Area Forecast Discussion
Issued by NWS Chicago, IL

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634
FXUS63 KLOT 201817
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1217 PM CST Mon Nov 20 2017

.SHORT TERM...
100 AM CST

Through tonight...

High pressure is across the Ohio/Tennessee valleys early this
morning, with low pressure digging southeast across the Canadian
prairies. The high will only slowly shift to the mid Atlantic
today, while the low will deepen as it moves toward Lake Superior.
The upper air pattern with WNW flow will support some cirrus
drifting through that will thin out with time today, otherwise the
weather story of the day will be the mild conditions and breezy
southwest winds that result from the increased gradient between
the two surface features. There is a pretty strong inversion which
will limit mixing, but winds will be potent enough not far off
the ground to support gusts to 30-35 mph this afternoon. Look for
temperatures to peak near 50 or so. The low will continue through
Canada just to the north of Lake Superior, therefore breezy
southwest winds will continue, and with some increasing clouds
overnight lows will hold in the mid to even upper 30s.

KMD

&&

.LONG TERM...
100 AM CST

Tuesday through Sunday...

While the forecast largely remains dry, the temperature roller
coaster ride will continue to be the story in the long term. A
sharp positively tilted shortwave trough will shift through
Wisconsin, and will steer a decent cold front through our region
through the day. Models do not paint much precip with the front
other maybe some isolated light showers or sprinkles, with better
(though still lower) chances across northwest/north central
Indiana in the afternoon. Breezy southwest winds do make a sharp
shift to the northwest and we get another shot of cold air. Cold
low in the 20s Tuesday night give way to highs again only in the
mid/upper 30s Wednesday. Winds will slacken through the day
Wednesday as high pressure returns to the mid Missouri valley,
with the ridge axis extending across northeast IL/northwest
Indiana.

The pattern from earlier in the week repeats itself for
Thanksgiving as the high will shift to the Tennessee valley on
the Thanksgiving holiday while low pressure will return to the
Canadian prairies. It should be a decent day with sunshine/high
clouds, and highs will be a tad below normal, but winds will be
relatively light. Then the breezy southwest winds kick up for
Friday as the high will cruise off to the mid Atlantic and the
Canadian low follows a similar path north of Lake Superior on
Friday. This time the low is deeper and bit closer to the region,
so we may see some more clouds ahead of it. Low level temps are
even warmer than what we expect for today, so it will again be
mild with highs in the 50 ballpark. There are some low precip
chances with the associated cold front. Temps again take a tumble
for the weekend.

KMD

&&

.AVIATION...
For the 18Z TAFs...

Only forecast concern this period are the winds. Southwest winds
in the 14-18kt range with gusts in the mid 20kt range with some
occasional higher gusts are likely today. With a +/- 3mb/3hr
pressure rise/fall couplet persisting through late tonight, wind
speeds/gusts will likely not begin to diminish until early
Tuesday morning. Winds will diminish for the morning hours as a
frontal trough moves through the area, but will then increase
while shifting to nwly in the afternoon following the passage of a
cold front. Even though winds should remain gusty winds at the
surface...low level wind shear will be possible as winds just
above a surface based inversion will be arnd 50 kt at 1.5 kft.
Dry/vfr conditions are expected through the period with just
increasing high clouds which will slowly lower to a mid deck
overnight.

&&

.MARINE...

205 am...A large area of high pressure over the southeast U.S.
this morning will move off the Atlantic coast tonight. Strong low
pressure will move from southern Canada this morning...across
Ontario tonight...and into Quebec Tuesday. The gradient between
these two features will tighten this afternoon and tonight with
southwesterly gales expected over Lake Michigan. A trailing cold
front from this low will move across the lake Tuesday morning...
shifting winds to the northwest. Its possible gale force winds may
continue into early Tuesday evening. A large area of high
pressure will build across the plains Tuesday night and then move
east across the Ohio Valley Wednesday night and Thursday. Another
strong low is expected to move across Ontario Friday with
southwesterly gales possible again Friday afternoon into Friday
evening. cms

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Gale Warning...LMZ777-LMZ779 until 3 PM Tuesday.

     Small Craft Advisory...IL nearshore waters until 6 PM Tuesday.

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

&&

$$

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