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LMZ741 Expires:202406160930;;089262 Fzus53 Klot 160156 Nshlot
nearshore marine forecast national weather service chicago/romeoville il 856 pm cdt Sat jun 15 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.
lmz740-741-160930- winthrop harbor to wilmette harbor il- wilmette harbor to northerly island il- 856 pm cdt Sat jun 15 2024
Tonight - Southeast winds 10 to 20 kt becoming south. Mostly cloudy. Waves 2 to 4 ft. Sunday - South winds 15 to 20 kt. A chance of showers and Thunderstorms in the morning. Waves 2 to 4 ft. Sunday night - South winds 15 to 20 kt becoming southwest 10 to 15 kt. Mostly clear. Waves 1 to 3 ft. Monday - South winds 10 to 15 kt increasing to 15 to 20 kt. Slight chance of showers and Thunderstorms in the afternoon. Waves 1 to 3 ft.

LMZ700
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FXUS63 KLOT 160003 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 703 PM CDT Sat Jun 15 2024

KEY MESSAGES

- There is a low (20-30%) chance for a few thunderstorms tomorrow morning mainly across northern Illinois.

- A period of warm and humid conditions will start Sunday and last through at least Tuesday. Monday will be the warmest day with peak heat indices near or above 100 degrees.

- Chances for showers and storms Monday onward are relatively low (20% or less), with expected limited coverage.

DISCUSSION
Issued at 248 PM CDT Sat Jun 15 2024

Through Sunday night:

The Western Great Lakes are positioned between a 1020mb surface high pressure system in southern Ontario and a 1002mb surface low pressure system over the Northern Plains, both beneath inflection points of a developing upper-level ridge centered over the Upper Mississippi River. A series of upper-level shortwave are propagating northeastward from the Plains toward the Great Lakes on the southern periphery of the ridge, with the first now lifting into Minnesota and the second located in southern Kansas. Locally, filtered sunshine, modest humidity levels, and a southeasterly breeze are making for a pleasant mid-June day.

Tonight, the second upper-level shortwave will lift into Iowa and Missouri and induce the development of a broad low-level jet into the Great Lakes. As a result, we should not decouple in spite of relatively low humidity levels, and the night will be marked by breezy southerly winds with lows in the mid to upper 60s.

Tomorrow, the upper-level shortwave is expected to lift into Wisconsin carrying the majority of any remnant overnight convection (initially in Iowa) with it. However, ongoing isentropic ascent along the eastern fringe of the moisture plume shifting eastward into the Great Lakes and along the northern edge of a plume of steep mid-level lapse rates may nevertheless allow for isolated to scattered thunderstorms to develop as far south as northern Illinois. Model guidance typically struggles with the southward extent of thunderstorm LLJ/lapse rate-driven convection, and accordingly, felt introducing 20-30% chances for thunderstorms across northern Illinois tomorrow morning would be prudent. The far more likely scenario is that most areas stay dry tomorrow morning.

Tomorrow afternoon, rising heights in the wake of the shortwave should lead to low-level capping in spite of rising humidity levels and temperatures. As a result, we currently do not favor any (renewed) development of showers and thunderstorms beyond anything that occurs in the morning. Forecast 925mb temperatures around +25C should support afternoon highs in the low to mid 90s. A southwesterly breeze won't offer much respite from the humidity levels and partial sunshine, with max heat indices expected to peak from 95 to 100F during the afternoon. After sunset, winds will relax but high humidity levels will hold overnight lows to the balmy mid to upper 70s.

Borchardt

Monday through Saturday:

The upper ridge that is currently building across the southern CONUS is expected to be lifting into the Mid-Atlantic on Monday as a broad trough digs into the Pacific northwest. This pattern will place stout southwest flow across much of the Mississippi Valley and Great Lakes generating a period of above normal temperatures and high humidity. The warmest day during this period continues look like Monday where high temperatures will likely be in the mid-90s with overnight lows in the mid to upper 70s. Though, there continues to be the possibility for high temperatures to overperform into the upper 90s (20-30% chance)
if there is more sun than currently forecasted. Regardless, the combination of these temperatures and dew points in the upper 60s to around 70 should yield peak heat indices in the 95- 105 degree range Monday afternoon. Given that these values are below our local Heat Advisory criteria (widespread heat indices greater than 105), no heat headlines are anticipated. With that said, we still encourage those who are sensitive to heat to limit time outdoors and remain hydrated.

Furthermore, some guidance (namely the GFS) continues to hint at the potential for scattered showers and storms to develop Monday afternoon. However, all guidance continues to show modest high rises occurring overhead in combination with the mid- levels looking rather dry, which I suspect will prohibit any shower/storm activity despite the robust instability. While I do expect the area to remain dry on Monday, I have maintained a 20% chance for showers/storms Monday just in the off chance something is able to develop. It should be noted that if a stray shower or storm does develop, then high temperatures in those location may verify slightly cooler than forecasted.

Heading into Tuesday, the western extent of the ridge is progged to get shoved east as a trough ejects into the northern Plains. While this will allow more moisture to advect into the southern Great Lakes, the resultant increase in cloud cover should also keep high temperatures a few degrees cooler, in the lower 90s, for Tuesday. Though, peak heat indices in the upper 90s to near 100 are still expected. Additionally, the increase in moisture in conjunction with stout diurnal instability and modest upper height falls will support the potential for isolated to scattered showers and thunderstorms to develop Tuesday afternoon. Though, the lack of more defined forcing may mean most areas stay dry.

Beyond Tuesday, guidance continues to attempt to break down the ridge and develop a more zonal (east-west oriented) pattern. As a result, a frontal boundary is forecasted to be near the Mississippi River and may move through northern IL during the later half of the week, breaking the heat and humidity.
However, there continues to be a wide range of timing differences amongst guidance members on when the front would arrive and the coverage of any associated showers and storms.
Therefore, I have decided to maintain the advertised 90 degree temperatures and 30-40% rain chances offered by the NBM through the end of next week which seem reasonable at this range.

Yack

AVIATION /00Z TAFS THROUGH 06Z MONDAY/
Issued at 704 PM CDT Sat Jun 15 2024

Generally VFR conditions are expected through the TAF cycle with the one possible exception being a low chance of SCTD SHRA and TSRA (20-30% chance) Sunday morning. Upper level disturbance responsible for TSRA over central Plains will move into the region Sunday morning and could lead to some ISOLD to SCTD TSRA, though air mass will be drier and less conducive for storms, so chances appear low. Have kept precip mention out of the Chicago TAF sites for now.

Otherwise, southeast winds tonight will become south-southwest and gusty Sunday morning and remain moderately strong and gusty until sunset Sunday evening.

- Izzi

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...Air Quality Alert from 1 AM Sunday to 1 AM EDT Monday for INZ019.

LM...None.

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