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LMZ646 Wind Point Wi To Winthrop Harbor Il- 905 Am Cst Fri Dec 1 2023
.small craft advisory in effect through this evening...
Rest of today..Northeast wind 15 to 20 knots. Rain showers. Waves 4 to 6 feet.
Tonight..Northeast wind 15 to 20 knots easing to 10 to 15 knots late in the evening, then becoming northeast 10 to 15 knots after midnight. Rain showers through around midnight, then showers likely after midnight. Waves 4 to 6 feet subsiding to 3 to 4 feet in the late evening and overnight.
Saturday..Northeast wind 10 to 15 knots veering east late in the afternoon. Cloudy. Waves 2 to 4 feet subsiding to 1 to 3 feet late in the afternoon.
Saturday night..East wind 10 to 15 knots easing to 5 to 10 knots late in the evening, then veering southeast after midnight rising to 10 to 15 knots early in the morning. Rain likely after midnight. Waves 1 to 2 feet.
wave heights are for ice free areas.

LMZ600
No data

FXUS63 KLOT 011141 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 541 AM CST Fri Dec 1 2023

SHORT TERM
Issued at 301 AM CST Fri Dec 1 2023

Through Saturday...

Key Messages:

* Storm system will bring widespread rain this morning with periods of light showers and drizzle this afternoon

* Rain/snow mix develops across portions of far north and northwestern Illinois late this afternoon and evening. Small chance (around 20% to 30%) of some slushy accumulations if surface temperatures can become cold enough

* Otherwise cloudy and seasonable temperatures through Saturday

A negatively tilted shortwave trough continues to advance northeastward across southern Missouri this morning with 1003 mb surface low in tow. Ahead of this system a broad area of light to moderate rain is advancing across Missouri and central Illinois with even some periods of drizzle being noted over the southern 1/3 of our forecast area. The expectation is for this area of rain to continue to spread across northeastern Illinois and northwest Indiana through the morning as the shortwave slides across the area. While the majority of any robust rainfall is expected to occur between daybreak and noon, periods of lighter showers and drizzle will persist through the afternoon even as the shortwave pushes east of our area. Given that surface temperatures have remained in the upper 30s and lower 40s overnight and will remain nearly constant through this afternoon, all of the precipitation with this initial shortwave will be in the form of rain.

As this initial wave exits the surface low is expected to lag behind and be slowly propagating across central Illinois through the afternoon. A second shortwave, currently moving across the southwest CONUS, is expected to catch up to the low as we head into the evening hours which will allow another wave of widespread rain to redevelop across the area, but especially northern Illinois. As this occurs winds should become northerly and begin to advect in colder air from Wisconsin which may allow temperatures to fall near if not just above freezing. Forecast soundings do show temperatures aloft to be well below freezing which should allow some snow to mix in with the rain primarily for areas along and north of a line from Sterling, IL to Woodstock, IL. Given that surface temperatures and resultant wet bulb temperatures are expected to remain in the mid-30s as the snow begins I suspect that snow accumulations should be very limited with maybe just some slushy amounts of a tenth or two at best.

However, some recent guidance is starting to indicate that upright instability may develop in the mid-levels for a period late this evening which may aid in a brief period of increased snowfall rates if sufficient moisture in the DGZ can linger long enough. If this was to occur, the briefly more robust snowfall rates could overcome the marginal surface temperatures and lead to some slushy accumulations upwards of an inch or possibly a little higher. Given that a lot of ingredients need to come together and the fact that most guidance keeps the bulk of any accumulating snow more into far northwest IL and southern WI, I have decided to maintain a forecast for a rain/snow mix with no accumulation for now. But will need to keep a close eye on trends this afternoon especially across portions of northern Winnebago County for signs of this scenario establishing.

Regardless, as the second wave and surface low exit the area tonight, rain and snow will gradually taper with dry conditions expected to return by Saturday morning. Unfortunately lingering mid-level moisture will maintain overcast skies through the day on Saturday. Though light southeasterly winds should still advect in enough warm air to keep highs in the low to mid-40s.

Yack

LONG TERM
Issued at 301 AM CST Fri Dec 1 2023

Saturday night through Thursday...

