Sunrise 5:37 AM   Sunset 7:58 PM Moonrise 4:41 AM   Moonset 6:32 PM

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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- Northerly Island To Calumet Harbor Il- 946 Pm Cdt Mon May 6 2024
Rest of tonight - East winds 10 to 15 kt becoming southeast late. Mostly clear late 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 070133 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 833 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.

UPDATE
Issued at 833 PM CDT Mon May 6 2024

Generally the forecast for the next 24 hours remains on track with very little changes made this evening.

A broad negatively tilted upper trough continues to spread across the central CONUS which has developed a line of strong to severe thunderstorms from South Dakota to Oklahoma. This trough will continue to progress east overnight which will shove the associated line of thunderstorms east as well. As far as northern IL is concerned, this line of storms is forecast to reach the area around daybreak Tuesday morning but in a notably weaker and decaying state. Therefore, the threat for any true severe weather Tuesday morning continues to look virtually zero; however, modest mid-level lapse rates and stout winds aloft may still promote a threat for locally gusty winds with these storms.

The bigger question for northern IL and northwest IN continues to be how quickly the environment will destabilize before an upper level wave and associated 75-80 kt jet streak pivot overhead. Latest guidance trends continue to show destabilization occurring during the late morning and early afternoon hours on Tuesday which should yield around 1000 J/kg of MLCAPE and support at least isolated to widely scattered thunderstorms. However, exact placement of the storms remains uncertain though it does appear that the guidance consensus is to favor areas along and south of I-80 with the greatest coverage. Regardless of where the highest coverage occurs, the aforementioned 500 mb jet will generate 60-65 kts of shear overhead which is more than sufficient to promote storm organization and a threat for severe weather. That said the greatest threat with any severe storms Tuesday afternoon continues to be damaging wind gusts and hail, but modestly strong low-level shear may also support brief tornadoes to occur where surface instability is maximized. While the severe weather timing also remains uncertain, the general consensus is for the greatest threat to be within the 11 AM to 4 PM CDT timeframe.

Additionally, there is also a signal for a second round of strong to possibly severe storms Tuesday evening, mainly across northern IL. Obviously this secondary threat largely depends on the afternoon round so details remain much muddier at this time.
Given the aforementioned uncertainties we recommend staying updated with the latest forecast which will continue to reflect the latest thinking and provide better timing and coverage areas. In the meantime, the going forecast seems to have a decent handle on the latest trends and therefore remains virtually unchanged.

Yack

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 /00Z TAFS THROUGH 06Z WEDNESDAY/
Issued at 621 PM CDT Mon May 6 2024

Relatively quiet conditions characterized by east to southeasterly winds of 10-15kt and occasional bouts of SCT to BKN VFR cloud cover will continue through the first 12 hours of the TAF.

Toward or just after daybreak, a line of decaying showers and thunderstorms is expected to move over the terminals from southwest to northeast. Incoming model guidance appears to exhibit a very modest slowing trend with the arrival of the morning activity. For now, opted to shave off the first hour of the inherited 3-hour TEMPO groups to target the 13-15Z window in the Chicago terminals, plus or minus one hour at GYY and RFD, respectively. Once the line fully develops in the Plains overnight, later TAF packages may offer refinement in the arrival of the morning activity. With the decaying showers and storms expected to be rooted above a near-surface stable layer, confidence is lower than average in whether the morning activity will be accompanied by a wind shift. For now, felt maintaining the inherited southwesterly wind shift with gusts to 25kt was a course of least regret.

Immediately before and after the line of morning showers, MVFR stratocumulus clouds with bases between 2000 and 2500 feet are expected to prevail. Chances for a second round of showers and storms in the vicinity of DPA/ORD/MDW/GYY in the 18-22Z timeframe appear to be increasing (currently around a 40-50% chance), though felt it would best to wait one more cycle of CAM guidance before adjusting the inherited PROB30 groups.

Behind all convective activity tomorrow afternoon, southwesterly winds will prevail as MVFR cigs erode.

Borchardt

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

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