Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 222004

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
304 PM CDT FRI JUL 22 2016

304 PM CDT

Through Saturday...

Have made no changes to current heat headlines for Saturday and no
major changes to the low chances for storms tonight and Saturday
morning, mainly in north central Illinois and then slowly
increasing chances Saturday afternoon.

Influence of last night`s and this morning`s MCS can still be seen
on visible satellite this afternoon. The outflow from this arcs
northward across central/eastern Iowa with a zone of 20-25kt
850-925mb warm advection across that same area. Have seen isolated
storms develop in that area and expect an uptick through late
afternoon as a subtle short wave disturbance moves eastward near
the MN/IA border and the cap is eroded. The instability gradient
and thicknesses slope southeastward and would expect any organized
convection to propagate that way, possibly clipping north central
Illinois during the evening hours and east central Illinois in the
late evening/overnight hours. Confidence is low on this, but
somewhat higher confidence than in recent days that the Chicago
area and far northwest Indiana will remain dry.

Afternoon dew points have recovered into the lower to mid 70s
across the area and may inch up a couple more degrees into this
evening. The humid air mass will not allow the temperatures to
drop much, but they could reach the temperatures for a few hours
which under the surface ridge could set the stage for patchy
shallow fog or even some haze. So have added that into the
overnight forecast. There may even be stratus development based
on the lowest level profiles forecast by some of the high-
resolution guidance.

For Saturday outside of morning fog/stratus possibility and the
small chance of festering convection in the south/southwest, the
morning should be sunny. Given the 925mb temperatures forecast to
be 26-27C, highs in the mid 90s are possible, but at this time
have gone lower 90s in most locations. Light onshore flow is
expected to develop across northeast Illinois and far northwest
Indiana which should keep them cooler. Heat index values may be
close or reach advisory criteria for some counties not currently
under a headline for Saturday, but given the uncertainty on
clouds/afternoon storms have collaborated on waiting for any new
headline issuance or changes.

As for Saturday afternoon storms, as the higher theta-e returns
from the southwest there are increasing chances reflected. Also,
if convection can sustain itself from the Dakotas/Minnesota late
tonight and Saturday morning, this would possibly reach into or
near the CWA by mid-late afternoon Saturday. Some guidance
indicates this and being in the zone of ridge-riding complexes,
this is a possibility at some points late Saturday or into the



342 AM CDT

Sunday through Thursday...

A cold front swings through Sunday with a chance of showers and
storms ahead of it.  Thinking Sunday will be the final day of heat
and humidity. A heat advisory may be needed for most of the warning
area. Heat indices vary from 100 to 110.

Slightly cooler and drier conditions move in Monday and Tuesday with
highs in the 80s.  A more active pattern returns mid and late in the
week with periodic chances of storms.  High temps look to remain in
the 80s.



For the 18Z TAFs...

The concerns for the Chicago area airports are the potential for a
lake breeze shift to easterly winds late this afternoon, as well
as a chance for light haze/fog tonight. Finally, east winds may
reach 7-10 kt for a period Saturday afternoon.

Westerly winds have continued to be stout enough across northeast
Illinois to prevent a lake breeze development thus far. The
somewhat local pressure gradient in place is anticipated to weaken
and allow a lake breeze to develop, though it should be slow to
move inward and have backed up the timing of an easterly wind
shift. It is possible that this wind shift does not arrive at all
or that the speeds are below 7 kt when it does, and confidence is
medium in its arrival and low on the specifics.

Isolated storms are possible across eastern Iowa late this
afternoon though the expected organization in these if they
develop is low. Because of that, the potential to move over the
TAF sites tonight is even lower, especially the Chicago airports.

Saturday will see a high pressure ridge over the area allow for
easterly to southeasterly winds across the TAF sites. These could
see an increase to 7 to 10 kt during the afternoon especially for
Chicago area sites.



259 AM CDT

Winds this morning should ease as high pressure gradually shifts
east over the lake later today into tonight. A weak cold front
will also move south across the lake today but winds should remain
relatively weak. The high will shift east Saturday, and this
coupled with low pressure that will move to south of Hudson Bay
Sunday will allow a warm front to lift north across the lake
Sunday. Winds may gust to 25 possibly 30 kt Sunday. A weak cold
front will return Sunday night followed by high pressure through
mid week.

Waves are still a bit disturbed this morning following
thunderstorms last night, but expect that they will continue to
ease this morning.



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

     Heat Advisory...ILZ003-ILZ004-ILZ005-ILZ006-ILZ011-ILZ012-ILZ013-
     ILZ014 UNTIL 7 PM Friday.

     Heat Advisory...ILZ020-ILZ022 UNTIL 7 PM Saturday.

IN...Heat Advisory...INZ001-INZ002 UNTIL 7 PM Friday.

     Heat Advisory...INZ010-INZ011-INZ019 UNTIL 7 PM Saturday.




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