Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231148

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

306 AM CDT

Through tonight...

Two questions for today and tonight involve precip chances
through daybreak and again toward late morning into the afternoon.

Short term CAMs have had a decent handle on the activity over WI
and the lake during the overnight hours although the RAP has so
far been a little too eager to develop convection across northwest
IL. A couple of small cells have finally started to pop up so a
continued slight chance for that area and downstream seems like a
prudent approach through daybreak. With such a light pressure
gradient at the surface there is little in the way of low level
convergence, and moisture transport into the area appears to be
minimal as well. Satellite WV loops do suggest the presence of a
weak shortwave aloft, so maybe that will be enough to support
isolated activity this morning.

Later in the morning into early afternoon the surface boundary
associated with the present activity over WI makes its way into
the area. Again the convergence at low levels appears to be weak
though upper shortwave support looks to be a bit more favorable.
Scattered convection is likely to develop in NW IL toward late
morning and traverse toward the southeast through the afternoon
and into the evening. Given the overall lack of widespread
strong forcing it is also likely that some areas will remain dry

If convection does develop today, SPC has the southern half of
the forecast area in a slight risk. This looks reasonable given
the anticipated location of activity at this time, decent deep
layer shear, plenty of low level moisture ahead of the boundary,
and the advantage of peak heating during the afternoon.

By this evening into the overnight hours the boundary will be
pushing south and out of the area. This will bring an end to any
lingering convection and also begin to usher in drier and cooler
air which will give Monday a considerably different feel than
what we have been experiencing these past few days.



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 12Z TAFs...

Concerns for the day involve timing and extent of convection as
well as development of an afternoon lake breeze.

Confidence is low regarding convective chances but do believe
there will be a window of scattered showers and thunderstorms
starting in the late morning in NW IL and moving toward the metro
area by early afternoon. Extent of this activity may be greater
south of the Chicago terminals but opted to include a couple hours
of VCSH to avoid the impression that the entire day would be
smooth sailing locally. Activity also could develop a bit earlier
than shown in the TAF or persist a bit later.

With a relatively light wind gradient it seems likely that local
thermal effects would become more dominant into the afternoon, so
latest forecast includes a lake breeze passage that flips winds
around to the northeast at ORD and MDW. Local convection may also
have an influence on wind directions, to the point that it
overrides lake effects. Confidence is thus low regarding afternoon
winds, but overall speeds should remain fairly light except near
thunderstorm outflow.



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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