Severe Storm Outlook Narrative (AC)
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788
ACUS01 KWNS 270450
SWODY1
SPC AC 270449

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1149 PM CDT Wed Apr 26 2017

Valid 271200Z - 281200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM PORTIONS OF
THE GREAT LAKES REGION SOUTHWARD TO THE SOUTHEAST STATES...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS PORTIONS
OF SOUTHEAST CO...WESTERN/SOUTHERN KS...THE TX/OK
PANHANDLES...NORTHERN OK...

...SUMMARY...
Isolated severe wind gusts may accompany thunderstorms from portions
of the Great Lakes region to the Southeast States today into this
evening, and across portions of the central and southern Great
Plains late this afternoon into this evening.

...Synopsis...
Deep surface low pressure is forecast to advance from the vicinity
of Lake Michigan northward into Ontario, associated with a
northward-advancing shortwave trough embedded within the eastern rim
of broadly cyclonic flow covering the Central and Western States. A
cold front will arc south from the low to the north-central Gulf
Coast vicinity and westward. The South-Central States portion of the
front is forecast to weaken and develop northward in response to
surface lee cyclogenesis over the central High Plains -- enhanced by
the emergence of a midlevel speed maximum from the central/southern
Rockies vicinity.

...Portions of the Great Lakes region southward to the Southeast
States...
Comparatively stronger deep ascent accompanying the
northward-advancing shortwave trough is forecast to become
increasingly displaced to the north of richer moisture within the
low-level cyclone`s warm sector. This will tend to marginalize the
severe potential through the period. Nevertheless, storm coverage
and intensity are forecast to gradually increase through the diurnal
heating cycle along and ahead of the front, with an extensive
corridor of eastward- and northeastward-moving clusters of storms
across the Marginal Risk area. Isolated damaging wind gusts should
be the primary hazard with this activity.

The strongest pre-frontal buoyancy related to the richest low-level
moisture characterized by middle/upper 60s dewpoints will be
confined to portions of central SC into GA and vicinity. While
effective shear around 30-40 kt and modest low-level directional
shear may support a few rotating updrafts, weakening low-level
ascent will tend to limit updraft sustainability and keep any severe
risk isolated.

Farther north, diurnal gains in buoyancy will be muted by cloud
coverage and more scant moisture return northward to the Great Lakes
region and vicinity. However, sufficient low/midlevel flow could
encourage momentum transport capable of supporting locally damaging
wind gusts.

Storms across the Marginal Risk area will weaken during the evening
as they spread across an area from western NY southward to the SC
piedmont.

...Portions of southeast CO, western/southern KS, the TX/OK
Panhandles, northern OK...
Strengthening deep ascent associated with the aforementioned speed
maximum, including isentropic ascent related to lee cyclogenesis,
will support the development of convective clusters in the
steep-lapse-rate environment across the High Plains by late
afternoon. Convection will subsequently spread eastward along a zone
of frontogenesis through the evening hours. A dearth of moisture
return will greatly stunt buoyancy. However, long/looping hodographs
amid sufficient buoyancy and the steep lapse rates may support a few
bowing convective segments capable of isolated severe wind gusts.

..Cohen/Leitman.. 04/27/2017

$$



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