Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 260828

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
328 AM CDT Tue Sep 26 2017

325 AM CDT

Through Tonight...

One more day - this week - of temperatures making a solid run to
near record levels mainly for locations along and east of I-55.
There remains a minimal chance for isolated showers today and
this evening, but overall forcing for any storms/high impact
weather looks low.

The main cold front this morning is draped from western Wisconsin
through Kansas City and slowly inching eastward. Outflow emanating
from showers last night reached northwest/north central Illinois,
triggering a few showers. Spotty light showers could pop in this
same area into the morning as 20-25 kt south-southwest flow
continues over the old outflow.

The challenge with temperatures today will be clouds. Early
morning satellite imagery depicts high clouds spreading over
northern Illinois. Some of this looks to thin later this morning
into early afternoon before re-increasing later, although
predicting mid-high cloud trends is challenging. The approach of
the boundary and increasing precipitable waters to 1.5 inches
should mean some off and on high clouds for the forecast area
today, with north central Illinois likely seeing more clouds than
sun. Warm advection areawide and a mild starting point will allow
temperatures to at least reach well into the 80s, and eastern
locations if enough sun will have a fair chance to reach 90 given
low-level thermal profiles. Boundary layer southwest wind speeds
should be enough to prevent lake breeze cooling today, so
lakefront areas including downtown Chicago should see just as warm
as inland.

As the front moves through the area later today and early this
evening, isolated showers along the general convergence zone are
possible. Ahead of the front in eastern areas, afternoon
temperatures may reach the convective temperatures, but will
depend on dew points. Cannot rule out a stray earlier shower, but
the best chance...which is still quite low...should be along the
boundary. Thunder chances are even lower given the general
synoptic pattern of clouds shielding the convergence zone as well
as weak elevated lapse rates.

Northwest winds will pick up some tonight behind the front
steering in gradually cooler and drier air.



325 AM CDT

Wednesday through Monday...

Noteworthy items for this part of the forecast are another cold
front moving through late Thursday night with a more seasonable
shot of air, but that looks quite brief with increasing potential
for another warm-up into the first week of October.

Northwest winds of 10-15 mph will filter in cooler air on
Wednesday. Morning cloud cover looks to ease in the afternoon due
to a stout subsidence inversion and drying. Lake effect
stratocumulus, at least patches, are likely into northwest
Indiana. Both Wednesday and Thursday should see highs in the
lower-mid 70s for most.

A strong upper short wave trough will drop southeast over Ontario
and the Great Lakes region Thursday night into Friday. Looking at
the model trend of this, it has been further northeast in global
solutions in the past 24 hours. For our area, that means even less
chance for any rain with the system`s cold front, as well as a
quicker passage of the cool air.

Friday looks breezy behind the cold front with lake effect
stratocumulus very likely into northwest Indiana and possibly
northeast Illinois. Moisture depths are shallow, but some
sprinkles or light showers may need to be added to lakeside areas.
Highs are forecast to fall short of 70 in northern areas Friday
and for most on Saturday. Depending on precise high placement,
Friday and Saturday nights should see decent radiational cooling.
Some outlying favorable locations could dip into the upper 30s if
dew points are low enough.

Global solutions are trending warmer for at least the first half
of next week, with upper ridging building back into the area. The
forecast pattern on both the 00Z GFS and EMCWF, as well as the
locally dry ground and increased sensible heat flux, could favor
highs reaching or topping 80 already by Monday.



For the 06Z TAFs...

Light southerly winds will become southwest after daybreak with an
increase in speed to 9-10 kt by midday. Isolated showers are
possible around RFD this afternoon with the approach of a cold
front. This front will pass the RFD area during the late afternoon
and the Chicago airports during the early evening, turning winds
to the northwest. Centered around this passage time is a window
for all TAF sites where there could be an isolated shower, but
overall the forcing along this front is meager. Behind the frontal
passage, 4000-6000 ft cloud bases are expected. Chances for MVFR
look small through tonight.

Confidence in TAF details is medium-high.



325 AM CDT

A cold front passage tonight will end the quieter marine weather,
at least temporarily. Winds will turn northwest behind the front
and increase in speed with gusts around or a little above 25 kt
for much of the open water. A Small Craft Advisory will be needed
for Wednesday along the Indiana nearshore for winds and waves and
possibly the Illinois nearshore for winds. Winds and waves will
ease some Wednesday night and early Thursday.

A second and more stout cold front will move south across the lake
on Thursday night. This will bring northwesterly 30 kt gusts for
the open water for Friday, and these winds will turn more toward
due north by Friday evening it appears. This means quickly building
waves for the Indiana nearshore by Friday morning and the Illinois
nearshore shortly after. Winds and waves look to diminish by
Saturday morning or early afternoon at this time.






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