Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 242048

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
348 PM CDT Mon Oct 24 2016

135 PM CDT

Through Tuesday...

High pressure will move overhead tonight for light winds and some
patchy fog. Before cloud cover arrives, we could have some patchy
frost in outlying areas as well. Low pressure will approach later
Tuesday. Expect clouds to gradually lower through the day with
increasing lift ahead of the system. At this point the low levels
(below 10k ft) are dry on forecast soundings and supports the
pattern of a drier ese wind. Given the strong warm frontogenesis
and associated lift there could be a few sprinkles late, but
expect better precipitation chances arrive Tuesday night and more
so Wednesday. Expect similar highs to today on Tuesday.



306 PM CDT

Tuesday night through Monday...

The main concern is with rain and thunder potential and
breezy/windy conditions late Tuesday night through Wednesday as
low pressure moves across the region. Additional focus was on
temperatures Wednesday, with uncertainty based on low pressure
track, followed by warm temperatures Friday.

Large scale pattern through the week will be driven by persistent
deep mid and upper troughing off the Pacific Northwest coast, and
from eastern Canada into the Northeast. The result will be fast,
progressive flow with brief cool downs followed by warm ups as
ridging continues to rebuild back into the center of the country.

Short-wave energy associated with convection over the Desert
Southwest today will spur surface cyclogenesis over the Central
Plains later Tuesday. Lift from strong 850-700 mb frontogenesis
will be pivoting across the area Tuesday evening, but a lot of
this will likely go toward saturating what will start out as a
very dry column, so kept PoPs low in the early evening. Rain
showers will then increase overnight in northern Illinois,
especially north of I-88 as strong f-gen continues to translate
across the area with strengthening WAA from stout LLJ. Model
soundings show some minimal elevated instability above 800 mb, so
a few embedded rumbles of thunder are possible. East-southeast
winds will pick up overnight as pressure gradient tightens
between departing high pressure and approaching low pressure,
particularly on the IL lake shore where gusts near/above 30 mph
are possible by daybreak Wednesday and the gusty southeast winds
continuing through at least mid-day.

Models continue to exhibit fairly significant variance with
respect to the track of the 1010-1012 mb low pressure area on
Wednesday, with the 12z GFS and GEM in the northern camps
along/north of the WI border, NAM a bit south of those, and ECMWF
staying consistent with a track near/south of I-80. This lowers
confidence in the temperature forecast. Clouds and precipitation
will limit temperatures overall, but the farther south path would
lock areas north of I-80 into the 40s much of the day along with a
strong east wind. For what it`s worth, many of the GEFS and
Canadian ensemble members are farther south, so farther south of
the current operational runs may be somewhat more likely to occur.
With this being said, confidence is still low so tweaked slightly
below consensus blend in the north to low 50s, increasing to mid
50s near I-88 and low 60s south of I-80.

Bumped up 12-hr PoPs to categorical for most of the area except
the far south, with sufficient MUCAPE to maintain slight chance
thunder mention in the grids. After a brief shot of seasonal to
slightly below cool air on Thursday, guidance is in decent
agreement today with a quick return to above normal warmth on
Friday. This will be driven by gusty south-southwest winds as low
pressure passes to the north. Still have low PoPs Friday
afternoon, but these may be able to be lowered with later
forecasts. With less clouds and strong WAA, would not be surprised
some locations reach the low 70s on Friday. For now, bumped up
temps slightly from previous forecast and slightly above blend to
mid to upper 60s.

Confidence again lowers on Friday night into Saturday as the 12z
operational ECMWF added a wrinkle to the forecast in delaying cold
front passage to Saturday night. This scenario would result in
Friday night likely remaining dry and MUCH warmer (70s)
temperatures on Saturday. The GFS and GEM still have good support
from their ensembles so did not make any big changes to the
weekend forecast, with Friday night FROPA and cool down to
seasonable temperatures, followed by another warm-up to start
next work week.



For the 18Z TAFs...

- Wind speed/direction
- Precip chances later Tuesday

Winds have been somewhat variable between 330 and 040 this
afternoon as high pressure moves in from the west. As the high
continues east...coupled with a lake breeze...will generally shift
the winds to an ENE direction through the afternoon with speeds of
7-10 kt. The high will move overhead tonight for light winds and
some patchy fog. Low pressure will approach later Tuesday. Expect
clouds to lower through the day with increasing lift ahead of the
system. At this point the low levels (below 10k ft) are dry on
forecast soundings and supports the pattern of a drier ese wind,
therefore have kept the ORD 30 hr TAF dry for the Tuesday
afternoon period. Given the strong lift there could be a few
sprinkles though this will be of low impact. Better chances arrive
after the current TAF cycle.



347 PM CDT

The main concern is the potential for gale force winds/frequent
gusts on Wednesday into Wednesday evening. Strong high pressure
will move over the lake tonight into Tuesday, causing winds to
quickly diminish and remain generally 15 kt or less into Tuesday
evening. The high will then depart to the east Tuesday night as
low pressure develops to the west. There remains uncertainty on
the exact track of the low on Wednesday, but good consensus on it
crossing over or south of the south portion of the lake. The tight
pressure gradient between the departing high and approaching low,
along with cool air mass over the mild waters, favor a period of
gales for at least the central and southern portion of the lake
into Wednesday evening.

This will be followed by a period of brisk north and northwest
winds later Wednesday night into Thursday as high pressure spreads
back into the region. The pattern will stay active on Friday into
the weekend as another low passes over or north of the lake,
resulting in brisk northerly winds returning. Depending on the
track and strength of the low pressure, gale force winds are
possible in this period as well.

Regarding headlines, have extended the Small Craft Advisory for
the Indiana nearshore through late this evening to account for
still hazardous waves subsiding. Issued a Gale Watch for the
southern and central open waters on Wednesday. Held off with a
watch for the near shore for now, due to uncertainty in the low
pressure track lowering confidence in occurrence and duration of
gales for the IL nearshore. The IL shore is where a Gale
headline would most likely be issued for if one is needed. Will
need at least a Small Craft Advisory Tuesday night through
Wednesday afternoon or evening for the entire near shore.



LM...Small Craft Advisory...INDIANA NEARSHORE WATERS UNTIL 11 PM Monday.

     Gale Watch...LMZ777-LMZ779...7 AM Wednesday TO 7 PM Wednesday.




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