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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- Northerly Island To Calumet Harbor Il- 911 Am Cst Wed Dec 6 2023
.small craft advisory in effect from 6 pm cst this evening through late tonight...
Rest of today..Northwest winds 10 to 15 kt becoming southwest 15 to 20 kt in the afternoon. Mostly cloudy. Waves 1 to 3 ft.
Tonight..Southwest winds to 30 kt diminishing to 15 to 25 kt late. Mostly cloudy in the evening, then becoming partly cloudy. Waves 2 to 4 ft.
Thursday..Southwest winds 10 to 20 kt. Mostly Sunny. Waves 1 to 2 ft.
Thursday night..South winds 10 to 20 kt increasing to 20 to 25 kt. Partly cloudy in the evening then clearing. Waves 1 to 3 ft building to 2 to 4 ft.

LMZ700
No data

FXUS63 KLOT 061719 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1119 AM CST Wed Dec 6 2023

SHORT TERM
Issued at 257 AM CST Wed Dec 6 2023

Through Thursday...

Key Messages:

* Precip-free conditions with warmer temperatures on Thursday.

The main forecast issue for today centers around cloud trends which remain a bit unclear. Expansive low stratus is sloshing back eastward across the region, while a relatively defined back edge is apparent in nighttime microphysics RGB loops approaching the Mississippi River. While this clearing line should continue to march eastward, persistent modest warm advection/upglide on the back side of the stratus shield may yield stratus re-development through the day. Copious high-deck will also stream overhead in advance of an incoming disturbance embedded within the northwest flow aloft. Have continued to side with the cloudier guidance as a result, although suspect some holes in the stratus layer will evolve with time, particularly across our west. High temperatures should be similar to yesterday.

A robust LLJ will briefly drift across the region tonight, likely yielding a period of southwesterly breezes with gusts to 20-25 mph. These will ease towards daybreak Thursday. With an increase in moisture/mixing ratios just off the deck, some signal exists for some light mist (BR) developing Thursday morning. Mechanical mixing associated with lingering 25 kt flow towards 925 mb will curtail any fog threat, however.

Through Thursday afternoon, 850 mb temperatures will jump towards +10C although this will remain solidly out of reach with a formidable subsidence inversion in place. 925 mb temperature climatology does, however, support highs rising towards the 50s.

Carlaw

LONG TERM
Issued at 257 AM CST Wed Dec 6 2023

Thursday night through Tuesday...

Key Messages:

* Unseasonably warm temperatures expected for Friday and possibly again on Saturday.

* Storm system forecast to move into the Great Lakes this weekend and bring more precipitation to the area.

* Seasonable temperatures return next week.

The upper-level ridge will be in the process of exiting the area Thursday night into Friday as a deepening trough begins to eject into the central CONUS. This will put our area solidly under the influence of stout warm advection that will support at least one more day of unseasonably warm temperatures for Friday where high temperatures are forecast to be in the mid to upper 50s and possibly near 60 in spots. While the current forecast remains under the daily record highs for Chicago and Rockford of 64 and 60 respectively, if the combination of warm advection and diurnal heating allow temperatures to over preform then we could get close especially at Rockford. However, cloud cover is expected to gradually increase through the day on Friday as the trough inches closer which could limit diurnal heating and keep temperatures a tad cooler if clouds arrive quicker than forecast. Given that winds are expected to be breezy on Friday (gusts of 20 to 25 mph)
from the southwest and the anticipation for the thicker clouds to hold until Friday evening, I have decided to once again trend the temperature forecast towards 75th percentile of guidance.

Beyond Friday, our attention turns to the aforementioned trough that is forecast to continue to progress eastward through the Great Lakes over the weekend. As the trough enters the area, it will force a frontal boundary through the area sometime on Saturday which looks to serve as the breading ground for an area of low pressure to develop along with its associated area of precipitation. The 00z guidance suite continues to be in overall good agreement on the evolution of this system through the weekend, but varies greatly on the speed of the surface frontal boundary and therefore the exact track of the developing low center which will largely dictate precipitation duration and type.
While the general consensus is for most of the precipitation to fall in the form of rain as temperatures should still be within the 40s and 50s ahead of the front on Saturday, there is the potential that a transition to a rain/snow mix or even all snow could occur in the wake of the front if temperatures can cool quick enough prior to drier air arriving. Given the uncertainty in the speed and track of the system I have decided to maintain the NBM forecast for rain transitioning to a rain/snow mix at this time, but may be able to refine with future forecast packages if a more consistent solution arises.

Additionally, there is also the chance for some thunderstorms to develop within the warm sector of this system if the system dynamics are able to overlap with favorable diurnal heating and increasing low-level moisture to generate sufficient instability.
Not to mention that if instability is able to materialize it does appear that a strengthening kinematic field would overlap the warm sector and could support more robust updrafts and possibly stronger storms. Obviously there are a lot of conditions that have to come together for any thunderstorms to materialize, but I do feel confident enough in this potential to maintain the near 20% thunder chances offered by the NBM though confidence in storm intensity is low.

Depending on the ultimate speed of the aforementioned system, dry conditions could return to the area as early as Sunday and look to linger into at least early next week. However, the upper-level pattern is forecast to favor northwesterly flow through much of next week and possibly beyond. Thus, periodic chances for precipitation could materialize at times when disturbances and sufficient moisture overlap. Regardless, more seasonable temperatures for early to mid December are expected through next week.

Yack

CLIMATE

Issued at 216 PM CST Tue Dec 5 2023

Unseasonably warm temperatures are expected Thursday through Saturday and here are the record high temps for Rockford and Chicago.

Rockford...
Friday, December 8th.....60 in 1980 Saturday, December 9th...55 in 2020

Chicago...
Friday, December 8th.....64 in 1946 Saturday, December 9th...62 in 1879

The record warmest low temperatures for the calendar day, December 8th are,

Rockford...37 in 1987

Chicago....53 in 1946

AVIATION
For the 18Z TAFs...

Forecast concerns for the terminals...

* MVFR ceilings persist into this afternoon before improving prior to sunset

* Threat for LLWS for about a 6 hour period mid to late evening into the overnight

MVFR ceilings with bases as low as 1800 feet will continue to blanket northeast Illinois and northwest Indiana into this afternoon. We likely are not to see much improvement until after 21z this afternoon. Shortly thereafter, VFR conditions are expected to return for the remainder of the forecast period.

Westerly winds this morning will become southwesterly through the afternoon as a surface high settles in across the Tennessee Valley. Wind speeds should generally remain around 10 kts through the remainder of the day. Speeds are expected to increase this evening as a strong 45 to 50 kt low-level jet develops about 1500-2000 ft off the surface. While some surface gusts of 20 to 25 kts are possible as this low-level jet develops through the evening, the gustiness may be more occasional in nature. For this reason, we have opted to add a formal mention of LLWS to the TAFs for about a 6 hour period from 03-09z tonight for the Chicago area terminals, and a couple hours earlier at KRFD. The threat of Low- level wind shear and gusty surface winds will wind down after 09z, with a light (up to around 10 kt) southwesterly surface wind expected through the day on Thursday.

KJB

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM
Small Craft Advisory
IL and IN nearshore waters...6 PM Wednesday to 3 AM Thursday.

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