Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230726

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

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

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.



226 AM CDT

Monday through Saturday...

The first and likely last parts of this coming week look much
quieter and less muggy than much of what July has provided so far.
During midweek, particularly sometime later Wednesday through the
day Thursday, a cold front will bring a chance for storms from
northwest to southeast across the area.

Northeast winds on Monday will steer in cooler boundary layer air,
with 850mb temperatures of 12C-13C. This will keep highs in the
mid to upper 70s in most areas, while onshore flow and possibly
areas of morning low clouds provide lower 70s along the immediate
lake shore. Tuesday will see less synoptic wind speed allowing
for a lake breeze push. Temperatures inland will warm back up to
around or a little above 80. More sun should be seen on Tuesday
than Monday.

The upper ridge, while dampened from last week, will attempt to
build into the area later Tuesday into early Wednesday, before a
strong upper wave across Canada cuts into it. An associated
surface cold will be moving southeast with quite a bit of warming
expected Wednesday across our area. If current frontal timing
stands and not until Wednesday night/early Thursday, think
forecast highs on Wednesday could be a tad underdone, with
possibly low 90s more prevalent. High dew point air, not tough to
come by in late July in the Corn Belt, will return ahead of the
front, assisting in modest to high instability. While frontal
speed cannot be latched onto with a high degree of certainty at
3-4 days out, guidance is in fair agreement with the boundary a
ways north of the CWA during peak heating Wednesday. So the storm
chances presently peak Wednesday night and early Thursday, which
may keep the severe weather threat more limited. There could be
somewhat of a heavy rain threat with this despite it being cold
frontal driven, as the steering winds become unidirectional to the
boundary as it and its associated high PWAT plume pass. Also some
anafrontal rain characteristics of the front indicated on
guidance as it slowly creep south on Thursday. Something we will
have to keep an eye on, but 2-3 days of drying ahead of time will
be well received.

Beyond we look to repeat Monday and Tuesday`s weather later
Thursday into the start of next weekend per recent runs of global



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.



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.






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