Sunrise 5:37 AM   Sunset 7:50 PM Moonrise 3:51 AM   Moonset 3:53 PM

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LMZ745 Expires:202405040915;;578502 Fzus53 Klot 040142 Nshlot
nearshore marine forecast national weather service chicago/romeoville il 842 pm cdt Fri may 3 2024
for waters within five nautical miles of shore on lake michigan
waves are provided as a range of significant wave heights, which is the average of the highest 1/3 of the waves, along with the average height of the highest 10 percent of the waves which will occasionally be encountered.
lmz743>745-040915- calumet harbor il to gary in-gary to burns harbor in- burns harbor to michigan city in- 842 pm cdt Fri may 3 2024
Rest of tonight - North winds 5 to 10 kt becoming east after midnight. Mostly clear. Waves 1 ft or less.Saturday - East winds 5 to 10 kt becoming southeast in the evening. Partly cloudy. Waves around 1 ft. Saturday night - South winds 5 to 10 kt becoming northwest 10 to 15 kt overnight. Scattered showers and Thunderstorms. Waves around 1 ft. Sunday - North winds 10 to 15 kt. Decreasing cloudiness. Waves 1 to 2 ft building to 2 to 4 ft.

LMZ700
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FXUS63 KLOT 040549 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1249 AM CDT Sat May 4 2024

KEY MESSAGES

- A dry and cool night is in store with overnight lows in the mid to upper 40s.

- Tomorrow (Saturday) will be warm with highs in the upper 70s to lower 80s, and increasing chances (60 to 70%) for showers and thunderstorms in the evening especially across northern Illinois.

- Periodic shower and thunderstorm chances next week, with some potential for strong to severe storms Tuesday.

DISCUSSION
Issued at 200 PM CDT Fri May 3 2024

Through Saturday night:

A surface high pressure system continues to meander through the Lower Great Lakes region and is leading to mostly sunny skies, light north to northeasterly winds, and generally pleasant temperatures (at least away from Lake Michigan).

With our area on the western flank of the surface high tonight, quiet conditions will prevail. Light winds, clear skies, and a dry airmass will facilitate efficient radiational cooling and allow for overnight lows to fall into the mid to upper 40s.

Tomorrow, low-level flow will turn southeasterly and eventually southwesterly as the surface high pulls further away to the east and an upper-level short-wavelength trough approaches from the west. Low- level moisture advection will allow for humid air across central Illinois and Indiana to quickly return northward and into our area by mid-afternoon. Even with filtered sunshine, the advection of a plume of seasonably warm 850mb temperatures near +13C (into which the boundary layer will build) will help boost afternoon highs into the upper 70s to lower 80s.

Chances for showers and thunderstorms will increase during the afternoon hours in two apparent waves. The first will be with an axis of remnant convection (originating from the Plains tonight), due to arrive sometime in our area early tomorrow afternoon. An ensemble of CAM guidance suggests the leading axis of showers and storms may outrun the best forcing provided by the upper-level shortwave and an associated cold front lagging to the west, and fight a pocket of dry mid-level air in place across the Lower Great Lakes. As a result, chances for showers and storms appear higher later in the afternoon and toward evening along the cold front as it moves across the area (50 to 70% chance from west to east ahead of the cold front). With the upper-level shortwave and associated jet streak expected to lift northeastward toward the Upper Peninsula of Michigan (e.g.
remain somewhat displaced from the instability axis further to the south across our area), any convective cell tomorrow afternoon and evening should behave in a "pulse" like fashion with overall limited storm-scale organization. As a result, the main threats with any storm tomorrow will be locally gusty winds, small hail, downpours, and lightning strikes, similar to yesterday (and consistent with a level 1/5 threat for severe weather). In all, tomorrow looks like another quintessential spring-like day.

Coverage of showers and storms will decrease after sunset as the cold front approaches northwestern Indiana. Temperatures behind the front will fall into the upper 40s (northwest) to mid 50s (southeast) early Sunday morning as northwesterly winds become predominant.

Borchardt

Sunday through Friday:

Next week is shaping up to be a fairly wet and active pattern.

