Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 270013

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
713 PM CDT Tue Oct 26 2021

713 PM CDT Tue Oct 26

No changes planned to the Frost Advisory at this time. Some
consideration was given to expand the advisory west into Iroquois
County, but opted to hold off. A pocket of higher surface
dewpoints into the mid 40s from lake-modified air was analyzed
over the southern CWA from roughly around the Kankakee River
Valley into Iroquois and Ford counties. This area of higher
moisture will become rather shallow through the night as drier air
advects into the area on gradually veering flow. However, the
combination of the higher dew points with recent rain and clear
skies in the vicinity of a the surface ridge supports some
development of patchy shallow fog overnight, primarily in river
valleys or fog-prone sheltered areas. Because of this, frost
potential appears lower in Iroquois County than areas to the south
and east. Will maintain just a mention or patchy frost in the



240 PM CDT

Through Wednesday night...

We issued a frost advisory for our northwestern IN counties for
tonight into early Wednesday morning.

A surface ridge of high pressure is overhead, and is resulting in
much lighter winds than what we experienced yesterday. While
sunshine has been abundant this afternoon across the Rockford
area, the same has not been the case farther east into the Chicago
metro area. Instead, a deck of Strato Cu shifting off of Lake
Michigan has resulted in more cloud cover across much of eastern
IL and northwestern IN. Temperatures this afternoon will top out
in the mid 50s.

Our surface ridge axis will shift eastward over Lake Michigan and
northern IN tonight. This will generally result in light winds
along with a period of clearing skies tonight, especially over
eastern IL and into northwestern IN tonight. Frost development
appears most probable over IN, where the combination of reduced
cloud cover and light winds should allow temperatures to drop off
into the low to mid 30s overnight. We have thus issued a frost
advisory for this area for tonight. Farther west into IL,
overnight low temperatures look to be a bit warmer due to some
increasing higher level cloudiness, so the overall threat of
widespread frost is lower across much of northern IL tonight.

Another dry day is expected on Wednesday, with high temperatures
perhaps another degree or two warm than today`s. We should see
more higher level cloudiness around during the day in advance of
our next storm system over the Plains. It appears that any rain
from this approaching system will hold off across most of the area
until the daytime on Thursday.




Thursday through Tuesday...

251 PM...Main forecast concern remains rainfall amounts Thursday
through Friday.

Overall, no significant changes to the forecast. The next storm
system will track from the central Plains, across the Ohio Valley
and to the mid Atlantic region. Rain is expected to spread across
the local area during the daytime on Thursday with some timing
differences still remaining. Appears the best time window for rain
will be Thursday night into Friday morning, when some lake
enhancement is possible, which shows up in the model qpf amounts.
The highest totals, which approach an inch, are centered over the
Chicago metro area. Blended qpf amounts have trended up over the
metro area, and are generally in the half inch to three quarter
inch range. This amount of rainfall may lead to minor rises on
area rivers, but it will depend on how fast it falls which
currently appears to be more of a moderate rainfall. Showers look
to continue a bit longer, through Friday afternoon, then finally
tapering off from northwest to southeast across the local area
Friday evening.

The gradient will also tighten late Thursday night into Friday and
gusts into the 25-30 mph range remain possible. The stronger north
winds will also lead to high waves on Lake Michigan, but still
will be lower than the event this past Monday.

The weekend continues to look dry with a weak cold front moving
across the area Sunday afternoon or evening. The models have
struggled with the evolution of a system early next week along
this frontal boundary but both the GFS and ECMWF now develop
precipitation along the front Monday night into Tuesday. Though
they differ on the amount of colder air that advects into the
region on the north side of this precipitation. Blended pops are
rather low, in the slight chance range and that is probably fine
for now but if these trends continue, higher pops will be needed
for this time period. In addition, if colder air does indeed
arrive as the precip is ongoing, there may be at least a rain/snow
mix early Tuesday morning. Confidence is very low from this
distance, however and for now have continued with just rain
mention. cms


For the 00Z TAFs...

Aviation Forecast Concerns:

* East-southeast winds through the period.

* Low-probability for MVFR ceiling development Wednesday evening,
  with greater potential beyond 06Z end of current ORD/MDW

Early evening surface analysis depicts weak north-south oriented
high pressure ridge extending from eastern WI into the IL/IN state
line region. This ridge will continue to drift east tonight,
allowing surface winds to become light southeast. Skies have
generally cleared across the area, though lake-induced stratocu
continues south of Lake Michigan from KGYY eastward. This will
dissipate as winds shift southeast, though model forecast
soundings do indicate the potential for some patchy (FEW) 2000-3000
foot clouds off the lake into far northeast IL overnight.

With the surface ridge off to the east on Wednesday, and surface
low pressure drifting east across the central/southern Plains,
southeast winds will increase to around 10 kts within a gradually
tightening pressure gradient. KRFD, farthest to the west may see
some gusts into the mid-teens of so. Otherwise, VFR high clouds
will develop/thicken overhead. Persistent southeast winds turn a
bit more east-southeast Wednesday night, with guidance trends
indicating an eventual increase in low level cloud cover
especially beyond 06Z. HRRR/RAP are a little faster in indicating
MVFR ceilings during Wednesday evening for KORD/KMDW, with
probability increasing just beyond the end of the current 30 hour



IN...Frost Advisory...INZ001-INZ002-INZ010-INZ011-INZ019...midnight
     Wednesday to 9 AM Wednesday.

LM...Small Craft Advisory...LMZ743-LMZ744-LMZ745 until 9 PM Tuesday.


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