Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 220803

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
303 AM CDT Sun Oct 22 2017


Through tonight...

300 am...Forecast concerns include precip timing and temps.

A cold front is just about to cross into northwest IL early this
morning with its associated precip slowly dissipating. Additional
scattered showers are developing ahead of the front from PIA to
FEP and short term guidance has been handling these trends well.
This activity is expected to continue moving northeast and
increasing in coverage through daybreak. Still some uncertainty as
to how far east it will move...perhaps into the western suburbs/
I-55 corridor by mid morning. At that time there may be some
weakening of the showers with additional and more widespread
activity developing by early/mid afternoon over the center of the
cwa...then moving east this evening. Its during this time
period...afternoon/evening...that most of the area will see
showers/periods of rain. Best chance for thunder is likely this
morning across the western cwa but just slight chance mention.
Confidence is low for when precip ends tonight and if it may
continue across the southeast cwa. Some guidance continues showers
across the eastern third or so of the area all night and into
Monday morning as an upper low moves north across the Ohio Valley.
Thus maintaining some likely pops across these areas is
reasonable from this distance.

Southerly winds have remained gusty early this morning and
combined with cloud cover have kept temps in the 60s. Highs in the
west will be this morning as temps fall behind the front. But
temps likely to warm into the lower 70s from I-55 east...maybe
mid 70s in northwest Indiana. As the front continues across the
area this afternoon...temps will fall back into the 50s. Low
confidence for precip tonight also affects low temps...mainly in
the west. If clouds clear across parts of the area...temps could
be cooler than currently the mid 40s. MET guidance
drops lows in the west into the upper 30s so trends will need to
be monitored later this evening. cms



Monday through Saturday...

300 am...Forecast focus is on a much cooler pattern through next
weekend with periodic chances for showers.

Another cold front will move across the area Monday afternoon into
Monday evening with a burst of much cooler air for the middle of
the week. There is some uncertainty regarding how this next front
and following upper low interacts with the upper low moving
across the Ohio Valley. Latest ECMWF merges these systems with
quite a bit of precip across the area. Nam/Gem/Gfs all move precip
off to the northeast with just a chance of showers...likely
diurnally driven Tuesday afternoon/evening and probably scattered.
Currently several periods with pops and these will likely be able
to be trimmed and refined as this time period approaches. Temps
on Tuesday likely to struggle to reach 50 with lows in the mid 30s
by Wednesday morning. These temps may lead to some patchy frost
but didn`t include mention in this forecast due to uncertainty
regarding winds/cloud cover.

Still some potential for isolated showers Wednesday with similar
high temps...upper 40s/lower 50s. A short lived and fast moving
ridge moves across the area Thursday...though timing still varies
and this will likely lead to a little warm up on Thursday with
highs perhaps back to 60. But another stronger cold front will
move across the area Thursday night into Friday. Latest Ecmwf is
looking similar to its run 24 hours ago with a system developing
over the region and the front slowing with periods of showers
before the front moves east. Gfs is similar but more progressive
with the front. Too early to pin down specifics but what this
front currently looks to do is bring an even colder blast to the
region for next weekend. cms


For the 06Z TAFs...

A cold front is located over eastern Iowa this evening with a line
of thunderstorms pushing east across the Mississippi River Valley.
Models continue to be in good agreement indicating diminishing
intensity and coverage as the line pushes into northern Illinois
overnight, and this is supported by recent satellite and radar
trends late this evening. RFD will still have the best chance for
seeing precipitation on station overnight, but activity will
become increasingly spotty thereafter and lower coverage will
preclude anything more than a vicinity mention for the Chicago
area terminals overnight. Breezy south to south-southwest winds
will prevail overnight into midday Sunday. Showery coverage will
begin to increase again across the area late Sunday morning into
the afternoon immediately ahead of the front and have include a
tempo for rain during this time frame. As the cold front sweeps
across the region mid afternoon, winds will turn sharply to the
north-northwest and expect more stratiform rain and lower
ceilings to develop. There is a high likelihood that ceilings will
drop to low MVFR and IFR is a possibility. Rain will begin to
exit east of the terminals late evening with winds backing some
and dropping off.



301 AM CDT

An active weather pattern will be in place across the Great Lakes
through the upcoming week. Today, low pressure will lift across
Manitoba to Hudson Bay while a trailing cold front pushes across
Lake Michigan. Southerly winds to 30 kt with an occasional gale
to 35 kt possible will be in place out ahead of the front, then
winds will turn west to northwest behind the front late this
afternoon through the evening. While there may be a very brief
window of northwest winds to 25 or 30 kt along the front, winds
are expected to fairly quickly diminish behind the front.
Southwest flow overspreads the lake again by Monday as another
front moves into the far upper Midwest. This second stronger
front will move across Lake Michigan Monday evening and overnight
resulting in winds swinging back around to the northwest to north
across Lake Michigan. The cold front and associated trough is
expected to merge with another developing low lifting north from
the Ohio Valley into the central and eastern Great Lakes Tuesday.
As the low deepens and colder air filters into the region, winds
will increase to gale strength on Tuesday and continue into
Tuesday night. There are still large model differences in how
quickly and deeply the low will be which will impact the strength
of the gales. For now, not making big changes to the forecast due
to lower confidence, but this is certainly a period to keep an
eye on in the coming days as the details come into better focus.



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




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