SPC Sep 18, 2017 0730 UTC Day 3 Severe Thunderstorm Outlook

SPC 0730Z Day 3 Outlook

Day 3 Convective Outlook  
NWS Storm Prediction Center Norman OK
0227 AM CDT Mon Sep 18 2017

Valid 201200Z - 211200Z

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS FROM EASTERN
IOWA NORTHEASTWARD INTO THE UPPER PENINSULA OF MICHIGAN...

...SUMMARY...
A few severe storms are possible across portions of Wisconsin,
eastern Iowa, and vicinity Wednesday afternoon.

...Synopsis...
Strong, cyclonic flow aloft will persist across the West and extend
into the northern Plains and Upper Mississippi Valley throughout the
forecast period.  A vigorous mid-level disturbance will lift
northeastward from Minnesota into western Ontario during the
afternoon.  Meanwhile, another trough over the Pacific Northwest
will amplify while migrating southeastward into the western Great
Basin.  Ridging will extend from Texas to the Mid Mississippi
Valley, while TC Jose lingers near/south of southern New England.

At the surface, a cold front will extend from eastern Minnesota
south-southwestward to central Kansas.  The Minnesota portion of
this front will migrate eastward into western Wisconsin before
stalling in the afternoon and into the overnight.  Farther south,
this front will begin to retreat as a warm front overnight over
portions of the central Plains.  An anticyclone will remain anchored
across the southeast throughout the forecast period, and another
high will be centered across the southern Rockies.  The overall
pattern will aid in maintaining a warm/moist airmass just ahead of
the aforementioned cold front, which will aid in a Marginal risk of
severe storms in the afternoon.

...Wisconsin/eastern Iowa and vicinity...
A broad area of showers and thunderstorms should be ongoing across
portions of Minnesota near the cold front early in the morning. 
Models suggest that these storms will weaken and lift north into
Canada throughout the day.  As the front advances east, an
increasingly unstable airmass - supported by 60s to low 70s F
dewpoints - will develop along an axis just ahead of the front. 
Although the bulk of mid/upper forcing will be lifting away from the
region, convergence along the front amidst a weakly capped airmass
will foster a few thunderstorms, and vertical shear profiles support
some updraft rotation if storms indeed form.  Marginal (5%) severe
probabilities have been introduced, and an upgrade may be needed in
later outlooks - especially if later observations/model guidance
suggests that morning convection will not negatively impact the
severe risk later in the afternoon.

..Cook.. 09/18/2017

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