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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- 443 Pm Cst Sat Dec 9 2023
.small craft advisory in effect through Sunday morning...
Tonight..West winds 20 to 25 kt. Intermittent flurries. Waves 1 to 3 ft.
Sunday..Northwest winds 20 to 25 kt diminishing to 10 to 15 kt. Mostly cloudy. Waves 1 to 3 ft.
Sunday night..Northwest winds 10 to 15 kt diminishing to 5 to 10 kt. Gradually clearing skies. Waves 1 to 2 ft.
Monday..West winds 10 to 15 kt becoming southwest in the afternoon. Mostly Sunny. Waves 1 to 2 ft.

LMZ700
No data

FXUS63 KLOT 092327 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 527 PM CST Sat Dec 9 2023

SHORT TERM
Issued at 142 PM CST Sat Dec 9 2023

Through Sunday Night...

Key Messages:

* No impactful weather is expected in the short-term period.

* Breezy west winds will continue overnight and ease tomorrow.

* Outside a few flurries overnight and tomorrow and a lake effect snow shower or two tomorrow afternoon east of Portage, IN, no precipitation is expected.

Discussion:

The Great Lakes region remain under the influence of a 998 mb surface low pressure system currently located near the Straits of Mackinaw lifting northeastward into southern Ontario. Strengthening cold air advection and the arrival of a secondary surface pressure rise/fall couplet will maintain breezy west winds with gusts occasionally tagging 25-30 mph this evening and overnight. In addition, low-level stratus will thicken in response to steepening low-level lapse rates and support a few flurries from time to time, particularly near RFD where moisture depth will be greater and closer to the DGZ.

As the surface low pulls away and the surface pressure gradient relaxes tomorrow, winds will gradually lose their gusto. Highs will be some 10 to 15 degrees cooler than today and in the low to mid 30s. The approach of an upper-level shortwave trough tomorrow morning should provide a reinforcement to low-level stratus and development of a few flurries, particularly near Lake Michigan downwind into northwestern Indiana. In fact, a few lake effect snow showers may develop east of Portage. Above-freezing surface temperatures and light snow rates (e.g. less than a tenth of an inch per hour) will prevent any snow from sticking.

The advection of dry air behind the upper-level wave should begin to erode stratus from west to east tomorrow afternoon, but just how fast is always a point of low confidence this time of year.
Regardless, gradually clearing skies and slackening surface winds (along the northern periphery of a surface ridge) should make way for temperatures tomorrow night to fall into the lower 20s, if not colder.

Borchardt

LONG TERM
Issued at 142 PM CST Sat Dec 9 2023

Monday through Saturday...

Key Messages:

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

* Seasonable temperatures will start the workweek followed by a warm-up toward the end of the week.

Discussion:

Ensemble model guidance advertises a series of upper-level waves tracking north of our area early next week before a relative stagnant zone develops over the Great Lakes ahead of aggregate troughing to our southwest to close the workweek. As a result, the long-term period looks remarkably quiet for mid-December. High temperatures Monday through Wednesday should be close to average with highs in the mid to upper 30s. Lows will similarly close to average and in the lower 20s, though values Tuesday night may tank as a 1040mb surface high slides overhead (it wouldn't be shocking to see temperatures in the typical cold spots make a run toward the single digits). Otherwise, a gradual warming trend appears on track to arrive toward the end of next week as a pool of seasonably warm 850mb temperatures sloshes eastward into the Great Lakes. The next opportunity for precipitation appears to be next weekend with ensemble model guidance already favoring a precipitation type of rain.

It's simply too hard *not* to talk about the signal for no impactful snow between now and late December. Of the 100 EPS/GEFS/CMCE members from the 12Z suite, only 30 to 40 show >0.1" of a snow, and fewer than 5 show more than 2", through December 24th at both Chicago O'Hare and Rockford. Unless the weekend system manages to phase much more deeply and outside what is depicted in the ensemble model envelope, there are no signs of an impactful winter storm in the region anytime soon.

Borchardt

AVIATION
For the 00Z TAFs...

* Blustery west winds with gusts 25+ kts this evening. Winds gradually turn northwest late tonight/Sunday, with gusts easing.

* Ceilings lower to MVFR this evening, and MVFR likely persists for a good portion of Sunday before scattering late.

* A few flurries possible at times tonight (just a few flakes in the air, no vis reduction and no accumulation).

Deep surface low pressure (995 mb) was just east of Lake Superior early this evening, with a cold front trailing through the central Great Lakes into the lower Mississippi Valley/east Texas. Cyclonic low level flow lingered across the mid-upper Mississippi Valleys, with blustery west to northwest winds gusting 25-30 kts. Within this region of low-level cold advection, extensive MVFR stratocu was noted across western WI, MN and IA. Snow showers across MN/WI are expected to lift northeast and not affect the forecast area tonight, though spotty flurries (no vis reductions, no accums) within the shallower cloud deck spreading into IL/IN can't be ruled out at times, especially later this evening. Current VFR ceilings across the terminals should lower into the 1500-2000 foot MVFR range later this evening, and are not expected to improve significantly until late Sunday. Confidence in this timing is somewhat low, as some model forecasts scatter MVFR ceilings as early as Sunday afternoon, while others hold onto a thin MVFR deck into Sunday night. Strong subsidence develops Sunday evening as surface high pressure spreads east into the area however, and that will likely help clear out the lower clouds during the evening.

West winds (250-270 deg) gusting 25-30 kts this evening will very gradually turn more west-northwesterly (280-300 deg) early Sunday morning, with speeds/gusts also gradually diminishing.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM
Small Craft Advisory
Winthrop Harbor IL to Gary IN until 9 AM Sunday.

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

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