Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221851

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
151 PM CDT Sat Jul 22 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.



402 AM CDT

Sunday through Saturday...

After some very wet and active weather, Sunday begins a transition
to a cooler and drier airmass, at least for a couple of days.  By
Sunday morning winds should be veering west and northwest at all
levels.  While this will begin to usher in the new airmass, the
afternoon passage of an upper trough over residual warmth and
moisture at low levels may support a few additional showers during
the afternoon.

By Monday morning northerly flow has become established with the
approach of a surface high into WI and the western Great Lakes. This
high will support cooler temperatures and inhibit moisture return
through early Wednesday as it slowly makes its way through the Great
Lakes and toward New England. Highs Monday in many locations will
not make it out of the 70s, and Tuesday looks only be a few degrees

By Wednesday the high is far enough east that southeasterly return
flow again becomes established.  Models also suggest the approach of
a broad frontal zone stretching from the southern Plains into the
central Great Lakes by Wednesday afternoon or evening which then
shows signs of lingering over the area through late in the week.
Moisture convergence along this boundary would support increased
precip chances for the second half of the week, with possible
clearing by the weekend as the front finally moves south away from
the area.  Confidence decreases considerably toward later in the
week regarding the motion of this boundary.



For the 18Z TAFs...

Concerns center on lake breeze arrival, and on small convective

Radar shows the lake breeze confined near the shore in northern
Cook county, with some inland headway in southern Cook. Movement
has been slow and conversely short term guidance has slowed the
easterly wind shift. Confidence is only medium on arrival time,
and it could be delayed, but weak pressure pattern would support
movement inland.

Initial frontal boundary is south of the terminals in an area
currently capped but likely to lose the cap later this afternoon.
Some convection is possible but forcing is limited, and expected
to be well south. This activity may strengthen late tonight but
still remain south. There is a cold front across MN and it will
move close to the area late tonight. Some weakening showers may
impact area near the WI border overnight but chances locally are
small. The front will pass southeast through the area in the
morning. With the main upper forcing well north expected shower or
storm coverage to be limited and have kept the TAFs dry. Winds
shift to NW behind the front.



400 AM CDT

A light gradient today will generally support northerly flow but
local thermal effects likely will promote lake breeze formation.
Winds remain light on Sunday and somewhat variable. A slightly
stronger gradient ahead of an approaching high will support
northerly flow again Monday and the long fetch may promote enough
wave growth on the southern end to prompt small craft advisories.






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