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LMZ741 Expires:202312031015;;802220 Fzus53 Klot 030258 Nshlot
nearshore marine forecast national weather service chicago/romeoville il 858 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>742-031015- winthrop harbor to wilmette harbor il- wilmette harbor to northerly island il- northerly island to calumet harbor il- 858 pm cst Sat dec 2 2023
Rest of tonight..East winds 5 to 10 kt becoming south late. Rain after midnight. Waves 1 to 3 ft.
Sunday..Southwest winds 10 to 15 kt becoming west 15 to 20 kt in the afternoon. Rain mainly in the morning. 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 030404 AAA AFDLOT

Area Forecast Discussion...UPDATED National Weather Service Chicago/Romeoville, IL 1004 PM CST Sat Dec 2 2023

UPDATE
Issued at 538 PM CST Sat Dec 2 2023

The next weather system is approaching from the southwest with a growing area of radar returns across southern Iowa and much of Missouri. With nearly neutral low-level temperature advection tendencies and limited frontogenetical forcing, nearly all forcing will be tied to the diffluent right exit region of an approaching 150kt 250mb jet streak and a coherent region of mid- level DCVA. As a result, our expectation is for radar returns to further expand while moving into Illinois this evening, reaching I-39 perhaps as early as 10 PM and the Chicago metropolitan area around or just after midnight.

With heavy cloud cover (and even signs of developing drizzle) as well as light winds, current temperatures in the lower 40s may not move much over the next 12 hours. The exception will be along and north of I-88 where efficient evaporative cooling into a dry layer from roughly 850 to 600mb (sampled recently by AMDAR soundings from MDW) will support temperatures falling into the low to mid 30s. As a result, a transition from rain to snow appears increasingly probable across far northern Illinois (again mainly north of I-88). Such a transition from rain to snow is already occurring across far northern Missouri, per recent AWOS and ASOS station reports. Even with the majority of lift above the DGZ, gradually steepening mid-level lapse rates will support increasing convective augmentation to snow rates, which may approach or even exceed 0.5"/hr at times overnight. As a result, slushy snow accumulations appear increasingly probable on grass and decks, as well as potentially bridges and overpasses. Such a threat is a modest departure from our earlier forecasts and matches increasingly snowier trends in model guidance. Elsewhere, a cold rain is expected.

Peak forcing for snow will occur from about midnight to daybreak, which is a somewhat ideal time of relatively low travel.
Regardless, opted to issue a targeted Special Weather Statement for Lee, Ogle, Winnebago, Boone, DeKalb, Kane, and McHenry counties to highlight the threat for patchy slick spots tomorrow morning. An associated targeted impact-based graphic was also shared on our social media feeds.

Updated products have been sent.

Borchardt

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 00Z TAFs...

Aviation Concerns:

* Increasing likelihood for SN at RFD and associated LIFR cigs/vis overnight into Sunday AM (potentially VLIFR at times)

* SN mixing in at ORD/DPA overnight with LIFR cigs and IFR vis (potentially LIFR vis at times)

* Mainly all rain at MDW/GYY with IFR cigs and MVFR vis

Have adjusted 06Z TAFs to account for latest model trends toward more snow across northern/northwest Illinois. A larger period of all SN has been introduced at RFD from 7-14Z. Have currently capped the vis and cig reductions to low-end LIFR though additional targeted TEMPO groups of VLIFR vis may be warranted with later updates. Will closely monitor upstream observations this evening. This could result in a slushy 1 to 2 inches of snowfall.

Further to the southeast toward DPA and ORD it does look like snow will mix in at times overnight into Sunday morning and have introduced a period of RASN from 9-15Z. A targeted TEMPO for SN may also be needed along with lower vis, especially at DPA. The rain/snow line looks to reside somewhere near the I-88 corridor.
Accordingly, MDW and GYY TAFs remain all rain with this update.

Ceilings will be slow to increase during the day as the precipitation transitions to mainly a drizzle/flurries late morning, eventually returning to MVFR by the afternoon.

Winds will be light southeasterly this evening then becoming light and variable as the system moves overhead overnight. As the system shifts to the east late Sunday morning winds prevail westerly around 10 kts.

Petr

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

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