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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- Northerly Island To Calumet Harbor Il- 900 Pm Cst Thu Dec 7 2023
.small craft advisory in effect from 3 am cst Friday through Friday afternoon...
Rest of tonight..South winds 15 to 20 kt increasing to 20 to 25 kt. Partly cloudy. Waves 1 to 3 ft building to 2 to 4 ft.
Friday..Southwest winds 20 to 25 kt decreasing to 15 to 20 kt in the afternoon. Mostly cloudy. Waves 2 to 4 ft subsiding to 1 to 3 ft.
Friday night..South winds 10 to 15 kt increasing to 15 to 25 kt. Scattered showers and areas of drizzle in the evening, then showers overnight. Waves 1 to 3 ft.
Saturday..Southwest winds to 30 kt becoming west 15 to 25 kt. Scattered showers in the morning. Waves 2 to 4 ft.

LMZ700
No data

FXUS63 KLOT 080510 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1110 PM CST Thu Dec 7 2023

UPDATE
Issued at 854 PM CST Thu Dec 7 2023

No significant changes made to going forecast this evening, other than to raise temps and dew points a smidge per obs trends.

Evening UA and GOES vapor data depict a fairly sharp mid-level short wave trough over the northern and central Plains. Surface reflection of this feature was evident as a 990 mb low over central North Dakota, while a southwesterly low level jet (50 kt at 850 mb in SGF 00Z RAOB) was noted from eastern TX into the mid- MO valley. This low level jet is expected to continue to develop northeast across the region tonight, as the short wave continues to wrap up into the upper Midwest.

Locally, surface winds from the south around 10 mph this evening will increase over the next few hours as the low level jet maxima spreads overhead, with some sporadic gusts 20-25 mph possible overnight. Temperatures which had settled into the mid-40s in most spots after sunset should remain fairly steady through the remainder of the night given the increasing south-southwest low level flow and associated warm advection. Starting to see some low-level stratus development into far southwest MO on the nose of the stronger low- level moisture advection, and would expect this area of cloud cover to expand quickly northeastward overnight, making for increasing cloud cover into (especially western parts)
the WFO LOT forecast area by morning. Breezy, mild (mid-upper 50s)
and mostly cloudy conditions are then anticipated during the day Friday.

Going forecast has all of the above handled reasonably well as of mid- evening, so no significant adjustments have been made. Did trend surface dew points and temps up slightly based on obs trends through early evening, but generally forecast remains on track.

Ratzer

SHORT TERM
Issued at 255 PM CST Thu Dec 7 2023

Through Saturday Night...

Key Messages:

* Unseasonably mild and breezy (gusts from southwest to 25-30 mph)
on Friday

* Scattered showers and areas of drizzle developing by mid-late Friday evening (30-60% chance of measurable precip) and then more widespread showers (70-100% chance) overnight/early Sat.

* Showers ending and breezy on Saturday morning with a short period of strong winds (gusts up to 35-40 mph from south to southwest), then drying out, breezy, and turning colder Sat. PM

Another fairly stout nocturnal low-level jet will bring occasional 20-25 mph gusts out of the south tonight, but more notably prop temperatures up above normal highs for the date (forecast lows in low-mid 40s). Expect increasing cloud cover after a mostly clear start to the evening.

There's uncertainty regarding the timing of stratus development on Friday. As the gradually weakening low-level jet draws in increasing moisture beneath the inversion, should see at least pockets of stratus form over parts of the area as early as the mid to late morning. With signs pointing to at least some filtered sun, breezy southwest boundary layer flow, and the unseasonably mild start to the day (and air mass in place), continue to anticipate highs in the mid to upper 50s, with a few 60F readings feasible, especially if it's less cloudy than expected through early afternoon.

