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Day 1 Outlook Convective Tornado
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Z Outlook
  
Images courtesy of the NWS Storm Prediction Center
 Forecast Discussion

  
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ACUS01 KWNS 231256
SWODY1
SPC AC 231254

Day 1 Convective Outlook  
NWS Storm Prediction Center Norman OK
0754 AM CDT Thu Mar 23 2017

Valid 231300Z - 241200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM PARTS OF THE
CENTRAL PLAINS TO THE SOUTHERN HIGH PLAINS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PARTS OF
CENTRAL/SOUTHERN FLORIDA...

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

...Synopsis...
A progressive and somewhat highly amplified mid/upper-level pattern
will cover the contiguous U.S. this period, as a synoptic-scale
ridge shifts eastward across the Mississippi River Valley to the
Great Lakes and Southeast.  In the difluent upper pattern southeast
of that ridge, a weak shortwave trough was evident moving
southeastward across northern FL and the northeastern Gulf. This
feature will proceed southeastward across the rest of FL through
tonight.  Out west, split flow will predominate around an intense
trough now located from eastern NV to northern/central Baja.  A
basal vorticity max -- now apparent in moisture-channel imagery over
northern Baja -- will eject northeastward to eastern/southern NM by
00z then wind itself into a closed 500-mb low over southeastern CO
and southwestern KS overnight.  By 12Z, the accompanying, positively
tilted mid/upper trough should extend from that low south-
southwestward across the TX Panhandle and Llano Estacado to the
Trans-Pecos and central/southern Chihuahua.

At the surface, a diffuse cold frontal zone -- associated with a
mid/upper trough that has departed the East Coast, will sag
southward across the southern FL Peninsula, extending as a
decelerating baroclinic zone over the northeastern/north-central
Gulf.  The western limb of this frontal zone will shift or
re-develop rapidly northeastward across the southern/central Plains
today into a zone of warm frontogenesis over portions of NE and MO,
as a lee cyclone deepens over eastern CO.  A lee trough was analyzed
from a low over southeastern CO southward across eastern NM and
blending with a dryline there.  The dryline will mix eastward across
the CO/KS and TX/NM borders today, likely prior to severe-level
growth of any associated convection.  The trough and dryline will be
overtaken from the west by a Pacific cold front crossing the
southern Rockies this evening and overnight.  By 12Z, the combined
boundary should arch from a primary surface low over southwestern KS
across western OK and west-central TX to the Edwards Plateau, with a
warm to quasistationary front from the low northeastward across IA
to WI.

...Central Plains, southern High Plains...
Widely scattered high-based thunderstorms are expected to develop in
a regime of intense/deep mixing near the dryline and move
northeastward across the central/southern High Plains from late
afternoon into early evening.  Activity will cross a narrow corridor
of relatively maximized MLCINH corresponding to strong heating,
steep low/middle-level lapse rates, increasing large-scale lift, and
a proximal surface moist axis characterized by dew points generally
50s F.  Damaging wind and large hail will be the main concerns,
amidst vertical wind profiles suitable for both supercells and
organized multicellular structures.  Some of this activity may
congeal into a broken band of convection this evening as the
overtaking Pacific front supplies additional low-level forcing;
however, weakening near-surface instability with time and eastward
extent will help to mitigate lingering severe threat therewith.

Farther north, a mesoscale reservoir of relatively maximized
low-level moisture -- characterized by dew points mid-upper 50s F on
11Z-12Z surface charts and 0.8-1.1-inch PW in GPS readings -- was
evident over northwestern OK and southern KS, and was sampled to
some extent by the 12Z DDC sounding.  This area of moisture may
reach the vicinity of the developing frontal zone over
southern/central NE relatively unperturbed by strong mixing, while
underlying a regime of cold air aloft with 8.5-9 deg C/km midlevel
lapse rates this afternoon, amidst weakening MLCINH and
strengthening lift. That combination may support a pocket of
2000-2500 J/kg MLCAPE amidst 50-kt effective-shear magnitude,
suitable for supercell character with any relatively
sustained/discrete storm(s) that can develop and move into the
regime.  While coverage of any such convection is in question,
precluding a greater unconditional probability line at this time,
isolated very large hail or even a tornado cannot be ruled out.

...FL...
In addition to ongoing convection near the east-central coast,
widely scattered to scattered thunderstorms are expected to develop
throughout the day.  Convection will move through a regime of
broadly confluent/convergent, easterly to northeasterly flow in
low-middle levels, in and south of the frontal zone, amidst subtle
large-scale lift related to the northwest-flow perturbation aloft. 
Activity should translate generally southwestward with the
low/middle-level currents, into a diurnally destabilizing and at
least marginally moist air mass, with dew points mostly in the 60s
F.  A long-trajectory residual version of an elevated mixed layer
will supply steeper than climatological midlevel lapse rates to the
region today, atop that increasingly well-mixed preconvective
boundary layer, with insolation leading to erosion of already weak
MLCINH.

All that should lead to pre-storm MLCAPE in the 1000-2000 J/kg
range, with much of the buoyancy, largest LIs and strong storm-scale
internal parcel UVV located in icing layers ideally suited for hail
generation aloft.  With forecast vertical wind profiles yielding
weak low-level and deep-layer shear, the predominant mode should be
multicellular, limiting the potential for very large hail; however,
large volumes of small to marginally severe hail may be generated in
the most intense cores.  For midday to afternoon convection, the
projected well-mixed/inverted-v character to the subcloud
thermodynamic profile supports evaporative downdraft-parcel
accelerations and strong to locally severe winds as well.

..Edwards/Broyles.. 03/23/2017

$$