Arlington Heights, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

486
FXUS63 KLOT 180459
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1059 PM CST Fri Jan 17 2025

.KEY MESSAGES...

- Rain develops mid to late this evening near and east-southeast
  of I-55. The rain may change over to a short period of wet
  snow overnight before ending. Any slushy accums are expected
  on cold surfaces and mainly south and east of I-55.

- Coldest airmass of the season arrives Saturday night and
  lingers through Tuesday night, with an extended duration of
  sub-zero wind chills (Sat night through Wed AM for much of if
  not the entire area).

- Dangerously cold wind chills (frostbite potential in ~30
  minutes) possible early Sunday, probable Sunday night-early
  Monday, and again probable Monday night-early Tuesday.

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

&&

.DISCUSSION...
Issued at 242 PM CST Fri Jan 17 2025

Through Saturday:

A potent and fast moving surface low (~985 mb) across southern
southern Ontario is inducing gusty and mild southerly winds in
advance of an approaching cold front shifting across the Upper
Midwest this afternoon. Temperatures have over performed,
especially in the snow free areas of northern IL. In this area
readings have climbed into the upper 40s to lower 50s. These
mild and breezy conditions will continue through the remainder
of the afternoon.

The approaching surface cold will begin to sweep eastward into
northern IL this evening, then across my northwestern IN counties
overnight. Increasing low-level moisture in advance of the front
is expected to foster the development of a west-southwest to east-
northeast oriented band of rain later this evening (likely after
8 PM). It looks to initially develop somewhere near, or just east-
southeast of the I-55 corridor, before gradually settling east-
southeastward with the front overnight. The Chicago metro area
looks to reside right along the northwestward periphery of this
developing area of precipitation. For this reason, northern and
northwestern sections of the metro may remain dry. Finally, as
colder air begins to move into the area overnight, the
precipitation may briefly mix with a period of wet snow before
ending from northwest to southeast, but little to no accumulation
is anticipated.

Blustery northwest winds will setup across the area in the wake of
this cold front, and persist during the day Saturday. Strong cold
air advection on these winds will result in early morning high
temperatures in the 20s, with nearly steady or slowly falling
temperatures through the afternoon. Wind chills will be turning
colder, with readings likely falling below zero into Saturday
evening (additional details below).

KJB


Saturday Night through Friday:

Temperatures and Wind Chills Sat Night-Tue Night:

An impressively long duration of sub-zero wind chills is in store
(likely areawide) tomorrow night through midday Wednesday.

The upstream arctic airmass will make its initial concerted push
towards the region Saturday night into Sunday morning, with 850 mb
temps plunging to about -20C by early Sunday. While the core of
coldest air will remain to our northwest through this period, a
stout pressure gradient will be in place as an impressive mid
1040 mb high begins to build into the western Dakotas. Given the
strong cold air advection and tight pressure gradient, modeled
wind speeds came up into the 10-20 mph range, with gusts to 25-30
mph. When combined with air temperatures meandering down through
the single digits, this will likely yield wind chill values in
the -15 to -25 degree range by daybreak Sunday, coldest across
interior northern Illinois north of I-80. We`ll likely need to
consider a Cold Weather Advisory for at least portions of the
area. The primary points of uncertainty are exactly how low air
temps get Sunday morning, which will determine the lowest wind
chill magnitudes. Confidence is medium-high in the wind forecast
portion of the wind chill equation.

Steep low-level lapse rates on Sunday along with the continued
tight surface pressure gradient looks to maintain gusts up to 30
mph. With air temperatures moving little through the day, wind
chill values will not "warm" meaningfully even in the afternoon.
In addition, as the reservoir of cold air spills into the region,
a fairly robust shortwave will push overhead into Sunday morning.
An attendant brief f-gen circulation will develop, driving an
increase in mid and upper level saturation, mainly above 750 mb.
Guidance is quiet on the QPF front, and with forecast soundings
revealing fairly dry conditions below 800 mb, have left things dry
in the gridded forecasts, although will continue to keep an eye
on this time period.

The core of the arctic airmass will build into the region
during the Sunday night through Monday night time frame in two
waves. Sub-zero air temperatures are forecast across the entire
forecast area both Sunday and Monday nights, with Monday night
looking like the coldest of the stretch as an impressive core of
approximately -30 C air at 850 mb arrives across northern
Illinois. Minimum wind chills during this period look to fall
solidly into the -15 to -25 F range, with some localized
occurrences of -30 F values certainly in play depending on wind
trends. Given current model output and ongoing trends (or lack
thereof), we`ll in all likelihood be headed for additional Cold
Weather Advisories for the Sunday evening through Tuesday morning
time frame (should an advisory be issued for Sunday morning). If
there is a decided trend toward -30F or colder wind chills in any
parts of the area during this stretch (20-40% chance early
Tuesday for interior far northern Illinois), then an Extreme Cold
Watch may be needed.

Tuesday night will also be a cold one with the center of the
1040+ arctic high sliding just to our south, however winds may
end up a smidge lighter than previous nights. Radiational cooling
also won`t be as effective given overall lack of snow cover.

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 is particularly true Saturday night into Sunday nights before
surface winds back more westerly and southwesterly. Have held
onto some low-end PoPs into Tuesday across Porter Co. as a result.
Concern is there for relatively persistent but primarily light
snow showers focused into the northeastern portion of the county.
The cold temps will make road treatments less effective, so even
only an inch or two of snow (in northeast Porter Co) could make
travel hazardous.

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.

Wednesday-Friday:

Temperatures will moderate back into the 20s on Wednesday and
possibly near or above 30F Thursday-Friday. The primarily dry
pattern will continue, though can`t rule out a few low amplitude
fast- moving disturbances in the Wednesday night-Friday morning
timeframe capable of producing narrow swaths of light snow
accumulations, and flurries elsewhere. Some guidance has another
modest reinforcing surge of CAA into Friday, though if this
doesn`t come to fruition, chances of a stretch of temps at or
above freezing will increase considering the lack of snow cover.

Castro/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...None.
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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