Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 251802

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

101 PM CDT

Through Wednesday...

Cloudy and cool conditions are the rule this afternoon as high pressure
departs to our east and cloud cover overspreads the region ahead
of low pressure across the plains.

Mid level lift will increase this evening as the approaching system
arrives. The increasing lift is evident in increased deeper
clouds on IR and echoes aloft on regional radar mosaics. While
some sprinkles are increasingly possible this evening, forecast
soundings maintain drier low level through this evening for most
of the area that and the forcing from the low level jet and mid
level frontogenesis is not quite strong enough to overcome the dry
air. There is increased agreement in a batch of rain showers after
midnight as the f-gen ramps up considerably in the warm advection
wing ahead of the surface low. This batch does not appear to
impact all areas, but the I-80/88 corridor northward will be
favored during this time frame. Lapse rates aloft do not appear to
support thunder during this period in spite of the strong low
level jet and mid level forcing.

After this wave moves through, it appears there will be a lull in
precipitation (for areas that actually receive rain overnight)
early Wednesday and the NAM/GFS are in good agreement on this
general idea, but there will likely still be some lingering
rainfall. The NAM is still a northern outlier with the track of
the surface low, and have tended towards the global and ensemble
guidance which continues to support an eastward track across the
I-80 corridor. Southeast winds continue to ramp up ahead of the
eastward moving surface low as the strong departing surface high
holds across and east of the Appalachians. Then as the upper
forcing arrives, expect rain/rain showers to increase from west to
east mid to late morning and spreading over the remainder of the
area Wednesday afternoon. Even with the northern low track,
instability is still suggested to be on the lower side and
therefore just maintained a slight chance thunder mention in spite
of high chances of rainfall. North of the low expected just a
cool, rainy and windy day, with conditions being more showery in
nature south where the thunder chances are a bit higher.



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 18Z TAFs...

- Winds becoming gusty out of the SE later tonight and becoming E
Wednesday afternoon
- Scattered Showers late tonight becoming then more organized
Wednesday mid to late morning through the afternoon.
- Isolated thunder chances, mainly south
- Associated lower vis and cigs in rain, with best chance for
lower cigs late in the TAF period.

Ceilings continue to slowly lower to the west as increasing mid
level lift from the approaching system arrives. While some
sprinkles are increasingly possible this evening, forecast
soundings maintain drier low level through this evening for most
of the area. There is increased agreement in a batch of rain
showers after midnight with a band of strong mid level
frontogenesis in the warm advection wing ahead of the surface low.
This batch does not appear to impact all areas, but the I-80 and
more so the I-88 corridor north will be favored which includes
most of the TAFs except GYY. During this period conditions
generally look to remain VFR except maybe brief MVFR.

Winds today are generally SE, and may shift to more of an easterly
direction for a time especially for the Chicago terminals, but as
the surface low approaches SE winds will continue to ramp up
tonight into Wednesday. Ensembles and global guidance support the
surface low tracking south of the terminals Wednesday afternoon
which would maintain an E wind, but the NAM brings the low farther
north which would hold the SE wind a bit longer. Have maintained
an E trend with support from the global guidance for now, but will
need to watch this. Strongest winds are morning through early

There appears to be a break in the  precipitation after the overnight
showers, and then lift increases later Wednesday morning into the
afternoon. This is the more likely timeframe for widespread rain
showers. Could be some embedded thunder south, but have not added
anything at this time given the more favored low track would not
support much instability farther north, but there will likely be
MVFR cigs and vis, and possibly some IFR in heavier shower



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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