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Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0747 AM CDT Thu Apr 21 2022

Valid 211300Z - 221200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER PARTS OF THE
NORTHWESTERN OKLAHOMA TO CENTRAL KANSAS REGION...

...SUMMARY...
Large, damaging hail, severe gusts, and perhaps a tornado are
possible with any thunderstorms that can develop over parts of the
northwestern Oklahoma to central Kansas region.

...Synopsis...
A substantial mid/upper-level pattern change is underway across the
CONUS, increasingly emphasizing cyclonic flow across the West.  This
will occur in response to a large synoptic-scale cyclone --
initially centered approximately 500 nm west of the mouth of the
Columbia River -- and the troughing that extends southward past 30N
in the northeastern Pacific.  As several vorticity maxima orbit the
mid/upper low, the cyclone will move inland over the Pacific
Northwest tonight.  An intense, basal shortwave trough will move
eastward and onshore central/southern CA tonight as well.

By 12Z tomorrow, the resulting major 500-mb trough will extend from
southern BC through the low, to coastal southern CA, and farther
south, to the west of much of Baja.  In response to these
developments, heights should rise over much of the Great Plains
through the period, while falling west of the Rockies.  A very
subtle shortwave trough -- evident in moisture-channel imagery over
parts of southern NM, far west TX and Chihuahua, should move slowly
northeastward today, its northern fringe reaching parts of
southwestern KS and the TX/OK Panhandles around 00Z, in even weaker
form.

The 11Z surface analysis showed a low near AMA, with a weak cold
front westward over northeastern NM and a wavy warm front over
southwestern, central and northeastern OK, to northwestern AR.  The
low is expected to drift northward to northeastward and weaken
through the afternoon, while the warm front moves northward to
southern KS.  As lee cyclogenesis occurs overnight in CO, the front
will move northward across eastern CO and central/northern KS.  A
dryline was analyzed near a CDS-HOB-INK line and southward over the
TX Big Bend region.  This boundary will mix eastward through late
afternoon, as far as southwestern KS, northwestern OK, to northwest
TX, to near 6R6, before retreating rapidly northwestward across the
southern High Plains and western KS overnight.

...Central/southern Plains to parts of MO/IA...
Isolated to widely scattered, surface-based thunderstorms are
possible late this afternoon and this evening over parts of western
OK into central KS.  Any sustained convection could beco*e
supercellular, with up to significant (2+ inch diameter) hail sizes,
damaging gusts, and a brief/conditional window of tornadic
opportunity.  Severe potential in this regime is conditional, and
the "slight" area has been pared down to where the parameter space
is most favorable (below) and largest hail sizes possible, should
storms form.

In the area from northwestern OK to central KS, the already meager
large-scale effect of the weak, approaching southern-stream
shortwave trough may be offset by ambient synoptic height rises.  As
such, little (if any) mid/upper support is expected, and surface-
based convective potential will depend on sustained lift related to
the front and dryline beneath a weakening EML inversion.  This may
be sufficient for initiation anyway, given 3-4 hours of essentially
no MLCINH present in unmodified soundings from multiple synoptic
models and CAMs.  These progs, unsurprisingly in such a subtly
balanced scenario, appear:
1) Largely underdone on warm-sector surface heating and
2) Very inconsistent on convective production/duration.

The parameter space will be favorable, with surface dewpoints
generally in the upper 50s to mid 60s F and steep mid/upper-level
lapse rates supporting MLCAPE in the 2000-3000 J/kg range.  Low-
level flow will veer well with height, along and within about 100 nm
south of the front, contributing to 35-45-kt effective-shear
magnitudes.  Hodographs should enlarge considerably during the early
evening as boundary-layer moisture increases somewhat, with
effective SRH as high as the 350-450 J/kg range.  A 1-2 hour early-
evening time window will exist for large hodographs while effective-
inflow parcels are still surface-based -- if a storm can develop and
mature to take advantage.

More-certain, denser-coverage, but mostly less-intense convection is
expected tonight north of the front, in a broad, elevated low-level
warm-advection regime with steep preconvective lapse rates aloft. 
Isolated severe hail and perhaps a strong-severe gust may occur with
that activity atop a boundary layer that is still somewhat dry as
well as stable -- before convection/precip beco*e overly expansive
and messy in mode.  The greatest potential for large and perhaps
briefly significant hail will be on the southern rim of this
activity, closest to the surface warm front, where inflow of
relatively high-theta-e air atop the frontal surface will be least
impeded.

..Edwards/Dean.. 04/21/2022