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LMZ741 Expires:202405230915;;693984 Fzus53 Klot 230205 Nshlot
nearshore marine forecast national weather service chicago/romeoville il 905 pm cdt Wed may 22 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>742-230915- winthrop harbor to wilmette harbor il- wilmette harbor to northerly island il- northerly island to calumet harbor il- 905 pm cdt Wed may 22 2024
Rest of tonight - West winds 10 to 15 kt becoming southwest 5 to 10 kt. Clear. Waves 1 to 3 ft subsiding to around 1 ft. Thursday - Southwest winds 5 to 10 kt becoming east 10 to 15 kt. Sunny. Waves around 1 ft. Thursday night - Southeast winds 10 to 15 kt. Clear. Waves around 1 ft. Friday - Southeast winds 10 to 15 kt increasing to 15 to 20 kt. Isolated showers and Thunderstorms in the morning, then numerous showers and Thunderstorms in the afternoon. Waves 1 to 3 ft.

LMZ700
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FXUS63 KLOT 230541 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1241 AM CDT Thu May 23 2024

KEY MESSAGES

- Tomorrow will be quiet with highs in the upper 70s to lower 80s. A lake breeze will turn lakeshore locations cooler.

- Scattered thunderstorms are likely on Friday afternoon/early evening, a few of which may be strong to perhaps severe.

- For the holiday weekend, a dry and pleasant Saturday, waves of showers and scattered t-storms (some severe threat) on Sunday, then cool, breezy, and possibly showery conditions are expected on Memorial Day.

UPDATE
Issued at 812 PM CDT Wed May 22 2024

Only minor adjustments to the forecast this evening, mainly to lower overnight low temperatures a bit especially in our typically cooler spots. Otherwise, mainly clear and pleasant conditions are expected into Thursday.

Evening surface analysis depicts the cold front,which pushed through the area last night and early this morning, now stretched from near Cleveland to the Missouri Bootheel. Much drier air has spread into our area in the wake of the front, noted by surface dew points mixing out into the upper 30s to low 40s in many spots across the cwa this afternoon. Weak surface high pressure will move east across the forecast area later tonight, allowing winds to diminish significantly and set up good radiational cooling conditions under clear skies. With this in mind, have lowered overnight lows outside of the core of the Chicago metro into the lower 50s in many spots, and some upper 40s in our typical cool locations west of Chicago.

Winds will shift light southerly across the area Thursday as the bubble high moves east, with temperatures expected to warm into the lower 80s in most areas under sunny skies. The gradient will remain weak enough however, to allow a lake breeze to develop in the afternoon and keep temperatures lower (upper 60s)
close to the lake shore. Our going forecast has all of this handled nicely, so other than lowering overnight temps as indicate above, no significant changes were made to the forecast.

Ratzer

DISCUSSION
Issued at 228 PM CDT Wed May 22 2024

Through Thursday night:

There are no weather concerns through the short-term period.
Breezy west winds this afternoon will ease after sunset and even become calm by daybreak Thursday. The passing of a surface high pressure system will keep winds weak throughout the day on Thursday, and allow for a lake breeze to surge inland during the afternoon. Sunny skies will boost highs into the upper 70s to lower 80s. Tomorrow night will similarly be quiet with light winds and clear skies.

Borchardt

Friday through Wednesday:

With dry high pressure overhead through Thursday night/early Friday, there are question marks on how quickly an elevated instability plume can advance eastward across the MS River and into our area. Some of the faster guidance brings decaying convection into the western half of the CWA or so as early as the mid to late morning hours, while the slower (ie. most recent ECMWF and Canadian) guidance hangs onto dry conditions through the early afternoon.

