Sunrise 5:27 AM   Sunset 8:07 PM Moonrise 12:22 PM   Moonset 2:10 AM

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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- Northerly Island To Calumet Harbor Il- 309 Am Cdt Wed May 15 2024
Today - North winds 10 to 15 kt. Decreasing cloudiness. Waves 2 to 4 ft.Tonight - North winds 10 to 15 kt becoming east 5 to 10 kt. Mostly clear. Waves 2 to 4 ft subsiding to 1 to 3 ft. Thursday - Southeast winds 5 to 10 kt. Scattered showers and Thunderstorms in the afternoon. Waves 1 to 2 ft. Thursday night - South winds 5 to 10 kt. Scattered showers and Thunderstorms. Waves 1 to 2 ft.

LMZ700
No data

FXUS63 KLOT 150802 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 302 AM CDT Wed May 15 2024

KEY MESSAGES

- Today will be pleasant with clearing skies and highs in the lower 60s (lakeside) to lower 70s (inland). A few showers cannot be ruled out east of I-57 this afternoon.

- Chances for showers and thunderstorms will return on Thursday, and peak between 40 and 50% areawide during the afternoon and evening hours.

- Periods of showers and storms may occur Friday into Saturday, but looking mostly dry for the weekend.

- Active pattern looks to develop next week bringing several opportunities for showers and storms.

DISCUSSION
Issued at 302 AM CDT Wed May 15 2024

Through Thursday night:

Regional radar imagery and a recent hand surface analysis depict several areas of showers rotating counter-clockwise around a broad region of surface low pressure centered on the Ohio River.
Seasonably cool air moving southwestward atop the relatively moist waters of Lake Michigan continues to support areas of stratus across northeastern Illinois and northwestern Indiana.
With all of that said, it's a nice and quiet morning.

Today, the vertically-stacked low pressure system across the Lower Great Lakes will continue drifting eastward away from our area, allowing for gradually clearing skies. A few showers rotating around the broad cyclonic circulation may graze areas east of I-57 this afternoon, though today looks dry for most of our area.
Continued northeasterly winds will maintain a lake shadow in temperatures, with highs generally expected to top off in the low to mid 60s east of I-355 and north of the Kankakee River, compared to readings in the low to mid 70s further inland.
Tonight will be similarly quiet as a short wave-length upper- level ridge and associated area of relative surface high pressure slides overhead. Overnight lows will fall into the upper 40s to lower 50s, which is close to average for this time of year.

Our break in waves of showers and thunderstorms today will be short- lived, as a pair of upper-level vort maxes (seen readily in water vapor imagery this morning over central Idaho and Wyoming) merge together and propagate overhead on Thursday. Surface pressure falls across the Upper Mississippi River Valley (associated with a surface low beneath the aforementioned merging upper-level vort maxes) will cause winds to gradually turn southeasterly and then southwesterly throughout the morning, allowing for a reservoir of relatively humid air with surface dew points in the low to mid 60s positioned across eastern Kansas and central Missouri to slosh northeastward into northern Illinois and northwestern Indiana by mid-afternoon. With forecast soundings depicting minimal capping and 500-750 J/kg of MLCAPE by mid-afternoon, widely scattered showers and thunderstorms appear poised to develop across the area in "popcorn" fashion.

Now, the vigor and coverage of showers and thunderstorms Thursday afternoon and evening is not overly clear. For one, upper-level cloud cover "debris" from expansive convection in the Plains tonight will be poised to spread northeastward over northern Illinois and northwestern Indiana during "peak heating" tomorrow afternoon, and may limit the amount the instability that develops at least across parts of our area. At the same time, any convective-induced MCV that develops tonight will be poised to lift northeastward into Illinois tomorrow afternoon, which may provide enough localized forcing to offset limited instability. For now, felt that broad-brushing 40-50% chances for scattered thunderstorms from tomorrow afternoon into early Friday morning was the safest bet in favor of making refinements in later forecast packages.

Borchardt

Friday through Tuesday:

The upper trough currently trying to organize off the coast of southern California is forecast to be traversing across the mid- Mississippi and Ohio River Valleys Friday into the day on Saturday. While there continues to be some timing differences amongst the 00z guidance suite, there is a decent signal that south-southeasterly low-level flow should allow for warm-moist advection to persist. Thus, temperatures and dew points in the mid to upper 70s and lower 60s respectively should promote destabilization Friday afternoon and allow for the development of shower and thunderstorm activity across at least the southern half to 2/3rds of our forecast area. Depending on which timing solution comes to fruition, Saturday could see either another period of showers and storms (primarily in our south and southeastern counties) or be mostly dry as mid-level height rises start to move into the region. However, a more robust upper trough is expected to be pivoting across the Upper Midwest on Saturday which looks to force a cold front through our area Saturday night. Guidance is not overly impressive with rain occurring along the front as the anticipated timing (after sunset) should reduce the effective instability available, but a few isolated to scattered showers and perhaps rumble of thunder cannot be ruled out especially for areas near the IL-WI line.

Regardless of how Saturday pans out, Sunday should be dry as the aforementioned height rises are expected to be overhead and we will be in the presence of northwest winds behind the front.
Though, temperatures are still forecast to be in the seasonably warm category with highs in the upper 70s to lower 80s Sunday afternoon. The exception, however, will be for those along the lake where a lake breeze is progged to keep temperatures notably cooler in the 60s to around 70.

Heading into the beginning of next week, guidance continues to show that a more active pattern will take shape across the CONUS. During this period the northern jet stream is forecast to dig into the Pacific Northwest and phase with a developing sub- tropical jet across the southwestern US. In doing so, a broad trough looks to develop across the western CONUS which will allow the aforementioned sub-tropical jet to intensify and nose into the southern Plains and mid-Mississippi Valley. Not only will the sub-tropical jet aid in enhancing the southwest low- level flow over the Plains and Midwest supporting seasonably warm and humid conditions, but it will also provide an avenue for shortwaves to break off the trough and eject into the Great Lakes virtually unabated. This combination of synoptic forcing, warm-humid air, and potential wind shear looks to make for a period of rather stormy weather across our area which may include the threat for severe weather at times next week. While this signal seems fairly consistent amongst guidance, it still should be noted that specifics on timing, location, and intensity of any potential storms (and severe weather) still remains very uncertain. Therefore, we recommend keeping an eye on next week with subsequent forecasts and in the meantime review those severe weather plans just in case.

Yack

AVIATION /06Z TAFS THROUGH 12Z THURSDAY/
Issued at 1229 AM CDT Wed May 15 2024

Forecast concern for the terminals...

- Low-end MVFR to IFR ceilings tonight mainly across northwest IN and central IL

A broad area of low pressure will continue to track eastward across the Ohio River Valley tonight which will allow an area of low clouds to pivot across northwest IN and adjacent areas of IL overnight. Latest observations show that the majority of these low clouds are within the 1500 to 2000 ft range, but some pockets of 900 ft IFR ceilings have been noted at times. The expectation is for these MVFR ceilings, and occasional IFR, to remain primarily across northwest IN (i.e. GYY) overnight, but I cannot completely rule out a brief MVFR ceiling at ORD or MDW.
Therefore, have maintained a SCT025 mention at ORD and MDW.
Ceilings will improve back to VFR Wednesday morning (around daybreak at ORD/MDW and 15z at GYY) as the low pulls away with VFR conditions prevailing through the rest of the period.

Otherwise, expect winds to maintain an east-northeasterly direction with speeds generally around 10 to 15 kts through Wednesday afternoon. Though speeds are expected to ease to around 5 kts Wednesday evening.

Yack

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

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