Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 232309

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
609 PM CDT Tue Oct 23 2018

152 PM CDT

Through Wednesday...

Conditions will be, generally, quiet through the short term forecast

For the remainder of the afternoon and into the early evening,
following the passage of a secondary cold front, surge of cooler,
drier air, high pressure is spreading across the Middle and Upper
Mississippi Valley.  The cooler north flow over the relatively
warmer lake, lake effect cloudiness is moving into nwrn Indiana.
With the pressure gradient weakening over nrn IL as the high builds
east, winds are weakening and taking on a more northeasterly
component as the the lake exerts some influence on the wind field.
latest satellite imagery indicates some diurnal stratocu developing
over far nern IL, mainly near the lake.  Under ample sunshine,
temperatures have been able to reach into the lower 50s across the
area even as the region sits under deep layer cold advection with
850mb temps dropping to -2 C late this afternoon.  Into the
overnight hours, forecast soundings suggest that a layer of moisture
trapped under a subsidence inversion could spread into nern IL,
bringing some increasing cloudiness, mainly to nern IL/nwrn IN,
while more inland locations still remain mostly clear.  Temperatures
overnight, away from any cloud cover, should drop into the lower to
middle 30s with some isolated upper 20s possible in sheltered, low-
lying areas.  Under the cloud cover and around the urbanized Chicago
Metro area, lows should bottom out in the upper 30s to lower 40s.
Highs tomorrow should be in the lower to middle 50s.  There is a
slight chance for a few isolated lake effect rain showers late
tonight and early tomorrow, with some of the guidance suggesting
some low level convergence developing over southern Lake Michigan as
the center of high pressure moves across Lake Michigan.  Any shra
coverage should be isolated to scattered and confidence in any
specific area receiving any wetting rain is very low.


233 PM CDT

Wednesday night through Tuesday...

Seasonably cool conditions will continue across the Great Lakes
region as longwave troughing remains a feature across eastern Canada
into next week.

Wednesday night and Thursday: Upper ridging will build across the
region as surface high pressure slowly dissipates over the Lower
Great Lakes. Mostly clear skies Wednesday night should give way to
increasing high clouds Thursday ahead of an approaching trough
across the northern and central Great Plains.

Thursday night through Saturday: The pattern begins to change by
week`s end as the southern jet stream and remnants of eastern
Pacific hurricane Willa interact with a relatively weak shortwave
trough shifting SE into the northern and central Great Plains.

The initial trough will drift across the Upper Mississippi Valley
Thursday night. Phasing between this trough and the southern stream
will be a bit disjunct during this time. A fairly stacked weak mid
to upper-level trough will translate across the CWA Thursday night
and Friday as the NW extent of the southern stream brush the SE CWA.
Dry low-levels under relatively weak forcing aloft will likely
prevent precipitation from reaching the ground across much of the NW
2/3 of the area, with virga likely prevalent during this time.
Across the SE 1/3, isolated to scattered showers will be possible.
Friday night into Saturday, weak mid-level deformation may allow for
a enough additional top-down saturation for a few rain showers, but
most of the area looks to remain dry.

In the extended: 00Z Sunday through 00Z Wednesday
At the start of the forecast period, according to the 12Z run of
the GFS, there are two low pressure centers to our west: one
located in the Northern Great Plains of about 1006mb, and the
other in the Central Great Plains of about 1010mb. At about 06Z on
Sunday, the two low pressure centers will merge together with the
low now centered over MN and a deep trough axis extending down
through western IA, northwestern MO, and eastern KS. The low
pressure center begins to enter the CWA around 15Z Sunday morning
which will setup the potential for isolated showers northeast to
numerous showers southwest throughout the day on Sunday. 12Z GFS
indicates overcast skies for most of the day on Sunday.
Precipitation chances in the CWA drop off after 06Z Monday as high
pressure builds in from the west.

Beyond Sunday, dry NW flow will result in mostly clear skies into
Tuesday, with some light lake effect rain showers/clouds possible
across NW Indiana on Monday under NW/NNW low-level winds.



For the 00Z TAFs...

Surface high pressure will dominate across the western Great Lakes
through the period. Overall this will result in primarily
easterly winds at the terminals through the period. Some lake
effect clouds, with bases as low as 2500 ft AGL may also develop
across the eastern terminals by daybreak Wednesday, and an
isolated sprinkle or shower can`t be ruled out. For the time
being I have left the mention of precip out of the taf due to the
low possibility, and have also remained low end VFR (3500 ft AGL)
for the CIGs at ORD and MDW Wednesday morning.



LM...Small Craft Advisory...Gary to Michigan City IN until 10 PM Tuesday.




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