Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 221535

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1035 AM CDT Sun Oct 22 2017

1018 AM CDT

Continue to monitor precip trends this morning, with latest radar
imagery depicting showery development moving through portions of
northern Illinois. This has been occurring with several pieces of
shortwave energy lifting across the region, ahead of upper level
trough still to the west. Weak instability axis is overhead and
has supported scattered thunderstorm development. Despite
instability in place, likely weakening forcing has allowed much of
the lightning to diminish at this time. May see coverage of
showery development lower over the next couple of hours but as the
upper level trough and stronger vort lobe over the
central/southern plains inch closer, should see coverage and
intensity of showers increase once again going into the afternoon.
Focus will be centered into northeast Illinois and then areas east
this afternoon. Lightning still a possibility, and continued
slight chance thunder for the remainder of the day. Now through
the afternoon, will see cold front push through the CWA. Ahead of
it, temps have risen to the upper 60s to low 70s. However, will
see temps fall to the 50s behind the front.




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 12Z TAFs...

A cold front is pushing east across the Mississippi Valley early
this morning with areas of showers and thunderstorms overspreading
northwest and north central Illinois out ahead of the front.
Guidance has consistently shown this area diminishing in coverage
and intensity and it pushes east towards the Chicago area
terminals this morning, so do continue to expect a relative lull
with prefrontal precipitation becoming more isolated to widely
scattered mid to late this morning. By late morning to early
afternoon, coverage will once again increase over Chicago.
Thunderstorms are certainly possible but given the relatively weak
instability continue to favor just a shower mention in the TAF.
The front will sweep across the terminals late this morning at RFD
and early afternoon for the Chicago terminals with moderately
strong south winds turning northwest and gradually diminishing.
Stratiform post frontal rain will be in place for several hours
behind the front along with IFR or low MVFR ceilings. Winds will
drop off this evening and potentially become light and variable
overnight, though will likely favor some sort of westerly
direction. Some models hint at IFR ceilings setting up overnight
while others improve back to VFR.



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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