Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211742

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1142 AM CST Tue Nov 21 2017

928 AM CST

A narrow band of precipitation will continue to move across
portions of northern Illinois and northwest Indiana through midday
and early afternoon. The drier air in place had limited overall
development for much of the early morning hours, however, have
really noted stronger development over the last 1-2 hours. Lead
shortwave energy ahead of a digging upper level trough as well as
surface reflection and associated boundary are in place, but the
main driving force for this precip is an area of strong/deep
frontogenesis moving overhead. This has allowed this drier air to
be overcome, with light rain occurring across many locations in
northern Illinois at this time. Cooling to the wetbulb while
strong CAA is in place is also allowing for a wintry mix to
develop. Although this mix should remain brief/limited and
confined to rain/sleet, additional cooling may also support some
snow. Don`t anticipate any accumulation, as this forcing and
precip. slide to the east into remaining areas in northern
Illinois and northwest Indiana through midday and the early



107 AM CST

Through Wednesday...

The story of the next few days will be the temperature swings
along with occasional windy periods. Today we take the ride back
downward as deep low pressure across Lake Superior will steer a
cold front across the region today.

There is a lead shortwave across southern MN and several others
in the NW flow on the south side of the low driving some mid and
upper level cloudiness, but the focus will be on the deeper trough
axis across northern MN. Model guidance is in good agreement in
driving this energy southeastward today. We do not think this
will be a big deal for our area precip-wise, but several short
term guidance sources paint some light QPF with this wave noted by
some 850 mb fgen, compact omega/rh signal as the front shifts
through. The main focus is along and southeast of Chicago into
northwest Indiana and across east/central Illinois, but some virga
or very light precip may fall across northeast IL as well. Low
level positive energy forecast suggests enough low level warm air
lingers for rain to be the dominant p-type, but cannot rule out a
few snowflakes toward the tail end as the colder low level air
arrives. And the cold air will be noted after a mild Monday, as
the colder air will cruise on into north central Illinois not long
after daybreak and spread southeast through the day.

The wave will quickly depart this evening, but the cold northwest
winds will remain in its wake. Lows will fall to near 20 in the
north, mid 20s southeast and in Chicago. The surface ridge will
shift over the area on Wednesday. Winds will slacken some, low
level warm advection will ensue, and there should be plenty of
sun, but highs will struggle to reach 40.



107 AM CST

Wednesday night through Monday...

Another much weaker wave will slide through Wisconsin Wednesday
night with precip staying north as the ridge axis will stretch out
southeast of the area across the Ohio/Tennessee valleys into the
mid Atlantic. A secondary high will be in place over the area for
Thanksgiving Day, which will allow temps to recover to the lower
40s. This is still below normal, but with the high resulting in
likely our lightest wind day of the week, all in all pleasant
weather conditions for the holiday.

Friday will be a repeat of yesterday (Monday), as gusty southwest
winds will develop ahead of deepening low just north of Lake
Superior. The low level thermal fields support even warmer highs
than Monday, which were in the low to mid 50s area wide. The only
kicker will be there may be a bit more cloudiness as the upper
forcing is a bit farther south with this wave. Models still don`t
paint much precip again and very well could end up dry, but have
some close bay to warrant not removing a model blend of low
chances. The cold air behind this system does not come slamming in
that quickly but gets reinforced through the weekend, thus highs
Saturday will reach the 40s, then only the 30s Sunday. High
pressure will mean weakening winds, dry conditions, and sun.



For the 18Z TAFs...

Main forecast concern for the period will be wind trends with the
passage of a cold front.

A narrow band of pcpn is moving east of the terminals at issuance
time, with only some lingering light rain likely at GYY for
another hour or so. With the pcpn, cloud bases should lower to
lower end vfr, 4-5 kft for the remainder of the afternoon. In the
strong cold advection and pressure rises following the frontal
passage, the gusty swly winds are shifting to nwly and should
remain gusty to 20-25 kt through the remainder of the afternoon.
By arnd 00z, the gradient should relax as high pressure begins to
spread across the Middle Mississippi Valley, allowing winds to
drop off overnight. As the colder, drier air filters across the
region, sky cover should diminish to mostly clear overnight with
no operationally significant cloud cover. As the high pressure
continues to spread across the region tomorrow, conditions should
be dry, with relatively light, westerly winds.



205 am...Deep low pressure just north of Lake Superior early this
morning will move into northern Quebec tonight as it slowly
weakens. A trailing cold front will move across Lake Michigan this
morning shifting southwest winds to the northwest. The gales will
likely diminish toward 30kts with the front but a period of low
end gales is possible behind the front...which may last into early
this evening...but confidence is low. A large area of high
pressure will build across the plains Tuesday night and then
extend east into the Ohio Valley Wednesday night. Strong low
pressure will move across Ontario Friday and Friday night with a
tight southwest pressure gradient developing over the lakes
region. Southwest gales are possible again Friday morning into
Friday evening...with a cold front shifting winds northwest by
Saturday morning with gales still possible behind the front. cms


LM...Gale Warning...LMZ777-LMZ779 until 3 PM Tuesday.

     Small Craft Advisory...IL nearshore waters until 9 PM Tuesday.

     Small Craft Advisory...IN nearshore waters until 9 AM Wednesday.




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