Sunrise 5:38 AM   Sunset 7:56 PM Moonrise 4:17 AM   Moonset 5:13 PM

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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- Northerly Island To Calumet Harbor Il- 252 Pm Cdt Sun May 5 2024
Tonight - Northeast winds 5 to 10 kt. Mostly cloudy. Waves 1 to 3 ft. Monday - East winds 5 to 10 kt becoming northeast. Mostly Sunny. Waves 1 to 2 ft. Monday night - East winds 10 to 15 kt becoming southeast overnight. Mostly clear. Waves 1 to 3 ft. Tuesday - Southeast winds 10 to 15 kt becoming south in the afternoon. Showers and a chance of Thunderstorms. Waves 2 to 4 ft.

LMZ700
No data

FXUS63 KLOT 052310 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 610 PM CDT Sun May 5 2024

KEY MESSAGES

- No impactful weather is expected over the next 36 hours.

- Waves of showers and thunderstorms are expected Tuesday morning through Sunday, though there will be plenty of dry hours.

- Confidence in severe weather in our area on Tuesday is lowering, but remains for Wednesday especially along and south of I-80.

DISCUSSION
Issued at 126 PM CDT Sun May 5 2024

Through Monday:

No impactful weather is expected over the next 36 hours as the center of a surface high passes through Wisconsin and Michigan.
Lows tonight will fall into the mid to upper 40s beneath broken to overcast upper-level clouds and with nearly calm winds.
Tomorrow will be partly sunny with highs in the low to mid 70s, except the low to mid 60s near Lake Michigan where easterly onshore flow will prevail. A shower or two cannot be ruled out during the afternoon mainly south of I-80 near a weakening upper-level wave passing through the Ohio River Valley (only a 10-15% chance).

Borchardt

Monday Night through Sunday:

A strong upper level low will strengthen overnight Monday into Tuesday, with a deepening surface low over the Northern Plains.
A strong negatively tilted short wave on the eastern side of the upper-level low is expected to propagate northeastward into the mid-upper Mississippi Valley and western Great Lakes region Tuesday morning with a strong 80-90 kt jet aloft. With increased forcing and isentropic ascent over an northward propagating warm front, showers and thunderstorms are expected to move into Illinois from daybreak through mid Tuesday morning. The risk for severe weather with this first wave of storms is low given model soundings projecting only modest instability.

The forecast for Tuesday afternoon continues to evolve. The main uncertainty with Tuesday afternoon will become how much clearing happens behind the first line of showers to destabilize and allow for storm redevelopment associated with a weak secondary area of lower pressure developing over southern Wisconsin. Of note, ensemble models have trended weaker and weaker with that low developing since Friday of last week, leading to continued southeastern trend to the instability axis and a lower moisture return. That may be what the 12Z run of the NAM is keying in on. It has a slightly later arrival of the morning wave of showers (mentioned in the previous paragraph)
with almost no low developing in Wisconsin. If the NAM scenario plays out, the area of afternoon redevelopment could be as far away as Central and Southern Indiana and have limited to almost no storms in the afternoon/evening for Northern Illinois. And yet, the 12Z GFS & Euro is still suggesting a slightly (relative to the NAM) stronger surface low that can assist with better destabilization that could provide a second round of storms in the afternoon. Should that play out and some clearing in the cloud cover behind the morning line, over 1500 J/kg CAPE over the region with upwards of 50 knots of deep layer shear can be realized and there could be a second round of showers and storms that may potentially involve severe weather. In short, the Tuesday forecast has higher confidence in showers and non-severe storms in the morning, with now lower confidence in the potential for showers and thunderstorms developing in the afternoon given the disparate model projections.

Models have still remained consistent with showing the upper level low start to weaken, wobble and transition into a positively tilted trough through Wednesday. The surface low over the Northern Plains occludes and weakens as a secondary low over Oklahoma will strengthen and lift with an associated wave aloft to the northeast. As it moves, southwesterly winds will allow better moisture to move over Illinois with a warm front pushing northward. Where that boundary sets up will determine the overall storm threat for the day, with models suggesting that it could set up somewhere around or just south of I-80.
With model soundings suggesting another potential for 1500-2000 J/kg of CAPE and deep layer shear around 60 knots, storms could be strong to even severe, including some localized flooding potential along and south of the boundary.

The upper level wave will broaden through the end of the week becoming a positively tilted long wave trough over eastern Canada. Persistent cool air from northwest flow will allow for temperatures to fall back into the 60s for the end of the week.
Weaker and smaller waves may pass around the longer wave Thursday and Friday providing the ability to perturb the environment along with diurnal heating to create some scattered showers in the afternoon through at least Saturday, though exact time and location of showers at this scale would not be understood until higher res guidance can have a go at trying to resolve it. Looking at the extended long range, models are suggesting that that upper level wave moves east and sets up along the Atlantic coastline. Meanwhile, a long wave ridge will slowly grow and park itself over the Rockies, thereby placing Chicago underneath weaker northwest flow. With the exception of some smaller scale features changing it, days with general showers are certainly still possible, but diminishes any larger severe signal for most of the rest of the month.

DK

AVIATION /00Z TAFS THROUGH 06Z MONDAY/
Issued at 610 PM CDT Sun May 5 2024

No forecast concerns for the terminals as high pressure will remain in control through the period. Expect SCT to BKN VFR cirrus to increase tonight and linger into Monday morning before diminishing Monday afternoon. Winds will remain easterly through the period with speeds around 5 kts overnight increasing to around 7 to 9 kts Monday afternoon.

Yack

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

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