Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 250855

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
355 AM CDT Tue Oct 25 2016

311 AM CDT

Through tonight...

Extensive mid-high level cloudiness expected today, but no real
significant wx with similar temps to yesterday. Tonight, WAA
will quickly intensify as nocturnally enhanced 45kt+ low level jet
takes aim on the region. Except for the NAM, remainder of
guidance, including Hires-WRFs, show WAA wing of showers
developing later this evening and spreading east across mainly far
northern IL. Cannot totally rule out an isolated t-storm, but
better chance looks to lie to our west across IA & SW WI.



310 AM CDT

Wednesday through Monday...

GFS/ECMWF/GEM all have sfc low Wed tracking right along I-80,
while NAM remains a northerly outlier still. Given the strong
agreement with the typically stronger performing models have
trended forecast more in their direction. The result should be a
easterly wind driven rain north of I-80 in the deformation zone of
this cyclone through much of the day Wednesday, perhaps becoming a
bit more showery later in the afternoon. Have undercut the model
blended cocktail temps north of I-80 with temps struggling to
reach 50F away from the warmer lake waters. South of I-80 look for
milder temps, more showery precip regime with a slightly better
chance of isolated t-storms. Will need to monitor wind
direction/speed off the lake, should gales be more easterly than
southeasterly, then there could be some low end lakeshore flooding
problems. At this point, the most significant wave action looks to
take aim on SE WI and Lake Co IL, but will need to keep an eye on
this threat.

Quick shot of drier and seasonably cool air Thursday. The addition
of sunshine though could actually result in high temps Thurs a few
degrees warmer than Wed over northern CWA where rain/low clouds
hold highs in upper 40s.

GFS/ECMWF/GEM seem to be locking onto a more unified and
consistent solution for the track of the late week system. It is
appearing increasingly likely that a fairly strong northern stream
system will track across the northern tier of the states with a
strong surface low tracking north of the area Friday into
Saturday. The result will be increasingly strong southwest winds
and a healthy warm up starting Friday and likely continuing into

By 00z Friday, GFS & ECMWF both have 925mb temps reaching into
the mid teens celsius, which would support highs in the 70s.
Timing of how quickly the warm air surges in is unclear, with
925mb temps a good 4C+ cooler at 18z opted to remain a bit
conservative with highs and only bumped up the model blended
consensus temps slightly into the mid-upper 60s Friday. If warm
air is a bit quicker, then 70F+ would be a good bet.

Friday night looks to be an unseasonably warm night with thermal
ridge overhead and tightening gradient leading to southwest winds
remaining elevated into the night. The fairly strong southwest
winds and WAA could result in temps falling very little during the
evening, likely holding in the 60s, which would raise concerns
about the potential for high winds given the some mixing with
50kt+ LLJ progged to be overhead. Forecast lows Friday night are
likely too cool, but would like to see one more day of consistency
in the models before jacking temps up further.

Saturday, the GFS and ECMWF both have us remaining in the warm
sector. Its possible that we could be starting the day Saturday
with temperatures not far from 60F. With continued strong SW or
WSW winds and 925mb temps well into the teens we stand a much
better chance at seeing highs in the 70s Saturday. MOS guidance
from ECMWF has 74F for ORD and MEX has 73F, ran out of time to
collaborate with neighbors to raise the blended model guidance
temps much, but its possible we could have highs Saturday well
into the 70s to perhaps not far from 80 southern CWA!

Late afternoon/evening cold front should knock temps back down
closer to seasonal norms Sunday. A fairly zonal and progressive
pattern looks to have us back in WAA regime Monday with potential
for more unseasonably warm weather. Depending on timing of warm
fropa, we could be looking at more potential 70F warmth Monday
and/or Tuesday next week. Given uncertainties at this distance,
forecast in the grids/text products remain more conservative with
"only" mid-upper 60s Monday.



For the 06Z TAFs...

Anticipate VFR conditions for the forecast period as high
pressure has settled overhead. Expected system later tonight into
Wednesday morning will bring a period of showers on Wednesday, but
confidence of when it will begin is low. It will likely be right
around the end of the forecast period, and so have just included
VCSH and a PROB30 for light showers. East southeast winds for much
of the period with lake influence taking them more east northeast
at around 10 KT later this afternoon.



345 AM CDT

Main forecast concerns/challenges are with anticipated gales over
much of the lake on Wednesday. In the near term, surface high
continues to build across the region, helping the winds and waves
to ease. Expect lighter winds to be in place for much of today and
then turn attention towards approaching low over the central
Plains. Expect this low to move towards the southern end of the
lake by Wednesday evening. As this occurs, winds will quickly
increase late tonight into Wednesday. With higher confidence of
gales occurring over the southern half of the lake, did issue a
gale warning for this location as well as the Illinois nearshore
waters for Wednesday into Wednesday evening. Gales are quite
possible over the Indiana shore but with slightly lower
confidence, have only issued a gale watch at this time. Do expect
speeds to quickly ramp up late tonight into Wednesday morning and
so have not issued a small craft advisory ahead of the gale
headlines. However, hazardous conditions will likely develop late
tonight. Will also need to monitor for the potential for gales
over the northern portions of the open waters during this same
period. Low confidence of this possibility at this time, but will
need to monitor for at least a shorter period of gales. Anticipate
speeds to diminish Wednesday night as this low departs to the



LM...Gale Warning...LMZ740-LMZ741-LMZ742-LMZ777-LMZ779...7 AM
     Wednesday TO 10 PM Wednesday.

     Gale Watch...LMZ743-LMZ744-LMZ745...7 AM Wednesday TO 10 PM




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