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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- Northerly Island To Calumet Harbor Il- 141 Pm Cst Sun Dec 10 2023
Tonight..Northwest winds 10 to 15 kt becoming west 5 to 10 kt. Mostly cloudy. Waves 1 to 2 ft.
Monday..West winds 5 to 10 kt becoming southwest 10 to 15 kt in the afternoon. Partly cloudy in the morning then clearing. Waves around 1 ft.
Monday night..Southwest winds 10 to 20 kt. Mostly clear. Waves 1 to 3 ft.
Tuesday..West winds 15 to 20 kt. Mostly Sunny. Waves 1 to 3 ft.

LMZ700
No data

FXUS63 KLOT 110156 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 756 PM CST Sun Dec 10 2023

UPDATE
Issued at 756 PM CST Sun Dec 10 2023

No significant changes were made to forecast this evening with the exception being to the sky cover. Earlier guidance had cloud cover diminishing overnight into Monday morning. While there is a little pocket of improvement over northwest Indiana, most of the sky remains shrouded in clouds. Therefore the forecast was adjusted to keep cloudy conditions overnight before gradually becoming partly cloudy Monday afternoon. Overnight lows were adjusted up a degree due to the increased cloud cover, but it did not alter the overall low temperature forecast of expecting mid 20s overnight. Otherwise, no other changes were made to the extended forecast listed below.

DK

SHORT TERM
Issued at 101 PM CST Sun Dec 10 2023

Through Monday night...

Key Messages:

* A pocket of fine drizzle/mist will continue moving east- southeastward this afternoon and evening followed by drying/clearing overnight

* A period of quiet weather is shaping up tomorrow onward.

Discussion:

A subtle 850-700mb vort max and associated area of fine drizzle/mist continues to pinwheel through northeastern Illinois. With surface temperatures right at or just above freezing, the mist/drizzle has not contributed to any additional icing like what occurred in northwestern Illinois this morning. The shallow wave is expected to continue into northwestern Indiana this afternoon and depart the area this evening (the RAP 850mb RH appears to be a good proxy for the back edge).

Light northwesterly winds continue to transport an extensive stratus deck across the area though there are signs of thinning across eastern Iowa and far southwestern Wisconsin. With continued subsidence behind the low-level wave responsible for the ongoing drizzle as well as modest thermal mixing induced by the low-angle sunshine, the hole in stratus may continue growing while moving into northwestern Illinois this afternoon and evening. The HRRR, which appears to be handling the ongoing stratus well, shows a similar thinning trend this evening into the overnight hours. As a result, our forecast will feature a gradual decreasing trend in clouds overnight.

If we clear up quickly, overnight lows may fall into the lower 20s if not upper teens. However, if clouds manage to hold on (as would be suggested by the RAP/HRRR), overnight lows will hold steady near or just below the freezing mark. Interestingly, the RAP/HRRR also show splotches of QPF overnight which would manifest as freezing drizzle. However, forecast BUFKIT soundings do not look supportive of precipitation given a lack of, well, any forcing mechanism.
Besides, any remaining cloud cover may simply be too thin to support precipitation. For these reasons, as well as the expectation for at least some clearing overnight, we'll keep our official forecast dry.

Tomorrow looks pleasant with at least filtered it not complete sunshine and light southwesterly winds. Highs will be near seasonable values and in the mid to upper 30s. As a surface high pressure system slides overhead tomorrow night, clear skies and calm winds will facilitate efficient radiational cooling. Overnight lows appear poised to tank into the teens, if not colder.

Borchardt

LONG TERM
Issued at 101 PM CST Sun Dec 10 2023

Tuesday through Sunday...

Key Message:

* No impactful weather is expected in the long term period.

Discussion:

The long-term period looks remarkably quiet. A surface high pressure system will lead to light winds and periods of sunshine Tuesday through Friday. Highs temperatures Tuesday and Wednesday will be similar to tomorrow and in the mid to upper 30s. A building upper- level ridge will allow for temperatures to warm Thursday into the weekend with highs returning to the upper 40s and potentially 50.
The next chance of precipitation looks centered on the Sunday/Monday timeframe, though ensemble spread is vast in location and amount.
Until a more meaningful signal materializes, our official forecast will be completely dry through Sunday.

Borchardt

AVIATION
For the 00Z TAFs...

Aviation Forecast Concerns:

* MVFR ceilings persist into Monday afternoon.

Region remains within weakly cyclonic low level flow beneath an upper trough early this evening. An extensive area of MVFR ceilings remain in place across the upper Midwest, and though the clearing line extends from eastern IA into central IL, the northwesterly trajectory of our low level flow suggests this clearing will not make it into the terminals tonight. MVFR ceilings have rising to around/just above 2000 ft and this is expected to prevail this evening, though guidance does indicate bases settling back below 2000 ft again after midnight and into Monday.
Models continue to show some spread in timing of improvement to VFR, though generally indicate VFR conditions developing at KRFD around midday/early afternoon and across the Chicago terminals during the afternoon.

Winds are expected to be 10 kts or less through the period, initially northwest tonight, then gradually backing westerly Monday morning and flipping southwest by midday.

Ratzer

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

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