Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 222334

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

303 PM CDT

Through Monday...

Main forecast concerns/challenges are with precip trends this
afternoon through Monday, with an extended period rain drizzle
appearing likely over much of the CWA during this time.

In the near term, strong/sharp front steadily moving through
northeast Illinois at this time and is clearly evident on latest
radar/surface obs. Ahead of it, still noting mild temps in the 70s
with gusty southerly winds. Directly behind, conditions change
rapidly with temps falling to the 50s, gusty north northwest
winds, and steady rain or drizzle. While precip noted along the
front, most of it is staying confined post fropa. Any dry
conditions remaining in east central Illinois and northwest
Indiana will see this precip over the next hour or two as the
front continues to the east. Large scale ascent has really ramped
up over the last hour and has been noted with the expanding precip
shield over northern Illinois. This increase in forcing is due to
the arrival of the upper level trough to the west and vort lobe
to the south/southwest. Guidance still struggling with the phasing
of these features, and trends with precip through tonight and
early Monday. Some guidance steadily pushing precip shield to the
east through the evening, while some guidance keeps it in place
over much of the CWA tonight. Despite the variability with the
phasing, confidence is high with the slow movement of these upper
level features. It would then make sense for an extended period of
precip, and so have gone more pessimistic and increased pops
significantly through tonight and Monday morning. Much of the CWA
will see rain or drizzle into this evening, with maybe the
Rockford area beginning to get into the western edge of this
precip shield. Slowly shift highest pops east tonight but do think
I still may be too aggressive, with this precip shield possibly
still in place Monday morning over much of the CWA. Cold Advection
will continue to usher in a colder air mass, with low temps
tonight getting down to the 40s. Did lower temps slightly on
Monday, but with the possibility of this precip shield to remain,
may need to lower them even further.



303 PM CDT

Monday night through Sunday...

Amplified and active pattern still anticipated at the start of
the period with a large upper level trough situated across the
central CONUS. Anticipate several lobes of energy to round the
base of this trough Monday night into Tuesday and as this occurs,
expect additional periods of showers. Also expect a push of colder
air during this time with the overall conditions on Tuesday not
looking the greatest. Temps in the 40s, gusty northwest winds, and
precip are expected during this time. A rather chilly airmass in
place by Tuesday night and early Wednesday with night time lows in
the low to mid 30s appearing probable. Late Tuesday night into
Wednesday morning will be a period to monitor as have included
mention of rain/snow in the grids. Guidance showing another strong
wave and surface reflection dropping southeast through the upper
Midwest during this time, and looks to return moisture/forcing to
the region. Column is appearing cold enough for at least a chance
of a rain/snow mix. Air mass will then moderate some through the
end of the work week, but with another push of colder air expected
by the start of the weekend.



For the 00Z TAFs...

Main aviation concerns are MVFR/occasional IFR cigs in
rain/drizzle tonight. Wind direction also a bit of a concern
with potential to vary from 340-020 degrees this evening.

Slow moving surface cold front has cleared the terminals as of
early this evening, with winds shifting sharply to the north
behind the front. While gusts in excess of 20 kts were common with
the strong pressure rise immediately behind the front, speeds have
diminished closer to 10 kts or so and should remain there tonight.
Some high res guidance depicts winds may veer as far as 010-020
degrees at times this evening, as gradient weakens a bit as weak
surface high pressure ridge moves across northern IL/southern WI.
Confidence in details/timing of any NNE winds is low.

Another low pressure wave will lift northeast along the front from
the lower Ohio Valley this evening into lower Michigan on Monday.
This will slow the eastward progression of the front, and is
expected to keep especially the furthest east terminals (Chicago
area) in occasional rain/drizzle through the night. At least
occasional IFR cigs are likely at times this evening/overnight
before low-level drying strengthens and rain ends early Monday
morning. Conditions should improve to VFR by mid-morning.



316 PM CDT

Southerly winds up to 30 kt have been observed over much of the
lake today including the nearshore waters, as surface trough/front
remained to the west of the lake. However, both these features
continue to move east this afternoon and are currently moving
across the entire lake. Although do expect a diminishing trend
with the winds tonight, the initial shift to the north/northwest
will also be noted with gusts up to 30 kt. This quick change in
direction and speed will likely bring an increase in waves. Also,
it`s now appearing that winds over the south end of the lake may
stay somewhat stronger, longer tonight. This would likely keep
waves elevated along the south end of the lake and especially the
nearshore. Did extend the small craft advisory for both the
Illinois and Indiana nearshore waters through this evening. Will
need to monitor the Indiana side, as an extended period of
northerly winds may keep waves higher. The small craft advisory
may need to be extended in this location. Still monitoring Tuesday
into Tuesday night with stronger winds and possible gales
returning to the lake. However, still too early for headlines.



LM...Small Craft Advisory...nearshore waters
     until 10 PM Sunday.




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