Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211129

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
629 AM CDT Sat Oct 21 2017


Through Sunday...

251 am...Forecast concerns include precip timing/chances tonight
and Sunday as well as high temps today and Sunday.

Fairly thick mid/high clouds early this morning which are helping
to keep temps in the upper 50s/lower 60s. Mid deck also producing
some sprinkles across northwest IL and will add mention for early
this morning. This cloud cover appears to thin just a bit by this
afternoon and if that trend materializes...highs likely to reach
near the warmest guidance values in the upper 70s. As the gradient
tightens...southerly winds will become breezy today with gusts to
30 mph.

A line of thunderstorms is expected to develop well west of the
area later today and will weaken as it moves into northwest IL
later this evening. This activity will move across the western
cwa overnight in a weakening phase. Thunder will be possible but
still only slight chance mention. Its also possible that
additional showers will develop over the area during the early
morning hours.

A cold front will move across the area on Sunday and the best
chance for showers will likely be along/ahead of this front...
which will be dependent on frontal timing. Some of the hi-res
guidance suggests a lull in precip Sunday morning with activity
redeveloping across the eastern cwa Sunday afternoon. At this
point...not confident enough to make too many changes to pops but
some changes may be needed if current trends continue. And if
precip coverage is less Sunday morning...then temps are likely to
be warmer and could be in the lower 70s over the eastern cwa.
However...these highs would be midday/early afternoon with temps
likely falling by mid/late afternoon as the front moves across
the area. cms



Sunday night through Friday...

251 am...Main forecast concerns are two cold fronts and
associated cold bursts/precip.

There remains uncertainty regarding how fast precip will exit the
southeast cwa Sunday night or Monday morning. The Canadian remains
the most aggressive with precip across the region as an upper low
moves north across the Ohio Valley Monday...while the GFS moves
precip east of the area by Monday morning. Confidence is too low
to make any changes during this time period with some low pop
mention necessary until a consensus emerges. How this evolves then
affects the next system...another upper low diving across the
midwest on Monday and settles over the Great Lakes region Tuesday.
Much colder air will spread across the region with a chance of
showers as well as lake effect showers downwind of Lake Michigan.

As this upper low departs midweek...a short-lived ridge builds
across the region Wednesday night into Thursday which may make for
a milder day on Thursday before another cold front moves across
the area in the Thursday night time frame. cms


For the 12Z TAFs...

Moderate southerly flow will continue across the region today
ahead of a cold front expected to sweep across the terminals on
Sunday. Early this morning, a lead upper level disturbance is
lifting across the mid Missouri Valley with a few high based
showers and an isolated thunderstorm out ahead of this wave over
western and northwest Illinois. RFD may see a few drops of rain
from these showers, but it is unlikely that any thunder would
occur on station nor any limitations to visibility or ceilings.
Beyond these early morning sprinkles, expect partial clearing to
occur later today in the wake of this lead wave with sunshine
helping to promote deep mixing. Wind gusts will increase mid to
late this morning into the afternoon, eventually topping out in
the mid 20 kt range throughout the afternoon hours.

Forecast uncertainty grows late this evening and overnight as the
cold front moves east across Iowa. Showers and thunderstorms will
be ongoing along the front over Iowa mid to late this evening,
but should begin to diminish in coverage and intensity as they
move east across the Mississippi River. There is the possibility
that a few of these decaying showers may reach the terminals after
midnight, especially at RFD, but many areas may also stay dry as
coverage wanes. Better chances for showers along with MVFR clouds
will occur as the front begins to move across northern Illinois
Sunday morning and afternoon.



318 AM CDT

Expect an active stretch of weather across Lake Michigan through
the upcoming week as a series of cold fronts move across the
region. A moderate southerly gradient will be in place today and
tonight with south winds increasing to around 30 kt as low
pressure lifts from southern Manitoba to James Bay. A few gale
force gusts are possible on Sunday ahead of a trailing cold front,
mainly over the northern half of the lake. The front will sweep
across Lake Michigan on Sunday turning winds west to northwest for
several hours Sunday evening and night. Winds will back to the
southwest again by early Monday as another low tracks east across
the Canadian Prairies. A stronger cold front associated with this
low will move across Lake Michigan Monday night with winds turning
north to northwest. As cold air spills south across the region
behind the front Monday night into Tuesday, winds will increase to
gale force and remain there into Tuesday evening or overnight
before tapering back below.



LM...Small Craft Advisory...nearshore waters
     until 4 PM Sunday.




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