Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 171854
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
154 PM CDT Tue Oct 17 2017

.SHORT TERM...

Through Wednesday...

154 pm...Only forecast concern are the winds Wednesday afternoon.
Under sunny skies with gusty southwesterly winds...temps are
currently pushing 70 in many areas. Wednesday will be a carbon
copy of today with temps maybe a degree or two warmer...in the
lower 70s. Winds will once again steadily increase during the
morning and should be similar to today through midday. But by
afternoon...winds are bit stronger in the low levels and how much
of this is able to make it to the surface will depend on how deep
the low levels will mix. Its possible wind gusts may approach
35-40 mph by mid/late afternoon...especially across the northern
cwa...but confidence is low and trends will need to be monitored.
Temps will quickly cool through the 50s early this evening with
lows likely in the lower/mid 40s with perhaps a few/usual cooler
spots. cms

&&

.LONG TERM...
154 PM CDT

Wednesday night through Tuesday...

With the large dome of high pressure parked across the southeast
CONUS continuing to block Gulf moisture return, the stretch of
nearly identical days continues Thursday except with a small bubble
of high pressure moving overhead and supporting considerably less
wind than is expected Wednesday afternoon. Morning lows Thursday
also will be just a bit warmer than on Wednesday morning, though
afternoon highs in the upper 60s north to lower 70s south will be
similar to the pattern on Wednesday.

Friday also looks very similar to the going trends except for a few
degrees more warmth in the afternoon.  Highs should reach the 70s
across the area.

Moisture and associated cloudiness begin to increase Friday night
and will not allow lows Saturday morning to be quite as cool as seen
so far this week.  A start in the mid 50s areawide will yield to mid
70s by afternoon and rain chances will be on the increase.
Deterministic models agree on bringing a cold front across the area
during the weekend, but disagree on the timing of its passage and
the strength of the associated trough.  Current guidance does agree
on the possibility of some prefrontal showers Saturday morning in
western portions of the area, so latest forecast update has included
chance POPs to account for this.  The best chance for widespread
activity would appear to be late Saturday into early Sunday, and
periodic showers could linger through Tuesday under the postfrontal
upper trough and vort maxes.

Saturday easily looks like the warmer of the two days this weekend,
with Sunday and Monday cooling back into the 60s, followed by even
cooler air in the 50s for Tuesday as the deepest part of the upper
trough settles overhead.

Lenning

&&

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

1233 pm...Gusty southwesterly winds will continue through sunset
with gusts into the mid 20kt range. While prevailing speeds will
likely diminish to under 10kts this evening...its possible there
could be some periods of 10-15kts overnight and toward sunrise.
Wind directions will turn a bit more south/southwest Wednesday
morning with similar speeds/gusts steadily increasing in the
morning. But by mid afternoon through sunset Wednesday evening...
its possible that gusts may approach 30kts depending on how deep
the low levels mix. Dry/vfr through the period. cms

&&

.MARINE...
259 AM CDT

The main weather concern over Lake Michigan this week is the
increasing potential for a period of high end southerly gales over
the northern half of the lake on Wednesday and Wednesday evening.

An expansive area of high is expected to become extended from the
Lower Ohio Valley to the Mid-Atlantic today, and then remain
dominate here through much of the week. Meanwhile, a couple of
storm systems will shift eastward across Canada through mid week.
The associated surface pressure gradient over the lake will
continue to be favorable to support a prolonged period of stout
southwest winds through mid to late week.

The first storm system, is currently shifting eastward across
northern Ontario. This system is currently supporting
southwesterly winds to 30 KT. These wind speeds will gradually
ease through the day as this system shifts into northwestern
Quebec later today.

The second storm system is expected to be the main story of the
week, however, as it is forecast to be a much more potent area of
low pressure. As such, this will set up a very strong gradient
over the lake on Wednesday and Wednesday night. During this period
high end south-southwesterly gales of 40 to 45 kt will be
probable, especially over the northern half to 2/3rds of the
lake. With this in mind, the current gale watch will likely need
to be converted to a gale warning later today.

Yet another period of enhanced southerly winds look possible late
Friday into Saturday as another storm system shifts northeastward
into south central Canada. Wind speeds with this event could
again approach gale force.

KJB

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...nearshore waters until 7 PM Tuesday.

&&

$$

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