Area Forecast Discussion
Issued by NWS Chicago, IL

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FXUS63 KLOT 032024 CCA

Area Forecast Discussion...CORRECTED
National Weather Service Chicago/Romeoville, IL
224 PM CST Sun Dec 3 2023

Issued at 223 PM CST Sun Dec 3 2023

Through Tuesday...

Key message...

* The next winter system arrives Monday night into Tuesday morning.
  Increasing chances for snow north of I-80, with slushy
  accumulations less than an inch likely (70%). A rain/snow mix for
  the south and eastern portions of the forecast area.

Only meaningful change to the forecast tonight into Monday was to
remove the slight chance pops for tonight across southern CWA.
Guidance has generally trended a bit farther south with tonight`s
compact shortwave. Otherwise, prospects for meaningful clearing
appear pretty low through Monday. A pocket of clearing opened up
across Iowa this afternoon, but as system passes to our south
tonight, our low level westerly flow will turn more northerly
advecting clouds south from Wisconsin. Kept skies mostly cloudy
tomorrow, but confidence in that is only about 60%, so there is a
appreciable chance that could be clearing. If skies do clear out,
then high temps would likely be a couple/few degrees warmer than

On Monday night, the next wave in the form of an Alberta Clipper
will move southeastward across the region into Tuesday morning. With
models trending temperatures for areas north of I-80 below freezing,
there is increasing confidence that precipitation will be all snow.
Snow ratios are expected to be on the wetter side (less than 10:1),
so accumulations are expected to be slushy, generally less than an
inch. With warmer temperatures south of I-80, a rain/snow mix is
expected overnight. Since models have trended slightly warmer to the
south, accumulations were pared back to just north of I-80. As the
sun rises Tuesday morning and temperatures increase, despite the
widespread cloud cover, any snowfall should transition to
rain/drizzle before the system slowly exits to the east.



Issued at 223 PM CST Sun Dec 3 2023

Tuesday night through Sunday...

Key messages...

* Unseasonably warm temperatures likely Thursday and Friday
* Somewhat cooler temperatures along with chances (30-60%) for
  rain over the weekend

In the wake of the Monday night/Tuesday system, the parent long
wave trough will begin to quickly shift east of the region with
100-150m 500mb height rises in its wake. This will herald the
pattern change as we transition to a more zonal flow with some
ridging over the central and eastern U.S. late this week.

Strongest warm air advection Wednesday will take place to our west
across the mid section of the nation. By Wednesday night and
Thursday, southerly flow will increase across our area
accelerating the warm air advection. Confidence is growing that
On Monday night, the next wave in the form of an Alberta Clipper
will move southeastward across the region into Tuesday morning.
Thursday will be unseasonably warm and have adjusted high
temperatures up about 5F from the NBM. Medium range models depict
the axis of higher low level moisture (and likely stratus) will
remain well west of our area Thursday, meaning a good deal of
sunshine is likely. The sunshine, moderately gusty southerly
winds, lack of snow cover all favor temps ending up on the higher
end of the 925mb high temperature spectrum, which would has highs
in the upper 50s to around 60. Didn`t go quite that warm yet, but
if guidance remains consistent, highs could end up even warmer
than today`s forecast of mid-upper 50s.

It does appear as though stratus could spill east into the area
Thursday night into Friday as the low level moist begins to spread
east into the area. Stratus and gusty southerly winds should keep
temps unseasonably mild Thursday night with lows in the 40s
(warmer than avg highs). The very mild start to the day Friday
should set the stage for another unseasonably mild day. If there
are breaks in the cirrus, temps could be even warmer on Friday
with even temps reaching 60 not out of the question. Given the
better chance of stratus on Friday, opted to not bump up temps
over NBM yet.

Upper trough is progged to track north of the region next weekend,
which should beat down the upper ridge and allow cold front to sag
southward across the area bringing somewhat cooler temps to the
area. Chances of rain should ramp up some as well, but still lots
of spread in the guidance regarding the details (track, timing,
etc) of individual shortwaves.

- Izzi


Issued at 1131 AM CST Sun Dec 3 2023

For the 18Z TAFs...

Main aviation concerns...

* IFR cigs lingering through Sunday afternoon. Improvement to MVFR
  after 00Z, with lower confidence in exact timing at KRFD

* West winds through the afternoon with occasional gusts up to 20

* There is a low chance (10%) in brief periods of NNE for
  terminals closer to Lake Michigan Monday morning.

The rain system is slowly moving east. Terminals in or closer to
Indiana may experience light drizzle through 19Z, but high
confidence for a drier trend for the remainder of the current TAF
period. The main impacts in the short term is the lower cigs.
There have been noticeable improvements near MVFR cigs at ORD/MDW
recently, but lower IFR conditions remain entrenched to the west.
There was lower confidence in cigs above 1000 feet persisting
which led to maintain IFR through 00Z. However, there moderate
confidence for conditions to improve to MVFR through the overnight
and into Monday.

Winds will remain out of the west through 00Z. Increased
prevailing winds to around 15 knots, though gusts up to 20 knots
is possible through 00Z. Winds will gradually diminish into the
night and become north to northwesterly. There is a non-zero
chance for a brief period of NNE winds Monday morning at ORD/MDW.
If NNE winds do develop, the current thinking is that it would
not be for an impactful duration of time, but there is lower
confidence. Winds are forecast to return to the west Monday





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