Severe Storm Outlook Narrative (AC)
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890
ACUS02 KWNS 210541
SWODY2
SPC AC 210540

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1240 AM CDT Thu Sep 21 2017

Valid 221200Z - 231200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM EASTERN SOUTH
DAKOTA INTO NORTHERN MINNESOTA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM THE
SOUTHERN HIGH PLAINS NORTHWARD TO THE EASTERN DAKOTAS AND NORTHERN
MINNESOTA...

...SUMMARY...
Scattered thunderstorms are expected to pose a severe risk from the
eastern Dakotas to Minnesota Friday morning through early Saturday,
with a more isolated severe threat southward into the southern High
Plains.  Hail and gusty winds are the primary threats, although some
tornado potential may exist from South Dakota into Minnesota.

...Synopsis...
A longwave mid/upper trough will remain nearly stationary throughout
the day across the West, while stout ridging remains anchored across
the Mid-Mississippi Valley and Great Lakes region.  Between these
two systems, strong, meridionally oriented mid-level flow will
extend from the southern Rockies northward to the Dakotas and
Minnesota.

At the surface, a nearly stationary boundary will extend from
northwestern Minnesota south-southwestward to western Nebraska, and
a weak low will migrate northeastward along this surface boundary,
reaching western Minnesota during the evening.  Farther south,
broadly confluent low-level flow will exist from eastern Colorado
into southeastern New Mexico along a weak surface trough.
Ahead/east of these surface boundaries, a modified maritime airmass
maintained by southerly low-level flow will remain in place.
Farther east, high pressure will dominate most areas from New
England southward to the Lower Mississippi Valley, while TC Jose
lingers just southeast of the New England coast.

...Eastern Dakotas into Minnesota...
Convection should be ongoing in the early morning hours within a
warm-advection regime as 30-40 kt southerly low-level flow continues
to interact with the surface boundary across the region.
Thermodynamic and kinematic profiles suggest potential for isolated
instances of 1"+ hail with the strongest activity, and some of these
elevated storms will extend as far west as central North Dakota.
This hail threat should persist into the afternoon, although the
bulk of this activity should migrate northeastward into Canada
during that time frame.

Despite weak upper forcing/subtle height rises across the region,
models suggest that surface-based convection will develop during the
afternoon along the surface boundary and just ahead of the western
Minnesota surface low.  Near 70F dewpoints and steep lapse rates
will foster moderate to strong destabilization within the
pre-convective airmass, and vertical wind shear will be supportive
of organized convection with updraft rotation.  Large hail, gusty
winds, and a few tornadoes will be possible with this activity
(primarily within the Slight risk area).  This severe threat will
persist into the overnight hours, although storms should gradually
become more elevated above a cooling boundary layer with time.

...Nebraska south-southwestward to eastern New Mexico...
Isolated to scattered storms are expected across this region
during/after peak heating hours.  Although mid-level lapse rates may
be a bit less steep compared to areas farther northeast, modest
destabilization will occur due to surface warming and boundary layer
mixing, and veering vertical wind profiles may foster updraft
rotation.  Isolated damaging wind gusts are the primary threat,
although a few instances of large hail are also possible.

Overnight, convergence on the nose of an increasing low-level jet in
western Kansas will foster an area of elevated convection in
portions of western and central Nebraska from 03Z onward.  An
isolated hail threat may exist with this convection.  Low (5%)
severe probabilities were expanded northward/westward to address
this threat.

..Cook.. 09/21/2017

$$



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