Downers Grove, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

132
FXUS63 KLOT 180841
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
241 AM CST Sat Jan 18 2025

.KEY MESSAGES...

- Dangerously cold wind chills (frostbite potential in ~30
  minutes) expected early Sunday across interior northern
  Illinois, and probable Sunday night - early Monday, and again
  Monday night - early Tuesday area-wide.

- Northeast Porter County may be clipped by light lake effect
  snow accumulations and associated travel impacts today through
  Sunday night, with the lake effect then shifting east for the
  rest of the cold snap. Flurries are possible areawide Mon PM.

&&

.DISCUSSION...
Issued at 241 AM CST Sat Jan 18 2025

Through Sunday:

Analyzing surface observations, winds to the west have already
switched to the northwest behind a cold front moving through the
forecast area. Wind gusts behind the front are strong between
30 to even 35 mph. Gusts will diminish slightly in the
afternoon, but remain above 20 mph. Weather conditions are
mostly dry behind the front. With temperatures dropping quickly,
it is not impossible that even the thinnest of cloud layers
could develop some minor, non-accumulating flurries today.
However, looking upstream into Minnesota on satellite imagery
and seeing less cloud cover lowered confidence for a formal
mention in the forecast at this time, but could potentially be
re-added tactically.

Despite drier conditions behind the front, there are still
returns on radar out ahead of it. Most of the surface obs that
are reporting rain, if not drizzle, are over in northern
Indiana. However, there is a chance for rain/drizzle lingering
in northwest Indiana through daybreak. As temperatures drop,
there is the chance for some snow to mix in. But considering
that its still 37/38 in Jasper and Porter Counties at the time
this discussion was posted, confidence is lowering and
accumulations should be fairly small, if at all.

The northwesterly winds behind the front will remain through
the day. With decent fetch, a lake surface to 850 mb temperature
difference greater than 12 degrees and lake induced instability
over 100 J/kg, there is the potential for lake effect snow in
northwestern Indiana, though primarily Porter County. It is
possible PoPs will need to be boosted depending on where exactly
the lake effect band sets up.

Cold, arctic air will continue to filter in through Saturday
and into Sunday morning. Even 24 hours away, guidance still has
a wide spread for what temperatures will bottom out at on
Sunday morning (some staying well above zero and others
dropping below zero). And there is still uncertainty regarding
cloud cover and how this will impact ambient air temperatures.
Nevertheless, strong northwesterly winds and cold temperatures
are expected to create wind chill temperatures down to -10 in
Benton County Indiana to -20 (if not colder) in Winnebago
county. We have issued a Cold Weather Advisory for late tonight
through Sunday morning for Winnebago, Ogle, Boone, McHenry, De
Kalb, and Kane counties to cover this initial period of wind
chill values down to around -20. The cold will expand and
intensify Sunday night through Tuesday night. For more on that
and additional headline decisions, see the discussion below.

DK

Sunday Night through Friday:

The dump of arctic air will be in full swing Sunday night with a
lobe of roughly -27 to -30 C 850 mb air waiting in the wings just
to our north and west, and this will traverse northern Illinois
fully on Monday night. From a climatology perspective, this is
just about as cold as we`ve seen (at 850 mb) in the nearby upper
air database (Quad Cities and Lincoln, IL). Out of curiosity, went
back and took a look at the arctic outbreak from this time last
year (January 14 - 17), which featured low temperatures in the -5
to -15 degree range and wind chills solidly down towards -30 F
(and even lower than that on short time periods). 850 mb
temperatures in this case were actually notably "warmer",
generally around -20 C. They key difference was a widespread
and dense snowpack which we obviously don`t have this time
around, which just goes to show the power of snowpacks in
altering these arctic airmasses. In this case, little/snow in
place, even amidst near-record cold just off the deck, the peak
of this arctic episode looks to wind up a bit under where we
found ourselves one year ago.

With this in mind, there weren`t any significant changes to this
forecast iteration. With air temperatures already at a low
starting point Sunday afternoon, persistent cold advection will
help send these down into the negative single digits into Monday
morning. On Monday morning and afternoon, CAA is set to relax a
bit, and this will likely allow air temperatures to wander their
way back into the +5 to +12 F range, although winds will likely
tick upwards a bit through the day with increasing mixing. As the
core of the arctic airmass drifts overhead Monday night, temps
will fall to their lowest values of this episode, with some
negative teens not out of the question across interior northern
Illinois.

