Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230645
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
145 AM CDT Sun Jul 23 2017

.SHORT TERM...
150 PM CDT

Through Sunday...

Quieter weather is in place with surface high pressure overhead.
The surface boundary is anchored along our southern counties.
Southerly flow ahead of the boundary and some cloud breaks should
lead to destabilization across the area. Hi resolution guidance
does still attempt to break out some convective activity in the
WAA atop this boundary but it would fairly limited in coverage at
least in our area and more likely confined across Central and
southern IL/IN. DCAPE and high PWATs suggest a wind and heavy rain
threat.

A fairly broad mid-upper level trough will push southeast through
the Great Lakes on Sunday. Ahead of this trough, convection is
expected across the Upper Midwest this evening, with the majority
shorter term guidance weakening this activity as it nears IL,
though some sprinkles or isolated showers would not be unheard of.

The associated cold front will pass through the area during the
morning into early afternoon. Convective coverage is still a bit
uncertain at this time, given the main upper forcing is north in
the less unstable air, and the cold front will largely be through
a good portion of the area in the afternoon. Some convection
remains plausible both north and ahead of the wave, so held onto
some lower chances at this time. Areas east and south stand the
best chance for isolated severe storms later in the day. Generally
speaking most times will be dry, and it will be at least a little
less muggy with dewpoints in the 60s instead of 70s.

KMD

&&

.LONG TERM...
217 PM CDT

Sunday night through Saturday...

Looking at a cooler and drier upcoming week.

An upper level wave swings overhead Sunday, and we may have
lingering showers and storms mainly along and east of I-55 Sunday
night.

Northerly winds set up Monday bringing in cooler and less humid air.
Highs Monday are forecast to be in the low 70s along the lake to
around 80 south of I-80.  High pressure moves over the lake Monday
night leading to dry conditions through Wednesday morning.
Temperatures slowly climb back into the mid to upper 80s by
Wednesday.

The next chance of precip arrives Wednesday afternoon and evening in
advance of a weak low and cold front. Showers and storms linger into
Thursday.  Cooler air moves in behind the cold front with highs in
the low 80s away from the lake Thursday.  Another high moves over
the midwest late next week.

JEE

&&

.AVIATION...
For the 06Z TAFs...

Very light winds overnight into the predawn hours should
eventually take on a west to northwesterly component toward
daybreak and into the morning, then very gradually move toward
a northerly direction by evening. Development of a lake breeze
also is a good possibility with such a weak pressure gradient,
but it is not clear if this would push all the way to the
terminals. The other concern would be development of isolated to
scattered convection during the late morning to early afternoon
hours as the forcing for existing activity across WI moves toward
the area. Considered adding a VCTS but elected to wait for
additional information given the present degree of low confidence
on timing and extent of this activity.

Lenning

&&

.MARINE...
145 AM CDT

Broad low pressure and an associated diffuse and mainly light wind
field will prevail over the lake this morning. Isolated showers or
storms may continue over the lake during today associated with
this low. As the low pressure shifts southeast, an associated cold
front will progress south this afternoon and evening, turning
winds from westerly to northerly. This front/wind shift should
reach the far southern tip of the lake by midnight.

On Monday, the northerly winds look to occasionally gust over 20
kt over the open water and southern nearshores. This fetch will
build waves to at least close (3-5 ft) to Small Craft Advisory
criteria in the Illinois and Indiana nearshore beaches late
tonight and throughout Monday.

The next similar oriented cold front will move southward across
the lake on or near Wednesday evening. This could have some gusty
storms in advance of and along its passage. Behind the passage,
another tight pressure gradient looks to drive northerly winds and
potential Small Craft Advisory criteria waves, and maybe even
winds, for Illinois and Indiana on Thursday.

MTF

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.
&&

$$

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