Sunrise 5:39 AM   Sunset 7:59 PM Moonrise 4:43 AM   Moonset 6:34 PM

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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- Northerly Island To Calumet Harbor Il- 333 Pm Cdt Mon May 6 2024
Tonight - Northeast winds 10 to 15 kt becoming southeast late. Mostly clear in the evening, then becoming mostly cloudy. Waves 1 to 3 ft. Tuesday - Southeast winds 10 to 20 kt becoming south in the afternoon. Showers with Thunderstorms likely in the morning, then chance of showers and Thunderstorms in the afternoon. Waves 2 to 4 ft. Tuesday night - Southwest winds 10 to 15 kt. Slight chance of showers and Thunderstorms in the evening. Waves 1 to 3 ft subsiding to around 1 ft. Wednesday - Southwest winds 10 to 15 kt becoming east in the afternoon. Slight chance of showers and Thunderstorms in the afternoon. Waves around 1 ft building to 1 to 3 ft.

LMZ700
No data

FXUS63 KLOT 062033 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 333 PM CDT Mon May 6 2024

KEY MESSAGES

- Thunderstorms are likely tomorrow morning and may produce gusty winds and/or small hail.

- A second round of thunderstorms appears probable during the afternoon, with the highest confidence in storm redevelopment being east of a Pontiac, IL to Chicago, IL line. A few of these afternoon storms could become severe with large hail and damaging winds as the primary hazards.

- Another round of storms may develop late Wednesday, some of which could be strong to severe, mainly south of I-80 into central IL/IN.

DISCUSSION
Issued at 332 PM CDT Mon May 6 2024

Through Tuesday night:

All eyes are upstream on the central/southern Plains, where a significant severe weather outbreak is underway and will continue well into the night. Strong forcing associated with an ejecting upper-level trough and an attendant cold front at the surface will promote rapid upscale growth across Kansas and Nebraska, with the resultant elongated QLCS then expected to track into Iowa and Missouri late this evening and eventually into Illinois late overnight. By the time this strongly-forced line of storms reaches our forecast area, it will likely be elevated and on a general downward trend in intensity, so prospects for severe weather in our forecast area with this early morning QLCS appear to be minimal. Near-surface stability won't be overly strong, however, and MUCAPE could still be as high as about 1000 J/kg (highest in our southern counties) as this line of storms rolls through, so there could still be some gusty winds and/or small hail with any of the stronger individual updraft cores within this line.

The remnants of the weakening QLCS should skitter out of our forecast area from west to east by about 11 AM CDT or so, and rapid destabilization efforts will begin as soon as it departs.
Cold mid-level temperatures associated with the aforementioned trough pivoting overhead coupled with strong upper-level forcing from DPVA and divergence within the left exit region of an upper-level jet should not have much of a problem with steepening mid-level lapse rates in the wake of the morning convection, which is not likely to cause substantial convective overturning of the troposphere in the first place. Warm/moist advection will also modestly increase temperatures and dew points at the surface going into the afternoon, with pockets of sunshine likely to provide a boost to air temperatures as well.
Taken together, it appears that at least part of our forecast area will end up realizing 1000+ J/kg of uninhibited MLCAPE around come the early-mid afternoon, with the greatest degree of instability (potentially up to 2000 J/kg of MLCAPE) focused across our southeastern CWA The aforementioned large-scale forcing, plus the presence of a low-level jet and isentropic ascent, thus appears likely to reignite an additional round of scattered to potentially numerous thunderstorms in at least part of our forecast area during the afternoon tomorrow, and the majority of the latest 12Z CAM suite seems to be on board with this idea.

