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000
FXUS63 KLOT 040520
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
1120 PM CST Sun Dec 3 2023

.SHORT TERM...
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
forecast.

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.

DK/Izzi

&&

.LONG TERM...
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

&&

.AVIATION...
For the 06Z TAFs...

Aviation concerns:

 * MVFR cigs expected through the majority of the period

 * Next round of -SN arrives late Monday night

Cloud bases are expected to remain mainly MVFR overnight. There
continues to be a signal for clouds briefly scattering out over
the Chicago area terminals Monday morning, though confidence on
the timing is too low to incorporate into the TAFs and have
maintained MVFR through the period. Skies then fill back in by the
afternoon ahead of the next weather system.

Have introduced -SN at ORD/MDW beginning 7Z Tuesday with
associated MVFR vis and near IFR cigs. There remain model
differences on the placement of the axis of higher snowfall rates
(which would accordingly result in lower vis/cigs), though there
has been a southward trend in the latest guidance.

Winds will be light through the period (<10kt), generally out
turning NW to N overnight and returning to the west Monday
afternoon. Winds then turn southerly to southeasterly as the snow
moves in though speeds will continue to be light.

Petr

&&

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

$$

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