Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 200730

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
230 AM CDT Fri Oct 20 2017


Through tonight...

230 am...No forecast concerns this period. High pressure will
remain across the east coast/mid Atlantic with a trough of low
pressure across the plains. The gradient will slowly tighten with
southerly winds increasing to 10-20 mph with some higher gusts
possible this afternoon. Temps are currently in the upper 40s to
lower 50s and probably won`t drop much more before sunrise.
Guidance highs for today are all in the upper 70s and its likely
several areas will tag 80. Temps will drop into the 60s this
evening with lows by Saturday morning likely staying in the 50s.
Increasing high clouds today which will thicken a bit more
tonight. cms.



Saturday through Thursday...

230 am...Forecast concerns include rain chances Saturday night
through Sunday evening...then two cold fronts and associated cold
air burst behind each front.

The gradient will tighten further on Saturday with southerly winds
into the 15-25 mph with higher gusts. There will likely be more
cloud cover Saturday...perhaps in the afternoon and this lowers
the confidence for high temps which for now are near guidance
highs but its likely that highs on Saturday could be similar to
Friday...especially across eastern areas.

A cold front will move across the area on Sunday with showers
developing ahead of the front late Saturday night and continuing
for much of Sunday. Timing appears to have slowed just a bit with
the best chances for rain from daybreak Sunday through mid/late
afternoon. Thunder chances look fairly low and will continue with
slight chance mention. QPF amounts look to be mainly under an
inch...storm total but there could be band/axis of heavier
rainfall though too difficult to pin down where this may occur
just yet. Models differ on end time across the eastern cwa Sunday
night into Monday morning and trends will need to be monitored as
to the exact end time of precip.

The first of two stronger cold fronts will arrive Monday as an
upper low closes off over the Lakes region Tuesday into Wednesday.
This will spread much cooler air across the region through
midweek along with showers and potentially lake effect showers in
northwest Indiana though too early for exact locations which will
be dependent on wind direction. Also currently looks to stay warm
enough that this precip should remain all rain but if precip can
become heavy enough and cool the column enough...then some mixed
snow can`t be completely ruled out.

This midweek upper low shifts east by Thursday with a short-lived
ridge building across the area Thursday then another strong cold
front moves across the area from late Thursday afternoon through
Thursday night...still some timing differences. This cold blast
will likely be colder than the first and while its just beyond the
end of this also looks to be short lived as temps
begin to moderate going into next weekend. cms


For the 06Z TAFs...

No significant changes were made to going forecast. High pressure
is centered just east of the terminals and will continue to
drift away from the region through the period allowing a modest
southerly breeze to develop through much of the day Friday. A
150-180 wind direction overnight will veer slightly to 180-210
Friday with gusts in the mid to high teens throughout the
afternoon, with a few sporadic gusts starting late morning.
Around and after sunset Friday evening, a modest low level jet is
expected to develop along the Mississippi Valley. Winds at 020 are
expected to increase to 35-45kt at RFD which may result in LLWS
for a few hours Friday night. For now the setup looks very
marginal and surface winds look to stay up just over 10kt at RFD
so will not make any mention of LLWS at this time. VFR conditions
will prevail with only FEW to SCT cirrus expected.



302 PM CDT

Lighter winds in place across the entire lake today, as gradient
has generally relaxed. This is most noticeable across the south
half where surface ridge axis has moved overhead. Across the north
half, fairly tight gradient still in place and allowing for
slightly elevated winds to persist. However, only anticipate these
winds to 30 kt to only last through this afternoon and then
diminish back towards the 15 to 25 kt range. Southerly winds
expected to be in place over the entire lake through much of the
weekend ahead of approaching surface trough/front. Expect winds to
increase back to 30 kt over the open waters and initially only
for the north half, but will see this trend observed over the
south half. Although currently not forecasting gales, am
monitoring the potential for gales late Saturday into Saturday
night mainly over the north half. Across the nearshore waters,
conditions hazardous for small craft are again possible by this






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