Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 212043

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
343 PM CDT THU JUL 21 2016

340 PM CDT

Through Tonight...

Will continue the heat headlines as is for the area with an
Excessive Heat Warning for northern Illinois and a Heat Advisory
for northwest Indiana.

Immediate subsidence in the wake of the now re-strengthened MCS
across northern Indiana should keep much of the CWA dry through
sunset outside of Jasper and Benton Counties Indiana which will
have a localized strong/severe threat for a couple more hours.
Residual outflow boundary is draped northwestward from the west
Chicago metro through Janesville, Wisconsin and northwest from
there to scattered convection in west central Wisconsin. There
has been some agitation of cumulus along this boundary but with
the warm thermal nose near 850mb atop northwest/north central IL
and southwest Wisconsin, the expectation is for this area to
remain slightly capped. As for the storms in western Wisconsin, a
short wave helped to instigate these and that looks to move more
east of south into eastern Wisconsin. However, steering vectors
for anything organized would bend propagation more southerly. The
next couple hours should tell if storms will become more organized
there and pose a larger potential to work their way into northern
Illinois this evening, or if in fact just continue to fester in a
scattered state along the surface boundary. Again being at the
edge of a strong heat ridge with high CAPE/PWAT air is a
precarious place to be and one of lower confidence for
thunderstorm specifics. There would be a gusty wind and heavy
rainfall threat with any storms tonight.

While outflow has cooled northeast Illinois, already quick
recovery of the low-level air mass is occurring with almost 90
degrees already observed again in Lake County, Illinois as of 330
pm. Dew points CWA-wide outside of the rain-impacted area are in
the upper 70s with a few lower 80s, and even in the rain-cooled
area remain primarily in the 70s. So certainly a very muggy and
warm evening is in store. It is somewhat uncertain how much
downtown Chicago will recover late this afternoon with still winds
off the lake, but temperatures may actually warm this evening.
Below will note some of the uncertainty regarding tomorrow (which
will be similar to today), but feel the best message is to
continue the heat headlines as is.




To be issued shortly.


For the 18Z TAFs...

The concerns with the Chicago area TAF sites are the northeast
winds and how long they will last this afternoon before shifting
back southerly, along with a small chance of thunderstorms through
the rest of this afternoon and tonight and a slightly better
chance Friday morning. Also there is potential for a wind shift to
north-northeast again on Friday afternoon.

The primary storm activity is east of the Chicago area TAF sites
as of 1830Z, with outflow having shifted winds north or northeast.
Based on behavior of the winds upstream, expect the winds to
slowly veer east and then southeast before turning south. How
quick the shift from east to south happens is low confidence and
may take an hour or two longer than in current TAF.

With weak forcing in place and somewhat of a worked over air mass,
am not anticipating much for redeveloping storms through sundown.
Tonight forcing improves to the north of the area along an
approaching front. Some storms may evolve southward into the area
Friday morning, but this is low confidence. The more storm
activity along the boundary the more likely it will help nudge the
front and bring a wind shift to north-northeast.



302 AM CDT

A strong south to southwest breeze is in place across Lake
Michigan early this morning and will continue through the day and
portions of the overnight hours. Late tonight into Friday, a cold
front will gradually move down the lake with showers and
thunderstorms expected along the front. High pressure will
briefly build across the region Friday night into early Saturday,
then low pressure will move across the Canadian Prairies with a
warm front lifting back north. The low will continue to Hudson Bay
late Sunday with a trailing cold front sweeping across the lake
Sunday Night. High pressure builds back across the Great Lakes
Monday and Tuesday.



IL...Excessive Heat Warning...ILZ003-ILZ004-ILZ005-ILZ006-ILZ011-
     ILZ012-ILZ013-ILZ014-ILZ020-ILZ022 UNTIL 7 PM Friday.

     Excessive Heat Warning...ILZ008-ILZ010-ILZ019-ILZ021-ILZ023-
     ILZ032-ILZ033-ILZ039 UNTIL 7 PM Saturday.

IN...Heat Advisory...INZ001-INZ002-INZ010-INZ011-INZ019 UNTIL 7 PM




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