Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211148

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
648 AM CDT Tue Aug 21 2018

311 AM CDT

Through tonight...

Primary short term concern is with upper low moving across the

Upper low is over far northern Illinois early this morning with
bands of showers and t-storms pivoting around the upper low.
Greatest coverage is to our east and north at this time, but as
upper low continues eastward toward look for some of the
convection over southern WI to pivot southeast across our CWA this
morning into the afternoon. Not expecting severe threat, but could
see some very heavy downpours and perhaps a couple funnel clouds.

Another northern stream shortwave trough which is kicking our
upper low east, will sweep across the western Great Lakes region
this evening. Been a fairly consistent signal for a small
convective cluster moving across Chicago area into NW IN this
evening in association with this feature. Some lake enhancement to
the t-storms is expected as lake induced CAPE values hover around
1500+ j/Kg. Waterspout potential with the convection this evening
looks pretty high and will need to keep an eye to make sure no
spouts attempt to sneak onshore into NW IN. Trough axis moves
across the region around 06z-ish time frame bringing an end to the
convective threat, though some lingering much weaker lake effect
showers could linger a bit into the overnight hours over Porter

- Izzi


311 AM CDT

Wednesday through Monday...

Wendesday and Thursday look spectacular. Sunny skies, low
humidity (Tds in 50s, perhaps 40s over Chicago metro), high temps
in the 70s.

Another (likely last one for a while) vigorous trough/closed mid-
upper level low will track across the western Great Lakes Friday
into early Saturday. Good bet we`ll see one or two rounds of
showers and t-storms in association with this wave, though timing
details are still a bit murky, Friday into Friday night time frame
looks most likely now.

In the wake of that trough, heights begin to build over the
central and eastern CONUS and all indications are that a fairly
significant and potentially long lived hot spell is in the offing
this weekend through potentially much of next week over the
eastern CONUS. The exact strength and positioning of the upper
ridge will be key is determining where the "ring of fire" (ROF)
will set up and our area looks close enough to potentially be
impacted by some of this convection. If one or more of the MCSs
associated with the ROF affects the area then it will likely
temper the heat some. Otherwise, potential exists for multiple
days with highs in the 90s with high humidity as well. Forecast of
lower 90s Sunday through Tuesday could even end up being a bit
conservative, but is a good start in messaging the potential
impending hot spell.

- Izzi


For the 12Z TAFs...

An upper-low currently crossing northern IL is producing LIFR
ceilings at ORD/DPA/RFD with the potential for at least IFR ceilings
in the coming hour or so at MDW/GYY. Anticipation is that these
ceilings will gradually rise into the MVFR range over the next few
hours with the onset of daytime heating. BKN MVFR ceilings should
ensue through the remainder of the day behind a cold front that is
expected to arrive by mid morning at RFD and late morning elsewhere.
Scattered SHRA should accompany this, with a small chance of
isolated TS. Confidence in TS was low, so did not include with this
TAF issuance. Otherwise, gusty winds favoring a N or possibly NNW
direction is expected through much of the day. Another batch of SHRA
off Lake Michigan may clip ORD/MDW and pass over GYY this evening.



311 AM CDT

An unseasonably strong low pressure system over northern IL early
this morning will track across southern Lake Michigan from roughly
Kenosha to Saugatuck through mid morning. N winds to 30 knots can be
expected across much of the lake through tonight before gradually
diminishing to under 20 knots by Wednesday afternoon as high
pressure begins to build across the region SW winds will then
increase to 25 knots across the north half by Thursday evening, and
become southerly to 30 knots across all of the lake Friday afternoon
through into Saturday ahead of another low pressure tracking NE
across the Upper MS River Valley.

Also of note today, any thunderstorm will be capable of producing a
waterspout across central Lake Michigan this morning, and southern
Lake Michigan this morning and afternoon.



IL...Beach Hazards Statement...ILZ006-ILZ014 until 1 PM Wednesday.

IN...Beach Hazards Statement...INZ001-INZ002...1 PM Tuesday to 7 PM

LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742...10 AM Tuesday to
     11 AM Wednesday.

     Small Craft Advisory...LMZ743-LMZ744-LMZ745...1 PM Tuesday to 7
     PM Wednesday.




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