Starting Sunday, as an upper level low drops southward into Northern California from the Bay of Alaska (more on this later), a surface low over Hudson Bay is forecasted to continue to move east drawing the cold front east out of the forecast area. There may be some lingering showers on Sunday morning, particularly over northwest Indiana, but potentially extending as far back to I-55. However, improving and drier conditions are expected through Sunday. Temperatures are expected in the upper 60s and 70s away from the lake, but with cooler northerly air over Lake Michigan, the immediate shoreline may remain in the 50s.

On Monday, the low over Northern California is expected to weaken slightly and passes over the Rocky Mountains. Ahead of of it, another upper level wave will pass over Northern Illinois. Models are disagreeing on the overall strength of the wave. The GFS is leaning deeper with an brief closed low aloft developing, where the Euro and the Canadian depict broader waves. Moisture availability seems limited though over most of the area north of I-80. With some weak lobes of vorticity that pass over, there is a chance for some light showers to pass over, but confidence remains low as it should mostly stay south of the area as temperatures warm back into the 70s, with low 60s on the shoreline.

There is fairly good model agreement that the upper level low will eject out of the Rockies over west central Montana overnight Monday into Tuesday morning while deepening. Strong surface cyclogenesis is expected to take place in the Northern Plains on Tuesday; however, where and how the low develops will largely drive what unfolds on Tuesday, and models are wildly disagreeing on that. There is a strong 300 mb jet that will develop just to the south of the upper level low, with its left exit region around or just north of northeastern Illinois, which will could assist in providing additional synoptic lift, though the exact placement of the jet will have to be monitored over time. But even models are disagreeing with the strength of the jet in addition to its exact placement. Regardless, strong mid level steering flow will draw up anomalously large precipitable water values into Illinois with dew points temperatures expected in the low to mid 60s.

After a weak boundary pushes through Tuesday morning providing the first chance for rain, temperatures are projected to increase behind the front into the upper 70s and 80s. With better instability, showers and thunderstorms are expected Tuesday afternoon and evening. While models are suggesting the strongest shear and instability a little farther south in Central Illinois and the Ohio River basin, there is the potential for strong to even severe storms locally on Tuesday.

By Wednesday, the upper level low over the Northern Plains is projected to eventually start to weaken; however, a positively tilted trough axis will remain over Plains. With persistent southerly warm air advection, there is a risk for another round of showers and storms on Wednesday. Once again though, models are still keeping higher moisture content and stronger instability to the south over the Ohio River Valley as the jet core aloft shifts slightly over Southern Wisconsin. Perhaps the risk for stronger to severe storms will remain south of I-80 on Wednesday, but it will be close enough to be monitored.

Beyond Thursday, there is much more uncertainty on exact timing of showery activity. However, both Euro and GFS ensembles are suggesting the upper level trough to continue to broaden and trudge eastward from Thursday through Sunday. With lingering moisture in the area and lobes of vorticity passing around the upper level trough, there could be periods of showers from Thursday through Saturday. It is not expected to be a complete wash out as there should be breaks from the rain at times, but it remains too far out for complete understanding on timing, so the forecast maintained a 20 to 40 percent chance for rain through Saturday morning.

DK

AVIATION /06Z TAFS THROUGH 12Z SUNDAY/
Issued at 1236 AM CDT Sat May 4 2024

Key Aviation Messages:

- Low probability for shallow fog through sunrise, mainly away from Chicago.

- Scattered showers and thunderstorms likely this afternoon and evening, some of which could produce gusty winds.

- Light southeast winds become south-southwest by midday, with possible lake breeze wind shift late afternoon for KORD/KMDW/KGYY.

Surface high pressure was centered northeast of the forecast area at midnight. Farther west, an area of weak low pressure was over the central Plains. This low will move east-northeast to the western Great Lakes region by evening, with its cold front moving east of the area late. Light easterly (or variable) winds are expected early this morning, turning southeast around sunrise and then south-southwest by midday. Several recent high- res model runs indicate a southeasterly lake breeze will develop, and may affect KORD/KMDW/KGYY late this afternoon. This return to easterly winds appears to be short-lived however, as an area of scattered showers and thunderstorms are expected to move east across the area later this afternoon and early evening, possibly producing westerly outflow winds of 25-30+ kts as they move into the Chicago metro area toward evening.
Thunderstorms would likely be in a diminishing trend as they move into the metro.

The cold front associated with this system will bring a shift to northwest and eventually north-northeast winds later in the evening, which would continue overnight. Guidance also indicates a period of MVFR ceilings is possible behind the front.

Ratzer

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

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