Sunrise 5:24 AM   Sunset 8:09 PM Moonrise 4:28 PM   Moonset 3:26 AM

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LMZ741 Expires:202405200330;;499629 Fzus53 Klot 192031 Nshlot
nearshore marine forecast national weather service chicago/romeoville il 331 pm cdt Sun may 19 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>742-200330- winthrop harbor to wilmette harbor il- wilmette harbor to northerly island il- northerly island to calumet harbor il- 331 pm cdt Sun may 19 2024
Tonight - Northeast winds 10 to 15 kt becoming southeast 5 to 10 kt. Partly cloudy in the evening, then becoming mostly cloudy. Waves 1 to 2 ft. Monday - South winds 10 to 20 kt increasing to 15 to 20 kt in the afternoon. Showers and Thunderstorms in the afternoon. Waves 1 to 3 ft. Monday night - Southwest winds 10 to 15 kt becoming south 5 to 10 kt. Showers likely and a chance of Thunderstorms in the evening, then chance of showers and slight chance of Thunderstorms overnight. Waves 1 to 3 ft. Tuesday - South winds 10 to 15 kt increasing to 30 kt. Chance of showers and Thunderstorms. Waves 1 to 3 ft.

LMZ700
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FXUS63 KLOT 192015 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 315 PM CDT Sun May 19 2024

KEY MESSAGES

- Period of showers and thunderstorms Monday afternoon, some possibly severe with locally heavy rainfall.

- Summer-like warmth expected through Tuesday night, interrupted only by showers/storms Monday and Tuesday.

- Breezy to windy on Monday and Tuesday, with southwesterly gusts up to 30 mph on Monday, and southerly to south- southwest gusts up to 30-40 mph Tuesday afternoon and evening.

- Scattered (mainly non-severe) thunderstorms possible Tuesday morning and then after a break, thunderstorm chances return Tuesday evening/night, including some threat for severe weather, primarily for the western CWA (near/west of I-39).

DISCUSSION
Issued at 315 PM CDT Sun May 19 2024

Through Monday Night:

A cooler and much drier airmass continues to work in across northern IL on east-northeasterly winds along the southern periphery of a surface high centered over southern WI. However, the higher theta-e airmass, which has been destabilizing all morning, is not that far away. As if this writing, it resides in closer proximity to a west-to-east oriented surface frontal boundary near or just south of US-24 across central parts of IL and IN. Regional satellite imagery has been showing some building CU near this boundary over the past couple hours, with some of the better CU development currently located across southern Iroquois east-northeastward into Benton county, IN. This is the general area we will be watching for isolated thunderstorm development over the next couple hours as any remaining weak capping continues to quickly erode.

While weak flow through the troposphere will curtail any threat for organized storms into this evening, a high amount of DCAPE (in excess of 1,000 j/kg) and very steep low-level lapse rates will support the potential for some pulse type storms capable of producing isolated instances of gusty winds up to 45 mph. The weak steering flow will also result in nearly stationary to very slow moving storms (at 10 mph or less), so some very localized instances of heavy rainfall could materialize. These isolated storms should diurnally weaken after sunset this evening.

The only other area of showers and storms we will have to keep an eye on into this evening is the area of weakening convection currently moving across IA. Fortunately, this activity is expected to continue weakening with eastward extent evening as it moves into a drier and more hostel environment across northern IL.
We should thus end up with a relatively quiet night for the area.

Our weather will begin to turn active on Monday as the frontal boundary to our south lifts northward and opens up the gates for the moist and unstable airmass to our south to return northward.
Meanwhile, a notable mid-level impulse is expected to become convectively enhanced tonight by a severe complex of storms over KS. This feature is expected to be a key player with our threat of thunderstorms Monday afternoon. It appears that some of the decaying remnants and outflow from the Plains complex will be shifting eastward across eastern IA by mid to late Monday morning. Assuming that lingering cloud debris and outflow from this dyeing MCS does not hinder the northward return of the high theta-e airmass to the south, renewed thunderstorm development is likely (70%+ chance) as the airmass over northern IL destabilizes through the morning. Thunderstorms could develop over northern IL as early 1 or 2 in the afternoon.

Deep layer flow, and the associated deep layer shear, will increase modestly for a period as the main impulse shifts over southern WI into the afternoon. Deep layer shear is not expected to become overly impressive. However, with the prospects for up to 30 to 35 kt deep layer bulk shear, it certainly seems plausible that some organized multi-cell clusters and possibly a few supercells will materialize. Instances of damaging winds and hail would be the main threat from these storms Monday afternoon. Some locally heavy rainfall will also be possible with these storms as PWATs increase to around 1.5". While significant widespread flooding is not likely, some minor instances of flooding could result. This would be particularly true over the Chicago urban areas, along with any areas that experience a period of east- northeastward training of storms. The threat for these storms will wane into Monday evening following the eastward departure of the main mid- level impulse.

