Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 152347

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

300 PM CDT

Through Thursday...

Quiet and very dry weather is in store through Wednesday and then a
quick warm-up on Thursday albeit with continued low humidity. This
will be immediately ahead of an active stretch Thursday
evening/night-Friday with severe weather potential and even flooding
a possibility despite the dry antecedent conditions. For full
details on that, see the Long Term section below.

Main item of note this evening into Friday morning is lake enhanced
onshore winds bringing high wave action and strong currents to the
beaches. With a slower onset of stronger north-northeast winds, and
not much wave response yet as of this writing, will be pushing the
Beach Hazards Statement start time back to 7pm.

Surface high pressure over the northern Lakes will ridge southward
tonight, bringing diminished winds, clear skies for good radiational
cooling during the very short nighttime hours. Forecast lows in
favored outlying spots are from around 50 the lower 50s, with
isolated upper 40s readings possible west of the Fox Valley.
Elsewhere outside Chicago, looking at mid 50s, with upper 50s to
around 60 in the city.

Wednesday will be a near carbon copy of today though a hair
cooler in the lake cooling footprint, and with less diurnal Cu in
the exceptionally dry for June air mass (afternoon dew points
bottoming out in the 30s-lower 40s). Despite warm advection aloft,
otherwise favorable radiational cooling conditions will yield low
50s in cool spots from interior northeast and east central
Illinois into northwest Indiana. A weakening MCS should be
tracking south-southeastward to the west of the MS River late
Wednesday night-early Thursday, spreading some mid and high cloud
cover into the far west and northwest CWA, keeping things a bit
milder than overnight tonight (mid 50s).

As the surface high exits east on Thursday, strong thermal ridge
will advect eastward, though not yet low level moisture as surface
dew points will only slowly climb from a low starting point. Low
heat capacity of the still dry air mass and plenty of sun will
allow high temperatures to reach the upper 80s to lower 90s, with
localized mid 90s readings in the drought area of northwest/north
central Illinois. It looks like the convective threat should hold
off until after 00z, so have a dry 7am-7pm CDT forecast for the
entire CWA.



300 PM CDT

Thursday Night through Tuesday...

Main concern for this period is a severe thunderstorm and flash
flooding potential Thursday night across northern Illinois and
Friday afternoon into central Illinois.

Thursday night and Friday: A long-wave trough shifting southeast
through central Canada will suppress the northern extent of broad
ridging centered over the Four Corners region on Thursday. An
associated cold front will track SSE across IA and WI Thursday
afternoon. Robust isentropic ascent with a developing LLJ in an
increasingly moist environment will likely result in rapid
development of deep convection northwest of the forecast area early
in the evening. Strong mid-level flow of 40-50kt in a highly
unstable environment supports a severe storm potential at onset to
the northwest of the area, with a continued threat into northern
Illinois in the evening and early overnight. With the stronger mid-
level flow somewhat parallel to the boundary sagging into northern
Illinois, propogation vectors will diminish considerably later in
the evening. Given PWATs potentially nearing 2", backbuilding
convection and an inherent flooding potential exists in the region
conditional on sufficient growth of convection earlier in the

Friday`s forecast remains highly conditional on the evolution of
convection Thursday night and the ultimate placement of the
effective boundary. Current expectations are for the boundary to
push well south of I-80, but possibly not clear the southern tier of
counties in the southern CWA prior to the afternoon. South of the
boundary, a highly unstable airmass will develop ahead of the
approaching trough axis. While some capping exists, the combination
of the mid-level trough and low-level convergence on the existing
boundary may be enough to allow for the development of isolated
explosive severe convection in the afternoon. Again, this is a
highly conditional threat, but anything that forms in Friday`s
environment will likely be severe.

Saturday through Tuesday: A break in the precip potential is
expected Saturday before concensus guidance brings the boundary
back north across the area on Sunday and Monday ahead of another
longwave trough in south-central Canada and the northern Great
Plains. While timing is unclear this far out, a couple of day of
unsettled weather in the region appear likely. Drier weather
should follow on Tuesday.



For the 00Z TAFs...

There are no significant aviation forecast concerns for this TAF
period. Winds are still sporadically gusting to around 20 kts at
the time of this TAF issuance, but will soon settle down as the
boundary layer decouples with loss of diurnal heating. Northerly
to northeasterly winds won`t pick up much tomorrow as surface
high pressure exerts its influence over the area, but another lake
breeze is expected to push inland and turn winds more easterly at
ORD, MDW, and DPA by mid-afternoon. Winds will become very light
and likely variable at some point during the late afternoon or
evening on Wednesday as the center of the surface high tracks near
the terminals.



IL...Beach Hazards Statement...ILZ006-ILZ103-ILZ104 until 7 AM

IN...Beach Hazards Statement...INZ001-INZ002 until 7 AM Wednesday.

LM...Small Craft Advisory...nearshore waters
     until 4 AM Wednesday.



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