Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 211921

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
221 PM CDT Sun Oct 21 2018

148 PM CDT

Through Monday...

Ridge of high pressure stretches from the eastern Great Lakes
southwest across Texas this afternoon and is providing fair
weather across much of the Midwest. Skies will remain mostly
clear overnight, but a modest southwest breeze will prevent us
from strongly decoupling and help prop up overnight lows. Temps
should bottom out in the low to mid 30s for most of the CWA, but
the city of Chicago will remain in the upper 30s to near 40 within
the urban heat island.

A sheared shortwave trough will dig into the western Great Lakes
through the day Monday with attendant surface front dropping
across the CWA through the afternoon. Orientation of the surface
ridge axis to our south will prevent moisture return into the
region, thus front should pass through with only a gradual wind
shift back to the northwest and perhaps a few passing mid and high
clouds, but otherwise no changes to sensible weather conditions
are expected with frontal passage.



207 PM CDT

Monday night through Sunday...

Mid week will feature dry but continued seasonally cool
temperatures. No rain chances until later Thursday.

An upper trough over the western Great Lakes Monday night will
shift to the northeastern United States on Tuesday. This trough
will send a weak cold front across southern Lake Michigan Monday
night and knock our highs down back a bit below normal Tuesday,
with upper 40s to mid 50s. Surface high pressure will be centered
over the northern Great Lakes Wednesday, which will keep our lower
level flow out of the northeast and keep temperatures below normal,
but with a building upper ridge expect plentiful sunshine and dry

Our next weather maker will be the upper trough currently off the
California coast. This trough will slowly meander across the
central US this week. Several pieces of energy from this trough
will arrive in the late Thursday into early Friday time frame.
Surface high pressure to our east will act to weaken this energy a
little, and therefore guidance is fairly modest with the chances
of precip, and only shows light QPF/mainly light rain showers.

An upper trough off in the northern Pacific will eventually shift
across our region at some point next weekend. There are
timing/location differences and what the impacts would be from
this, but the generally upper pattern suggests we remain cooler
than seasonal norms for most of the extended.



For the 18Z TAFs...

VFR conditions and southwest to west flow expected through the
TAF period. Expect modest uptick in winds through the afternoon
with some gusts into the mid and high teens, then surface winds
will drop off after sunset this evening. Earlier model runs were
suggesting winds aloft increasing to around 45 kt late this
evening and overnight, but have since backed off. NAM, which has
the strongest winds, now only indicates winds may briefly tag 40
kt while other models keep winds aloft below 40 kt. Have opted to
pull LLWS from the forecast as a result of these changes.






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