Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 201256
SWODY1
SPC AC 201254

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0754 AM CDT Fri Apr 20 2018

Valid 201300Z - 211200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS OVER A SMALL PART
OF THE LLANO ESTACADO AREA OF EASTERN NEW MEXICO AND TEXAS...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ELSEWHERE ACROSS
PARTS OF EASTERN NEW MEXICO AND WEST TEXAS...

...SUMMARY...
Severe hail and gusts are possible today from thunderstorms over
portions of the southern High Plains, mainly mid-afternoon into
early evening.

...Synopsis...
In mid/upper levels, a progressive/split-flow pattern will continue
over the contiguous U.S.  The dominant and most convectively
pertinent feature is a pronounced cyclone now centered over southern
UT, with trough southward to Sonora.  The accompanying 500-mb low is
forecast to reach south-central CO by 00Z, with trough south-
southwestward to northern Chihuahua.  By 12Z, the cyclone aloft
should cover much of the central/southern High Plains, with trough
southward over the TX Big Bend region.

At the surface, 11Z analysis showed an occluded/cold front arching
southeastward from a low over northwestern CO, through a weak low
over northwestern NM, then southward over western NM and
southwestward over northern Sonora.  A dryline was developing ahead
of the front over south-central NM.  The diffuse western remnants of
an old frontal zone -- better defined over the Gulf -- extended
across northeastern MX and far west TX into south-central NM.  Each
of these boundaries is forecast to move eastward/northeastward
today, with a narrow corridor of convectively favorable return flow
between them that will shift into eastern NM and the lower Pecos
Valley region.

...Southern High Plains...
Two primary convective episodes, in meridional bands, are forecast
over the region today -- in chronological order:

1.  Thunderstorms forming from late morning through at least
mid-afternoon across the Panhandles and perhaps as far south as
parts of the Permian Basin -- a southern extension of a longer
elevated convective regime also forming across portions of the
central High Plains.  This convection will outpace the eastward
extent of surface-based destabilization, but at least from the
Panhandles northward, should persist for a few hours while moving
northeastward toward western OK and central KS.  This activity
should be supported by a layer of elevated low-level warm advection,
low/middle-level moistening, and weak CINH.  MUCAPE generally
300-800 J/kg is predicted, rooted near 700 mb.  Isolated subsevere
hail may reach the surface, but with steeper lapse rates remaining
farther west, and effective-shear magnitudes expected to be only in
the 25-35-kt range, organization should be too weak in this regime
for an organized severe threat.

2.  Convection forming mid/late afternoon near the dryline/cold
front and moving east-northeastward to northeastward across the
narrow (but also eastward-shifting) sliver of favorably heated/
surface-based warm-sector air.  A few supercells are possible with
damaging gusts and large hail.  Lapse rates aloft will steepen with
time throughout the afternoon as the mid/upper trough approaches and
related large-scale DCVA move over the area.  Below that, within
about 75 nm east of the boundary, and south of I-40, surface dew
points 40s F and heating with temps into the 60s and low 70s should
be sufficient at that elevation to support preconvective MUCAPE
reaching 1000-1500 J/kg.

The small area of 15%/slight-risk upgrade represents the most
densely concentrated/unconditional severe threat, with buoyancy/
theta-e decreasing to the north, deep lift and convective coverage
decreasing to the south, and the most favorable overlaps in the
CAPE-shear parameter space generally between HOB-TCC.  Activity will
move with an eastward component faster than the translation of the
narrow surface-based inflow plume, ultimately outrunning it east of
the TX/NM border.  A slight eastward shift of the eastern rim of the
marginal-risk area is made, given conditional potential for activity
to reach part of the I-27 corridor this evening before weakening
below severe levels.

..Edwards/Mosier.. 04/20/2018

$$


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