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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- Northerly Island To Calumet Harbor Il- Calumet Harbor Il To Gary In- 324 Pm Cst Fri Dec 8 2023
.small craft advisory in effect from midnight cst tonight through Saturday evening...
Tonight..Southwest winds 15 to 25 kt early becoming south 10 to 15 kt after Sunset and then increasing to 30 kt overnight. A few 35 kt gale gusts possible late. Periods of showers in the late evening, then showers and slight chance of Thunderstorms after midnight. Waves 1 to 3 ft building to 3 to 5 ft.
Saturday..Southwest winds to 30 kt becoming west 20 to 25 kt. A few 35 kt gale possible early. Showers likely in the morning. Waves 3 to 5 ft subsiding to 1 to 3 ft.
Saturday night..West winds 20 to 25 kt becoming northwest 15 to 25 kt. Mostly cloudy. Waves 2 to 4 ft.
Sunday..Northwest winds 15 to 25 kt diminishing to 10 to 20 kt late. Mostly cloudy. Waves 2 to 4 ft.

LMZ700
No data

FXUS63 KLOT 082107 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 307 PM CST Fri Dec 8 2023

SHORT TERM
Issued at 300 PM CST Fri Dec 8 2023

Through Saturday Night...

Key Messages:

* Showers (possibly with some drizzle) increasing later this evening with more widespread showers (peak 80-90+% coverage)
overnight into early Saturday

* A few showers may be capable of producing briefly stronger winds late tonight along with a slight (15%) chance for a few t-storms

* Southerly wind gusts up to 30-35 mph and sporadically up to 40 mph in the pre-dawn hours ahead of a strong cold front, shifting to west-southwest gusting up to 30-35 mph Saturday morning

There are no big changes in the overall forecast thinking with the fast moving short-wave, surface low, and its trailing strong cold front late tonight into Saturday. The 1003 mb surface low currently over the southern Plains will track across central and northern Illinois overnight, reaching central Lake Michigan by around daybreak, and the UP of Michigan at ~995 mb by mid day Saturday.

Low cloud cover, which has been generally slow to increase today, will more rapidly increase this evening as the low level jet intensifies and strengthens poleward moisture advection.
Corresponding increase in isentropic and large scale ascent should also start to wring out showers and even some drizzle out of the lowering and thickening stratus later this evening. The most dynamic portion of the system will be immediately ahead of the cold front in the overnight/pre- dawn hours when sufficient MUCAPE may be realized for a couple thunderstorms (15% chance), and more robust and potentially gusty showers. Our PoPs peak in the categorical range, though hourly details will likely need some refining this evening as trends emerge. Showers will quickly come to an end after sunrise Saturday, clearing the CWA by 9am-11am (at the latest).

The main change in the forecast (along with addition of slight chance thunder overnight) was to bump up the southerly winds ahead of the front. 6-hour pressure falls up to 5-6 mb, the strong momentum just off the surface, and mild temps in the lower and even mid 50s overnight should be able to tap into some of the strong flow at the base of the low-level jet. With this in mind, forecast gusts were increased to near/slightly over 35 mph over the southeast half or so of the CWA from a south to south- southwest direction. During the strongest synoptic winds (~2am-8am), sporadic gusts to around 40 mph may occur.

As the front sweeps across the area early to mid Saturday morning, right on the front, there could also be a "pop" of westerly gusts up to 35-40 mph due to stout pressure rises and cold air advection. Expect westerly gusts up to 25-30 mph the rest of the day and remaining breezy through Saturday night. There will likely be fairly extensive strato-Cu through early Sunday after any brief clearing behind the front Saturday morning amidst persistent CAA. If enough moisture is available, could see a few sprinkles Saturday afternoon and a few flurries Saturday evening, but will leave this to be assessed for subsequent updates.
Temperatures will drop into the 40s behind the front and then flat-line, if not rise slightly diurnally, before steadily dropping to the upper 20s-lower 30s by early Sunday. Pre-dawn Sunday wind chills will be in the upper teens to lower 20s, a reminder that it is indeed December.

Castro

LONG TERM
Issued at 300 PM CST Fri Dec 8 2023

Sunday through Friday...

A trailing northern stream upper trough will send a secondary cold front across the lower Great Lakes late Saturday night. With a slightly colder air mass and a low level flow direction out of the west-northwest (290-310 deg), some lake effect precipitation may be in the offing. A perusal of forecast soundings indicate that moisture depths and equilibrium levels are not all that impressive. Boundary layer temperatures are also marginal for accumulations as well, and the bulk of the forcing is below the main snow production region. At this time areas east of Porter County would be favored. NBM probabilities of accumulation are limited to 15-20% in eastern Porter County, and with surface temperatures above freezing, impacts would be likely mitigated.

More seasonal conditions will return for early next week behind this front, and this will be reinforced one final cold frontal passage on Tuesday. This front will pass through dry thanks to sprawling high pressure that will set up from the central Plains, across the Ohio Valley and to the eastern seaboard. This feature will remain anchored in place for most of the upcoming week.
Southwest flow aloft will set up ahead of developing low pressure across the southwest. The result for the Midwest will be dry and mild conditions through the latter portion of the week.

KMD/Castro

CLIMATE

Issued at 300 PM CST Fri Dec 8 2023

The record warmest low temperature for the calendar day, December 8th in Rockford is 37 degrees in 1987, which appears likely to be set officially at midnight tonight.

AVIATION
For the 18Z TAFs...

Key Aviation Concerns: - Increasing coverage of MVFR ceilings by early evening lowering into IFR levels late tonight.
- RA/DZ transitioning to SHRA tonight.
- Increasing south winds overnight, with a period of 30 knots gusts (possibly higher) 09-13Z Saturday.

A strengthening low pressure system over Oklahoma this afternoon will track to just northwest of the Chicago metro late tonight.
Increasing low-level moisture and weak warm-air advection this afternoon will begin to promote the development of MVFR ceilings before ceilings gradually lower into low-end MVFR levels with- RA/DZ late this evening into the early overnight hours. A band of -SHRA with IFR ceilings is then expected to cross the area late tonight ahead of a cold front. -SHRA will diminish through the morning Saturday while MVFR ceilings likely persist through the day.

SSW/SW winds gusting over 25 knots early this afternoon will diminish into early evening, settling S/SSW around 10 knots by 00Z. The northwest fringe of a developing 50 knot LLJ will bring increasing S winds late this evening and especially overnight.
Gusts to 25 knots will back just east of south (170 degrees) late this evening before shifting back west of south (190 degrees) and increasing with gusts to around 30 knots in the 09-13Z window.
While some low-level stability at a diurnally unfavorable period for mixing will likely limit the upper potential for gusts, some sporadic gusts to 35 knots will be possible. Winds will then veer WSW/W behind the front (around 12-13Z for ORD/MDW) with gusts to around 25 knots through the day Saturday.

Kluber

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

Small Craft Advisory
Winthrop Harbor IL to Gary IN
midnight Saturday to 10 PM Saturday.

Small Craft Advisory
Gary to Michigan City IN
midnight Saturday to 4 AM Monday.

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