Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 272355

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
655 PM CDT Tue Sep 27 2016

218 PM CDT

Through Wednesday...

For lingering fire weather concerns, see the fire weather section

Round 1 of the airmass change occurred Sunday night as we
transitioned from our maritime tropical to our continental polar
air mass, but conditions have been fairly benign from a cloud and
precipitation perspective. Up to this point we have felt the wind
impacts and a cool down to more seasonal temperatures, but round 2
is on its way as it will now look and feel a bit more fall-like
Wednesday. Deep upper level low pressure continues to spin across
Lake Superior with an extensive cloud shield associated with it.
Most of the precipitation with the low is still in northern and
central Wisconsin.

Model guidance suggests the cloud shield will fill in this
evening, with precipitation increasing more so late this evening
and overnight as the upper low moves overhead. It is during this
time that rain/drizzle will become a bit more widespread.

On Wednesday the upper low will pull to the east which will,
generally, decrease shower coverage, but expect a generally cloudy
day all in all. Have high temperatures near 60/low 60s. Upstream
in Wisconsin highs have barely gotten out of the 50s. We will not
be that cold as the core of cold temperatures will not be on top
of us during the day, but some highs in the upper 50s are not
unreasonable. The caveat to the decrease in rain is with the low
in the area, coupled with a cold airmass moving over the warm
lake, rain showers will likely pivot around the back side of low
with an uptick in the afternoon as low level winds/convergence
favor increase lake effect parameters across northwest Indiana and
northeast Illinois. Have maintained the waterspout mention along
with some chance of thunderstorms close to the lake as lake
induced equilibrium levels allow parcels to cross the charge
separation region.



322 PM CDT

Wednesday night through Tuesday...

The pesky upper level low pressure will continue to be the main
driver of our sensible weather into the weekend before finally
pivoting northeast away from the region on Sunday. This will mean
cool and periodically showery weather, especially for northeast
and east central Illinois and northwest Indiana. The upper low
will drive fairly far south into the Ohio Valley on Wednesday
night into Thursday, keeping shower chances focused farther east
(mainly along/east of I-55), including the lakeshore where
impressive lake induced thermodynamics will be in place much of
the time. Kept a slight chance of thunder near the lake into
Wednesday evening. With cold temperatures aloft and resulting
steep low level lapse rates and minor SB CAPE Thursday afternoon,
added a slight chance of thunder. The upper low will then
retrograde north-northwest Friday into Friday night, spreading the
chance for scattered showers farther west, with again a slight
chance for isolated thunder for areas mainly east of I-55.

With the lake effect parameters remaining quite favorable and
persistent onshore flow into Saturday, will need to monitor for
enhanced low level convergence and potentially heavy showers at
times near the lake.

Afternoon temperatures will be several degrees warmer on Thursday
given the farther south upper low position, then cool at least a
few degrees on Friday. For now, stuck with the blended
initialization on Saturday, but the most recent guidance suggests
potentially similar conditions to Friday, suggesting cooler
temperatures and slightly higher PoPs than in current forecast
could be in store.

For Sunday through Tuesday, the departure of the upper low Sunday
will enable temperatures to moderate with lower shower chances.
Monday and Tuesday will be primarily dry with warm advection and
mid-level height rises, ahead of the next mid/upper trough and
associated surface system that may affect the area Tuesday night
into Wednesday, beyond Day 7.



For the 00Z TAFs...

Deteriorating conditions expected tonight as upper low drops
south into the region. Band of rain over WI will gradually pivot
south into northern IL very late this evening or more likely
during the overnight hours. Initially dry low level airmass should
keep CIGS VFR initially, but as rain falls it should gradually
saturate low levels and combined with cooling air mass should
allow CIGS to build down to MVFR overnight. Seems like a pretty
good bet there will be at least periodic IFR early Wednesday
morning before CIGS slowly lift diurnally to MVFR and potentially
low end VFR by later in the afternoon. The confidence in lifting
to VFR is lower for ORD/MDW where lake influence could keep CIGS
lower. Steady rain from overnight should largely end Wednesday
morning with a break in precip before lake effect showers develop
mid-late afternoon Wednesday and likely continue into Wed evening.
Generally light west winds will shift north tonight, then trough
will move through mid-late afternoon Wednesday with wind shift to
northeast, at which times wind will likely quickly ramp up and
become quite gusty.



341 PM CDT

Westerly winds will quickly diminish this evening, though elevated
waves will continue east of Gary Indiana. Strong northeast winds
generally to 30 kt will develop Wednesday and then continue
through Friday night as high pressure locks in north of the lake
and low pressure locks in southeast of the lake. Forecast
soundings suggest that maximum speeds/gusts will occasionally be
near/around gale force Wednesday night through Friday, though do
not yet have enough confidence for any gale headlines. Winds and
waves hazardous to small craft will be in place for the entire
Illinois and Indiana near shore area mid day Wednesday through
late Friday night. The weak low pressure will move across the lake
Saturday and Sunday, resulting in much lighter north and northeast



IL...Red Flag Warning...ILZ012-ILZ013-ILZ014-ILZ019-ILZ020-ILZ021-
     ILZ022-ILZ023-ILZ032-ILZ033-ILZ039 UNTIL 7 PM Tuesday.


     Small Craft Advisory...LMZ740-LMZ741-LMZ742-LMZ743...1 PM
     Wednesday TO 4 AM Saturday.

     4 AM Saturday.




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