SPC Jun 12, 2017 0730 UTC Day 3 Severe Thunderstorm Outlook

SPC 0730Z Day 3 Outlook

Day 3 Convective Outlook  
NWS Storm Prediction Center Norman OK
0228 AM CDT Mon Jun 12 2017

Valid 141200Z - 151200Z

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS WEDNESDAY
AFTERNOON AND EVENING ACROSS PARTS OF THE UPPER MIDWEST...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS ACROSS
SURROUNDING AREAS OF THE UPPER MISSISSIPPI VALLEY AND GREAT
LAKES...AND ACROSS A BROAD AREA FROM THE SOUTH CENTRAL PLAINS INTO
THE MIDDLE MISSISSIPPI/OHIO VALLEYS...AS WELL AS ACROSS PARTS OF THE
SOUTHERN MID ATLANTIC COAST STATES...

...SUMMARY...
An evolving cluster of thunderstorms, accompanied by a risk for
severe weather, is possible across parts of the Upper Midwest
Wednesday afternoon and evening.  Other strong thunderstorms may
impact portions of the southern Plains through middle Mississippi
and Ohio Valleys, and parts of the southern Mid Atlantic Coast
states, accompanied by at least some risk for severe weather.

...Discussion...
A remnant lower/mid tropospheric cyclone, within the southern branch
of split mid-latitude westerlies, appears likely to progress north
of the Canadian/U.S. border, into central Manitoba and adjacent
portions of Saskatchewan/northwest Ontario during this period.  In
the wake of this feature, mid/upper flow is expected to remain
broadly cyclonic and modest in strength across the northern Plains
through the upper Great Lakes region, while trending broadly
anticyclonic across the Pacific Northwest into the northern Rockies.

Subtropical ridging will remain influential across the southern tier
states, particularly areas adjacent to the Mexican Plateau.  At the
same time, sharp southern stream ridging is forecast to shift across
eastern portions of Ontario and the lower Great Lakes region, in
phase with the eastern flank of broad northern stream ridging
encompassing much of interior Canada, while large-scale downstream
troughing continues to dig southeast of the north Atlantic Coast.

Models indicate that a frontal zone associated with the troughing
off the Atlantic Seaboard may become quasi-stationary across the Mid
Atlantic Coast into the Great Lakes region.  Another front,
associated with the central Canadian closed low, may advance
into/through the upper Mississippi Valley and central Plains before 
weakening.  Seasonably high boundary layer moisture content
along/south of these features is expected to provide the focus for
the development of corridors of moderate to strong instability by
late Wednesday afternoon, particularly beneath a remnant plume of
elevated mixed layer air extending from the southern Plains through
the Upper Midwest at the outset of the period (where CAPE on the
order of 2000-4000 J/kg is possible).

This destabilization likely will be sufficient to support areas of
scattered strong thunderstorm development, some of which probably
will be accompanied by at least some risk for potentially damaging
wind gusts and severe hail.  At this point, severe weather potential
for Wednesday seems largely conditional, with limited potential for
appreciable organization where storms do form, in the presence of
generally weak flow/shear.  

One exception may be across Wisconsin and adjacent portions of the
Upper Midwest, where mid-level height falls to the southeast of the
closed low may provide a focus for considerable thunderstorm
development Wednesday afternoon into Wednesday evening.  In the
presence of at least moderate shear and strong instability, the
evolution of an upscale growing convective system appears possible.

..Kerr.. 06/12/2017

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