Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 182357

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
657 PM CDT Tue Oct 18 2016

140 PM CDT

Through Wednesday...

A cold front continues to push across the region this afternoon,
and stretches from roughly Gary through Pontiac at 18Z. Out
ahead of the front, temperatures in the upper 70s with dewpoints
in the mid 60s will contribute to close to 1000 J/kg MLCAPE by mid
afternoon, so cannot rule out a shower or isolated thunderstorm
developing but better chances will be off to our east where there
is very modest upper support associated a sheared shortwave
pushing across the Great Lakes. Behind the front, winds are
turning northwest and easing while skies are clearing out. Latest
runs of the RAP and HRRR continue to suggest a lake breeze will
form and push inland across NW Indiana and a little into Cook
County mid to late this afternoon which will hasten temperatures
dropping back through the 60s for those areas. Otherwise, expect
quiet weather this evening and overnight as high pressure builds
towards the mid/upper Mississippi Valley.

High pressure will continue to dominate most of the day Wednesday
providing light northeast flow and primarily dry conditions.
Despite the cold frontal passage, we will continue to experience
above normal temperatures Wednesday with mid to upper 60s north of
I-80 and low 70s along and south of I-80. An upper level
disturbance currently digging across the Inter-mountain West is
progged to lift across the central plains Wednesday. Weak mid
level height falls will begin to overspread the local area by mid
to late in the afternoon with increasing mid and high clouds.
Models do generate some QPF up through roughly the I-80 corridor
by 00Z tomorrow evening, though soundings show dry air persisting
below 700mb. Could be some high-based light showers or sprinkles
moving up from the south into the local CWA, but better chances
for showers will be closer to the baroclinic zone stretching from
downstate Illinois into southern/central Indiana.



210 PM CDT

Wednesday night through Monday...

A transition back to more seasonal weather is on tap for the extended

NAM/GFS both depict fairly strong frontogenesis with the cold
front Wednesday night in the clash between our  anomalously warm
airmass of late and the colder continental air arriving from the northwest.
This will be coupled with strengthening low pressure from the
left rear quadrant of an upper level jet stream and should result
in an increasing precipitation shield across the region Wednesday
night. At this point the better co-location of forcing and
moisture will remain draped south and east of our area, but there
should be enough for some bands of lighter to even moderate rain
across the I-55 corridor southeastward and possibly even extending
farther into northeast IL. This forcing continues through the day
Thursday with the jet in the region, an approaching pacific
shortwave trough axis and a strengthening surface low continuing
the deformation precipitation across much of Indiana with some
lower chances across Northwest Indiana. Expect more clouds than

While colder air slowly bleeds in during the day Thursday, a secondary
surge of colder air will be shoved southeastward by a portion of
the Canadian upper trough will reinforce the changing airmass.
Models depict this will be a largely dry frontal passage, but do
portray some lake effect rain showers across northwest Indiana.
The convergence signal is somewhat weak and progressive and
moisture is but the thermodynamic signal is robust to suggest
there will be some lake effect showers with fairly high
confidence, but lowered confidence on how widespread and how long
lasting. Areas away from the lake will have sunshine but cool
conditions and area wide highs in the 50s, some low 50s north.

High pressure returns Friday night and Saturday which look dry,
in spite of the EC painting some light precip along an elevated
warm frontal boundary as the high approaches. An upper wave will
pass by to our north on Sunday which will initially lead to
slightly warmer conditions, though there are better indications of
a backdoor cold front sometime later Sunday into Monday. The front
holds off till after sunset on the GFS, midday/afternoon on the
EC/GFS. This will setup a frontal zone across the local area which
may serve as a focus for increased precipitation chances Tuesday
or into the middle of next week.



For the 00Z TAFs...

Concerns with the aviation forecast are east-northeast winds on
Wednesday afternoon possibly reaching 8-10 kt, at least
occasionally, and the chance for light rain Wednesday evening.

A cool front that went through earlier had brought northwest
winds, but fairly weak. These will become light and variable
within the next hour and persist through Wednesday morning. In the
afternoon, a lake breeze or at least winds off the lake are
likely. Confidence on speed not exceeding 7 kt is medium. Mid-
level cloud cover will thicken late in the day into the evening. A
dry low-level may prevent any light rain from reaching the surface
and have leaned that way in the ORD TAF right now.



210 PM CDT

Today`s cold front has cleared the lake as low pressure across
northern Quebec exits the region. High pressure across the
Canadian Rockies will creep southeastward into the upper Midwest
and plains region over the next few days. Meanwhile, and area of
low pressure currently stretched across the Ohio valley and into
the southern Plains will lift northeast toward the latter half of
the week. In between these two systems a cold front will pass
south across the lake Wednesday night with a tightening pressure
gradient resulting in gusty north winds Thursday. With the
persistent north winds behind the front, expect waves to build on
the south end of Lake Michigan and result in dangerous conditions
for small craft Thursday into Friday. The high will weaken some
Friday as the low continues to strengthen across the northeastern
United States Friday. The high pressure ridge will move east of
Lake Michigan Sunday and will result in a shift to southerly winds
briefly before a backdoor cold front results in a shift back to
northeast winds.






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