Severe Storm Outlook Narrative (AC)
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000
ACUS11 KWNS 211832
SWOMCD
SPC MCD 211832
ILZ000-IAZ000-212100-

Mesoscale Discussion 1380
NWS Storm Prediction Center Norman OK
0132 PM CDT Fri Jul 21 2017

Areas affected...much of Iowa...northwest Illinois

Concerning...Severe potential...Watch likely

Valid 211832Z - 212100Z

Probability of Watch Issuance...80 percent

SUMMARY...Severe thunderstorms are expected to develop in the
19Z-21Z time frame with a risk for damaging winds, large hail, and a
tornado or two. Short term trends will be monitored and a watch is
likely prior to 21Z.

DISCUSSION...A composite frontal boundary extended east-west from
northeast Nebraska across central Iowa and northern Illinois as of
1815Z. Elevated thunderstorms continued in the vicinity of the
Iowa/Wisconsin border, and thunderstorms have recently developed
over northeast Nebraska/northwest Iowa.  A very moist air mass
(lower-mid 70s surface dew points) remains in place, and with
continued heating moderate-strong surface-based instability will
exist near the boundary this afternoon.  The discussion area remains
on the periphery of the stronger mid-level flow, with deep-layer
shear averaging 35-45 kts.

Surface-based thunderstorms are expected to develop in the 19Z-21Z
time frame as convective inhibition continues to diminish due to the
combined effects of heating and ascent associated with an
eastward-moving MCV over southern Minnesota/northern Iowa.  An
initial supercellular mode is expected with large hail and damaging
winds the primary threats.  Favorable low-level hodograph curvature
near the boundary will support a risk for a tornado or two.  With
time, one or two convective clusters should gradually evolve with a
more concentrated risk for damaging winds into the evening hours,
supported by a strengthening southwesterly low-level jet.

Given this expected scenario, one or two watches will likely be
needed for the discussion area, with an initial watch prior to 21Z.

..Bunting/Dial.. 07/21/2017

...Please see www.spc.noaa.gov for graphic product...

ATTN...WFO...LOT...DVN...DMX...FSD...OAX...

LAT...LON   42719413 42559250 42499170 42489129 42419016 42248961
            42108930 41808921 41488943 41318993 41129069 41139176
            41329378 41449530 41819601 42249617 42639571 42719413




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