Large scale ascent will increase on Friday evening in advance of our strong short-wave and its associated surface low. This ascent through a deepening moist layer should encourage the development of generally lighter showers with time in the evening, especially 9pm and onward, along with areas of drizzle, as the DGZ will remain dry during this time. The most recent model guidance remained in very good agreement into Saturday that the ejecting short-wave will consolidate rain focus near the surface low and its trailing cold front. The late Friday night-pre-dawn Saturday timeframe is when we have our highest PoPs (~70-100% as noted in key messages) for a 3-6 hour window of showers capable of producing occasional downpours, particularly for the northwest half or so of the CWA. Instability still looked to meager to keep thunderstorms officially out of the forecast, with NBM thunder chances of about 10% reasonable at this juncture.

Temperatures Friday night through early Saturday out ahead of the front will remain well into the 40s to lower 50s, 5-10F or more above normal *highs* for the date. Another item to note is the potential for fairly strong southerly winds out ahead of the cold front despite the unfavorable timing, especially southeast half of the CWA or so, with gusts up to 35 mph forecast and potential for up to 40 mph per some of the forecast soundings.

Lingering isolated to scattered showers behind the front Saturday morning should come to an end by late morning at the latest, with a dry and breezy afternoon expected. Strong pressure rises immediately in the wake of the front may enable a short period of west-southwesterly gusts up to 35 mph (upside potential to 40 mph), again primarily over the southeast half or so of the CWA.
Cold advection and continued pressure rises will keep breezy westerly winds going into the evening as temperatures steadily fall.

Lows Saturday night will be about 20 degrees colder than Friday night's mild readings. Cold advection will persist through the night and with cyclonic flow aloft, some flurries or light snow showers can't be ruled out across parts of far northern Illinois.
However, given uncertainty regarding sufficient moisture for any snow to be wrung out, maintained a dry forecast for now with some silent PoPs around 10%.

Castro

LONG TERM
Issued at 319 PM CST Thu Dec 7 2023

Sunday through Thursday...

* Chance of lake effect snow showers (~30%) for parts of primarily Porter County on Sunday

As flow turns northwesterly behind a reinforcing surge of cold air advection late Saturday night into Sunday morning, the global guidance has indicated a marginally favorable window for lake effect snow showers. Conservatively adjusted PoPs up into the chance range around 30% given the fairly marginal parameters and northwesterly winds being more supportive for areas just east of Porter County. If snow showers do occur, above freezing temperatures should preclude any accumulations. Highs on Sunday will only be in the mid to locally upper 30s with northwest winds gusting up to 20-25 mph adding to the chilly feel vs. the mild stretch we'll have come out of. Clearing skies Sunday night as a high pressure ridge shifts over the area could support some upper teens lows in favored cold spots, with low-mid 20s for the rest of the area.

Next work week will be dominated by a series of high pressure systems at the surface and split flow aloft likely both acting to keep us atypically quiet for this time of year. Temperatures will be near normal through Tuesday, though look to trend milder by Wednesday and Thursday, possibly warmer than in the official gridded forecast (40s).

Castro

CLIMATE

Issued at 248 AM CST Thu Dec 7 2023

Unseasonably warm temperatures are expected 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 06Z TAFs...

Key Aviation Concerns:

* There is a risk of LLWS through daybreak from a low level jet between 45 to 50 knots

* Chances for rain (30%) and lower cigs move into the area before 06Z Friday night. Rain chances increase significantly after 06Z through 12Z

Strong low level southwesterly winds between 45 to 50 knots around 1500 feet AGL are expected to continue through daybreak. Surface winds have been gusty upstream in Central Illinois and reached Chicago terminals, prompting their addition to the prevailing TAF line overnight. The LLWS threat is expected to diminish around 12Z before surface gusts increase up to 25 knots through early Friday afternoon. Winds will gradually diminish into Friday evening but remain out of the southwest through the TAF period.

Cloud cover will increase on Friday ahead of the next weather system. Confidence has not changed in adding lower clouds for Chicago terminals for Friday morning, only maintained a TEMPO for MVFR cigs at KRFD. As clouds increase Friday evening, MVFR cigs become more expected as the next weather system approaches Friday night. A PROB30 group for rain was maintained due to lower confidence on the exact start time. However, prevailing rain and lower cigs were added after 06Z. There is a chance for rain to end before 12Z, but exact timing for rain stoppage will be re-evaluated at later TAF packages.

DK

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

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