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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- Northerly Island To Calumet Harbor Il- 936 Am Cst Sat Dec 2 2023
Rest of today..Northeast winds 5 to 10 kt becoming east. Slight chance of drizzle in the afternoon. Waves 2 to 4 ft subsiding to 1 to 3 ft.
Tonight..East winds 5 to 10 kt becoming southeast overnight. Chance of rain in the evening, then rain overnight. Waves 1 to 2 ft.
Sunday..Southwest winds 10 to 20 kt becoming west in the afternoon. Rain in the morning, then chance of rain in the afternoon. Waves 1 to 2 ft.
Sunday night..Northwest winds 10 to 15 kt. Mostly cloudy. Waves 1 to 2 ft.

LMZ700
No data

FXUS63 KLOT 021814 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1214 PM CST Sat Dec 2 2023

UPDATE
Issued at 933 AM CST Sat Dec 2 2023

The region remains shrouded in a thick cloud deck. The higher moisture content, supported by cirrus clouds above the stratus deck, is mainly south of I-80. With weak forcing present, it would not be surprising if a spit of drizzle occurred this afternoon, but there is higher confidence that the drier air near the surface will win out. So, PoPs were adjusted down to below 10 percent north of I-80, confining the official mention of drizzle to the southern part of the region. Otherwise, no major changes were made compared to the forecast listed in the discussion below.

DK

SHORT TERM
Issued at 334 AM CST Sat Dec 2 2023

Through Sunday...

Key Messages:

* Lingering rain/snow showers this morning will give way to overcast skies and periods of drizzle this afternoon.

* Another storm system arrives tonight bringing more widespread rain for most and a rain/snow mix to portions of northwestern Illinois. Some slushy accumulations will be possible, especially if surface temperatures are cold enough.

* Persistent overcast skies and seasonable temperatures through Sunday afternoon

A robust shortwave disturbance continues to generate an area of rain and snow showers across portions of northern Illinois this morning. The shortwave is expected to exit the area around daybreak which should allow any lingering showers to diminish in coverage through mid-morning leaving most areas with dry, but cloudy skies through this afternoon. However, the area will be positioned in the diffluent upper flow between this departing wave and the trough that is currently ejecting out of the Four Corners Region. The combination of this modest forcing with the steep low- level lapse rates and 3500 to 4000 ft saturated layer at surface should allow for periods of drizzle to develop during the morning and linger through this afternoon.

The aforementioned trough will begin to move into the Mississippi Valley this evening and then track overhead through the night. As this occurs another area of rain is expected to develop over Missouri and spread into northeast Illinois and northwest Indiana late this evening. Similar to the last system, surface temperatures are expected to gradually cool as the rain moves in with most areas forecast to remain in the mid to upper 30s but with temperatures aloft expected to be well below freezing. This should once again allow for some snow to mix in with the rain late tonight into Sunday morning particularly for areas north of I-88 where temperatures look to be coldest. Though unlike the last system, forecast soundings do show mid-level lapse rates to be steeping as the system moves overhead which may allow for some upright instability to be realized in the 500 mb to 600 mb layer.
Coincidentally this layer just so happens to be within the saturated DGZ which means that if this upright instability can be realized then more robust snowfall rates should develop and may be enough to overcome the marginal surface temperatures. Should this materialize as some guidance suggests, then areas within this convective snow band could see some slushy accumulations potentially in excess of an inch.

Given that a lot of conditions which have to come together tonight for these slushy accumulations to occur confidence in the location and amounts are somewhat low. Though, guidance is in decent agreement that areas along and northwest of a Sterling, IL to a McHenry, IL line look to be the most favored area for any accumulating snowfall. Since yesterday's system was able to over perform with snow despite the lack of instability, I have decided to add a couple tenths of accumulation to the forecast with this system for the far northwest portion of our forecast area. While potential impacts should be minimal due to the generally above freezing temperatures, we will need to carefully monitor trends as the system approaches before we will know for certain so be sure to check back for updates this afternoon.

Tonight's system will exit the area Sunday morning which will allow conditions to dry out once again as a weak upper-level ridge begins to build into the area. Though lingering mid-level moisture does look to keep skies overcast through Sunday afternoon which will hold high temperatures in the upper 30s and lower 40s for Sunday.

Yack

LONG TERM
Issued at 334 AM CST Sat Dec 2 2023

Sunday night through Friday...

