Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 241903

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
203 PM CDT Tue Apr 24 2018

200 PM CDT

Through Wednesday...

Low pressure across the Ohio and Tennessee valleys will finally
pull away from the region tonight. Lower clouds and maybe an
isolated shower will found along and east of I-55. On the heels
of this departing low, a cold front will move through tonight
ahead of a northern stream shortwave that is presently shifting
through the upper Midwest and upper Great Lakes. High pressure in
wake of this wave will enable gusty northeast winds to usher in a
cooler airmass beginning late this evening and begin our modest
temperature roller coaster ride that will continue for a few days.

Therefore, after some warm or at least seasonably warm days,
temperatures will take a dip on Wednesday following the passage of
the cold front. Brisk northeast winds will hold readings some 10
degrees colder in spots, with lower 60s south and west, and
readings in the 50s as you head closer to the lake. The warm April
sun should help to make it feel a bit warmer.



241 AM CDT

Wednesday through Monday...

Fluctuating temperatures are in store throughout the extended period
as a series of cold fronts move across the region through the latter
half week and into the weekend. A big warmup may be in store by
early next week, though, as a strong upper ridge builds across the
Great Lakes.

Cold front that sags through the region Tuesday night will usher
in cooler temperatures on Wednesday. A modest north/northeast
gradient will be in place early in the day ahead of an expansive
ridge that extends from the High Plains into the Great Lakes.
Ridge axis will gradually build south across northern Illinois
and northwest Indiana by Wednesday evening/overnight allowing the
gradient to gradually relax through the day. Temperatures will be
cool on Wednesday, topping out in the mid 50s inland and 40s near
the lake with persistent onshore flow throughout the day. Weather
looks to be otherwise quiet Wednesday and Thursday under the
influence of the building ridge. Temperatures should moderate
behind the ridge axis on Thursday with afternoon highs near
seasonal norms in the low to mid 60s. There probably will be a
lake breeze on Thursday, but boundary layer flow increasing to
around 10 kt out of the southwest may result in minimal inland
progress, or a late day surge that won`t have much impact on high

Thursday night, another cold front is poised to sweep across the
region. Guidance has been in good agreement indicating a line of
showers and thunderstorms developing within a narrow corridor of
instability near the front Thursday afternoon across portions of
Wisconsin into Iowa. There has also been good agreement on frontal
passage across the local CWA overnight with a rapidly stabilizing
boundary layer which will limit the intensity of convection as it
reaches northern Illinois. Would expect a decaying line of showers
and not terribly confident that it will maintain through the
overnight hours across the entire CWA. Behind the front on
Friday, afternoon high temperatures drop back into the 50s with
40s near the lake.

Another ridge axis builds across the region over the weekend
bringing a dry spell and a warming trend back to seasonal norms
by Sunday. Meanwhile, strong upper ridge will be translating east
across the Great Plains over the weekend then across the Midwest
early next week. A strong thermal ridge will move across the
region with 850mb temps warming to 11-14C and 925mb temps of
16-20C Monday and Tuesday. Late April/early May climo has median
surface high temps in the upper 70s given those temperatures
aloft. Cloud cover could be a complicating factor, especially on
Tuesday as another cold front approaches the region but both days
look favorable to make a run at 80F.



For the 18Z TAFs...

ENE winds will continue through the afternoon, holding
in the 8-11 kt range. MVFR ceilings continue to erode from the
northwest this afternoon, though expect these clouds to linger
southeast of the terminals even into the evening hours across
east central IL and northwest IN. Any isolated showers would be in
this corridor. VFR clouds will linger across the Chicago

A cold front will usher in gusty N to NNE winds beginning mid-
evening. There may even be some expansion of the MVFR cigs farther
westward but at this point it appears any lower clouds would be
scattered. After gusty N to NNE winds tonight (to 25 kt or so),
expect continued NE winds in excess of 10 kt through the day on



241 AM CDT

Lake Michigan lies within a weak flow regime early this morning,
between systems, but a cold front will move down Lake Michigan
this evening and overnight with northerly flow strengthening in
its wake. Wind gusts are expected to peak around 30 kt late this
evening and overnight, then should very gradually come down
through the day Wednesday as a ridge of high pressure across the
region. A small craft advisory will be hoisted for the Illinois
and Indiana nearshore waters for this evening through Wednesday
evening. Winds flip to southwest behind the ridge axis on Thursday
and ahead of another approaching cold front. This next front is
expected to sweep across the Lake Thursday night bringing another
period of north winds gusting to around 30 kt.



LM...Small Craft Advisory...LMZ740-LMZ741-LMZ742...10 PM Tuesday to
     10 PM Wednesday.

     Small Craft Advisory...LMZ743-LMZ744-LMZ745...1 AM Wednesday to
     1 AM Thursday.




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