Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 232332

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
632 PM CDT FRI SEP 23 2016

229 PM CDT

Through Saturday...

A cold front, now stationary, was shoved south through the area today
 by strong high pressure centered across western Ontario. The
high will move southeast towards Lake Huron tonight, which should
help to keep the area locked in cool and moist southeast flow off
the lake. This will keep a low level inversion in place to keep
the stratus locked in or allow it to fill back in for much of the
night into early Saturday. We are seeing some clearing this
afternoon for our southern and western areas. Expect with some
heating occurring in the cloud breaks that there could be some
expansion of the clearing/lifting. Another mild night is expected
with lows in the 60s.

On Saturday the upper ridge will build overhead, and the low level
flow component should shift to more east-southeast. This should
generally allow for more and earlier clearing on Saturday as
opposed to today, though it will be slower-going in northern
Illinois and close to the lake in northeast Illinois. Expect only
modest temperature recoveries from today as well, but the airmass
is still fairly mild as average highs for mid to late September
are in the low 70s. We should see 70s northeast to the mid 80s
across Central Illinois.



244 PM CDT

Saturday night through Friday...

Unseasonable warmth remains possible through Sunday, then a cold
front will bring cooler and more seasonable temperatures back to
the region.

Saturday night into Sunday, strong upper ridge will still be in
place across the western Great Lakes region while an upper low
lifts from the northern Plains into the upper Midwest. Highly
meridional pattern will transport teens Celsius 850mb temps north
into Hudson Bay while low single digits wrap south into the
central plains. On the warm side of the baroclinic zone, surface
temps for Sunday will continue to run above normal with some
models suggesting low to mid 80s much of the forecast area.
Remained a little conservative based on the expected cloud cover
for the day which should be mostly cloudy to cloudy. Models
continue to zero in on precip timing on Sunday, favoring the
showers and thunderstorms along the cold front as it passes
through during the afternoon and evening hours. Cannot rule out
spotty showers earlier in the day with very modest elevated
instability in place during the morning and broad mid-level
southwest flow.

Cool down still on track for Monday as a strong shortwave digs
from the Canadian Rockies Sunday into the Midwest Sunday phasing
with the aforementioned upper low resulting in additional
deepening of the surface low and wrapping 2-3C 850mb temps into
the local area. The combination of the tightening pressure
gradient and cold air advection will drive strong winds during the
day Monday and bumped up winds quite a bit now expected sustained
winds peaking around 15 to 20 mph and gusts to around 30 mph. Any
clearing early in the day should quickly fill in with cold air
stratocumulus during the afternoon and evening hours. Expect only
mid 60s for high temps Monday. Precip on Tuesday looks more
favorable to our north under the departing upper low, then dry
conditions look probable for much of the remainder of the week as
surface high builds into the region and upper ridging builds back
into the mid section of the country allowing temperatures to
moderate back towards normal, even above normal late in the week.



For the 00Z TAFs...

The primary forecast concerns for the airports through Saturday are
low clouds and possible fog tonight into Saturday morning.

A front is draped just to the south of the area early this
evening with pooled moisture and areas of low clouds along and
just to the north of this. With east-northeast winds continuing
onshore flow within this air mass after dark, would expect the
clouds to fill back in along with lowering bases. Do expect there
to be areas of IFR ceilings, with already some indications of
patchy sub 1000 ft cigs at KUGN and on area webcams. Confidence on
IFR for ORD and MDW is low-medium.

The coverage of clouds should dictate how bad any fog is. The
higher threat area for fog looks to be mainly south and west of
the Chicago area airports, with RFD having increased potential for
IFR cigs and/or visibility into Saturday morning.

Given the strength of the inversion observed and forecast just
north of the front, would expect the clouds to be sluggish to
depart Saturday morning and possibly even the afternoon though
their should at least be afternoon lifting of the bases. The
confidence in rate and timing of clouds lifting on Saturday is
low-medium. East-northeast winds will increase a tad during the
day Saturday, once again around 10 kt at ORD and MDW with high



244 PM CDT

A broad area of high pressure is centered over Ontario this
afternoon and is resulting in fresh northeast breeze across Lake
Michigan this afternoon. This high will shift over the eastern
Great Lakes over the weekend allowing winds to diminish some
across Lake Michigan Saturday, but low pressure is expected to
lift into the northern plains causing southeast winds to
strengthen with gusts near 30 kt by late Saturday night and
through the day Sunday. The low will move east across Lake
Superior Monday with winds shifting to the west behind a trailing
cold front, but remaining strong.






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