Evanston, Illinois 7 Day Weather Forecast

Wx Forecast - Wx Discussion - Wx Aviation

229
FXUS63 KLOT 220500
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1100 PM CST Sat Dec 21 2024

.KEY MESSAGES...

- 30% chance of light freezing drizzle/drizzle late Sunday
  night through early Monday morning.

- Turning mild with on-and-off chances for rain during the
  Holiday week.

&&

.UPDATE...
Issued at 709 PM CST Sat Dec 21 2024

Have made some adjustments to the forecast this evening,
primarily to lower overnight (evening) min temperatures and
tweak sky cover per satellite trends. In short, expect single
digit mins this evening across parts of northern IL west/north
of Chicago urban heat island before mid-level cloud cover
increases later this evening and overnight.

Early evening surface analysis places 1034 mb high pressure
centered over east-central WI, with a ridge axis extending
southward into northern IL. A combination of calm or
light/variable winds within the ridge axis, our recent snow
cover and mostly clear to partly cloudy skies across northern IL
early this evening, has already allowed temps to dip into the
single digits in parts of the Fox River Valley west/northwest of
Chicago. Mid-level warm air advection was already developing
west of the ridge across the upper Midwest however, producing
fairly extensive mid-level cloud cover just to our west and
northwest which is expected to develop southeastward across the
forecast area later this evening and overnight. This will
likely cause temps to rise a few degrees from evening lows,
especially for areas west/northwest of Chicago.

Have therefore adjusted forecast to include single digit lows
near and west of the Fox Valley, with temps then rising slightly
late. Also adjusted sky cover down across these areas through
mid-evening based on satellite trends, then increased coverage
late evening and after midnight across the area as the expansive
mid-deck spreads in aloft.

Forecast otherwise looks to be in good shape into Sunday, so no
other significant changes were made at this time. Updated
digital and text forecast products available.

Ratzer

&&

.DISCUSSION...
Issued at 257 PM CST Sat Dec 21 2024

Through Monday Night:

LES continues to struggle across northwest Indiana due to a
combination of lowering inversion heights and continued dry air
advection. Additionally, augmentation of the background flow by
a land breeze early this morning has kept the main convergent
axis north and east of the forecast area. As a surface ridge
shifts eastward across the area tonight, remaining LES over
southern Lake Michigan will quickly shift farther north and east
of the forecast area. With the current location of any LES
removed to the northeast and the expected development of another
land breeze tonight, any remaining LES should be limited to
just some flurries near the Indiana shore through this evening.

Though winds will be primarily calm over snow cover for much of
the area tonight, temps should only slowly fall through the
night as a disjointed pocket of mid to upper-level moisture
within stronger NW flow aloft likely maintains a decent amount
of cirrus and/or altostratus over the area. If clouds do clear
more than expected, min temps may plummet well into the single
digits above zero across interior northern Illinois.

Some semblance of mid to upper-level cloud cover should persist
through the day on Sunday, leading to filtered sunshine on
increasing southerly winds. Afternoon temps will near the
freezing mark while winds gust up to 25 mph. Precip with a small
mid-level wave shifting ESE across southern Wisconsin during
the afternoon should be limited to virga given dry air below
10kft cloud bases.

A strong Pacific-based trough crossing the western Great Lakes
region on Monday will bring an associated cold front across the
forecast area Monday afternoon. Broad low-level isentropic
ascent within a shallow but moistening pre-frontal airmass
should spread light precip across at least northern portions of
the forecast area late Sunday night into Monday morning. A 1-2C
warm nose advecting into the area on 850-900hPa 50 knot flow
will yield liquid precip, though it is becoming more likely that
temps will not rise above freezing before the precip begins.
Have continued to increase the mention of freezing drizzle
across much of the area through at least mid-morning, with some
impacts from slippery surfaces possible for the morning commute.
Temps will rise above freezing by the afternoon with residual
post-frontal stratus and patchy drizzle into the early evening.

Kluber


Tuesday through Saturday:

There remains a strong signal for well above normal
temperatures next week (typically low-mid 30s) as the northern
stream upper jet lifts into Canada, particularly during the
latter half of the Holiday week when highs in the mid 40s to
lower 50s are forecast.

The upper pattern also looks rather messy overall as a series
of upper level waves and associated disturbances eject off the
Rockies beneath the aforementioned northern stream jet/longwave
ridge. This will bring several chances for rain to the region
Tuesday night (Christmas Eve) through the end of the week with
the warmer temperatures limiting the potential for any wintry
precipitation. While the official forecast has rain chances
nearly every period beginning Christmas Eve night through the
end of the week, there will be dry periods in between systems.
Pinpointing when these occur remains lower confidence at this
range given continued model differences with the timing and
track of each system.

Looking beyond into early January, long-range ensemble guidance
is pointing toward a pattern shift occurring toward the New
Year with upper troughing setting up across the eastern half of
the CONUS. This matches up well with the latest CPC guidance
which already has a 55-60% chance of below normal temperatures
occurring during the first half of January across northeast
Illinois and northwest Indiana.

Petr

&&

.AVIATION /06Z TAFS THROUGH 12Z MONDAY/...
Issued at 1100 PM CST Sat Dec 21 2024

Aviation Forecast Concerns for the 06Z TAFs:

- MVFR ceilings possible briefly at GYY around issuance time,
  as lake-effect clouds drift into the vicinity.

- Breezy SSE winds with gusts 20-25 kts develop midday Sunday
  and persist through the period. Strongest winds may occur
  Sunday night, with gusts above 25 kt possible.

- RFD/DPA/GYY may lose gusts with sunset, though would expect
  them to redevelop during the evening. If not, LLWS conditions
  likely as south winds ~45 kts develop above 1500 ft AGL.

- Low probability for -FZDZ to develop toward 12Z Monday. Prob
  too low for TAFs at this time, but may need a mention into
  Monday morning with later forecasts.

Scattered MVFR (2000-2500 ft) deck may briefly affect KGYY
around 06Z this morning, as lake-effect clouds drift west across
the airport. Winds above the surface will be shifting to the
south however, likely pushing these clouds back out over the
lake within a couple of hours.

Surface high pressure ridge over the area at issuance will
maintain calm or light/variable winds into early Sunday, before
winds eventually settle south-southeast and begin to increase
mid-morning. By midday, breezy conditions are expected with
gusts in the 20-25 kt range, as the gradient tightens ahead of
an approaching area of low pressure across the central/northern
Plains. Sites away from the heart of the metro (RFD/DPA/GYY) may
lose gusts with sunset Sunday evening, though at least some
sporadic gusts would likely redevelop during the evening as a
45-50 kt low level jet spreads overhead. If gusts do not
rematerialize, LLWS conditions would likely result given the
strength of the low level jet. (RFD may see a brief period of
LLWS just prior to surface gusts redeveloping). ORD and MDW
typically prone to maintaining gusts in these situations.
Strongest winds, with gusts occasionally above 25 kts, are
likely Sunday night beneath the low level jet.

VFR conditions are expected through the duration of the
forecast, with periods of mid-level ceilings. The only real
concern is for the currently low-probability potential for the
development of light freezing rain/drizzle toward Monday
morning. While this can`t be completely ruled out as early as
10-12Z, most model guidance currently favors this to develop
just beyond the current 12Z end time of the ORD/MDW 30-hour
forecasts. A freezing rain/drizzle may therefore need to be
introduced in later forecasts into Monday morning.

Ratzer

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...None.

&&

$$

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