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000
FXUS63 KLOT 302116
AFDLOT

Area Forecast Discussion
National Weather Service Chicago/Romeoville, IL
316 PM CST Thu Nov 30 2023

.SHORT TERM...
Issued at 316 PM CST Thu Nov 30 2023

Through Friday night...

Key Messages:

- A widespread soaking cold rain is expected (for most) late
  tonight through Friday morning. Rainfall amounts of 0.5" up to
  an inch are forecast.

- Conditions continue to favor the rain to mix with, or even
  change over to a period of wet snow for period Friday morning
  across interior sections of far northern Illinois, primarily
  near/west of the Fox Valley.

- There remains a chance (currently 40%) of some slushy wet snow
  accums up to a inch or so (mainly on non paved surfaces) within
  the narrow zone generally along and north of a Dixon to McHenry
  line. Closer to the WI border, there remains a low (~20%) chance
  of amounts around 2" if the wet snow comes down at a heavy
  enough rate to overcome marginal temps and mild ground conditions.

- Periods of light precipitation, primarily in the form of light
  rain and drizzle will persist through Friday afternoon (60-70%
  probs), with some wet snow probably (50-60%) continuing to mix
  in over portions of far northern Illinois.

- Precipitation will reinvigorate (thought with continued
  primarily light rates) Friday evening with 80-90% probabilities
  near and north of the Kankakee River. A similar zone of far
  northern Illinois to Friday morning will likely (>60% chance)
  have at least a mix with wet snow, but no additional accumulation
  is forecast.

Discussion to follow shortly.

Castro

&&

.LONG TERM...
Issued at 219 PM CST Thu Nov 30 2023

Saturday through Thursday...

Key Messages:

* Multiple chances for rain and rain/snow mix through Tuesday night

* Seasonal temperatures through mid week, but the potential for
  anomalously warm temperatures build late in the week

Mostly cloudy skies are expected on Saturday with temperatures in
the 40s. Mid-level subsidence and cold air advection will likely
keep the area drier as the previous short wave moves east. However,
the 500 mb long wave trough over the western Texas panhandle is
projected to move northeastward toward the Great Lakes through the
day on Sunday. Overnight low temperatures Saturday into Sunday are
expected to be above freezing for most of the region, but best
chances for subfreezing temperatures at the surface is projected
northwest of a Dixon to Woodstock line. Precipitation is likely (55%-
65%) to be a cold rain, with a wet snow for northwest Illinois
Sunday morning, through Sunday evening. A secondary shortwave
traversing around the long wave keeps confidence low on the exact
timing of precip ending and light rain/drizzle could linger into the
early part of Monday morning. Northwest flow behind the trough will
provide additional cold air advection for colder, drier conditions
Monday morning.

Yet another upper level trough is expected to drop down out of
Saskatchewan on Tuesday and deepen as it approaches Lake Michigan
providing another chance for rain or rain/snow mix. There is still a
lot of uncertainty with the timing and speed of the system, but it
looked like the blend was going too low with PoPs in comparison to
the suite of ensemble models. So PoPs were nudged upwards to have a
chance (30% to 40%) be displayed in the forecast. Lastly, the Euro
has a much broader negative tilt to the upper level trough, while
the GFS has a more meridional dip to it with a tighter gradient.
Winds were maintained with gusts around 20 mph, but if models trend
toward the GFS it is possible that the winds are underdone.

An upper level ridge over the intermountain west will grow and
strengthen as it drifts eastward over the Northern Plains.
Temperatures will be around seasonal normals for much of the week.
However as the ridge drifts east, higher pressure will grow and
there is a signal for the potential for temperatures to warm to
almost 10 degrees above normal for the end of the week.

DK

&&

.AVIATION...
For the 18Z TAFs...

The primary aviation weather concerns through the 18Z TAF period
are as follows:

* Rain showers late tonight into tomorrow morning, with more light
  rain or drizzle to follow in the afternoon. Associated vsby
  reductions

* MVFR cigs this evening with IFR expected late tonight and
  tomorrow

* At RFD, snow showers and a rain/snow mix expected tonight into
  tomorrow

A few scattered showers are expected overnight before widespread
rain moves in from the south not long before daybreak. A steady,
soaking rain is expected to pull vsbys down to MVFR with periods
of IFR for the better part of the event. The main swath of rain
will lift off to the northeast during the late morning with
additional light rain or drizzle possible during the afternoon.
Should the drizzle play out, vsbys could hold at IFR but should
otherwise improve. At RFD, primarily snow is expected through mid-
morning or so before a transition to rain or a rain-snow mix for
the late morning.

Additionally, MVFR cigs will move overhead late tonight and are
expected to drop to IFR as the widespread rain pushes in during
the predawn hours. IFR cigs are forecast for the remainder of the
period with LIFR possible in the afternoon with the drizzle
potential.

Meanwhile, gusty SW winds will continue through the bulk of the
afternoon before subsiding late this afternoon. Winds will turn NW
around mid-evening then go east of north late this evening. East
or NE winds near or under 10 kt will then continue through the
period.

Doom

&&

.LOT WATCHES/WARNINGS/ADVISORIES...
IL...None.
IN...None.
LM...Small Craft Advisory...IL and IN nearshore waters until 4 PM
     Thursday.

     Small Craft Advisory...Winthrop Harbor to Wilmette Harbor IL...4
     AM Friday to 10 PM Friday.

&&

$$

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