Area Forecast Discussion
Issued by NWS Chicago, IL

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099
FXUS63 KLOT 151149
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
649 AM CDT Tue Jun 15 2021

.SHORT TERM...
252 AM CDT

Through Wednesday...

Quiescent and extremely comfortable conditions--unusually so for
this time of year--are in store through Wednesday. While the
shortwave parade continues upstairs with several additional
disturbances slated to drop southward across the Great Lakes
Region through Wednesday, a very dry and subsident airmass will
inhibit precipitation chances. A region of weak isentropic upglide
centered along the 300 K theta surface is helping squeeze out some
increased mid-level cloud cover which will drift across our
forecast area through the rest of the early-morning hours, but
that`ll be about it. A diurnally-thickening cumulus field will
develop with time later this morning and afternoon. The lake
breeze today will get an assist from this latest incoming wave, so
should make deep inland progress across northern Illinois this
afternoon and evening. Temperatures today will be near or a degree
or two under what we hit on Monday.

Breezy north to northeast winds will push down the lake this
afternoon and overnight which will result in another period of
choppy wave action and an increasing risk for rip currents. Beach
Hazards Statements have been posted for the southern Lake Michigan
beaches as a result. With slackening winds inland and dewpoints in
the low to mid 40s away from the lake, think we might see a
handful of upper 40 degree low temperatures in our typical
cool/sheltered locales come Wednesday morning.

Altogether similar conditions will prevail on Wednesday, with a
bit less cloud cover. Guidance suggests afternoon dewpoints might
mix out into the upper 30s in spots away from the immediate
lakeshore!

Carlaw

&&

.LONG TERM...
325 AM CDT

Wednesday night through Monday...

The highly amplified upper ridge begins to compress as a fast
moving shortwave trough digs southeast across the U.S./Canadian
border into Thursday. In turn, surface high pressure will begin
to shift to our southeast bringing a return to southwesterly flow
as well as temperatures in the upper 80s and lower 90s on
Thursday.

Model guidance has been quite consistent in developing convection
off to our northwest during the Thursday afternoon/evening period that
then progresses southeastward across the area Thursday night into
Friday morning. The local area looks to remain capped with the
axis of better instability and shear remaining off to our west
through the afternoon keeping things dry here locally. This axis
however, will begin to shift eastward with time Thursday evening.
If convection is able to develop the timing of when this reaches
the area would be during a less favorable time diurnally, but
MUCAPE values still in the 1000-2000+ J/kg and deep layer shear of
40-50kts late in the evening would be supportive of MCS
maintenance and any associated severe hazards that bears watching. As
is often the case when we are still a few days out, the exact
placement of summertime mesoscale convective systems can be quite
variable.

Despite breezy west northwest winds on Friday that develop behind
the overnight convection, temperatures manage to warm back in to
the lower 90s thanks to a lingering axis of warm 850 temps that we
are able to mix into. Thunderstorm potential is lower on Friday
for areas north of I-80 with a strong CAP in place. In areas
south of I-80 additional thunderstorm development would be
possible depending on the placement of any remnant outflow
boundaries that set up.

Beyond Friday, daily lake breezes will keep temperatures cooler
near the lake over the weekend, with temperatures inland in the
mid 80s. The pattern looks to remain active with long range
guidance depicting a sub-1000mb low over the Great Lakes Sunday
night into Monday.

Petr

&&

.AVIATION...
For the 12Z TAFs...

Winds are beginning to return to northeasterly a bit earlier than
anticipated this morning at ORD with other Chicago area sites
turning northeasterly over the next hour or two. Expect a gradual
uptick in wind speeds to 10-12kt by 15-16Z. A reinforcing lake
breeze then pushes inland across the area this afternoon with an
increase in gustiness anticipated behind the boundary to around
20kt that persists through the afternoon. Winds ease this evening
remaining northeasterly overnight into Wednesday morning.

A thin mid-level cloud deck across north central Illinois will
continue to erode over the next couple of hours with scattered to
broken VFR cumulus expected to quickly take its place late morning
through the afternoon. Opted to include a FEW020 mention at RFD
to account for any residual smoke in the vicinity this morning.
It is possible this may be continued later in the day given the
northerly flow.

Petr

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Beach Hazards Statement...ILZ006-ILZ103-ILZ104...4 PM Tuesday to
     7 AM Wednesday.

IN...Beach Hazards Statement...INZ001-INZ002...4 PM Tuesday to 7 AM
     Wednesday.

LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...4 PM Tuesday to 4 AM Wednesday.

&&

$$

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