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LMZ743 Calumet Harbor Il To Gary In-gary To Burns Harbor In- Burns Harbor To Michigan City In- 316 Am Cst Sun Dec 3 2023
Today..Southwest winds 10 to 15 kt becoming west and increasing to 15 to 25 kt. Rain in the morning. Waves around 1 ft building to 2 to 4 ft.
Tonight..West winds 10 to 20 kt becoming northwest 10 to 15 kt overnight. Cloudy. Waves 2 to 4 ft subsiding to 1 to 3 ft.
Monday..North winds 10 to 15 kt becoming west in the afternoon. Mostly cloudy. Waves 1 to 3 ft.
Monday night..Southwest winds around 10 kt becoming south overnight. Chance of rain in the evening, then rain and snow likely overnight. Waves 1 to 3 ft.

LMZ700
No data

FXUS63 KLOT 030914 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 314 AM CST Sun Dec 3 2023

SHORT TERM
Issued at 314 AM CST Sun Dec 3 2023

Through Monday...

Key Messages:

* Lingering rain showers, with a rain/snow mix north of I-88, will continue through the morning.

* Some slushy snow accumulations upwards of 0.5 inches are possible on grassy and elevated surfaces north of a Dixon to McHenry line through daybreak.

* Persistent cloud cover expected through Monday with seasonable temperatures

A broad upper trough continues to churn overhead this morning which is supporting the broad area of precipitation lifting through northeast Illinois and northwest Indiana. While most of this precipitation continues to fall in the form of rain, sites north of I-88 are starting to see a rain/snow mix. So far snow accumulation across northwest Illinois has been sparse due to the gradual decay of synoptic forcing and surface temperatures remaining above freezing. However, a couple of robust snow showers have developed near the Mississippi River along the mid-level lapse rate plume and are expected to track into the northwest portion of our forecast area over the next few hours. Given that sites under these showers have seen a quick drop in temperatures, I suspect that some slushy snow accumulations on grassy and elevated surfaces is still in play. Therefore, I have maintained a couple tenths to half an inch of accumulation for areas along and north of a Dixon to McHenry line for this potential.

The precipitation is expected to gradually diminish in coverage through the morning as the trough pivots into the eastern Great Lakes leaving us with dry but cloudy conditions for our Sunday afternoon. As seen the past couple days, the clouds will limit any major improvement in temperatures this afternoon with highs expected to remain in the upper 30s and lower 40s.

While these dry conditions are expected to persist through the day on Monday as some shortwave ridging builds overhead, another potent shortwave disturbance is forecast to dive out of the central plains this evening. Guidance is in good agreement that this system should track into central Illinois tonight but there are some indications that some modest instability and lingering mid-level moisture may allow a couple of rain/snow showers to clip the far south and southwestern portions of our forecast area.
Given that this window should be limited to just a few hours tonight, I have decided to maintain a slight chance (around 20%)
POP south of the Kankakee River Valley for this potential.

Regardless, it does look like the low-level cloud cover will persist through Monday though there could be some breaks in the clouds Monday afternoon. Depending on how much of a break develops temperatures could possibly verify a few degrees warmer than the lower 40s currently forecast.

Yack

LONG TERM
Issued at 314 AM CST Sun Dec 3 2023

Monday night through Saturday...

Key Messages:

* A period of wintry precipitation, potentially all snow north of I-80, arrives Monday night. Some slushy accumulations looking more plausible.

* Above average temperatures expected to close out the week.

A compact yet robust shortwave will track across the region Monday night, yielding the next appreciable precipitation chances for the area. While the large run-to-run southward shifts in system track have eased, the amplified and unstable nature of the upstream flow across the Gulf of Alaska suggests continued track wobbles remain on the table. That said, deterministic and ensemble guidance is coming into decent agreement now, supporting high-end likely PoPs (70 percent) across pretty much all of the forecast area. Only reason we didn't boost all the way to categorical at this point is to leave some wiggle room for additional track shifts.

With it appearing more likely that stratus remains in place on Monday, temperatures are expected to start the evening out in the mid, perhaps upper, 30s. Broad warm advection will develop ahead of the incoming shortwave and attendant surface low/reflection, meaning little in the way of temperature movement, at least initially, but given the relatively stout nature of large scale ascent within the warm advection wing, the combination of a little dynamic cooling plus sublimation/evaporation now looks like it may be sufficient to send thermal profiles cold enough to support more in the way of wintry precip.

With this forecast package, have re-worked the NBM p-types grids to depict a more aggressive transition to all snow, particularly for locales north of about I-80. In fact, precipitation may even onset as all snow, contingent on how surface temperatures respond during the evening, and the currently-advertised transition may not yet be swift enough. As it typical with these N/S oriented warm advection wing-driven events, the window for deep saturation will be somewhat brief, likely less than 4-5 hours in most spots.
During this time, however, strong ascent residing near or just under the heart of the -12 to -18 C DGZ, coupled with diminishing static stability as the system dryslot approaches, should yield a brief window feature of a wet snow (ratios probably averaging out 7-9:1) Monday night. Have gently increased snow accumulations a bit all the way down to the Kankakee River with this forecast package. While marginal surface temperatures look like they'll curtail most main road accumulations, think there may be a window right ahead of the dryslot where rates could very briefly approach an inch per hour which might result in a quick coating on bridges/overpasses.

Will likely lose most cloud ice through Tuesday morning, turning lingering precipitation over to a mixture of snow and drizzle, or even all drizzle with temperatures meandering back into the mid and upper 30s. Looks like stratus will probably remain locked in through the day, yielding a muted diurnal temperature range.

Marginal lake effect showers (mostly rain) might focus briefly into NW Indiana into Tuesday night before winds veer to the WNW and kick any lingering moisture out of the area.

Transition to a more zonal flow or even low amplitude ridging is still expected late next week with high temps well into the 40s to lower 50s. As always this time of year, temp forecast will hinge on cloud cover, if warm sector doesn't become covered in stratus then highs could be at or even above forecast levels. Conversely, if there is a more rapid development/expansion of stratus then temps could end up a bit cooler than forecast. Regardless of exact temperatures, there is high confidence in trend to above average temperatures. The next meaningful precip chances arrive into next weekend.

Carlaw/Izzi

AVIATION
For the 06Z TAFs...

Aviation Concerns:

* Rain moving into the region late this evening through Sunday AM with associated vis and cig drops * Transition to snow likely (>70% chance) at RFD after midnight * Rain snow mix possible (50% chance) at ORD and DPA

PRECIP: Rain is overspreading the region late this evening reaching the Chicago area terminals near to shortly after 6Z. This will begin as rain areawide, though a transition to all snow is becoming increasingly likely (>70% chance) at RFD 2-3 hours after onset. It is also possible that there are periods of snow mixed in with rain at DPA and ORD. Continue to maintain all rain at MDW and GYY though a few wet snowflakes may manage to reach the surface at times. The character of the precipitation may turn more showery after daybreak and then transition to drizzle/flurries as the system exits to the east.

CIGS: For now have made no major changes to forecast ceilings which continue to favor low-end IFR to LIFR overnight, lowest in areas of all snow (RFD). Cigs begin to improve by mid morning and into the afternoon, returning to MVFR.

VIS: Expect IFR visibility in rain overnight into Sunday morning. Where snow becomes the predominant precipitation type LIFR vis (and potentially LIFR) is more likely (RFD).

WIND: Light and variable winds are expected overnight into the morning hours as the system moves through. In its wake winds prevail westerly late morning into the afternoon around 10 kts then easing Sunday night and turning northwesterly.

Petr

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

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