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000
FXUS63 KLOT 031118
AFDLOT

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

.SHORT TERM...
Issued at 314 AM CST Sun Dec 3 2023

Through Monday...

Key Messages:

* Lingering rain showers, with a rain/snow mix north of I-88, will
  continue through the morning.

* Some slushy snow accumulations upwards of 0.5 inches are
  possible on grassy and elevated surfaces north of a Dixon to
  McHenry line through daybreak.

* Persistent cloud cover expected through Monday with seasonable
  temperatures

A broad upper trough continues to churn overhead this morning
which is supporting the broad area of precipitation lifting
through northeast Illinois and northwest Indiana. While most of
this precipitation continues to fall in the form of rain, sites
north of I-88 are starting to see a rain/snow mix. So far snow
accumulation across northwest Illinois has been sparse due to the
gradual decay of synoptic forcing and surface temperatures
remaining above freezing. However, a couple of robust snow showers
have developed near the Mississippi River along the mid-level
lapse rate plume and are expected to track into the northwest
portion of our forecast area over the next few hours. Given that
sites under these showers have seen a quick drop in temperatures,
I suspect that some slushy snow accumulations on grassy and
elevated surfaces is still in play. Therefore, I have maintained a
couple tenths to half an inch of accumulation for areas along and
north of a Dixon to McHenry line for this potential.

The precipitation is expected to gradually diminish in coverage
through the morning as the trough pivots into the eastern Great
Lakes leaving us with dry but cloudy conditions for our Sunday
afternoon. As seen the past couple days, the clouds will limit any
major improvement in temperatures this afternoon with highs
expected to remain in the upper 30s and lower 40s.

While these dry conditions are expected to persist through the
day on Monday as some shortwave ridging builds overhead, another
potent shortwave disturbance is forecast to dive out of the
central plains this evening. Guidance is in good agreement that
this system should track into central Illinois tonight but there
are some indications that some modest instability and lingering
mid-level moisture may allow a couple of rain/snow showers to clip
the far south and southwestern portions of our forecast area.
Given that this window should be limited to just a few hours
tonight, I have decided to maintain a slight chance (around 20%)
POP south of the Kankakee River Valley for this potential.

Regardless, it does look like the low-level cloud cover will
persist through Monday though there could be some breaks in the
clouds Monday afternoon. Depending on how much of a break develops
temperatures could possibly verify a few degrees warmer than the
lower 40s currently forecast.

Yack

&&

.LONG TERM...
Issued at 314 AM CST Sun Dec 3 2023

Monday night through Saturday...

Key Messages:

* A period of wintry precipitation, potentially all snow north of
  I-80, arrives Monday night. Some slushy accumulations looking
  more plausible.

* Above average temperatures expected to close out the week.

A compact yet robust shortwave will track across the region Monday
night, yielding the next appreciable precipitation chances for the
area. While the large run-to-run southward shifts in system track
have eased, the amplified and unstable nature of the upstream flow
across the Gulf of Alaska suggests continued track wobbles remain
on the table. That said, deterministic and ensemble guidance is
coming into decent agreement now, supporting high-end likely PoPs
(70 percent) across pretty much all of the forecast area. Only
reason we didn`t boost all the way to categorical at this point is
to leave some wiggle room for additional track shifts.

With it appearing more likely that stratus remains in place on
Monday, temperatures are expected to start the evening out in the
mid, perhaps upper, 30s. Broad warm advection will develop ahead
of the incoming shortwave and attendant surface low/reflection,
meaning little in the way of temperature movement, at least
initially, but given the relatively stout nature of large scale
ascent within the warm advection wing, the combination of a little
dynamic cooling plus sublimation/evaporation now looks like it
may be sufficient to send thermal profiles cold enough to support
more in the way of wintry precip.

With this forecast package, have re-worked the NBM p-types grids
to depict a more aggressive transition to all snow, particularly
for locales north of about I-80. In fact, precipitation may even
onset as all snow, contingent on how surface temperatures respond
during the evening, and the currently-advertised transition may
not yet be swift enough. As it typical with these N/S oriented
warm advection wing-driven events, the window for deep saturation
will be somewhat brief, likely less than 4-5 hours in most spots.
During this time, however, strong ascent residing near or just
under the heart of the -12 to -18 C DGZ, coupled with diminishing
static stability as the system dryslot approaches, should yield a
brief window feature of a wet snow (ratios probably averaging out
7-9:1) Monday night. Have gently increased snow accumulations a
bit all the way down to the Kankakee River with this forecast
package. While marginal surface temperatures look like they`ll
curtail most main road accumulations, think there may be a window
right ahead of the dryslot where rates could very briefly approach
an inch per hour which might result in a quick coating on
bridges/overpasses.

Will likely lose most cloud ice through Tuesday morning, turning
lingering precipitation over to a mixture of snow and drizzle, or
even all drizzle with temperatures meandering back into the mid
and upper 30s. Looks like stratus will probably remain locked in
through the day, yielding a muted diurnal temperature range.

Marginal lake effect showers (mostly rain) might focus briefly
into NW Indiana into Tuesday night before winds veer to the WNW
and kick any lingering moisture out of the area.

Transition to a more zonal flow or even low amplitude ridging is
still expected late next week with high temps well into the 40s to
lower 50s. As always this time of year, temp forecast will hinge
on cloud cover, if warm sector doesn`t become covered in stratus
then highs could be at or even above forecast levels. Conversely,
if there is a more rapid development/expansion of stratus then
temps could end up a bit cooler than forecast. Regardless of exact
temperatures, there is high confidence in trend to above average
temperatures. The next meaningful precip chances arrive into next
weekend.

Carlaw/Izzi

&&

.AVIATION...
For the 12Z TAFs...

The main aviation weather concerns are:

* Lingering mixture of rain/drizzle this morning (all TAF sites)
  with intermittent non-accumulating -SN at RFD

* IFR to LIFR cigs this morning. Some improvement to around
  010/low-MVFR this afternoon and tonight

* NW wind shift tonight. Some potential for winds to go NNE at
  ORD/MDW Monday morning.

Expecting largely IFR conditions in rain and drizzle this morning
at all TAF sites with a window for LIFR conditions through mid-AM.
At RFD, loss of deeper saturation means that p-types will probably
be switching back and forth between drizzle and snow. Somewhat low
confidence on the cessation time for light precip/drizzle today
with low-level moisture stuck in place.

Through this afternoon, cigs should gradually lift towards 1 kft,
with low-MVFR cigs largely expected to prevail through the
remainder of the TAF periods. Some guidance suggest some clearing
may develop late tonight/Monday morning, but continue to lean on
the more pessimistic side of things at this point. Winds will
veer to the northwest behind a weak front tonight. On Monday,
winds may attempt to flop NNE at ORD/MDW, but have refrained from
a mention due to lower confidence at this time.

Carlaw

&&

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

$$

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