Key Messages:

* Another round of precip Monday night-Tuesday. Low confidence on p-types with marginal surface temperatures at this point, but have added snow chances to the forecast Monday night.

* Period of dry and unseasonably mild weather late next week

Precipitation will largely have ended across the region Sunday evening although there is a small signal that a little drizzle may linger ahead of a weak reinforcing front which will be dropping out of Wisconsin. Temperatures are expected to be at or above freezing, so not concerned with this signal at this time. Some disorganized lake effect rain shower activity will likely develop across parts of lower Michigan, but with generally W/WNW surface flow, expecting the bulk of this activity to remain relegated to the east of our forecast area.

A very robust shortwave is forecast to scoot rapidly eastward across Missouri and into central/southern Illinois late Sunday night. Guidance is in pretty good agreement with this feature's track at this point, holding the best overlap of deeper moisture and forcing for ascent south of the Kankakee River and moreso into central Illinois overnight. Have held onto some low PoPs for these areas with just rain advertised. Quite the impressive look from a forcing/Q-vector convergence and upright instability perspective with this feature...just meandering into a relatively mild airmass.

Shortwave ridging will (very briefly) build in overhead on Monday. It's always difficult determining stratus staying power this time of year, and the typically under-mixed NAM solution depicts stratus remaining entrenched across the region through Monday afternoon. With some increase in west/southwesterly flow late in the day, however, seems like there may be some potential for some degree of clearing which would further help boost temperatures, perhaps into the low to even mid 40s. More stubborn stratus, however, would hold temps in the mid and upper 30s.
Latest thinking is headed more towards the former, and showing highs either side of about 40 on Monday.

The next intense disturbance arrives in the region Monday evening and overnight. Guidance trends appear to be upwards regarding both the degree of moisture/saturation and ascent, particularly within the preceding warm advection wing. Have boosted PoPs a bit above the NBM offering with good multi-model support at this time.
Thermal profiles remain on the cusp of supporting either just a cold rain or a changeover to a wet snow, and surface temperature trends coming out of Monday afternoon will likely play a big role in the regard. With WAA intensifying through the evening, suspect temperatures really may not fall all that much and may even slowly rise through the late evening and overnight hours. For this forecast package, have generally limited the snow wording to slight chance/chance in favor of both milder temps on Monday and a muted temperature fall in the evening with no accumulations formally depicted, but there's plenty of guidance now showing the lead WAA wing arriving as mainly snow. Something we'll continue to monitor. Deeper saturation will be lost rapidly through daybreak on Tuesday turning most lingering precipitation over to a non- freezing drizzle.

After a brief period of lake effect precip (likely rain), we'll begin a moderating trend as mid-level heights rise and the main active northern stream temporarily heads north of the region.

Carlaw

AVIATION
For the 18Z TAFs...

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

* Continuing MVFR cigs dropping to IFR or lower tonight

* Rain showers overnight transitioning to more of a drizzle for Sunday morning, with associated vsby reductions

* At RFD, snow or a rain/snow mix tonight into Sunday morning

Easterly winds under 10 kt will persist for the remainder of the day under MVFR cigs. Late this evening, cigs are expected to drop readily with the arrival of another round of showers. IFR is appearing likely with LIFR possible, especially during the predawn hours and through the earlier half of Sunday morning. During this time, it's conceivable that cigs could dip into VLIFR territory.
Cigs should improve some heading into the afternoon but will likely hold at low-end MVFR, if not high-end IFR.

The rain is anticipated to arrive sometime around 06Z or so.
Steadier showers are forecast through the night followed by periods of light rain or drizzle during the morning. Expect vsbys for the better part of the event to be MVFR, if not lower at times. The rain should let up during the late morning. Meanwhile, winds will veer during the early morning from SE to westerly while remaining primarily below 10 kts through Sunday morning. They'll pick up a little bit of momentum for the afternoon to around or just over 10 kt.

At RFD, precip will begin as rain late this evening. Not long after 06Z, snow is expected to begin mixing in before likely going over to all or majority snow shortly thereafter. There is a good signal for vsbys to go at least IFR, but could very well go even lower.
Additionally, LIFR cigs are forecast for most of the night, but VLIFR is certainly on the table as well. Rain may mix back in during the morning before the precip moves away altogether mid-late morning.

Doom

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

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