Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 280108 AAA

Area Forecast Discussion...UPDATED
National Weather Service Chicago/Romeoville, IL
808 PM CDT Thu Oct 27 2016

808 PM CDT

Areas of low clouds are persistent this evening as feared by the
day shift. The 1026mb surface high is located directly over the
CWA with clouds presently trapped below an inversion characterized
by an 8C climb in just 1500 ft on the 00Z DVN sounding. The good
news is that high clouds moving southeast over the area should
help prevent holes in lower cloud cover from radiating too
quickly, and prevent any dense fog developing. Confidence in this
continues fairly high. Also, increasing 1000-2000 ft layer winds
should help advect the low clouds northeast. Latest 11-3.9 micron
imagery appears to be showing the back edge across northwest
Illinois already moving northeast, feeling the effects of these
developing winds.

So have only made minor tweaks to the going forecast, just
delaying the diminishing cloud trend by a handful of hours. Some
high clouds may be a little thick early in the morning but these
also are forecast to move east, and going forecast highs for
tomorrow still look on track.



146 PM CDT

Through Friday...

The morning RAOBs from ILX and DVN indicate a staunch inversion
above 900 mb with significant dry air above that level. Abundant
moisture below the inversion along with visible satellite only
showing a small thinning in the status will combine to keep cloudy
conditions present through the evening. The inversion has also
precluded any efficient mixing, so winds will remain at or under 12
mph. High pressure will enter from the southwest overnight, allowing
clouds to start scattering out and causing winds to become light and
variable. The last of the H85 thermal trough will be slow to exit
tonight, so we will experience one last cool night before the noted
warming trend through the weekend.

On Friday, 500 mb heights will be on the rise ahead of mid-level
ridging. This is in concert with a warming southwest 30KT low level
jet and return flow from surface high pressure moving east of the
forecast area. The warm advection regime will allow 850 mb temps to
rise to 17C which climatologically should correspond with surface
temps close to 70F. Have opted to keep the forecast on the higher
side of guidance given the warm air aloft and quality of mixing. The
main limiting factor to any higher temperatures is the decreased
insolation this time of year. Early day cloud cover will also play a
small role.

Pressure gradient between high pressure in the OH river valley and a
low developing in northern MN poses a challenge to the Friday
evening wind forecast. Hourly pressure drops near 1 mb/hr will
persist from 15 to 00z. The isallobaric effects should allow for
winds over 20mph for much of that time period and gusts in the 30-35
mph range. Forecast soundings show decoupling in the 21-00z
timeframe, although with 1,000 ft winds at 30-40 KT, it is still
possible to see some significant SSW wind gusts into Friday evening.
Have continued to hedge winds on the high end of guidance due to
uncertainty in models.



313 PM CDT

Friday night through Thursday...

The main forecast concerns during the period will continue to
focus on:
1) The unseasonable warmth continuing Friday night and Saturday.
2) Windy southwest winds Friday night.
3) Timing of a cold frontal passage and any associated potential
for showers and storms late Saturday into Saturday night.

The pressure gradient across the Great Lakes region will compress
Friday and Friday night as a weak, but very fast, eastward moving
area of low pressure over the Upper Midwest impinges upon high
pressure over the eastern CONUS. This will result in a period of
stronger winds into Friday evening, when the strongest isallobaric
pressure falls in excess of 8 MB/6HR is expected to overspread
the Upper Great Lakes. With winds in the 925 to 950 MB layer
expected to increase to 35 to 40 KT during the evening...some
strong wind gusts of this magnitude could occur over portions of
northern Illinois, though confidence on getting gusts above 35 KT
is low at this time. Given the strong WAA regime it is unlikely
that temperatures will drop much, if at all, with sunset. So, this
may help keep the lowest levels well mixed and allow the winds to
remain gusty.

These breezy southwesterly winds could continue into at least a
portion of the day Saturday, but this will be dependent on the
timing of an approaching cold frontal boundary, which the model
guidance has been struggling on lately. Current forecast guidance
now suggests a slower southward progression across the area
Saturday evening and night. If this slower progression does pan
out, it will likely result in a very mild day across the area on
Saturday (Highs in the low to mid 70s), as 925 MB temperatures are
expected to be around 16-19C (~90th percentile for the time of
year). The record high on Saturday at O`Hare (Rockford) is 78
(81) degrees. So, while it will be unseasonably mild across the
area Saturday, it may be hard to match these record values, as it
does appear there could be a decent amount of higher level
cloudiness around.

