabatic cooling.

Details of the low-level vertical thermodynamic profile that
typically are not well-resolved by operational models will regulate
the ability of gusts accelerating through the dry layer to maintain
strong/near-severe intensity through the diabatically cooled
near-surface air mass. For now, severe probabilities are kept
marginal overall, emphasizing hail early, transitioning to isolated
wind late overnight and into early tomorrow morning, with
elevated/preconvective MUCAPE potentially reaching 1000-1500 J/kg
amidst 50-60-kt effective-shear magnitudes. A more-concentrated
severe threat may develop within this corridor, depending on
specific path and translational speed of a cluster that is likely to
be narrower than the outlook, but track uncertainties preclude such
a determination this soon.

…Mid-Atlantic region…
Scattered to widely scattered showers and isolated thunderstorms may
produce strong/locally damaging winds along/ahead of the surface
front this afternoon. Mainly due to concerns over convective
organization/coverage, the potential for severe/50-kt gusts still
appears too uncertain for an unconditional outlook area.

Surface flow ahead of the front will be veering with time as the
mid/upper perturbation deamplifies and the surface low passes well
north of the area. This will offer an offsetting effect of
weakening both low-level shear and frontal convergence during the
time when (weak) instability should be relatively maximized.
Forecast soundings still depict small MLCAPE around 50-300 J/kg,
supported by surface dew points 40s to low 50s F and assuming
sufficient heating behind areas of morning clouds/precip. Some of
that buoyancy may reside atop a well-mixed boundary layer where
pockets of sustained diurnal heating occur, enabling some downward
momentum transfer from a layer of 40-50-kt winds between 850-700 mb.

..Edwards/Goss.. 03/10/2020

$$

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