Key Messages:

* Another round of widespread showers/rain Saturday night into Sunday. Some potential for non-impactful snow/rain mix across far NW IL Sunday AM.

* Additional precip (mainly rain) chances Monday evening-Tuesday.

Hemispheric moisture channel loops reveal numerous eddies and embedded perturbations stretching from the Pacific Northwest, westward across the Gulf of Alaska and back towards the Kamchatka Peninsula. This highly active northern stream will result in repeated disturbances traversing the CONUS through the weekend and next week.

The main feature of interest for our weather Saturday night into Sunday looks to be one the many regions of strong cyclonic shear vorticity currently traversing the Aleutian Islands. This feature will eject out across the Central Plains with some degree of amplification indicated across the guidance suite as it sweeps across the Great Lakes region. Model guidance is in pretty good agreement regarding the salient large scale features, especially considering how chaotic and unstable the upstream flow appears.
Forcing will increase Saturday night as jet divergence increases overhead and anticipating an increase in precipitation chances as a result. With just a little in the way of elevated instability present, precipitation may be a bit convective/showery in nature, and suppose a renegade lightning strike can't entirely be ruled out.

With this forecast package, have limited the spatial extent of snow wording into Sunday with surface wetbulb temperatures largely expected to remain in the mid 30s and above. Thermal profiles look like they'll get close to supporting a changeover to snow through Sunday morning across northwest Illinois, but by that point forecast soundings show a rapid loss of deeper saturation as the upper trough axis swings east of the region.
Eventually, precipitation may transition to spotty drizzle as saturation above 750 mb is lost entirely through Sunday afternoon.

Most midrange guidance depicts another shortwave following quickly on the heels of the Sunday system arriving somewhere in the vicinity of central or northern Illinois Sunday night/Monday morning. This could be a feature to monitor a little more closely since thermal profiles with its arrival will be a bit cooler and more conducive for supporting snow. In addition, while there's a fairly large N/S spread in its track across the guidance suite, very cold H5 temps near -30 C will yield a reservoir of more meaningful instability aloft. At this point, the multi-model consensus seems to favor a track perhaps just south of our forecast area.

Another robust disturbance will then approach the area on Monday with renewed precip chances Monday night through Tuesday. Once again, marginal thermal profiles suggest we'll be right on the cusp of liquid vs. frozen p-types in the region. Did not make adjustments to the NBM-delivered p-type output for this period as a result, which yields just rain at this point. Strong ascent within the initial WAA-wing Monday night might support a burst of snow, but it remains unclear if initial sub-cloud dry air will be eroded in time. Otherwise, after a brief period of lake effect precipitation into NW Indiana on Tuesday and Tuesday night, dry conditions are advertised through Thursday with warmer temperatures.

Carlaw

AVIATION
For the 12Z TAFs...

Aviation weather concerns are:

* IFR to LIFR cigs today and tonight * Periods of rain through tonight * Rain/snow mix at RFD mainly this evening, perhaps changing briefly to all snow

Widespread rain will persist across all of the terminals this morning with MVFR/IFR vsbys and cigs. Have recently noted a gradual improvement in cigs from near MDW, southeast towards GYY and parts of the Kankakee River Valley locales. Plausible that cigs temporarily improve over the next few hours as this region lifts northward. However, expectation is for largely IFR cigs to prevail today. Precipitation will trend a bit more towards intermittent showers and drizzle this afternoon.

Additional waves of showers will expand later this afternoon and evening. Cigs should gradually build down, with the greatest chances for LIFR cigs of the TAF period this evening and overnight. Potential even exists for VLIFR cigs/vsbys, but the latest signal for conditions this low is from about GYY and southward.

Thermal profiles will become more favorable for snow mixing in at RFD late this afternoon and evening. Latest guidance suggests a period of all snow will be possible as well, but this may be short-lived as deeper saturation will be lost through the late evening/overnight hours. With above-freezing temperatures forecast, only very slushy accums on grassy and very elevated surfaces expected.

Precipitation will gradually end area-wide tonight/Saturday morning, although some lingering pockets of drizzle will likely remain.

Carlaw

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM
Small Craft Advisory
Winthrop Harbor to Wilmette Harbor IL until 10 PM Friday.

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