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 Forecast Discussion


000
ACUS03 KWNS 200731
SWODY3
SPC AC 200730

Day 3 Convective Outlook  
NWS Storm Prediction Center Norman OK
0230 AM CDT Wed Sep 20 2017

Valid 221200Z - 231200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM NORTHEASTERN
NEBRASKA INTO MUCH OF MINNESOTA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS THE
SOUTHERN AND CENTRAL HIGH PLAINS NORTHWARD TO THE UPPER MISSISSIPPI
VALLEY...

...SUMMARY...
Scattered thunderstorms are expected to pose a severe risk from
Nebraska to Minnesota Friday afternoon through early Saturday, with
more isolated activity 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...
An amplifying longwave trough over the West will make slow, steady
eastward progress throughout the forecast period, resulting in
strong, meridionally oriented mid-level flow across much of the
center of the country.  East of this area, a ridge will remain
nearly stationary across the Midwest.  Meanwhile, TC Jose will
linger just southeast of New England.

At the surface, a cold front will extend from the eastern Dakotas
southwestward into northeastern Colorado, and a lee trough will
extend southward into southwest Texas.  East of these boundaries, a
modified maritime tropical airmass will be maintained by southerly
low-level flow, while convergence along these boundaries will foster
isolated to scattered thunderstorm development from Friday afternoon
onward.

...Northeastern Nebraska into Minnesota...
The combination of insolation and sustained low-level convergence
will foster scattered to numerous thunderstorms across this region
during peak heating hours.  Moderate to strong instability, steep
lapse rates, and 60s F dewpoints will foster a threat for damaging
wind gusts and large hail, while wind profiles suggest the potential
for supercells with some tornado potential.  The tornado potential
will ultimately be modulated by 1) the progress of the front, which
may tend to undercut convection and encourage linear organization
and 2) favorable storm mode, which may be in question given shear
vectors parallel to the initiating boundary and the potential for
numerous storm-scale interactions.  Nevertheless, severe storm
parameters are maximized in this region, and a Slight risk has been
introduced to address the overall threat.

...Central Nebraska southward to eastern New Mexico...
Low-level moisture will be comparatively lower (50s F dewpoints) in
this region, resulting in weaker overall buoyancy and higher-based
storms.  Nevertheless, models are consistent in developing at least
isolated convection in this region -- with a potential for higher
coverage in southwestern Nebraska on the northern extent of a
strengthening low-level jet.  Shear profiles are supportive of
updraft rotation, with damaging wind and hail being the main
threats.  Portions of this region may need to be upgraded to Slight
risk in later updates pending greater certainty of convective
coverage.

..Cook.. 09/20/2017

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