As the cold front sags southward into northerly Illinois late
Saturday afternoon or Saturday evening, it appears that some
showers and potentially some thunderstorms may develop over the
area. Guidance is indicating that moisture return ahead of the
front could be enough to result in some modest destabilization
supportive of some thunderstorms. However, uncertainty remains
as to how much rain may fall across northern Illinois Saturday
night. This will ultimately be determined on the actual strength
of the frontogenesis along the southward sagging baroclinic zone.
If the frontogenesis does remains strong into Saturday night, as
some guidance is indicating, then there will be the potential for
some decent rain amounts over portions of northern
Illinois/northwestern Indiana Saturday night, especially
considering precipitable water values could be near record values
(~1.4") for this time of year. At the present, we have remained
conservative, with only chance POPs and rather low QPF. However,
these may need to be bumped up with future forecast updates if
the current trends continue.

Any rain with this frontal zone boundary should end across the
area by midday Sunday. Cooler weather, back near average in the
upper 50s to low 60s, is then expected for the afternoon on Sunday
as a surface high builds into the western lakes region. This cool
down will be short lived, however, as another, more potent,
surface low tracks eastward from the Northern Plains Monday into
southern Ontario by Tuesday. This low could become sub 990 MB and
fast moving, which may result in stronger isallobaric pressure
falls across the Upper Great Lakes. As such, expect another breezy
day developing Monday and Monday evening. Temperatures will also
warm significantly again back well into the 70s for Monday, and
possibly Tuesday as well.

Although temperatures by the middle to end of next week look to
be near to slightly above normal, there is considerable
uncertainty on precip potential and timing during this period. A
cold front, emanating from the aforementioned strong low over
Canada, is expected to push over the the area by around the middle
of next week, before possibly stalling out over or near the area.
This could set the stage for a period or two of showers and
thunderstorms, though confidence on timing is on the low side at
this time.



For the 00Z TAFs...

634 pm...Forecast concerns include mvfr cigs through Friday
morning and strong/gusty southerly winds Friday afternoon into
Friday evening.

Mvfr cigs have scattered out across far northeast IL and far
northwest Indiana early this evening while a large area of mvfr
cigs remains from southern WI through central/eastern IL. Low
level winds will begin to turn light southwesterly as high
pressure currently over the area moves east this evening. These
low level southwesterly winds should help push the current mvfr
clouds back across the terminals later this evening with cigs
possibly continuing into mid Friday morning before finally
scattering out. But confidence with these trends and timing is

Low pressure will move across the northern Great Lakes region
Friday afternoon and the gradient will tighten quickly between
this low and the departing high pressure to the east. Once low
clouds scatter/dissipate...low level mixing will allow gusts
into the low/mid 20kts by early afternoon. These will steadily
increase toward 30kt gusts by late afternoon/early evening. The
low levels are expected to remain mixed Friday evening with
strong/gusty winds continuing into Friday evening. cms


356 PM CDT

A very active period of weather is forecast over the Great Lakes
over the next several days. There are two storm systems of
interest, which will likely have a large impact on the winds over
the lake.

The first storm system will be a fairly weak, though very fast
eastward moving area of low pressure, which is expected to track
from the Northern Plains late tonight to southeastern Ontario by
early Saturday morning. Given the fast movement of this system,
and the fact that high pressure will be dominate over the eastern
CONUS, the pressure gradient will become strong over the lake late
Friday and into Friday night. This should support higher end
southerly gales of 40 to 45 KT, especially over the northern half
of the lake late Friday afternoon through the evening. Farther
south southerly gales of 35 to 40 KT are still expected over the
open waters of the lake. Because of this, we upgraded the gale
watch to a warning. We decided to leave the near shore waters out
of the gale headline at this time. In spite of this, 30 KT
southerly winds are expected, with some gale force gusts still
possible, especially Friday evening. To cover this threat a small
craft advisory has been issued.

A cold front will begin to shift southward down the lake during
the day on Saturday, with winds becoming northerly, albeit
lighter, in its wake. A surface ridge of high pressure will then
build over the lake on Sunday. As this low passes to the east on
Monday of next week, forecast guidance is forecasting the
development of a much stronger low over the Northern Plains. This
low then looks to take a similar track to our Friday and Friday
night low. Because of this, it looks as though there will be good
chances for another high end southerly gale event Monday and
Monday night.



LM...Gale Warning...LMZ777-LMZ779...4 PM Friday TO 3 AM Saturday.

     Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743-LMZ744-
     LMZ745...4 PM Friday TO 3 AM Saturday.




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