Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 191250
SWODY1
SPC AC 191248

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
0748 AM CDT Thu Oct 19 2017

Valid 191300Z - 201200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER PORTIONS OF
SOUTHERN NEW MEXICO AND FAR WEST TEXAS...

...SUMMARY...
Isolated severe hail or thunderstorm gusts are possible this
afternoon into early evening, over parts of southern New Mexico and
far west Texas.

...Synopsis...
The upper-air pattern will amplify through this period, as strong
troughing now over the northeastern Pacific moves partly inland
across the West Coast overnight, and downstream ridging amplifies
while crossing the Mississippi Valley.  By 12Z, this evolution
should lead to:
1.  A well-defined, negatively tilted, synoptic-scale trough from
offshore the BC coast south-southeastward across the Pacific
Northwest to central CA.  This should include a leading shortwave
trough/vorticity banner from eastern WA to the northern Sierras.
2.  Northeastward ejection of a series of mostly minor/southern-
stream perturbations from the Southwest and northern Mexico, amidst
broader-scale height falls related to the first process.  This
includes a leading MCV produced by yesterday`s convection over
northern Chihuahua, now evident in IR and composite reflectivity
imagery over far west TX east of ELP.  The MCV should shift
northeastward away from the region and across the TX and OK
Panhandles through 12Z.  Meanwhile, a higher-amplitude shortwave
trough -- now evident in moisture-channel imagery over the lower
Colorado River Valley region -- will move eastward across the Four
Corners and much of NM.

At the surface, a long-lasting, quasistationary frontal zone was
analyzed at 11Z over central FL west-southwestward across the
central/western Gulf, and will contribute to low-end, general
thunder potential over parts of FL as it lingers in that area.
Another cold front was drawn from northwestern QC across Lake Huron,
southern Lake Michigan and northern IL, becoming quasistationary
southwestward across central KS and northeastern NM.  Most of this
boundary, from the Great lakes to the south-central High Plains,
should lose definition through the period.

...Southern NM and far west TX...
Scattered to widely scattered thunderstorms should develop during
mid/late afternoon and move eastward to northeastward across the
outlook area, offering the potential for marginally severe hail and
isolated wind damage from the most intense cells.

Clearing in the wake of the MCV should lead to diabatic
heating/destabilization of the boundary layer from southern NM
southward into Chihuahua through at least mid afternoon, amidst
surface dew points generally mid 40s to low 50S F and PW near an
inch.  Meanwhile, subtle large-scale ascent/cooling aloft should
begin over parts of NM in advance of the lower CO Valley shortwave
trough.  This should lead to peak MLCAPE of 500-1000 J/kg atop
well-mixed subcloud layers, contributing to potential for any
near-severe hail generated aloft to reach surface, as well as
suitable subcloud conditions for downdraft acceleration and locally
damaging gusts.  Deep shear should remain modest, with multicellular
modes dominant.  The threat should diminish after dark, as the
boundary layer stabilizes from a combination of diabatic cooling and
expansive outflow coverage.

..Edwards/Dean.. 10/19/2017

$$



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