Area Forecast Discussion
Issued by NWS Chicago, IL

Home | Current Version | Previous Version | Text Only | Print | Product List | Glossary Off
Versions: 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50

FXUS63 KLOT 262331

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
631 PM CDT Mon Oct 26 2020

350 PM CDT

Through Tuesday night...

This morning`s precipitation has cleared the area, and while a few
minor blips of light snow and/or drizzle are tracking eastward along
the I-80 corridor, dry conditions will prevail into the overnight
hours across most of the area with the exception of maybe our far
southern counties. While the frontogenetic axis is now positioned
southeast of the CWA, the northern fringes of the f-gen precip
shield currently overspreading central Missouri and western and
central Illinois could creep into the far southern portion of the
CWA later tonight. If it does, there could be some drizzle or a
light rain/snow mix in locales south of the Kankakee River Valley,
but the bulk of the precipitation should remain south of the CWA.
Otherwise, another cool night is expected with lows in the low-
mid 30s in the Chicago metro and south of I-80 and mid-upper 20s
elsewhere, although low 20s are not entirely out of the question
outside of the Rockford area if there is sufficient clearing.

While the f-gen axis will continue to drift off to the east, the
upper jet looks to have its strength replenished over the Great
Lakes tomorrow morning. The strengthening of the jet will place us
underneath the right entrance region of a jet streak that will
provide relatively weak, but seemingly sufficient forcing for some
light precipitation to develop in the area around dawn. Questions
remain over whether there will be enough low-level moisture to
support precipitation reaching the ground, but model trends have
suggested an increasing likelihood of at least some spotty precip
coverage southeast of I-55. The cool morning temperatures will
likely result in most of this precipitation being light snow, but
a light rain/snow mix and/or drizzle remains possible, especially
as you go farther southeast and later into the morning. Some lake
enhanced precipitation will also be possible over far northern
Porter County in Indiana owing to the cold air mass creating a
16-17 degree Celsius surface-850 mb difference over southern
Lake Michigan, but west to northwesterly flow will favor the east
side of Lake Michigan seeing the majority of any lake effect
precipitation that develops. As a result of all this, have nudged
PoPs up a bit for tomorrow morning across the southern half of the
CWA. However, no accumulating snow is currently anticipated as
any precip that does end up falling should be lighter and over a
shorter duration than the precip we saw today.

Aside from the morning light precip chances across our southeast,
the rest of the day should be dry, but seasonably cool once again
with highs only in the low-mid 40s. Any residual cloud cover will
likely clear by the overnight hours and allow for another night with
temperatures around or below freezing.



350 PM CDT

Wednesday through Monday...

We should not need to mention the snow word too much in the
forecast for the extended, including as we transition into

The upper level pattern will feature an upper low near the Texas
panhandle that will initially cut off from the more progressive
northwest flow over the upper Great Lakes. Meanwhile, some of the
tropical moisture from Hurricane Zeta will get fed into this upper
low. The midlatitude system will eventually take over the tropical
system as they merge, with a corresponding surface low, across the
Ohio valley and points eastward. As the trough axis shifts over the
lower Great Lakes late Wednesday into Thursday, along with a coupled
upper level jet stream, some rain will spread into at least portions
of the area. Areas along and south/east of I-80 are favored at this

Low level flow will turn northerly Wednesday night into Thursday in
response to the deepening low and also with some influence from a
cold front that will graze the lower Great Lakes. Highs Thursday top
out in the lower 50s with Friday featuring only 40s for highs. There
could some lake effect showers, or more likely sprinkles or just
clouds behind the departing low.

High pressure will be overhead Friday, and we will have some warming
and plenty of sunshine. The high will shift east Saturday, and this
will open the door to a weak cold front Saturday evening. The day
will be seasonally mild, readings in the mid 50s.

The colder air will only linger into Sunday, as the upper level jet
will head north into Canada or at least the upper Great Lakes. As
high pressure and milder southwest flow result for at least the
start of next week, and if latest trends pan out -- for several



For the 00Z TAFs...

The primary forecast element of interest in the TAFs is the
potential return of MVFR clouds near daybreak along with a
possible brief period of light rain/snow, mainly during 11Z
through 15Z.

The earlier MVFR clouds over much of the Chicago area have been
shoved south by northerly winds early this evening, though may be
off and on at MDW for a bit early this evening. As the jet stream
increases over the area late tonight, there will be support for
lift and for some of this low-level moisture to expand back
northeast. This will at the least encroach on the Chicago area and
is most likely to reach GYY with returning MVFR clouds. Light
precipitation should develop further north than where it is early
this evening in central Illinois, and some of this may reach the
Chicago area airports. This does not look strongly forced, so
precipitation should be light, however some of this could be a
light snow if saturation depth is deep enough. Surface
temperatures look to be 33-35 degrees during this event.
Confidence in precipitation occurring is low-medium at ORD and MDW
right now, and overall impacts look limited if it were to occur.

Any activity should scoot eastward by later morning with westerly
winds and VFR conditions taking hold.



LM...Small Craft Advisory...IN nearshore waters until 1 AM Tuesday.



Visit us at

Follow us on Facebook, Twitter, and YouTube at: is the U.S. government's official web portal to all federal, state and local government web resources and services.