Frost Advisory 2, NWS Chicago, IL

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
328 AM CDT Fri Sep 28 2018

.SHORT TERM...
327 AM CDT

Through tonight...

The two main items are rain is expected north of I-80 this
afternoon and there is a Frost Advisory for north-central IL
tonight.

A cold front is crossing over the forecast area and will be
southeast of the forecast area later this morning. Winds will
become northwest and may briefly gust to 20 to 25 MPH immediately
behind the front.

Mid-level frontogenesis combined with an upper level vorticity
streamer will lead to a band of rain this afternoon. I have
medium- high confidence that the majority of rain will fall north
of a Mendota to Midway Airport line. Areas south of I-80 will be
dry today.

Thick cloud cover, rain, and cooler air behind the front will
lead to high temps in the mid 50s to around 60 north of a Mendota
to Midway Airport line. Areas south of I-80 will warm into the 60s
with a few locations maybe reaching 70.

High pressure builds in behind the departing wave leading to a
relatively clear night. Temperatures should cool quickly tonight
away from the lake, and areas of frost are possible across north-
central IL where lows will dip into the 30s. A Frost Advisory is
in effect from late tonight through early Saturday morning.
Elsewhere lows will vary from the upper 30s to around 50 near the
lake.

JEE

&&

.LONG TERM...
327 AM CDT

Saturday through Thursday...

Overview...80s return next week, and there will be a few chances for
rain and storms.

High pressure shifts east allowing winds to become southwest. Highs
will only be in the upper 50s to mid 60s. Showers are possible ahead
of a surging warm front Saturday night. I think the showers will be
late enough that thunder is unlikely. Lows will be warmer and in the
upper 40s to low 50s.

Guidance suggests that the front will stall over the forecast area
Sunday forcing more showers Sunday afternoon and evening. Guidance
differs on where the showers will form, but I think the majority of
rain will once again fall north of I-80. Some models suggest I-88
may even be the cut off for rain. Temperatures will also vary widely
across the front with mid 60s north of the front and around 80 to
the south.

Highs will be in the mid 70s to mid 80s Monday as we will be in the
warm sector of the system. There is a chance of showers and storms
mainly Monday night.  Above average temperatures continue through
mid week until a cold front brings more rain to the region
Thursday.

JEE

&&

.AVIATION...
For the 06Z TAFs...

No significant changes to previous AFD thinking or rain timing
from the 00z set.

The main concern is with a period of rain on Friday that could
knock down ceilings and visibility into the MVFR range. Conditions
will be quiet and VFR through the night with CIGs gradually
lowering. A cold front will move across the terminals during the
morning, shifting winds to northwest. It's possible that there
could be a period of near due north winds during the mid to late
morning, with speeds around 10 kt if it occurs. Have added a
TEMPO for the increased wind speeds and a noted brief shift to
more NNW. Gusts should generally hold under 20 kt. After the
northwest to possibly north winds behind the cold front, direction
will settle into a 290-310 direction along with gusts in the
15-20 kt range, possibly a bit higher at GYY. Speeds will then
quickly diminish with sunset as high pressure overspreads the
region.

Regarding rain trends on Friday, the forcing mechanism for the
precipitation suggests that the area of rain will be rather narrow
with likely a sharp cut-off on the southern end of it. It's
possible this cut-off could be right over the Chicago area. Timing
still appears fairly on track, with a late AM start time
at RFD and early PM for Chicago area airports. Have highest
confidence in prevailing -RA at RFD, and likely at ORD/DPA/MDW.
Have left a VCSH mention at GYY. CIGs outside of rain should be
VFR and possibly even during it, so confidence is only medium in
MVFR CIGs where prevailing or TEMPO -RA is mentioned. VSBY may
drop to MVFR at times during the rain. Conditions will quickly
improve once the precip ends.

KMD/Castro

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Frost Advisory...ILZ003-ILZ004-ILZ005-ILZ008-ILZ010-ILZ011-
     ILZ012...3 AM Saturday to 8 AM Saturday.

IN...None.
LM...None.
&&

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