Sunrise 5:40 AM   Sunset 7:54 PM Moonrise 3:54 AM   Moonset 3:55 PM

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LMZ741 Expires:202405041545;;595323 Fzus53 Klot 040831 Nshlot
nearshore marine forecast national weather service chicago/romeoville il 331 am cdt Sat may 4 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.
lmz740>742-041545- winthrop harbor to wilmette harbor il- wilmette harbor to northerly island il- northerly island to calumet harbor il- 331 am cdt Sat may 4 2024
Today - East winds 5 to 10 kt becoming southeast 10 to 15 kt in the afternoon. Partly cloudy. Waves around 1 ft increasing to 1 to 2 ft.Tonight - Southeast winds 10 to 15 kt early in the evening becoming northwest 10 to 20 kt by late evening. Scattered showers and Thunderstorms in the evening. Waves 1 to 2 ft. Sunday - North winds 15 to 20 kt becoming northeast 10 to 15 kt. Partly cloudy in the morning then clearing. Waves 1 to 3 ft building to 2 to 4 ft. Sunday night - Northeast winds 5 to 10 kt. Partly cloudy. Waves 2 to 4 ft subsiding to 1 to 2 ft.

LMZ700
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FXUS63 KLOT 040814 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 314 AM CDT Sat May 4 2024

KEY MESSAGES

- A line of scattered thunderstorms is expected to cross the area late this afternoon into mid-evening, with locally strong to severe wind gusts to 60 mph possible.

- Periodic shower and thunderstorm chances are expected next week. There is a potential for strong to severe storms along with a locally heavy rainfall threat on Tuesday.

DISCUSSION
Issued at 314 AM CDT Sat May 4 2024

Through Sunday:

A retreating surface ridge extending southwestward across the western Great Lakes will give way to ongoing lee cyclogenesis ahead of a mid-level trough entering the central Great Plains early this morning. Robust convection from northern OK into western IA has likely enhanced the trough, with multiple MCVs evident along this corridor. An associated cold front just behind this convection will become the main focus for storm potential into our area later this afternoon into this evening.

Surface dew point trends today will be an important factor on the coverage and intensity of storms into the forecast area.
Friday's weak cold front has since stalled across central Illinois and will jump northward in response to diurnal warming and moistening today. However, the magnitude of moistening primarily from evapotranspiration remains less clear this afternoon. Current thought is daytime mixing into a very dry layer above the growing PBL will with offset rising dew points or at least reduce the depth of moisture sufficient for development and maintenance of convection. Have opted to use only scattered thunderstorm wording even as some CAMs depict higher coverage as a result of higher modeled dew points.

While it is expected that a more prominent MCV currently near Omaha will lift NE within the broader mid-level steering flow this morning, the extreme southern influence of this MCV may clip the northwest CWA late this morning. Capping should remain far too high to support convection this far southeast, but isolated attempts at convection may reach as close as far northwest Illinois.

Overall expectations are for a line of broken showers and storms to develop/intensify roughly along and around the Mississippi River by mid-afternoon within a pre-frontal ribbon of higher theta-e air. Coverage of thunderstorms should then begin a slow downward trend across the forecast area late afternoon through mid-evening as the drier pre-storm environment becomes less favorable with time. However, given an increasing reservoir of DCAPE with eastward extent across the CWA, the potential for strong to locally severe wind gusts may actually maximize across the west half of the CWA up to and through sunset. The antecedent dry air ahead of these storms combined with diurnal mixing of low-level moisture speaks to this wind risk, with the strongest winds likely occurring with any existing loosely organized multi-cell cluster or collapsing cores as storm intensity wanes with time.

After the storms and cold front clear the forecast area by early Sunday morning, dry and cooler conditions are expected through the remainder of the period.

Kluber

Sunday Night through Friday:

Quiet weather starts the period Sunday night, as surface high pressure is progged to be drifting east across the Great Lakes region. Farther south, guidance continues to depict mid-level short wave approaching IL late, though it is deamplifying as it moves into the short wave upper level ridge axis. Forecast soundings depict very dry low and mid-levels initially in place across the forecast area, though persistent isentropic ascent and associated warm/moist advection does eventually produce saturation and precipitation development into central IL/IN and the LOT/ILX border region after midnight. Mid-level lapse rates are unimpressive in forecast sounding guidance, suggesting little/no thunder threat. Showers are expected to persist into Monday across the southern parts of the cwa as the mid-level wave continues its transit of the area. NBM blended pop guidance appears to spread too far north in this scenario, and note that much of the EPS/GEPS/GFS ensembles are dry along/north of the I-88 corridor through the day across northern IL.

Farther west, guidance continues to depict a deep upper level trough across the western CONUS, within which a strong, negative-tilt short wave develops a closed upper low over the northern high plains by Monday night. Along with a series of short waves propagating around the southeast periphery of the upper circulation and upper level diffluence ahead of a strong cyclonically-curved upper jet, this sets the stage for southerly low level warm/moist advection from the western Gulf into the mid-Mississippi Valley and upper Midwest Monday night, with a northward lifting warm front spreading into the area into Tuesday morning. Thus shower and thunderstorm chances look to ramp-up into the forecast area from the west especially after midnight and into Tuesday morning. While the primary upper/surface lows will be well to our northwest Tuesday, guidance continues to depict a strong mid-level jet is progged to spread east across the area during the day, with a surface wave developing along an occluding cold front approaching the Mississippi Valley later in the day. Combined with a warm, moist and conditionally unstable low- level air mass, additional thunderstorms (potentially strong to severe) are likely within the region Tuesday afternoon/evening. In addition to severe weather, high precipitable water values around 1.75 inches will support locally heavy rainfall will be possible especially if storms train over a particular area. SPC and WPC outlooks for Tuesday for both severe and heavy rainfall potential continue to appear appropriate at this distance.

The upper trough is forecast to drift east across the upper Midwest and northern Great Lakes region during the mid and late week period, maintaining an active weather pattern across our forecast area. While the primary low-level theta-E axis will shift southeast of the forecast area Wednesday through Friday, a series of short waves rotating through the southern periphery of the upper Midwest upper low will likely produce additional periods of showers and thunderstorms, especially during the diurnally more favorable afternoon and evening hours.

Temperatures will remain above average through most of the period, with daytime highs in the 70s to low 80s (Tuesday and Wednesday have the potential to be the warmest days). Cooler, more average temperatures are expected by Friday, with highs in the mid-60s.

Ratzer

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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