Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 230755

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
255 AM CDT Mon Oct 23 2017


Through Tuesday...

255 am...Primary forecast concern is periods of rain.

An upper level low lifting across the Ohio Valley this morning
will merge with a large upper trough digging into the upper
midwest later today. This large merged upper low will then sit
over the western Great Lakes Tuesday and lift northeast Tuesday
night. As a result...a large area of rain will move across the
eastern two thirds of the cwa through midday and then slowly shift
east...but rain is likely to continue into this evening for all
of northwest Indiana. Stormtotal rainfall amounts across northwest
Indiana...through this evening...may reach into the 1 to 1.5 inch

The next area of rain then begins to develop to the northwest
this afternoon and spreads across the area this evening and will
persist into Tuesday. Rainfall amounts likely to be less and
precip may be more showery then steady rain but another third to
two thirds of inch of rain is likely overnight through Tuesday

High temps today are a bit tricky across the west where precip
will end this morning and there may be some partial sun midday.
Generally have lower 60s west to upper 50s east but these may need
some adjusting with precip/cloud trends later today. After lows
in the lower/mid 40s tonight...there won`t be much recovery in
temps on Tuesday...likely staying in the mid/upper 40s. And winds
on Tuesday will be a bit stronger...15-25mph with some higher there might actually be some wind chills in the upper
30s Tuesday. cms



Tuesday Night through Sunday...

255 am...Forecast concerns include much cooler temperatures...
the potential for a frost or freeze this weekend...and for
now...much less precip.

As the upper low departs Tuesday night into Wednesday...precip
will be ending across the area Tuesday evening and should end
before there would be a potential for any mixed precip. In
addition...thermal profiles are not as cold as previous runs.
Low temperatures by Wednesday morning will be in the low/mid 30s
but there remains uncertainty about cloud cover and wind speeds.
The ground will be wet and still somewhat no mention of
frost for this period.

A weak upper wave will move across the region Wednesday that for
now appears to just bring an increase in cloud cover so have gone
dry for Wednesday. And depending on cloud cover...temps likely to
recover slightly back to the lower 50s. A fast moving ridge
spreads across the area Wednesday night into Thursday ahead of low
pressure passing across the upper lakes Thursday into Friday.
This should allow for one mild/warmer day on Thursday as highs
likely push back toward 60...perhaps warmer if trends continue.

But by Thursday night...a trailing cold front from the low just
noted will move across the area. And while a passing shower can`t
be completely ruled out...the models have come into better
agreement with generally dry conditions from Wednesday through
Friday night. Precip chances then return over the weekend as an
upper low moves across the region...but confidence is fairly low
from this distance and it may just generate lake effect precip. If
precip were to develop during this time period...there would be
some potential for mixed rain/snow mainly at night. If the
forecast stays dry along with some clear skies/light winds...then
its possible a widespread frost/freeze may occur as MEX guidance
has lows below freezing. cms


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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