Severe Storm Outlook Narrative (AC)
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ACUS01 KWNS 211632
SWODY1
SPC AC 211631

Day 1 Convective Outlook
NWS Storm Prediction Center Norman OK
1131 AM CDT Fri Jul 21 2017

Valid 211630Z - 221200Z

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM IOWA INTO
SOUTHERN WISCONSIN...NORTHERN ILLINOIS AND INDIANA...

...THERE IS AN ENHANCED RISK OF SEVERE THUNDERSTORMS FROM THE
EASTERN DAKOTAS INTO CENTRAL MINNESOTA...

...THERE IS A SLIGHT RISK OF SEVERE THUNDERSTORMS FROM THE NORTHERN
PLAINS THROUGH A PORTION OF THE GREAT LAKES AND UPPER MIDWEST...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS SURROUNDING THE
SLIGHT RISK AREA...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS OVER A PORTION
OF MAINE...

...THERE IS A MARGINAL RISK OF SEVERE THUNDERSTORMS CENTRAL
VIRGINIA...

...SUMMARY...
Severe storms with mainly damaging wind and large to very hail are
expected to develop this afternoon and evening from the eastern
Dakotas across parts of Minnesota and possibly into Wisconsin.
Another corridor of severe storms with damaging wind might evolve
from Iowa into extreme southern Wisconsin, northern Illinois and
northern Indiana. A couple of tornadoes are also possible in these
areas.

...Dakotas through MN...

Storms in progress this morning near the international border are
developing in response to warm advection north of a warm front and
could pose some risk for hail. The atmosphere should become
moderately to strongly unstable in pre-frontal warm sector as the
boundary layer warms along eastern fringe of steeper mid level lapse
rates sampled by the BIS 12Z RAOB. Additional storms will develop
this afternoon along cold front. Strong effective bulk shear will
support supercells initially, but some upscale growth into one or
more clusters is possible as activity develops southeast and
eventually into MN. Large hail and damaging wind will be the main
threats, but a couple tornadoes are possible especially near the
warm front across northeast ND.

...Upper Midwest area...

A convectively reinforced quasi-stationary front extends from
central IA southeast through northern IL and IN. Large MCS persists
within zone of warm advection north of this boundary, and changes to
previous outlook are in response to modifications on the mesoscale
by this complex of storms. Primary change has been to shift the
enhanced category south. Current indications are that storms will
eventually redevelop and intensify along and just north of this
boundary supported by warm, moist inflow from the destabilizing warm
sector. Satellite data show a couple of MCVs. One is located over
southeast SD and the other over southwest MN, and these features
could play a role in storm initiation this afternoon. This region
resides along the southern fringe of the stronger flow aloft, and
0-6 km shear profiles around 35-40 kt will support organized storms
including supercells and bowing segments. This activity will
probably contribute to a corridor of wind damage in and near the
enhanced outlook from IA into extreme southern WI and northern IL.
Have introduced a 5% tornado area where low-level hodographs are
maximized along the migratory southwesterly low-level jet.

...Maine...

Ref swo1378 for information on this region.

..Dial/Coniglio.. 07/21/2017

$$



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