Severe Storm Outlook Narrative (AC)
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376
ACUS02 KWNS 200501
SWODY2
SPC AC 200500

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1200 AM CDT Sun Aug 20 2017

Valid 211200Z - 221200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS LATE MONDAY
AFTERNOON INTO MONDAY NIGHT ACROSS PARTS OF THE MID AND LOWER
MISSOURI VALLEY INTO THE UPPER MIDWEST...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS
SURROUNDING AREAS OF THE CENTRAL PLAINS...MISSISSIPPI VALLEY AND
GREAT LAKES REGION...

...SUMMARY...
Severe thunderstorms may impact a corridor from portions of the mid
and lower Missouri Valley into the Upper Midwest late Monday
afternoon into Monday night, accompanied by a risk for potentially
damaging wind gusts and perhaps some hail.

...Synopsis...
Within the primary belt of mid-latitude westerlies, amplification
commencing Sunday across the northeast Pacific into western Canada,
and adjacent portions of the U.S., will continue through this
period.  This may include at least some further building of a
large-scale ridge shifting east of the Canadian Rockies into the
Canadian prairies, in advance of a deep upper trough and embedded
closed low digging into the northeast Pacific.  Downstream of the
ridge, significant upper troughing is still forecast to evolve and
begin digging across the central Canadian/U.S. border area by late
Monday night.  This is expected to be accompanied by the southward
advancement of a cold front into and through much of the Upper
Midwest, lower Missouri Valley and central Plains, in the wake of a
developing frontal wave which may reach the Great Lakes region by
12z Tuesday.

...North central Plains into the Upper Midwest/Great Lakes...
Scattered thunderstorm activity, largely driven by lower/mid
tropospheric warm advection, may still be ongoing east of the mid
Missouri Valley into portions of the Upper Mississippi Valley at 12Z
Monday, before weakening while spreading toward the Great Lakes
region.  In the wake of this activity, models suggest that
seasonably high moisture content, beneath modestly steep mid-level
lapse rates, will support sizable CAPE in excess of 2000 J/kg by
late Monday afternoon, particularly near the nose of an axis of
stronger pre-frontal surface heating extending northeast of the
central Plains into areas near/east of the mid and lower Missouri
Valley.  Guidance suggests that this will precede an upper impulse
emerging from the southwestern monsoonal regime, before merging into
increasingly cyclonic mid/upper flow to the south of the main upper
trough, across the mid/lower Missouri Valley into the Upper Midwest.

While some timing differences still appear to exist among the
various model output, large-scale ascent associated with this
feature seems to provide the most obvious focus for potential
strong/severe storm development.  This includes the possibility of
an evolving mesoscale convective system in the presence of
strengthening westerly lower/mid tropospheric wind fields (to 30-40
kt).  Given this ambient flow, and potential for heavy precipitation
loading (aided by precipitable water increasing to 2+ inches),
convection may be accompanied by a swath of potentially damaging
wind gusts, possibly continuing into the Great Lakes region Monday
night.

..Kerr.. 08/20/2017

$$



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