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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- 918 Pm Cdt Tue Oct 3 2023
Rest of tonight..South winds 10 to 15 kt. Partly cloudy late in the evening, then becoming mostly cloudy. Waves 1 to 2 ft.
Wednesday..South winds 10 to 20 kt becoming southwest 15 to 25 kt. Chance of showers and slight chance of Thunderstorms in the afternoon. Waves 1 to 3 ft.
Wednesday night..Southwest winds 10 to 15 kt. Showers likely and slight chance of Thunderstorms. Waves 1 to 2 ft.
Thursday..West winds 5 to 10 kt becoming southwest. Chance of showers and slight chance of Thunderstorms in the morning, then slight chance of showers in the afternoon. Waves 1 to 2 ft.

LMZ700
No data

FXUS63 KLOT 040552 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1252 AM CDT Wed Oct 4 2023

SHORT TERM
Issued at 214 PM CDT Tue Oct 3 2023

Through Wednesday night...

Key Messages:

* Showers and a few thunderstorms Wednesday PM into Thursday as a cold front slowly moves across the region

Plentiful sunshine abounds across the lower Great Lakes this afternoon. High clouds will begin their notable increase this evening, and will maintain mild conditions in the 70s this evening and bottoming out in the mid 60s (upper 60s in Chicago).

Wednesday...This will provide a mild springboard again for Wednesday, in spite of quite a bit more cloud cover. Deeper moisture will slowly increase as the upper trough over the Plains makes slow eastward headway. Subtle waves in the southwest flow aloft will provide the trigger for a few showers Wednesday, mainly in the afternoon. Thunder chances are less than 20%, and confined toward the I-39 corridor.

Wednesday night into early Thursday...Conceptually, deeper southwest flow develop will ahead of the approaching upper trough.
This coupled with an increased upper level jet should support additional lift. With the upper height falls, model soundings also depict an uptick in mid level lapse rates at least initially in the evening hours which would suggest higher precipitation chances overall in the evening. The evening would also be the favored period for isolated to maybe widely scattered thunderstorms.
Forecast guidance though is still a bit mixed as to the timing and coverage of precipitation in the evening and overnight both given model handling of the movement of the front along with the degree of moisture availability.

Much of the high resolution guidance has pretty spotty coverage of shower activity in the evening, likely owing to moisture (which is one of the more poorly handled forecast variables). HRRR/RAP/HRRR also seem to be in the camp of the EC/Canadian in terms of frontal progression than the GFS/EC, which therefore affects the footprint of higher rain coverage overnight, the latter camp favoring an I-55 southeastward, the GFS/NAM and GEFS ensemble suite more over the Chicago area.

Given the aforementioned uncertainties, we will lean toward the NBM and overall ensemble suite of guidance for now which does still place likely precipitation chances in the evening across much of NE IL, and then this spreads into the I-55 southeastward corridor overnight. If the high res guidance has a good handle on the moisture, the evening precipitation coverage would be lower.

Heading into overnight and into Thursday, the front will get held up a bit, with some heavier downpours likely depending on where that occurs. The ensemble suite still quite mixed if that would occur over our area, or more for points into Indiana. The NBM which accounts for the whole suite of guidance favors the heaviest precip SE of our area.

KMD

LONG TERM
Issued at 214 PM CDT Tue Oct 3 2023

Thursday through Tuesday...

Key Forecast Messages:

* Chances for scattered showers on Friday

* Lake effect showers Friday night into Saturday into parts of northwest Indiana could be heavy at times with embedded thunderstorms

Drier air is expected to move in behind the frontal passage. Should the front move slower than expected, there is a chance for lingering showers in northwest Indiana Thursday evening, but otherwise drier conditions are expected.

As the initial 500 mb wave drifts northward into Ontario, a deeper trough will extend southward over Illinois Friday into Friday night driving a second cold front at the surface to pass through the region. Lower dew points and drier air will likely move into the area behind the front; however, colder air advection along with steepening mid level lapse rates will most likely lead to shallower topped convection. The lack of moisture would suggest shower coverage would be scattered in nature. As the system moves over the area, the pressure gradient will tighten up providing better chances for stronger winds, with wind gusts potentially over 30 mph.

Northwest winds are expected behind the front leading into Saturday.
With cold air advection making 850 mb temperatures down to near zero Celsius and Lake Michigan water temperatures around or above 60 degrees Fahrenheit, it looks like a fairly robust set-up for lake effect convection. With equilibrium levels above 21000 feet and CAPE over 1500 joules per kilogram, thunder and maybe even a waterspout over the lake could be possible. With the orientation of the northwest winds, best chances for lake effect showers will be over northwest Indiana, keeping much of the rest of the region in Illinois dry. Chances for showers will likely diminish Saturday night into Sunday morning.

Most ensemble models are suggesting a very slow progression eastward of the upper level trough early next week. While the chances for rain diminish on Sunday, persistent cold air advection allows for the chance for cool temperatures to hang around with drier air inhibiting precipitation chances.. An upper level ridge at 500 mb will attempt to build, but the trough slow progression will limit the eastward extent, preventing any warming trend until, at the earliest, the middle of next week.

DK

AVIATION
For the 06Z TAFs...

Main Concerns:

* Scattered SHRA later this afternoon into the mid evening with a chance of a TS or two

* SHRA and scattered TS may redevelop late tonight/early Thursday near the Chicago terminals

Southerly winds will become southwest 10-15 kt and gust into the 20-25 kt range from the mid morning through the afternoon today.
Some higher gusts are possible at times. Winds will then diminish this evening. Widely scattered SHRA are expected to develop well ahead of a slow moving cold front this afternoon. A few isolated TS are also possible, but confidence is low. Direct impacts to the terminals are uncertain, though did hold onto PROB30s at RFD and DPA.

The better chance of more widespread SHRA and scattered TS appears to be later tonight into early Thursday. However, even then the concentrated corridor of SHRA and embedded TS may set up southeast of the terminals. Given the uncertainty, introduced a PROB30 for TS at ORD and MDW from 06z-12z Thursday. Conditions will be primarily be VFR outside of any SHRA and TS effects today and tonight.

Castro

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

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