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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- 903 Pm Cst Fri Dec 1 2023
Rest of tonight..Northeast winds 10 to 15 kt becoming north. Rain and drizzle. Waves 2 to 4 ft.
Saturday..Northeast winds 10 to 15 kt becoming east in the afternoon. Chance of drizzle in the morning. Waves 2 to 4 ft.
Saturday night..East winds 10 to 15 kt becoming southeast 5 to 10 kt overnight. Slight chance of rain in the evening, then rain overnight. Waves 1 to 2 ft.
Sunday..Southwest winds 5 to 10 kt becoming west 10 to 15 kt in the afternoon. Rain, mainly in the morning. Waves 1 to 2 ft.

LMZ700
No data

FXUS63 KLOT 020103 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 703 PM CST Fri Dec 1 2023

UPDATE
Issued at 703 PM CST Fri Dec 1 2023

It's a soggy night with pockets of drizzle, scattered showers, and fog. Much of the same is expected overnight albeit with gradually decreasing coverage of showers in favor of fog and drizzle. As a result, temperatures overnight will be pretty similar to where they stand now and in the upper 30s (northwest) to lower 40s (elsewhere).

Tomorrow, the signal remains that a thick (2500-3000 feet deep)
layer of low-level stratus will hold tight beneath a strong inversion rooted near 800mb. Temperatures will, accordingly, hold pretty steady in the lower 40s. With steep lapse rates through the stratus layer, areas of drizzle may prevail particularly during the evening hours as the next system approaches from the southwest. For this reason, opted to explicitly mention a slight chance of drizzle in our official forecast all day tomorrow carried by 20% chances for measurable rain though some (many?)
hours may end up completely dry.

Updated products will be sent soon.

Borchardt

SHORT TERM
Issued at 328 PM CST Fri Dec 1 2023

Through Tonight...

Key messages:

* Precipitation increasing in coverage again this afternoon into this evening. Most locations will see all rain, but snow may mix in with the rain, or even fall as the outright precipitation type, roughly along and northwest of a Dixon to Woodstock, IL line.

* Where snow falls in our forecast area, road accumulations are unlikely, but areas north and west of Rockford may receive anywhere from a dusting to an inch or so of slushy snow accumulation on grassy and elevated surfaces.

The core of the upper-level disturbance that helped foster this morning's widespread precipitation has long since departed to our east. Lingering low-level moisture allowed for some areas of light rain/drizzle to hang around into this afternoon, and this precipitation is still being observed across a good chunk of our forecast area as of this writing. Meanwhile, a residual vort max hanging out back closer to the center of a surface low in west- central Illinois has helped an additional area of stratiform precipitation flare up across the northwestern portion of the state this afternoon. The expectation is that this area of precipitation will continue to expand in coverage through the remainder of this afternoon and into this evening as this vort max shears out and shifts east and as a trailing shortwave over the lower Missouri River Valley rides up a southwest-northeast oriented baroclinic zone into the area.

Surface temperatures predominantly in the 40s across much of our CWA will support rain remaining as the sole precipitation type at most locations. However, across our northwestern counties, temperatures will be cold enough to support snow mixing in at times and even falling as the outright precipitation type. Surface temperatures there are still expected to remain at or above freezing, which should help mitigate road impacts, but webcam imagery just across CWA lines in Freeport, IL has shown that the snow there has fallen at a steady enough clip to accumulate on grass at least, and a similar scene could be seen in extreme northwestern portions of our forecast area (roughly west and north of Rockford). Any snow accumulation that occurs within our forecast area would be unlikely to be much more than a slushy inch due to the marginal surface temperatures and limited residence time of the steadier snowfall rates.

Most or all of the precipitation is then expected to end during the overnight hours as the aforementioned trailing shortwave starts to peel away. Precipitation could acquire a more drizzle-like nature again before ending as forecast soundings indicate that cloud ice should be lost before saturation depths within the overhead stratus deck are slashed by a meaningful amount. Even then, some guidance indicates the stratus deck could remain deep enough to support patches of drizzle through and beyond sunrise tomorrow morning, though confidence in this outcome being realized at this time is only low-medium.