It`ll remain blustery and very cold Tuesday and Tuesday night,
but with the sprawling arctic high cresting to our south, winds
will flip out of the south on Wednesday yielding a notable
moderation in the temperature department--hastened by the lack of
snow cover as well. By Thursday and Friday, highs will be flirting
with the freezing mark.

Regarding any cold weather headlines:

We are very likely headed towards an expansion and extension
(in time) of the current Cold Weather Advisory to cover the
Sunday evening through Tuesday night time period, likely for our
entire forecast area. One lingering question though revolves
around temperature and wind chill trends on Monday (coming out
of the first really cold night area-wide), with indications that
wind chills could somewhat meaningfully rise back into the -10
to 0 degree range during the afternoon which could warrant
splitting Cold Weather Advisories up, as opposed to running a
single longer- duration advisory to cover the entire period. If
this moderation on Monday ends up being shorter-lived or less
acute than currently indicated, that might suggest handling with
a single headline. Furthermore, with the most significant push
of cold air not arriving until later Sunday/Sunday evening
(third into fourth period and beyond), agreed in coordination
with surrounding offices to allow for an additional forecast
cycle before hoisting additional headlines. At this time, while
some -30 F degree wind chills are in play, it seems like these
would be on a localized and temporally-limited enough basis to
preclude the need for Extreme Cold Watches/Warnings.

Snow Chances During the Cold Snap:

While generally precipitation-free conditions are expected, the
back edge of lake effect snow showers may intermittently build
across parts of Porter County throughout this arctic intrusion.
This remains particularly true Sunday night into Monday morning
before surface winds back more westerly and southwesterly, and we
continue to paint some low-end PoPs across Porter Co. through this
period as a result. Any persistent, light snowfall with the cold
temps will make road treatments less effective, leading to a
potential for slick travel.

In addition, as we`ve mentioned in the previous few discussions,
a vort max embedded within the fast cyclonic flow is forecast to
slice through the region Monday afternoon and evening. While
forecast soundings don`t depict particularly thick cloud layers,
just a bit of ascent through a near-saturated -30 C layer may be
enough to squeeze out some very fine snow. There`s even a signal
on Tuesday in the GFS to suggest some flurry activity continue
then as well.

From Wednesday through the end of the week, extended guidance
suggests, to varying degrees, a potential for some additional
precipitation chances as a broad trough swings across the central
CONUS. Significant spread in the guidance at this point precludes
making any changes to the dry NBM-delivered forecast, however.

Carlaw

&&

.AVIATION /06Z TAFS THROUGH 12Z SUNDAY/...
Issued at 1059 PM CST Fri Jan 17 2025

The main aviation weather concerns for the 06Z TAF period
include:

* At GYY, light rain possible through around 08Z, then a period
  of MVFR cigs Saturday afternoon

Gusts have waned late this evening, but southwesterly winds
remain near or just above 10 kt for the time being. After
veering to NW, gusts to 25 to near 30 kt will return not long
after 06Z and remain through Saturday morning. Gusts will ease
to closer to 20 kt for the rest of the day and through Saturday
night.

Meanwhile, a system of light showers is moving into southern and
southeastern portions of the Chicago metro this evening. GYY may
see a period of non-impactful rain through around 08Z. We look
to spend much of tonight and Saturday beneath low VFR stratus.
While VFR is certainly favored throughout, it`s possible that
cigs could drop to MVFR for a time over the Chicagoland sites,
especially during the morning hours. GYY has the best shot at
seeing a period of MVFR. Cigs should scatter out for the
afternoon.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...Cold Weather Advisory from 3 AM to noon CST Sunday for ILZ003-
     ILZ004-ILZ005-ILZ008-ILZ011-ILZ012.

IN...None.
LM...Small Craft Advisory until 4 AM CST Monday for Winthrop Harbor
     IL to Gary IN.

     Small Craft Advisory until 10 PM CST Monday for Gary to Burns
     Harbor IN-Burns Harbor to Michigan City IN.

&&

$$

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