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LMZ741 Expires:202312030445;;785872 Fzus53 Klot 022027 Nshlot
nearshore marine forecast national weather service chicago/romeoville il 227 pm cst Sat dec 2 2023
for waters within five nautical miles of shore on lake michigan
waves are provided as a range of significant wave heights, which is the average of the highest 1/3 of the waves, along with the average height of the highest 10 percent of the waves which will occasionally be encountered.
lmz740>742-030445- winthrop harbor to wilmette harbor il- wilmette harbor to northerly island il- northerly island to calumet harbor il- 227 pm cst Sat dec 2 2023
Tonight..East winds 5 to 10 kt becoming southeast late. Chance of rain in the evening, then rain after midnight. 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..West winds 10 to 20 kt becoming northwest 10 to 15 kt. Cloudy. Waves 1 to 2 ft.
Monday..Northwest winds 10 to 15 kt becoming west in the afternoon. Cloudy. Waves 1 to 2 ft.

LMZ700
No data

FXUS63 KLOT 022000 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 200 PM CST Sat Dec 2 2023

SHORT TERM
Issued at 200 PM CST Sat Dec 2 2023

Through Sunday...

Key Messages:

* Rain likely overnight and into Sunday morning for most of the region with a rain/snow mix expected in portions of NW Illinois.

* With persistent cloud cover expected through the night, surface temperatures are expected to be warm enough to limit the threat of snow accumulations.

What a grey Saturday! Widespread cloud cover remains over the area with seasonal temperatures. The higher moisture content has gradually moved away from the forecast area and sources for lift have diminished allowing PoPs to be trimmed down even farther during the morning update. There is still a non zero (less than 10 percent chance) chance for an isolated drizzle/sprinkles occur the rest of this afternoon, but confidence is low and impacts would be non- existent.

The next weather system moves in overnight. An upper level trough will move northeastward from the Southern Plains. Chances for precipitation increase through the night, though the main rainfall will happen after midnight and into Sunday morning. Most of the area south of a Waukegan to Peru line will likely have rain given the warmer temperatures. For areas north of that line, colder temperatures through the column will result in better chances for a wet snow to mix. However, due to the continued cloud cover, limited evaporational cool and additional warm air advection from the system moving in, the main update with this forecast package was surface temperatures were increased above freezing. There is a chance that isolated communities around the Rockford Metro area receives a trace, but snow accums have otherwise been removed from the forecast.

There is still a lot of uncertainty with the timing of the exit of the system. Higher res models are trending quicker with the ending of precipitation earlier on Sunday morning. While precipitation may lighten, higher PoPs were maintained through at least noon given the saturated column profiles.

DK

LONG TERM
Issued at 200 PM CST Sat Dec 2 2023

Monday through Saturday...

Key Messages...

* Rain or snow is likely Monday night, with a small, but increased, chance of some snow accumulations (10-20%)

* Above average temperatures likely late in the week

Guidance continues to show a compact, yet fairly vigorous, shortwave trough tracking east across downstate IL. Given the current forecast track, it seems likely precip with this wave will remain south of our CWA. Maintained a slight chance of rain/snow across the southern CWA just in case guidance were to trend farther north. While thermal profiles would probably lean more toward rain at first glance, strong upward vertical motion and weak static stability suggests at some dynamic cooling could flip precip to wet snow in spots. Again, this largely looks to be south of our CWA at this point, so only maintaining slight chances of precip.

Will stratus clear out Sunday night? Given the lack of any appreciable dry air advection and likely lingering moist layer beneath frontal inversion, it seems likely stratus will hang on through Sunday night. Based on this expectation, have raised forecast low temps a few degrees. Where and if stratus does manage to break up, then lows would likely end up cooler than forecast.
As overnight shift discussed in previous AFD, the stratus dilemma will continue Monday with potential for cloud cover to linger through the day. Maintained forecast highs closer to 40, leaning toward the more pessimistic (cloudier) solution, if skies clear out then highs could be a 5F warmer.

The next (and last) in this series fast moving, vigorous shortwave troughs is forecast to move quickly across the region Monday night. There has been a fairly sizable southward trend with this system in this morning's 12z guidance. The 12z operational runs of the GFS and ECMWF and 12z GEFS have all shifted the track of this system 100 to nearly 150 miles farther south. The more southward solution increases confidence in precipitation occurring, so continued to trend pops higher than NBM and a bit higher than previous forecast. In addition, the farther southward track raises of a snowier solution and even possible accumulations into our CWA Monday night. For now, just trended forecast toward rain or snow, but if subsequent runs remain locked in with this farther south track then the snow may need to be hit a bit harder in the forecast.

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.

- Izzi

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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