Severe Storm Outlook Narrative (AC)
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000
ACUS02 KWNS 211731
SWODY2
SPC AC 211730

Day 2 Convective Outlook
NWS Storm Prediction Center Norman OK
1230 PM CDT Thu Sep 21 2017

Valid 221200Z - 231200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM NORTHERN MN
THROUGH EASTERN SD TO NORTH-CENTRAL NE...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK FROM THE UPPER MIDWEST TO THE CENTRAL AND SOUTHERN HIGH
PLAINS...

...SUMMARY...
Scattered thunderstorms are expected to pose a severe risk from the
eastern Dakotas to Minnesota, mainly Friday afternoon into the
evening, while a more isolated severe threat is possible southward
into the central and southern High Plains.  Hail and gusty winds are
the primary threats, although some tornado potential may exist from
South Dakota into Minnesota.

...Synopsis...
A strong mid-upper level ridge will remain anchored from the
southern Plains to the Great Lake region, while a large-scale trough
across the western U.S. shifts to a more neutral orientation.  The
evolution of this mid-upper level flow pattern will result in
stronger winds aloft extending from the southwest states to the
northern Plains.  At the surface, a front should remain nearly
stationary during day 2 from northwest MN to north-central and
western NE, as flow aloft will be parallel to this boundary.  A weak
low will migrate northeastward along this surface boundary,
reaching western Minnesota Friday evening.  Farther south, broadly
confluent low-level flow will exist from eastern Colorado into
eastern New Mexico along a weak surface trough.  Along and east of
these surface boundaries, rather rich moisture attendant to a
modified maritime air mass will be maintained by southerly low-level
flow.

...Dakotas/MN/NE...
A shortwave ridge located across the Upper Midwest at the start of
day 2 is expected to weaken, being replaced by generally flat flow
across this region.  Models suggest weak mid-level impulses should
track through the southwesterly flow aloft, though they differ in
the timing of these perturbations moving northeast across the
position of the surface front.  Initial warmer mid-level
temperatures should cool some Friday afternoon and evening,
steepening mid-level lapse rates, as forcing for ascent with the
impulses spreads across western/central NE to the eastern Dakotas
and MN.  This should support storm development along the surface
front from the surface low position in eastern South Dakota
southward to north-central and perhaps southwest NE, where moderate
to strong instability is expected Friday afternoon into the early
evening.  Strengthening deep-layer shear will prove favorable for
organized storms with hail and damaging winds the primary severe
hazards, though a tornado threat cannot be ruled out near and
northeast of the surface low, where low-level shear should be
greater and cloud bases lower.

...Central and southern High Plains...
The CAPE/shear parameter space is expected to be sufficient for
organized strong to severe storms within the low-level confluent
flow regime across the central and southern High Plains Friday
afternoon into the evening.  However, even weaker forcing aloft,
than expected farther north this forecast period, suggests the
coverage of strong to severe storms should be more isolated.

..Peters.. 09/21/2017

$$



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