Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231842

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
142 PM CDT Sun Oct 23 2016

141 PM CDT

Through Monday...

We are currently enjoying what will be our warmest day of the week
as a thermal ridge ahead of a cold front and modest southwest
winds have pushed readings into the upper 60s to lower 70s area
wide...even some mid 70s in Central Illinois. These numbers are
some 10 degrees or so above normal for late October.

Changes in the temperature department are seen on the horizon with
a peek at visible satellite this afternoon. There is an area of
status clouds marching southeastward in the colder air mass behind
the cold front. Some of these clouds will move through our region
tonight, but with the low tracking across Lake Michigan toward
Detroit the bulk of the precipitation of the front will miss to
our north. North winds behind the front will likely result in some
lake effect clouds though no precipitation other than maybe a
sprinkle across NW Indy as a stout inversion at 850 mb holds.

High pressure rebuilds quickly Monday bringing mostly sunny skies
but a return to more seasonal conditions under north-northeast
winds, with highs expected to peak in the 50s.



300 AM CDT

Tuesday through Saturday...

Attention by Tuesday will quickly turn to the rather sharp
shortwave trough forecast to amplify as it tracks east into our
region by midweek. Warm frontogenesis will take place in advance
of this wave with guidance consistently breaking out WNW-ESE band
rain in the strengthening WAA regime. Have seen some run-to-run
waffling of the medium range guidance with respect to the exact
track of this system and location of where the WAA wing of precip
sets up. Latest guidance suggests most likely location of WAA
breaking out over southern WI into far northern IL later Tuesday
and especially Tuesday night as LLJ strengthens nocturnally.

Track of the surface low Wednesday will dictate how potentially
crummy of a day weather-wise Wednesday will be. The ECMWF/GEM have
trended farther north with the surface low track which would place
most of our CWA in the warm sector Wednesday, which would mean
largely dry weather with just some showers and perhaps t-storms
late as cold front moves through. The GFS tracks sfc low across
downstate IL and would keep CWA socked in with steady rain through
the day in the WAA wing then deformation zone of the system. The
GFS solution would likely mean highs barely reach 50 in the rain
over northern CWA, while the ECMWF/GEM solution would support
highs in the 60s in the warm sector. Will need to watch trend in
guidance the next day or two and will fine tune forecast as it
becomes clearer where this low will track.

In the wake of this midweek system look for seasonable temps and
dry conditions for the end of the week. Some decent size
differences in handling of potential northwest flow shortwave
which could bring some rain to the area in the Friday night
through Saturday night time frame. Medium range models haven`t
shown great run-to-run consistency and some big differences in the
amplitude of the wave exists between the ECMWF (more amplified)
and the GFS/GEM (weaker/less amplified). Blended model
initializations in the grids came up with very low pops and given
low confidence saw no reason to make any big adjustments one way
or the other for now.



For the 18Z TAFs...

-Gusty southwest winds shifting to west this afternoon
-MVFR cigs this evening
-North winds shifting to northeast on Monday

Mixing to about 2500-3000 ft today will support low to mid 20s gusts
this afternoon. Winds will be veering from SW to NW through the
afternoon and low pressure moves southeast through Wisconsin. As
this low moves east to Michigan tonight it will send a cold front
southward through the region. Model guidance depicts a continued
shift NW winds tonight, possibly going to northerly for at least a
period mid to late evening, especially closer to the lakefront.

There is an area of MVFR to lower VFR clouds in the cold air behind
the front across Minnesota. Many sources of guidance do not bring
a ceiling with these clouds as the low tracks away from the area
to the east, but there are hints from the NAM and the NARRE that
some of these MVFR conditions spread through the area mid to late
evening for at least a time overnight. Since a decreasing band of
clouds is forecast but with the NARRE showing a period of MVFR
this evening, have TEMPO`d that in to the TAFs closer to the lake.

High pressure will fill in behind the low on Monday which will result
in clearing skies and shifting winds to more of a NE direction
mid to late morning through the remainder of the day.



311 AM CDT

A ridge of high pressure centered over the central Great lakes
will shift to the east today as low pressure over the northern
Plains quickly moves southeast to southern Lake Michigan this
afternoon. With these two surface features moving through the
region today, expect widely varying wind directions and speeds. It
does appear that the strongest winds will initially be over the
southern end of the lake and nearshore waters for much of today.
With these higher winds, hazardous conditions for small craft will
develop this afternoon. This low will then quickly move east of
the lake tonight with winds shifting more northerly tonight into
Monday. Higher speeds of 15 to 25 KT and even 30 KT winds/gusts
will help waves build, with hazardous conditions for small craft
continuing tonight into Monday morning. Have issued an advisory to
cover these hazardous conditions today through Monday morning, but
could possibly see higher waves lingering slightly longer over the
Indiana shore on Monday. Winds will then diminish with building
high pressure Monday night into Tuesday. Next area of low pressure
expected to move towards the lake through mid week with higher
winds and waves returning. Will continue to monitor this period as
gales are still appearing possible on Wednesday.



LM...Small Craft Advisory...INDIANA NEARSHORE WATERS UNTIL 1 PM Monday.

     Small Craft Advisory...ILLINOIS NEARSHORE WATERS UNTIL 10 AM Monday.




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