KJB

Tuesday through Sunday:

Tuesday & Tuesday night:

Strong to severe convection is once again expected to develop over the central Plains Monday evening and probably congeal into an MCS that will track toward the mid-Mississippi Valley. Some weakening remnants of this MCS will bring a chance of widely scattered showers and embedded non-severe thunderstorms to mainly areas near and north of I-80, where our forecast has 30-40% PoPs. A warm front is progged to be surging rapidly northward Tuesday morning with better support for convection quickly shifting well north of our CWA by mid day. The GFS finally trended toward the general consensus of the remainder of the guidance (ECMWF, NAM, GEM), that would support a windy (southerly gusts up to 30-40 mph) and unseasonably warm (almost hot) Tuesday afternoon, with minimal rain chances in our area after any AM lingering AM convection dissipates.

Explosive severe thunderstorm development is likely over central or western Iowa into northwest/north central Missouri Tuesday afternoon. This convection likely wouldn't reach our western CWA until after sunset Tuesday, if not later. By the time it arrives, boundary layer will be nocturnally stabilizing with growing MLCINH and weakening MLCAPE. The most likely scenario would be storms being in a weakening phase as they arrive Tuesday evening.
Depending on the extent of mesoscale organization that takes place, still certainly can't rule out a threat for severe convection, primarily in our western CWA through mid evening.

Current thinking is similar to our previous forecast, that areas near/west of I-39 in line with SPC's day three level 3 (enhanced risk) are most favored for the evening severe threat. If there is a severe threat it would likely be in the form of damaging winds and perhaps a QLCS tornado threat, but again, this will depend heavily on strong mesoscale organization of convection that allows it to overcome growing and increasingly large MLCINH over our area Tuesday night. System dynamics and strong southerly low-level flow and warm/moist advection plus a well organized MCS may be enough to bring the severe threat farther east across the level 2 (slight risk) area. Though this appears less likely at this time with the current thinking on timing, faster eastward propagation of the expected MCS may keep the severe threat going deeper into the evening and early overnight farther east.

Progged 925mb temps continue to support high temperatures making a run at the lower 90s. Given uncertainties on potential debris cloudiness, didn't make any meaningful changes to the NBM's mid- upper 80s Tuesday, but quite plausible that some areas breaking the 90 degree mark. If temps do overperform, that would increase the chances of boundary layer moisture mixing out, potentially lessening instability/increasing inhibition a bit more Tues night.

Wednesday & Wednesday night:

The GFS and its ensembles (and the typically unreliable outer ranges of the NAM) continue to be on their own Wednesday with respect to our weather, in particular the precip chances. A closer look at the GFS and NAM reveals that it is forecasting Tuesday night's cold front to be more anafrontal in nature with the front at 850mb still displaced a couple hundred miles northwest of the sfc cold front. The GFS/NAM then seem to break out convection rooted between 825-875mb near the elevated front. This GFS/NAM solution is an outlier versus the ECMWF and GEM (and an overwhelming majority of their respective ensemble members), which continue to strongly support a cleaner frontal passage with breezy, seasonably warm (70s to around 80F), and dry conditions Wednesday. If the front slows a bit, portions of the southeastern CWA (primarily southeast of the Kankakee River Valley) could have a chance for a few isolated showers and thunderstorms to develop in the afternoon.

Thursday and beyond:

Thursday looks to be sunny and seasonably warm with comfortable humidity as the baroclinic zone moves well south of the area.
Next northern stream shortwave should result in increasing chances of showers and storms again, perhaps as early as late Friday, but more probable Friday night into Saturday. Timing, track, intensity of this system is still subject to change this far out, but varying chance range PoPs in the official forecast appear reasonable. It will be at least a few days until we can hone in on specifics for the all-important Memorial Day Weekend forecast regarding effects on the many planned outdoor activities.
Temperatures don't appear they will be cooler than normal, but not as warm as what we experienced this weekend (unfortunately, for pool and beachgoers).

- Izzi/Castro

AVIATION /18Z TAFS THROUGH 00Z TUESDAY/
Issued at 1236 PM CDT Sun May 19 2024

Key Aviation Messages: - Scattered strong TS expected across northern Illinois and northwest Indiana early afternoon through early evening Monday.

NE winds of 10-15 knots at TAF issuance will slightly veer ENE or E this afternoon. Winds will continue to gradually veer while diminishing under 10 knots tonight, with the direction settling SSW around sunrise Monday. Winds will remain SSW/SW through the day Monday while gusting to around 25 knots by late morning.

One or two waves of TS are expected to develop across the area during the early afternoon. Any TS will be capable of producing strong to potentially severe wind gusts. There is medium to high confidence of occurrence of TS in the area through much of the afternoon into early evening, but confidence in coverage remains low enough to preclude TEMPO TS at this time. However, opted to prevail VCTS at ORD/MDW beginning at 18Z as at least isolated TS coverage should develop west of the terminals around or shortly after this time.

Kluber

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

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