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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- Northerly Island To Calumet Harbor Il- 958 Am Cst Thu Dec 7 2023
Rest of today..Southwest winds 10 to 20 kt becoming south 10 to 15 kt. Mostly Sunny. Waves 1 to 3 ft.
Tonight..South winds 10 to 20 kt increasing to 15 to 25 kt. Partly cloudy. Waves 1 to 3 ft building to 2 to 4 ft.
Friday..Southwest winds 20 to 25 kt diminishing to 10 to 20 kt late. Mostly cloudy. Waves 2 to 4 ft subsiding to 1 to 3 ft. A small craft advisory will likely be needed.
Friday night..South winds 10 to 20 kt increasing to 20 to 25 kt late. Slight chance of rain in the evening, then rain overnight. Waves 1 to 2 ft building to 2 to 4 ft.

LMZ700
No data

FXUS63 KLOT 071701 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1101 AM CST Thu Dec 7 2023

SHORT TERM
Issued at 248 AM CST Thu Dec 7 2023

Through Friday...

Key messages:

* Warmer conditions today and Friday.

While a robust subsidence inversion centered around 850 will suppress mixing today, warming southwesterlies will allow temperatures to jump into the lower 50s under mostly sunny skies (for once) with just some periodic cirrus.

Another stout low-level jet will develop tonight and will start to pump slightly more appreciable moisture our way, with surface dewpoints crawling upwards into the 40s. A large spread remains in guidance regarding the magnitude and depth of saturation associated with this LLJ (and associated mid-level shortwave). In typical fashion, NAM/NAMNest output saturate things up solidly through 850 mb as the aforementioned vort lobe rotates through tonight, sufficient to potentially even squeeze out some drizzle.
The rest of the guidance suite remains more subdued in this regard, and have maintained this (dry) thinking in the gridded forecast.

The degree of stratus development will have some impact on Friday's high temperatures, and with model guidance depicting a bit more in the way of low-level moisture on Friday, continued to favor the blended offering for high temperatures on Friday generally in the mid 50s as opposed to going warmer. Some 60 degree readings are certainly on the table, particularly in our south and west, but model trends are enough to support maintaining a more middle-of-the-road forecast.

Carlaw

LONG TERM
Issued at 248 AM CST Thu Dec 7 2023

Friday night through Wednesday...

Key Messages:

* Storm system brings widespread rain to the area Friday night into Saturday with a low chance (around 10%) for a thunderstorm.

* Slight chance (10% to 15%) of lake effect showers for far eastern potions of northwest Indiana counties, mainly Porter.

* Seasonable temperatures through next week.

The main focus for the extended period continues to be the storm system that is forecast to move through the area Friday night into Saturday. While overall thinking remains largely unchanged in terms of local expectations there are a few notable changes in the 00z guidance that are worth discussing.

The first of these changes is in regards to the evolution of the leading shortwave trough which is currently traversing the Sierra Nevada. This wave is expected to lift northward into the Upper Midwest this afternoon and now looks to progressively slow as it does so. At the same time, a second wave is forecast to move onshore near the northern California coast tonight and eject into the Four Corners region on Friday. The second wave should then begin to deepen as it continues to progress eastward into the central CONUS and phase with the aforementioned leading wave Friday night into Saturday. As this occurs, a surface frontal boundary is expected to develop near the Mississippi Valley on Friday and get shoved eastward into our area Friday night as the trough catches up to the boundary. This will lead to the development of a surface low over northern Missouri which guidance now lifts northeastward directly overhead Saturday morning before moving into lower Michigan Saturday afternoon.

A broad area of precipitation will also take shape near the aforementioned low and lift into northeast Illinois and northwest Indiana Friday night. Precipitation is expected to continue through at least Saturday morning as the low passes overhead and then taper from northwest to southeast as the front gets dragged through the area. Given that guidance has come into very good agreement with this evolution over the past several runs and that temperatures are expected to remain in the 40s and lower 50s through the duration, confidence is high that rain will be the sole precipitation type for this event. As previously stated, rainfall is expected to be widespread, but amounts do not look to be overly impressive with most areas forecast to pick up two to three tenths of an inch. But locally higher totals could be realized if thunderstorms are able to materialize.

Speaking of thunderstorms, the thunder threat continues to look meager compared to what it was 24-hours ago, but there are some guidance members that produce a couple hundred joules of CAPE in the southern third to half of our forecast area. The latest NBM forecast has once again offered up thunder probabilities around 10% which seem reasonable based on latest trends so have left thunder out of the forecast for now. Though I still cannot rule out a stray storm or two developing Saturday morning, particularly south of I-80 where the better moisture and instability look to overlap. While the kinematic fields are expected to be increasing Saturday morning, the aforementioned marginal instability and unfavorable diurnal time look to keep the threat for any strong to severe storms low and south of our area at least for now.

Once the front clears the area Saturday afternoon, most areas are expected to dry out and remain as such through the remainder of the weekend. However, the upper trough is expected to still be over the Great Lakes which guidance suggests will allow a couple of disturbances to pass through the area and lead to the development of lake effect rain and snow showers on Sunday. Given that current wind trajectories look to be out of the west- northwest I suspect that the bulk, if not all, lake effect showers should be confined to northern Indiana and Lower Michigan. Though I cannot completely rule out a few of these showers brushing the far eastern portions of Porter county, thus have left the 10 to 20 percent POPs in the forecast for this potential.

Beyond the weekend, guidance is in good agreement that upper- level northwest flow will persist through the majority, if not all, of next week. While this pattern usually favors periods of active weather locally, guidance does show a broad high developing over the southern CONUS which may limit moisture return into our area and keep precipitation chances low if not zero through next week if not longer. Though we will need to monitor trends going forward before we can say for certain. Regardless, seasonable temperatures for the middle of December are expected for Sunday into next week.

Yack

CLIMATE

Issued at 248 AM CST Thu Dec 7 2023

Unseasonably warm temperatures are expected Thursday through early Saturday. While record temperatures appear safe, they are included here for Rockford and Chicago to provide added context.

Rockford...
Friday, December 8th.....60 in 1980

Chicago...
Friday, December 8th.....64 in 1946

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...

With persistent troughing across the central Untied States, southwest winds will prevail across the terminal airspace. A period of southwesterly LLWS remains expected overnight as a 45-50kt LLJ develops at around 2kft, peaking between 04-10Z.
Otherwise, there are no other aviation concerns.

Gradually increasing low- to mid-level moisture may support occasional waves of VFR clouds particularly overnight into tomorrow. There is a growing signal that MVFR stratus will develop toward the end of the 30-hour TAF window for ORD/MDW. For now, opted to leave all mention out in favor of a more complete treatment with the 00Z TAF package.

Borchardt

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

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