Sunrise 5:23 AM   Sunset 8:12 PM Moonrise 4:29 PM   Moonset 3:27 AM

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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- Northerly Island To Calumet Harbor Il- 906 Am Cdt Sun May 19 2024
Rest of today - North winds 10 to 15 kt. Mostly Sunny. Waves 1 to 3 ft.Tonight - Northeast winds 10 to 15 kt becoming southeast 5 to 10 kt overnight. Partly cloudy. Waves 1 to 2 ft. Monday - South winds 10 to 15 kt increasing to 15 to 20 kt. Slight chance of showers and Thunderstorms in the morning, then showers and Thunderstorms likely in the afternoon. Waves 1 to 3 ft. Monday night - South winds 10 to 15 kt. Showers and Thunderstorms likely in the evening, then chance of showers and Thunderstorms overnight. Waves 1 to 2 ft.

LMZ700
No data

FXUS63 KLOT 191736 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1236 PM CDT Sun May 19 2024

KEY MESSAGES

- A few additional isolated showers and storms possible this afternoon and early evening, mainly south of the Illinois and Kankakee River Valleys.

- Summer-like warmth expected through Tuesday night, interrupted only by lake cooling Sunday; then during any showers/storms Monday and Tuesday.

- Scattered thunderstorms likely Monday afternoon and evening across portions of northern Illinois, with a low end threat for a couple/few isolated severe storms.

- Scattered (mainly non-severe) thunderstorms possible Tuesday morning and then after a break, thunderstorm chances return Tuesday evening/night, including some threat for severe wx, mainly western CWA

DISCUSSION
Issued at 319 AM CDT Sun May 19 2024

Through Tonight:

A cold front is passing over northeastern Illinois this morning. It has just enough to lift and moisture for cloud development to show up on satellite. However, pre-frontal low layer dry air as seen on the 00Z DVN sounding (less than 45 percent surface RH) seems too have been too dry to overcome.
Given the convection on satellite, it seemed fit to keep PoPs at a slight chance, below 20 percent, along the front as it passes. And as dew point depressions continue to decrease into the morning, perhaps there are some brief moments of light rain, but confidence remains low and chances fizzle out after day break.

No major changes to the forecast in that the cold front is expected to gradually slow down and stall along and south of I-80. Winds behind the front are out of the northwest and gradually becoming east northeast through the morning.
Temperatures today will still increase and be unseasonably warm in the 80s for most parts, but the onshore flow will help cooler maritime air protrude deeper inland allowing Lake IL, Cook, Lake IN, and Porter County have better chances to remain in the 70s with 60s close to the shore.

North of the pseudo stationary front will be lower dewpoints in the 40s and 50s, while along and south of the boundary will be dew points in the low 60s. In an uncapped environment this afternoon, over 1000 J/kg of MLCAPE can be tapped into for some showers and even thunderstorms to develop in the afternoon, mainly south of the Illinois and Kankakee River Valleys. With such weak wind shear, severe thunderstorms are not expected. The next upper level trough moves over western Illinois after midnight driving a reintroduction of PoPs into the western forecast area before 6 AM ahead of the next weather system which will be discussed in greater detail in the discussion below.

DK

Monday & Monday night:

Well defined vort max evident on water vapor imagery near Las Vegas early this morning is progged to emerge out onto the central High Plains later tonight and should be the impetus for strong to severe thunderstorm development this afternoon over the central Plains. This convection is likely to organize into a MCS, fed by a 40-50kt low level jet tonight. Models continue to strongly suggest that the incipient shortwave trough will become convectively augmented/enhanced tonight as it moves into the Corn Belt.

Decaying/weakening MCS is likely to move into WI and northwestern IL Monday morning. Should see strong heating of moderately moist air mass in advance of this weakening MCS. Guidance is supportive of 1000-2000 J/kg of MLCAPE across the portions of our CWA that aren't convectively overturned or disturbed by whatever is left of the morning MCS. Monday afternoon would anticipate either re- intensification of the morning convection or new robust development along remnant outflow boundary.

Shortwave trough is expected to provide a glancing blow of synoptic ascent, mainly early in the afternoon. Shortwave and associated speed max are both expected to quickly translate east of our longitude during the afternoon, leaving quickly weakening shear profiles in its wake over our CWA Magnitude of the progged instability would still support a pulse/isolated/brief type severe weather risk Monday afternoon with more robust updraft. Convection should dissipate rather quickly Monday evening with the loss of daytime heating given the lack of synoptic support with heights rising in the wake of the departing shortwave.

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. Cannot rule out some weakening remnants of this MCS bringing a chance of rain to mainly our northwest CWA Tuesday morning, however, a warm front is progged to be surging rapidly northward Tues morning with better support for convection quickly shifting well north of our CWA by mid-late morning.

The GFS continues to be a quick outlier and consensus with neighboring offices this morning was that the GFS solution is likely too quick and has been largely discounted. The remainder of the guidance (ECMWF, NAM, GEM) all would support a windy, unseasonably warm (almost hot) day Tuesday afternoon with minimal rain chances in our area.

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 CWA until probably mid-evening 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, certainly can't rule out a threat for severe convection, mainly in our western CWA Tuesday evening/night. 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.

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:

GFS and its ensembles continue to be on their own Wednesday with respect to our weather, in particular the precip chances. A closer look at the GFS 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 then seems to break out convection rooted between 825-875mb near the elevated front. The GFS is all by its lonesome with this solution with ECMWF and GEM (and their respectable ensembles) strongly supporting a cleaner fropa with breezy, seasonably warm, and dry conditions Wednesday. Consensus with neighboring offices was to remove/dramatically lower pops Wed into Wed night.

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 over the weekend. Timing, track, intensity of this system still subject to change this far out, but certainly gradually increasing pops offered up by the NBM look very reasonable.

- Izzi

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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