Severe Storm Outlook Narrative (AC)
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000
ACUS11 KWNS 142301
SWOMCD
SPC MCD 142301
INZ000-ILZ000-150030-

Mesoscale Discussion 1714
NWS Storm Prediction Center Norman OK
0601 PM CDT Sat Oct 14 2017

Areas affected...Northern Illinois and adjacent portions of the
Upper Midwest

Concerning...Tornado Watch 501...

Valid 142301Z - 150030Z

The severe weather threat for Tornado Watch 501 continues.

SUMMARY...The risk for potentially damaging wind gusts and perhaps a
couple of tornadoes is expected to increase across much of northern
Illinois, including the Chicago metropolitan area through 7-10 PM
CDT.  An additional watch or watch extension will probably be needed
eastward into the Chicago metro area.

DISCUSSION...Within moderate to strong southwesterly mid/upper flow,
ahead of a significant short wave trough now turning east of the
Rockies, forcing for ascent associated with a smaller scale
perturbation is contributing to the evolution of at least one
distinct mesoscale convective system.  The bulk of this activity has
spread east of the Mississippi River and appears likely to continue
progressing northeastward into the Great Lakes region this evening.
As it does, inflow of weak to moderately unstable warm sector air
likely will support further convective intensification through the
01-03Z time frame.

Strongest storms seem likely to become focused along a remnant
convectively generated or enhanced boundary near/north of Interstate
74 into the Interstate 80 corridor of northern Illinois. This is
where a zone of low-level warm advection will enhance large-scale
ascent, and low-level shear will become maximized as southerly 850
mb flow continues to strengthen to 50+ kt, in conjunction with
further deepening of the surface cyclone migrating northeast of
central Iowa.

The risk for potentially damaging wind gusts seems likely to
increase with the evolving convective system across northern
Illinois during the next few hours, and could impact at least
southern portions, if not much, of the Chicago metropolitan area by
around 02Z.  Further, an isolated supercell or two within, or
perhaps forming discretely ahead of the line, may be accompanied by
potential for tornadoes as low-level hodographs enlarge.

..Kerr.. 10/14/2017

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

ATTN...WFO...LOT...ILX...LSX...DVN...

LAT...LON   41828877 42298805 41978687 40798714 40118887 39719019
            39969122 40629046 41828877




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