Severe Storm Outlook Narrative (AC)
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ACUS02 KWNS 200457
SWODY2
SPC AC 200455

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1155 PM CDT Thu Oct 19 2017

Valid 211200Z - 221200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS LATE SATURDAY
AFTERNOON AND EVENING ACROSS MUCH OF CENTRAL AND EASTERN OKLAHOMA
AND ADJACENT PORTIONS OF SOUTHEASTERN KANSAS AND NORTHERN TEXAS...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS ACROSS SURROUNDING
AREAS OF THE PLAINS...NORTHWARD THROUGH THE LOWER MISSOURI AND UPPER
MISSISSIPPI VALLEYS...

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

...SUMMARY...
Widespread thunderstorm development is expected late Saturday
afternoon into Saturday night, from portions of the upper
Mississippi Valley into the southern Plains.  Some of this activity,
particularly across the Plains, will be accompanied by a risk for
potentially damaging wind gusts, some hail and perhaps a couple of
tornadoes.

...Synopsis...
Models indicate that large-scale mid/upper troughing, currently
advancing inland of the Pacific Coast, will remain progressive
through this period, likely reaching Manitoba, northwest Ontario,
the upper Mississippi Valley and southern U.S. Plains by 12Z Sunday.
 Within this regime, the corridor of strongest mid-level height
falls are forecast to spread east/northeast of the Canadian Prairies
and northern U.S. Plains Saturday/Saturday night, associated with a
couple of significant embedded short wave impulses.  But another
digging impulse may contribute to mid-level height falls across the
southern Plains Red River Valley region Saturday night, while
beginning to split away from the base of larger-scale troughing to
the north.

In lower levels, a deep associated surface cyclone is forecast to
migrate from southern Manitoba into Hudson Bay, while a modest
trailing cold front advances eastward/southeastward through much of
the U.S. Plains and mid/upper Mississippi Valley region by the end
of the Period.

...Plains/Mississippi Valley...
Models generally indicate that the mid-level cold core and strongest
mid-level forcing for ascent may tend to lag to the west of the cold
front through the period.  And severe weather potential, in general,
may hinge on how fast the eastward and southeastward advancing cold
front tends to undercut the pre-frontal initiating convective
development, which remains unclear at this time.  However, it
appears that there will be at least a window of opportunity for
substantive pre-frontal thunderstorm activity, accompanied by a risk
for severe weather.

At least a narrow plume of seasonably moist air (characterized by
mid/upper 60s surface dew points) appears likely to precede the
front in a corridor from the southern Plains into the upper
Mississippi Valley.  Models suggest that low-level moistening will
occur beneath a plume of modestly steep lapse rates associated with
elevated mixed layer air, which may contribute to CAPE up to 1000+
J/kg, and 2000 J/kg across north central Texas into Oklahoma.

Although deep layer wind fields are not expected to be exceptionally
strong, 30-50 kts in lower/mid-levels (somewhat stronger across
parts of the lower Missouri Valley into upper Mississippi Valley),
should be more than sufficient to support organized severe weather
potential, given the instability.  Guidance appears suggestive that
vigorous storm development may initiate first across parts of the
east central Plains and middle Missouri Valley late Saturday
afternoon, before intensifying while increasing and spreading
northeastward toward the upper Mississippi Valley through Saturday
evening.  This could include isolated discrete supercells early,
before evolving into a broken squall line, with damaging wind gusts
becoming the primary severe threat.

Farther south, stronger instability, coupled with increasing forcing
for ascent (enhanced by increasingly divergent high-level flow) may
support storm initiation across parts of western/northern Oklahoma
and adjacent portions of the Plains by early Saturday evening.  This
may include discrete supercells initially, in a corridor ahead of
the cold front, and near its intersection with the dry line.
However, fairly rapid and considerable upscale convective growth
appears probable, with potentially damaging wind gusts becoming the
primary concern by mid to late evening, before tending to become
undercut by the southward advancing cold front.

..Kerr.. 10/20/2017

$$


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