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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-gary To Burns Harbor In- Burns Harbor To Michigan City In- 231 Pm Cst Tue Nov 28 2023
.small craft advisory in effect from 6 pm cst this evening through Wednesday morning...
Tonight..Southwest winds 15 to 20 kt increasing to 25 to 30 kt. Partly cloudy. Waves 2 to 4 ft.
Wednesday..Southwest winds 20 to 25 kt becoming west 15 to 20 kt. Partly cloudy in the morning then clearing. Waves 2 to 4 ft.
Wednesday night..Southwest winds 15 to 20 kt. Partly cloudy in the evening, then becoming mostly cloudy. Waves 2 to 4 ft subsiding to 1 to 2 ft.
Thursday..Southwest winds 15 to 25 kt. Slight chance of rain in the afternoon. Waves 1 to 3 ft.

LMZ700
No data

FXUS63 KLOT 282328 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 528 PM CST Tue Nov 28 2023

SHORT TERM
Issued at 212 PM CST Tue Nov 28 2023

Through Wednesday night...

Key Messages:

- Below normal temperatures will continue in the near term, but moderate from the coldest air of the season so far that occurred at the beginning of this week.

DISCUSSION: This afternoon, high pressure is in place along the lower and mid Mississippi Valley with ridging extending into portions of the Upper Midwest. This is providing fair weather conditions across northern Illinois and northwest Indiana, albeit well below normal with northwest flow aloft driving cold temperatures into the region. Surface ridge will retreat south this evening and overnight while a shortwave trough digs into the Great Lakes. Surface winds will back to the southwest in response to this feature allowing temps to begin to moderate.
Meanwhile, dry low levels will prevail with only an increase in mid and high level clouds expected with this wave. Any precip with this wave is expected to stay well north/northeast of the forecast area.

Deubelbeiss

LONG TERM
Issued at 212 PM CST Tue Nov 28 2023

Thursday through Tuesday...

Key Messages:

- Rain is likely (60-90 percent confidence) later Thursday through Friday morning south of I-80 with a sharp cutoff in precip somewhere near or north of I-80.

- There will be periodic chances for light rain Saturday through Tuesday, though timing details still need to be refined.

DISCUSSION: Attention turns to our southwest Thursday as a compact vort max lifts quickly from the Desert Southwest Thursday morning to the mid Mississippi Valley Thursday night.
Most of the day Thursday will be dry well ahead of this feature, and temperatures will warm into the lower to mid 40s. A deep layer of isentropic ascent along the 280- 295K surfaces will overspread northern Illinois and northwest Indiana Thursday evening and overnight resulting in rain across much of the area.
800-900mb layer max wetbulb temps from the GFS indicates most of the area have wetbulbs at or above freezing in that layer while surface temps are also expected to be above freezing for all but the far northwest corner of the forecast area near Rockford. The 0C layer max wetbulb isotherm is right along the northern edge of the precip shield yielding rain for the entirety of the event. NBM only indicates a small chance (around 10 percent or less) for any snow (>0.1 inches) accumulation with this event with those probs highest along the northern edge of the precip shield north of I-80 and west of the Chicago Metro area. QPF amounts, meanwhile, look to be decent for this event. NBM 50th percentile indicates that between 0.25 and 0.75 inches of rain is possible south of I-80 with a very sharp gradient/cutoff north of I-80. NBM 75th percentile shows areas south of I-80 receiving closer to an inch of rain and shifts the sharp gradient/cutoff line north to a Dixon to Waukegan line.
Forecast soundings indicate that steep lapse rates will be in place aloft which contributes to very weak but non-zero MUCAPE rooted between 700-800 mb which could allow an isolated storm or two to occur, or, if nothing else, a few embedded heavier convective showers supporting at least swaths of 1 inch of rain south of I-80.

Main upper wave will exit the area by midday Friday with the heaviest rain tapering off. A second sheared vort max will be right on its heels later Friday into Saturday morning. Given the positive tilt and weak DCVA, precip amounts should remain light, less than a tenth of an inch, with this secondary wave Friday night into Saturday morning. Thermal profiles continue to support rain.

Forecast details begin to become more nebulous over the weekend and into early next week as a series of fast moving, low amplitude disturbances move across the region. Timing differences with these waves result in the blended forecast guidance coming out with broad-brushed chance/slight chance PoPs Saturday through Tuesday. Primarily rain is expected with these waves but timing details will still need to be ironed out.

Deubelbeiss

AVIATION
For the 00Z TAFs...

No major forecast concerns for the terminals through this forecast period.

Predominantly VFR conditions are expected through the period as SCT cirrus pass overhead at times. However, there is a signal in guidance that some 2500 to 3000 ft clouds could develop Wednesday evening. Given that this is lower confidence have decided to add a FEW025 at the 30-hour TAFs for this potential.
Otherwise, breezy west-southwesterly winds are expected through Wednesday afternoon where occasional gusts of 15 to 20 kts will be possible. Though there could be an brief (only around 2 to 3 hours max) period of LLWS after midnight tonight as a low-level jet develops, but given the fact that the strongest winds should remain above 2000 ft and the short duration have decided to forego a formal TAF mention.

Yack

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM
Small Craft Advisory
IL and IN nearshore waters until noon Wednesday.

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