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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- Northerly Island To Calumet Harbor Il- 237 Am Cst Mon Dec 4 2023
Today..Northwest winds 10 to 15 kt becoming west in the afternoon. Mostly cloudy. Waves 1 to 2 ft.
Tonight..Southwest winds 5 to 10 kt becoming south overnight. Snow and rain overnight. Waves 1 to 2 ft.
Tuesday..West winds 5 to 10 kt becoming north and increasing to 10 to 20 kt in the afternoon. Chance of rain and snow in the morning, then slight chance of rain in the afternoon. Waves around 1 ft building to 2 to 4 ft.
Tuesday night..Northwest winds 10 to 20 kt. Slight chance of rain in the evening. Waves 2 to 4 ft.

LMZ700
No data

FXUS63 KLOT 041148 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 548 AM CST Mon Dec 4 2023

SHORT TERM
Issued at 300 AM CST Mon Dec 4 2023

Through Tuesday...

Key Messages:

* Round of snow arrives tonight (except a rain/snow mix in our far south). Most accumulations look light (less than a half inch) and relegated to grass and the coldest surfaces.

* A chance (about 35%) for a very narrow ribbon of locally "higher" accums to 1-2" somewhere in the vicinity of I-80, although confidence on the precise system track remains unusually low.

* Most roads should remain just wet, but if/where enhanced rates set up, a dusting is possible on bridge decks.

Our feature of interest appears nicely in simple water vapor RGB loops emanating out of a zone of intense cyclonic shear vorticity across southern Alberta and Saskatchewan. This feature will continue to dig and amplify today as it rapidly traverses the northern Plains. Intense large scale forcing for ascent will increase quickly late this evening and overnight yielding an expanding zone of precipitation across the entire forecast area.
Wobbles in the system track continue, but think we're seeing guidance stabilize (finally). 00z HRRR looked like it was too far south, with the 06z run coming more in line with the global guidance. Have boosted PoPs to categorical across parts of the forecast area with this in mind.

While p-types might initially start as rain, Bourgouin melting energy output suggests a changeover to all snow will likely occur shortly after onset north of about our Kankakee River/I-80 locales. Do think the NAM continues to look a bit too cold, however. As is typical with these types of systems, moisture won't be in great supply, so still looking like a lighter event for us.
Cobb SLRs look pretty good, and suspect these will average out around 7-9:1 or thereabouts. With this in mind, hourly snowfall rates where things do fully changeover to snow will be less than a quarter inch per hour. The one exception might be late tonight/Tuesday morning immediately north of the 700 mb westerly wind max. Ephemeral f-gen forcing might yield a very small corridor of locally-enhanced precip rates for a brief period.
Uncertain where this will set up, with some guidance as far north as I-90 and others as far south as a Moline to Kankakee line.

Regardless of the exact track, not really anticipating travel impacts with air temperatures at or just above freezing. The lone exception would be where/if aforementioned enhanced rates set up which could yield a dusting on the very coldest surfaces such as bridge decks yielding patchy slick travel.

Precip will wane through Tuesday morning, although soundings look conducive for continue spits of drizzle/poor-quality snow even into the afternoon.

Carlaw

LONG TERM
Issued at 300 AM CST Mon Dec 4 2023

Tuesday night through Sunday...

Key Messages:

* Lake effect rain showers in northwest Indiana Tuesday night into early Wednesday morning

* Dry and unseasonably warm conditions likely to develop towards the second half of the week

* Active weather pattern and more seasonable temperatures return next weekend into the early part of next week

A broad upper trough is forecast to be centered over the Great Lakes and much of the eastern CONUS Tuesday night, but will getting shoved eastward during the day on Wednesday. While any lingering precipitation from earlier in the day Tuesday should be just about over, lingering low-level moisture and northerly fetch down Lake Michigan will support some lake effect showers across portions of northwest Indiana. Forecast soundings do show that the bulk of the deep (~8000 ft) saturated layer should reside below the DGZ which should keep the showers mainly in the form of rain, but there does look to be a brief period Tuesday evening where some snow flakes could mix in. Though no accumulation is expected.
The better moisture and modest lake induced instability should begin to wane after midnight Tuesday night as winds become more northwesterly which should in turn push the lake effect showers east of our area into northern Indiana and southwest Lower Michigan prior to daybreak Wednesday morning. Thus, leaving us with dry conditions for our Wednesday but with some residual cloud cover and seasonable temperatures.

As the aforementioned trough exits the area on Wednesday, a stout 500 mb ridge is expected to develop over the central CONUS and advect eastwards into the Great Lakes by Thursday. This ridge will not only aid in suppressing any cloud cover, but also generate west-southwesterly mid-level flow which is forecast to warm 850 mb temperatures upwards of 6 to 8 C. These mid-level temperatures are expected to translate to unseasonably warm temperatures for Thursday and Friday where highs are forecast to be in low to mid-50s and mid to upper 50s to near 60 respectively.
Additionally, winds look to become breezy during this period as a deepening surface low traverses the northern edge of the ridge and tightens the pressure gradient. Depending on how deep we are able to mix during the afternoon hours, winds could possibly gust upwards of 30 mph each day.

However, the unseasonable warmth is not forecast to last long as a deepening trough begins to take hold across the western and central CONUS during the day on Friday and push eastward into the Great Lakes over the upcoming weekend. As the trough pushes east it will shove a cold front through the area which will serve as the focal point for more precipitation and seasonable temperatures in its wake. Though, exactly where this front will be when the trough ejects into the area remains very uncertain which will obviously dictate precipitation coverage and type across our area.
So while it is too soon to discuss these specifics, it can be said that more active weather is on the horizon for next weekend into early next week.

Yack

AVIATION
For the 12Z TAFs...

Forecast concerns for the terminals...

* Persistent MVFR/low-VFR ceilings through this evening

* Snow moves in tonight and lingers into Tuesday morning with some accumulation possible on grassy and elevated surfaces

* IFR ceilings and visibilities with the snow

A broad area of MVFR ceilings continues to reside across northern Illinois, but recent satellite trends have shown an area of VFR ceilings and clearing over southern Wisconsin. The expectation is for these improved conditions to move into the Chicago terminals during the morning and likely persist through mid-afternoon before MVFR ceilings return ahead of our next storm system.
However, confidence on the exact duration of VFR conditions is low given that areas near RFD look to remain under MVFR clouds throughout the day.

Our next storm system will arrive around midnight and bring with it snow and IFR conditions into Tuesday morning. Some accumulation is also expected mainly on grassy and elevated surfaces. While most of the snow should be of a lighter variety with visibilities generally between 1 and 2 SM, guidance continues to hint that a band of locally heavy snowfall (and resultant lower ceilings and visibilities) will develop across portions of northern Illinois overnight. Forecast trends continue to favor areas south of the terminals near I-80 as the focus for this band, but confidence on exact placement remains low. Regardless, snow is expected to gradually taper through Tuesday morning which should allow for improvement of ceilings and visibilities as well. However, confidence on how quickly this improvement occurs is somewhat low with a chance that lower ceilings may persist into Tuesday afternoon.

Otherwise, expect light winds through the period with speeds less than 10 kts. Though directions will gradually turn from west- northwesterly this afternoon to southerly this evening and eventually to east-northeasterly by Tuesday morning.

Yack

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

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