Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 231915

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
215 PM CDT Sat Sep 23 2017

209 PM CDT

Through Sunday...

Very warm temperatures continue to be the weather focus through
the near term forecast period this afternoon. Record temps
occurring again this afternoon, though with more pronounced lake
breeze now providing some relief locations near Lake Michigan.

Chicago O`Hare set another record high this afternoon in this
string of 90+ degree days in late September, reaching 95 degrees.
Lake breeze is about to push through both ORD and MDW as of 2 pm
CDT, which should allow temps to cool a bit, with current temps
along the lake shore in the upper 70`s and lower 80`s. Rockford
has also set a new record high temp for the 23rd, reaching 83
degrees this afternoon. Most other inland locations were
reporting temps in the lower 90`s.

While the upper pattern will remain similar across the CONUS
through Sunday, the core of the upper level ridge will shift a bit
to the east, with slightly lower thickness values across the
forecast area. 850 mb temps, which were +20C per DVN`s 12Z RAOB
this morning, are progged to cool slightly into the +16/18C range
Sunday, with correspondingly slightly lower daytime high temps.
Highs Sunday are still expected to be unusually warm (and near
records, which are 91 and 92 for CHI and RFD respectively) for
the time of year, ranging from 88-92 degrees based on low level
thermal progs. Light southeast low level wind field will again
support development of a cooling lake breeze across parts of the
Chicago area by afternoon, with winds becoming more easterly off
the lake.

Otherwise, cumulus field across western cwa could still produce an
isolated stray shower late this afternoon, though probability and
coverage are expected to be too low to carry pops.




Sunday night through Saturday...

215 pm...Continued little change to the forecast for early/mid
next week. Temps will remain well above normal until a cold front
arrives late Tuesday into Tuesday night. Models continue to trend
drier with precip chances as this front moves across the area and
its possible that many areas may see just a brief shower with the
best chances for precip across the northwest cwa Tuesday afternoon
into Tuesday evening. High temps continue to rise above guidance
highs and expect this trend to continue on Monday with highs near
90 most areas...with a lake breeze keeping the lakeshore several
degrees cooler. An isolated shower is possible Monday afternoon
mainly west of the cwa and chances remain too low to mention.
Light/patchy fog will also remain possible Sunday night into
Monday morning and again Monday night into Tuesday morning.

This cold front will drop temps into the 70s Wednesday and
Thursday but a stronger push of colder air will arrive Thursday
night into Friday and this will keep highs in the 60s Friday and
Saturday. Some lake effect rain showers are expected and with
primarily northwest flow...these would currently only reach into
far northwest Indiana. Overnight lows by next weekend will be in
the 40s with the usual cool spots possibly dipping into the upper
30s next Saturday night. cms


For the 18Z TAFs...

Aviation weather concerns remain minimal this afternoon, with lake
breeze passage and shift to a more solid east-southeast wind at
ORD and MDW the only real issue. Extrapolation of current lake
breeze boundary would indicate a timing of about 19Z or so at MDW
and 20Z or so for ORD. Light southeast winds should prevail
overnight into Sunday, with another east-southeast lake breeze
push Sunday afternoon.

Otherwise, VFR conditions to prevail, except for potential for
some light MVFR fog again for DPA toward sunrise Sunday.




215 pm...A series of weak low pressures will move from the north
central plains into Ontario and Quebec through midweek with with
the last low dragging a cold front across Lake Michigan Tuesday
night. Southerly winds will continue until this front arrives and
then winds will shift northwest and increase to 15-25kts with a
a short period or some gusts to 30 kts possible. Winds will
diminish by Wednesday night...turn southwesterly Thursday and then
another stronger cold front will move across the region Thursday
night...shifting winds back northwesterly with speeds likely
increasing to 30kts with a period of gales possible Friday
afternoon/evening. cms





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