Severe Storm Outlook Narrative (AC)
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000
ACUS01 KWNS 230536
SWODY1
SPC AC 230534

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1234 AM CDT Thu Mar 23 2017

Valid 231200Z - 241200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE THIS
AFTERNOON AND EVENING ACROSS CENTRAL AND SOUTHERN PORTIONS OF THE
HIGH PLAINS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS
SURROUNDING AREAS OF THE CENTRAL AND SOUTHERN PLAINS INTO THE MIDDLE
MISSOURI VALLEY...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS TODAY ACROSS
PARTS OF SOUTHERN FLORIDA...

...SUMMARY...
Strong thunderstorms are expected from parts of the central and
southern Plains into the mid Missouri Valley late this afternoon
into tonight, accompanied by potential for severe hail and damaging
surface gusts.  Thunderstorms with a risk for severe wind and hail
are also possible across parts of southern Florida.

...Synopsis...
Much of the nation will remain under the influence of split
westerlies emanating from the mid-latitude Pacific.  Within this
regime, one prominent northern stream trough is forecast to continue
to progress away from the Atlantic Seaboard.  As it does, models
suggest remnant southern stream troughing will linger over the
Bahamas/parts of southern Florida and the Caribbean, while short
wave ridging builds to its northwest (across the Southeast), ahead
of a more significant southern stream short wave trough forecast to
progress east of the southern Great Basin/lower Colorado Valley,
through the southern Rockies/High Plains by 12z Friday.

In association with this latter feature, models generally indicate
that significant surface cyclogenesis will initiate across parts of
northeastern Colorado during the day today, before redeveloping
southeastward into western Kansas tonight, in closer proximity to a
lower/mid tropospheric cyclone developing within the larger-scale
trough as it crosses the southern Rockies.  This should be
accompanied by considerable strengthening of southerly low-level
flow through portions of the Plains into Mississippi Valley,
particularly by tonight, when speeds around the 850 mb level are
expected to reach 50-70 kt across much of the southern into central
Plains.

Despite the favorable evolving large-scale flow, a substantive
return flow of moisture from the Gulf of Mexico appears unlikely
during this period due to recent drying across much of the Gulf
region.  This probably will limit the extent of the convective
potential today and tonight.  However, steep mid-level lapse rates
associated with a plume of elevated mixed-layer air emerging from
the southern Rockies/Mexican Plateau region may contribute to
sufficient destabilization to support at least some risk for severe
storm activity across the central and southern Plains into the mid
Missouri Valley.  A remnant plume of elevated mixed-layer air, which
has advected considerably south and east of the Plains, may also
contribute to some severe weather potential across parts of southern
Florida.

...Central and southern Plains into mid Missouri Valley...
The lack of substantive deep return flow of moisture, coupled with
downward mixing of dry lower/mid tropospheric air, will be
problematic with regard to severe thunderstorm potential today.
Guidance continues to suggest that the most substantive
boundary-layer moistening may be confined to a narrow corridor along
the lee surface trough across parts of western Kansas (perhaps as
far south as parts of the Texas/Oklahoma Panhandle region) into
southwestern Nebraska by the 21-23Z time frame this afternoon.  Even
this may be characterized by surface dew points only on the order of
55-60F.  However, beneath steep mid-level lapse rates, this may be
sufficient for a corridor of mixed-layer CAPE up to around 1000
J/kg, as a 50-70 kt southwesterly mid-level jet streak noses across
the region.

While the primary upper impulse may still be lagging near or to the
west of the Rockies, the environment along/east of the sharpening
dryline probably will be conducive to discrete supercell
development.  Some risk for at least weak/brief tornadoes may exist
with initial development, but severe hail may be the most prominent
risk with this activity as it develops northeastward/eastward
through early evening.

As the main impulse/developing mid-level circulation emerges from
the intermountain region this evening, storms with severe hail
potential may become focused within an area of strengthening
low-level warm advection across north central and northeastern
Nebraska.  Additional storm development may become focused on the
leading edge of stronger lower/mid-tropospheric cooling, from parts
of western Kansas into the Texas South Plains.  In the presence of
intensifying environmental wind fields, potentially damaging wind
gusts seem likely to be the primary threat with an evolving squall
line.  This could continue into the overnight hours, eastward toward
the lower Plains, but it remains unclear at this time if instability
will remain sufficient to support vigorous convection east of
western Kansas/Oklahoma and northwest Texas.

...South Florida...
Relatively steep mid-level lapse rates associated with remnants of a
plume of elevated mixed-layer air, and cold air aloft, could
contribute to an environment conducive to storms capable of
producing severe hail and wind gusts.  This may be aided by forcing
for ascent associated with an impulse digging into lingering upper
troughing across the region.  However, this remains at least one
point of considerable spread among the model output, and severe
weather potential remains too uncertain for more than 5 percent
severe probabilities at this time.

..Kerr/Dean.. 03/23/2017

$$


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