Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231128

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
628 AM CDT Mon Oct 23 2017


Through Tuesday...

255 am...Primary forecast concern is periods of rain.

An upper level low lifting across the Ohio Valley this morning
will merge with a large upper trough digging into the upper
midwest later today. This large merged upper low will then sit
over the western Great Lakes Tuesday and lift northeast Tuesday
night. As a result...a large area of rain will move across the
eastern two thirds of the cwa through midday and then slowly shift
east...but rain is likely to continue into this evening for all
of northwest Indiana. Storm-total rainfall amounts across
northwest Indiana...through this evening...may reach into the 1 to
1.5 inch range.

The next area of rain then begins to develop to the northwest
this afternoon and spreads across the area this evening and will
persist into Tuesday. Rainfall amounts likely to be less and
precip may be more showery then steady rain but another third to
two thirds of inch of rain is likely overnight through Tuesday

High temps today are a bit tricky across the west where precip
will end this morning and there may be some partial sun midday.
Generally have lower 60s west to upper 50s east but these may need
some adjusting with precip/cloud trends later today. After lows
in the lower/mid 40s tonight...there won`t be much recovery in
temps on Tuesday...likely staying in the mid/upper 40s. And winds
on Tuesday will be a bit stronger...15-25mph with some higher there might actually be some wind chills in the upper
30s Tuesday. cms



Tuesday Night through Sunday...

255 am...Forecast concerns include much cooler temperatures...
the potential for a frost or freeze this weekend...and for
now...much less precip.

As the upper low departs Tuesday night into Wednesday...precip
will be ending across the area Tuesday evening and should end
before there would be a potential for any mixed precip. In
addition...thermal profiles are not as cold as previous runs.
Low temperatures by Wednesday morning will be in the low/mid 30s
but there remains uncertainty about cloud cover and wind speeds.
The ground will be wet and still somewhat no mention of
frost for this period.

A weak upper wave will move across the region Wednesday that for
now appears to just bring an increase in cloud cover so have gone
dry for Wednesday. And depending on cloud cover...temps likely to
recover slightly back to the lower 50s. A fast moving ridge
spreads across the area Wednesday night into Thursday ahead of low
pressure passing across the upper lakes Thursday into Friday.
This should allow for one mild/warmer day on Thursday as highs
likely push back toward 60...perhaps warmer if trends continue.

But by Thursday night...a trailing cold front from the low just
noted will move across the area. And while a passing shower can`t
be completely ruled out...the models have come into better
agreement with generally dry conditions from Wednesday through
Friday night. Precip chances then return over the weekend as an
upper low moves across the region...but confidence is fairly low
from this distance and it may just generate lake effect precip. If
precip were to develop during this time period...there would be
some potential for mixed rain/snow mainly at night. If the
forecast stays dry along with some clear skies/light winds...then
its possible a widespread frost/freeze may occur as MEX guidance
has lows below freezing. cms


For the 12Z TAFs...

A cold front has stalled over Indiana overnight while a persistent
corridor of rainfall has fallen just behind the front over
northeastern Illinois and northwest Indiana. Rainfall will
continue through the rest of the morning for the Chicago area
terminals while RFD stays west of the main precip shield.
Meanwhile, low pressure over the upper Ohio Valley this morning
will lift north along the frontal boundary and deepen. As the low
continues north into Michigan this afternoon, winds will begin to
back to the northwest and will help to very slowly start to nudge
precip and IFR conditions eastward. Precipitation at the Chicago
terminals should end sometime early to mid afternoon and
conditions will improve to VFR.

This evening, an upper level disturbance will begin to dig across
the region. Precip chances ramp up again around or just after
sunset this evening as this occurs and lower ceilings are expected
to overspread the terminals once again. Rain may come in several
waves off-and-on overnight and through the day Tuesday as the
aforementioned upper wave deepens into a closed upper low that
meanders over the region through midweek.



304 AM CDT

Two areas of low pressure, one over the lower Ohio Valley and the
second over southern Manitoba, will merge over Michigan today and
quickly deepen through tonight. North to northwest winds will
increase to gale strength tonight over northern Lake Michigan then
spread south across the remainder of the lake early Tuesday.
Strong gales will be in place at times through the day Tuesday and
cannot rule out a few gusts to 50 kt, mainly over northern Lake
Michigan. Gale headlines have been hoisted across Lake Michigan
this morning in effect through Wednesday morning when winds are
expected to diminish back below gale force. Gales will be possible
again late in the week as another low moves across the Great
Lakes region.



LM...Small Craft Advisory...IN nearshore waters until 1 PM Tuesday.

     Gale Watch...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-LMZ745...1 PM
     Tuesday to 4 AM Wednesday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742...7 AM Tuesday to 1
     PM Tuesday.

     Gale Watch...LMZ777-LMZ779...9 AM Tuesday to 9 AM Wednesday.




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