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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- Northerly Island To Calumet Harbor Il- 258 Pm Cst Tue Dec 5 2023
Tonight..North winds 15 to 20 kt becoming northwest overnight. Scattered showers through the evening. Waves 2 to 4 ft.
Wednesday..Northwest winds 10 to 20 kt gradually becoming southwest through the day. Mostly cloudy. Waves 1 to 3 ft.
Wednesday night..Southwest winds 15 to 25 kt. Mostly cloudy in the evening, then becoming partly cloudy. Waves 2 to 4 ft.
Thursday..Southwest winds 10 to 15 kt increasing to 15 to 20 kt. Mostly Sunny. Waves 1 to 3 ft.

LMZ700
No data

FXUS63 KLOT 052328 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 528 PM CST Tue Dec 5 2023

SHORT TERM
Issued at 216 PM CST Tue Dec 5 2023

Key Messages:

- Light lake effect rain showers along the IL shore through early evening shifting into northwest Indiana and mixing with snow tonight.

- Cloudy and dry conditions Wednesday and Wednesday night.

Synoptically-forced precip has ended across the CWA as the slightly northwest-tilted upper trough begins to shift eastward. Marginally favorable 850hPa-SST delta temps of 12-14C within weak CAA and a developing inversion around 6-7kft has supported an increase in lake effect rain showers from southeast Wisconsin southward into Cook County early this afternoon. NNE low-level flow will continue to focus the showers into the eastern half of the Chicago metro through early evening before a veering low-level wind profile shifts a broken band of showers eastward into northwest Indiana this evening and overnight. While thermal profiles will remain warm enough to support all rain into early evening, especially for areas near Lake Michigan, the weak CAA will support rain mixing with or changing over to all snow farther inland into northwest Indiana. With inversion heights lowering to around 5kft by this time, precip intensity should remain light enough to limit any accumulations to anything but a couple tenths of an inch on grassy surfaces overnight.

A surface ridge extending from the southern Great Plains into the mid-Mississippi River Valley tonight will drift south and east of the CWA Wednesday through Wednesday morning, effectively shifting any remaining lake effect showers well east of the area by sunrise.
Gradual mid-level height rises ahead of a large ridge across the central CONUS will foster a gradually lowering subsidence inversion late tonight through Wednesday evening. Widespread stratus throughout the Upper Mississippi Valley will likely remain trapped below this inversion while advecting across the area tonight and into Wednesday afternoon, favoring a mostly cloudy to overcast day amid dry conditions. With winds backing SW and the inversion lowering through the residual saturated layer late in the day and evening, existing stratus will erode from the southwest while mid and upper-level cloud cover increases.

Kluber

LONG TERM
Issued at 216 PM CST Tue Dec 5 2023

Thursday through Tuesday...

Key Messages...

- Unseasonably warm temperatures Thursday, Friday and likely Saturday for part of the area, with highs well into the 50s.

- Breezy southwest winds Thursday through Friday with gusts into the 30 mph range.

- A strong storm system will affect the region this weekend. There are some increased chances (10%) of severe weather southeast of a Paxton to Valparaiso line. This system may also bring snow to areas northwest of Interstate 57 but uncertainty on snow occurrence and location is still very high 4 days out.

Overall, no significant changes to the forecast for Thursday into Friday night. Southwest winds will be gusting into the 30 mph range which will allow much warmer air to spread across the region. Highs on Thursday are expected to be in the lower/mid 50s and by Friday, highs in the upper 50s are expected with a few places possibly tagging 60. For the most part, these expected highs are below the daily records. See climate section below for specific high temps.

While there still remains quite a bit of uncertainty for the storm system this weekend, there are some trends in the models and their ensembles that are becoming more clear. Just about every ensemble member is now showing qpf across the local area, adding confidence for precipitation occurring. The extent of the colder air has always looked marginal, meaning any changeover to snow, at least across the local area, would most likely be driven by dynamic cooling which needs several variables to all come together to occur and produce snow. There is now a notable shift to the northwest with the track of this system and one of the main players for how all this evolves will likely be the expansive high pressure over the southeast U.S. and how fast or sluggish it is as it departs over the Atlantic Friday night into Saturday.

A track of the surface low across the cwa, as currently shown by all the operational models would support another warm day south of the warm front, with highs possibly pushing 60 in the southeast cwa, while across the northwest, temps may only reach the lower 40s and then possibly fall during the day on Saturday. This track of the surface low would be concerning for the potential for severe weather across the southeast cwa and have added thunder mention for the southeast cwa based on these trends.

There still is a snow potential on the northwest side of this system, wherever that ends up going. And given the uncertainty of its location, which could also be a rather narrow swath of snow, in addition to multiple variables coming together to produce any heavier amounts, its simply too early for specifics. The weather grids from the blended guidance transition to a rain/snow mix Sunday morning with snow for Sunday afternoon, if precipitation is still occurring at that time. A period of stronger winds is also possible Saturday night into Sunday, which will be dependent on the strength and track of the surface low. Much colder temps, relative to late this week, are expected early next week, but will be close to seasonal normals for mid December. cms

CLIMATE
Issued at 216 PM CST Tue Dec 5 2023

Unseasonably warm temperatures are expected Thursday through Saturday and here are the record high temps for Rockford and Chicago.

Rockford...
Thursday, December 7th...64 in 1916 Friday, December 8th.....60 in 1980 Saturday, December 9th...55 in 2020

Chicago...
Thursday, December 7th...64 in 1916 Friday, December 8th.....64 in 1946 Saturday, December 9th...62 in 1879

The record warmest low temperatures for the calendar day, December 8th are,

Rockford...37 in 1987

Chicago....53 in 1946

AVIATION
For the 00Z TAFs...

Aviation Forecast Concerns:

* Lake effect rain showers at KORD/KMDW/KGYY early-mid evening.

* Chicago metro terminals likely see period of VFR conditions overnight, with MVFR ceilings returning Wednesday morning.

* Breezy southwest winds with gusts 20+ kts develop by Wednesday evening, beneath a 45 kt low level jet.

Surface low pressure will continue to pull away to the southeast across the central Appalachians this evening, with surface high pressure developing across the Mississippi Valley and into the western Great Lakes region Wednesday morning. North-northwest low level flow behind the departing low, combined with low level cold advection, will support a few lake-effect rain showers for KORD, KMDW and KGYY early this evening, before winds shift more northwest later this evening and overnight. MVFR ceiling and vis conditions are expected with these showers.

While a large area of MVFR stratus is noted along/east of the Mississippi River, portions of eastern WI and northeast IL do look to have a period of VFR conditions later this evening/overnight.
KRFD, farther to the west, will likely remain MVFR. MVFR conditions will likely spread back across the Chicago terminals Wednesday morning however, due to west-northwest low level winds.
MVFR ceilings may even gradually lower during the day, beneath a lowering subsidence inversion within the high pressure region.
Eventually, surface winds will continue to back southwest, and erode low clouds from west to east during the afternoon.
Confidence is somewhat low on timing the improvement from MVFR to VFR. Some guidance indicates KORD/KMDW could go VFR as early as 18Z, while high-res HRRR/RAP indicate later improvement.

A strong (45-50 kt) southwest low level jet is expected to develop Wednesday evening. This would typically set up LLWS conditions, though it looks as if the surface pressure gradient will tighten enough to produce surface wind gusts in the 20-25 kt range. This would limit the magnitude vertical wind shear to below TAF LLWS criteria.

Ratzer

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

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