Wherever instability recovers appreciably by the time the core of an inbound mid-level vorticity maximum arrives, convection appears likely to develop, and deep layer shear will be plenty strong enough to be able to sculpt any mature convection into supercells, with the largely unidirectional shear profiles promoting both large hail and damaging downburst winds and the most likely severe weather hazards. While not particularly likely, some threat for tornadoes does also appear to exist, mainly in our far southeastern CWA, where low-level flow looks like it will be a little more backed than elsewhere. All that said though, if adequately strong instability is unable to be realized to balance out the very strong deep layer shear, then updrafts are likely to be sheared apart by the strong mid- and upper-level flow and not really be able to get well-organized, which would greatly limit the overall scope of the severe weather threat that may be realized. At any rate, the overall prospects of convective redevelopment and severe weather in our forecast area tomorrow afternoon appear to be greatest across our southeastern counties, where the Storm Prediction Center has delineated a Slight (level 2/5) Risk in their Day 2 Convective Outlook. Farther northwest in our CWA, confidence in adequate destabilization occurring in time for convection to develop and mature is lower, and hence, a Marginal (level 1/5) Risk appears to more appropriately characterize the overall severe weather threat there at this time.

After the afternoon convection has departed, there may be one additional opportunity for isolated to widely scattered thunderstorms to affect our forecast area in the evening as a secondary, more subdued vorticity maximum swings into the area and kick-starts additional convective development to our northwest. It's possible that this activity could trickle into our northern CWA near or after sunset and pose some threat for hail and/or damaging winds, but with gradually waning instability and sounding profiles looking noticeably drier by then, confidence in this occurring is relatively low at this time.

Ogorek

Wednesday through Monday:

In the wake of the Tuesday system the parent mid-upper low will remain situated over the Central Plains with the core of the northern stream upper jet directly overhead. Low-level trajectories from the Gulf of Mexico will allow for a quick moisture recovery across the Mid-Mississippi Valley on Wednesday. A semi-coupled upper jet structure will provide additional synoptic support for widespread thunderstorm development across this region then spreading east across the Ohio Valley. The question locally is how far north can this moist warm sector reach which will impact our severe thunderstorm potential. Latest trends in 12Z guidance keep the northern extent to the better instability just south of the area, confined mainly to central/southern IL/IN and areas along the Ohio River Valley. However, given the strong dynamics in play, if better instability can lift into the area, especially into our southern CWA (along/south of the Kankakee River Valley)
the environment would be supportive of all severe hazards. Will continue to monitor trends closely.

While the severe threat is more in question, showers and thunderstorms are expected across the area by Wednesday evening and overnight. As the surface low occludes and becomes more elongated, an east-to-west axis of cool wind-whipped rain likely continues into the day on Thursday, especially along/north of I-80. The cooler north to northeast winds off the lake in rain will limit our daytime warming on Thursday to the 50s to lower 60s.

The upper low begins to split, with the western lobe cutting off over the western CONUS and the eastern lob phasing with a wave extending across Ontario/Quebec with a northwest flow pattern setting up, with the next wave diving out of Canada bringing our next chance for showers an perhaps a few thunderstorms late Friday into Saturday morning. There are differences with the handling of the western CONUS cut-off upper low and resultant upper level pattern though it appears the pattern remains active with off and on precipitation chances continuing next week.

Petr

AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/
Issued at 1240 PM CDT Mon May 6 2024

Key Aviation Messages: - Line of TSRA with gusty winds early Tuesday AM - Scattered TSRA possible Tuesday afternoon (30% chance)

VFR conditions are forecast through this evening and early overnight hours with generally east to southeast winds around 10kt.

A line of thunderstorms is expected to cross the area early Tuesday morning, potentially as early as 10Z toward RFD and 12Z for ORD/MDW with associated vsby/cig reductions down to as low as IFR at times. This line of storms may be accompanied by a gusty westerly wind shift before settling back to a southeast direction.

Once the initial line of storms moves east of the area, conditions become favorable for renewed isolated to scattered storm development by Tuesday afternoon as winds turn south and then southwesterly. Confidence in storm coverage is lower for areas north of I-80 and accordingly opted to introduce this potential with a PROB30 from 18-22Z. If storms do develop they would have the potential to become strong to severe.

Petr

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

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