Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230529

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1229 AM CDT Mon Oct 23 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 06Z TAFs...

A cold front has stalled east of the terminals late this evening
while bands of showers continue to lift north across Illinois and
northwest Indiana in the wake of the frontal boundary. Rainfall
will likely continue through the overnight hours for the Chicago
area terminals with occasional waves of moderate rainfall that
will reduce visibility to around 4SM. Most of the rainfall will
stay east of RFD though cannot rule out a few sprinkles at times.
Low pressure will deepen as it lifts across Indiana into Michigan
through the day tomorrow. Winds will back towards the northwest
and west in response and eventually help push out the persistent
band of precip over the region. Expect rainfall to exit east
sometime early to mid afternoon with several dry and VFR hours
through the rest of the day. Tomorrow evening and night, an upper
level disturbance will drop across the region resulting in another
round of showers along with low MVFR and IFR conditions.



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...IN nearshore waters until 4 AM Monday.




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