Ogorek

LONG TERM
Issued at 223 PM CST Fri Dec 1 2023

Saturday through Friday...

Key Messages:

* Several chance of light precip, mostly rain, Saturday night through Tuesday morning * Period of dry and unseasonably mild weather late next

Saturday:

We'll be between weather systems Saturday, but with no push of drier air, stratus is expected to linger through the day. Model guidance varies with respect to the depth of the saturated (ie stratus) layer, generally between 3-5kft thick during the morning. Given the expected depth of the stratus, it wouldn't take much for some drizzle to occur and have introduced a chance of drizzle during the morning. During the afternoon, ceilings may lift a bit and stratus is expected to thin a bit, so held off on adding drizzle to the forecast during the afternoon, but wouldn't be hard to envision drizzle lingering later. The thick cloud cover generally neutral to slight cold air advection should result in temps basically flatlining all day Saturday.

Saturday night-Sunday night:

Progressive shortwave trough is progged to ripple across the area bringing a period of mostly rain, starting late Saturday evening and continuing into Sunday morning. Thick stratus and warm air advection in advance of the system should result in temps generally remaining steady Saturday night. Thermal profiles support all rain over most of the area, though far northwest CWA could could see rain mix with or change to wet snow as slightly drier low levels could allowing for a bit of wet-bulb cooling.
Temps should generally remain just above freezing at the surface, so at this point the threat of any accums on roads appears minimal (<25%).

Rain should taper off to drizzle or end during the day as the trough axis shifts east of the area. Fast on the heels of this wave, another shortwave trough is progged to track mainly just south of the CWA Sunday night into early Monday morning. Stronger forcing should remain mostly south of our CWA, but did hang onto the slight chance pops southern CWA offered up by the NBM.

Monday-Tuesday night:

Monday should be dry with seasonable temps followed by another quick moving shortwave trough, an Alberta Clipper, moving quickly across the region Monday night into Tuesday morning. There could be some showers Monday night associated with the warm air advection wing of this clipper. Once again, cloudiness and warm air advection should result in a precip type of primarily rain with any showers that do occur. Better precip chances with this wave look to stay north of our area with sfc low progged to track north of our CWA.

Wednesday-Friday:

Medium range guidance is in reasonably good agreement on pattern transitioning to more of a zonal flow late next week. Zonal flow this time of year tends to be a mild pattern. and with storm track associated with the northern stream jet expected to remain well north of our area, we could be headed toward a period of dry and unseasonably mild weather.

- Izzi

AVIATION
For the 00Z TAFs...

Aviation concerns:

* -RADZ with associated IFR vis and IFR/LIFR cigs tonight * -SNRA at RFD this evening

Rain and drizzle continues across the area this evening along with associated IFR visibility and ceilings and LIFR at GYY. This will gradually transition to mainly drizzle late evening into the early overnight hours. There is a corridor of improved vis and cigs approaching the Chicago area terminals that could result in at least a brief period of improved vis and cigs. Will continue to monitor trends, though model guidance still supports a return to drizzle and reduced visibility. The better signal for LIFR/VLIFR visibility in fog remains mainly south of I-80 and accordingly have have held onto 3SM for the Chicago area terminals. It is possible that light drizzle persists into the morning hours on Saturday (20 percent), though cigs will gradually lift to MVFR after daybreak into the afternoon.

The earlier band of snow has since shifted east of RFD with light snow/flurries and even a period of VFR visibility. There is another band near the Freeport area shifting east that could bring another round of brief vis/cig reductions in snow (potentially mixed with rain) this evening.

Northeasterly winds will trend northerly overnight then return to a northeast to easterly direction during on Saturday.

Petr

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

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