A relatively narrow plume of 60s dew points will be drawn northeastward ahead of a cold front. Along with temps in the low-mid 80s, and mid-level lapse rates around 7-7.5 C/km, minimally capped MLCAPE will build up to around 1500 J/kg. As large scale ascent overlaps the front, paired with frontal convergence, scattered mainly pulse-type thunderstorms are probable in the afternoon into the evening before shifting out of the area along with weakening with post-sunset stabilization.
SPC included our whole area in a level 1 (marginal risk) severe threat, mainly for isolated damaging winds (downbursts) and marginally severe hail. Aside from the convection onset uncertainty, the other item to monitor will be for any potential MCV to enhance wind fields and possibly increase the threat somewhat for organized severe storms, including supercellular structures.

For the all important Memorial Day Weekend forecast, the unofficial start of summer 2024, there's some good news and some bad news. First, the good news: underneath weak surface high pressure, Saturday looks pleasant, dry, and seasonable with highs in the 70s, except cooler near the lake due to lake breeze associated cooling. The pleasant conditions should persist through the evening for holiday weekend outdoor activities.

Now, onto the bad news: Sunday is looking inclement as an unseasonably strong synoptic system and surface low affect the region. Lead showers and scattered thunderstorms spreading northeast in the pre-dawn hours of Sunday will overspread the rest of the area through the morning hours. Warm/moist advection ahead of the surface low, as a warm front lifts north, will boost dew points back to the lower to mid 60s south of I-88, with the northward extent of the warm front and moist sector dependent on the exact storm track. While cloud cover and ongoing rain will limit destabilization, especially the farther north you go, there may be enough heating for temps to reach the 70s. Thus, near and south of I-80, despite, again, instability appearing to be a limiting factor, the moist adiabatic lapse rates could still support pockets of 1000-1500 J/kg of uncapped MLCAPE during the afternoon.

SPC already included areas south of I-80 within our CWA in day 5% severe probs (level 1), which appears reasonable given the current forecast thinking on instability. Potential hazards include damaging winds and brief tornadoes, whether from low- topped mini-supercells, which appear possible per forecast hodograph shape (small but with strong low-level shear), or from a QLCS mode (considering aforementioned strong low-level shear). A farther north track of the low-mid 990s mb surface low would also entail the brief tornado threat correspondingly shifting northward near the warm frontal zone, if sufficient 0-3 km CAPE can materialize.

Aside from the somewhat conditional severe weather threat, confidence was high enough to feel comfortable with the likely (60-70%) PoPs offered by the NBM. Outdoor plans could certainly be in jeopardy given the current forecast. Finally, seasonably high PWATs and fairly deep warm cloud depths may also support a localized flooding threat (WPC already has us included in its day 5 level 1 (marginal risk) ERO. Shower and thunderstorm coverage will wind down Sunday night as the surface low tracks into the central Great Lakes, though deformation type showers may hang on longer across the far northern CWA

Monday (Memorial Day) currently looks to have less than ideal conditions for holiday BBQs and pool/beachgoers, albeit with a much lower, if any chance for thunderstorms. Pattern wise, it appears unlikely to be a wash-out of a day, but on and off showers are plausible, with a bit higher coverage (up to 50%)
perhaps focused across the northeast half or so of the CWA In addition to the at times showery conditions, it will be mostly cloudy and cool, with highs in the upper 60s-lower 70s amidst northwesterly winds gusting up to 25-30 mph, if not stronger.
Another short-wave wrapping around the mid-upper trough over the region may bring another bout of showers Tuesday into Tuesday night, possibly followed by a quieter period on Wednesday as high pressure builds in. Cooler than normal temperatures and breezy conditions will likely continue both days.

Castro

AVIATION /06Z TAFS THROUGH 12Z FRIDAY/
Issued at 1223 AM CDT Thu May 23 2024

Aviation Concerns:

- Southwesterly winds hold throughout Thursday (except where lake breeze occurs), before everything turns southeasterly Thursday night.

- Lake breeze sets up and tracks through GYY, MDW, and ORD roughly 19-21z Thursday, turning southwest winds to more east/east-northeast.

A quiet aviation forecast for the period. Southwesterly winds across much of the region will persist tonight into Thursday as the area remains stuck in the middle of the large MCV and trailing boundary positioned to the northeast and south.
Expecting a lake breeze Thursday afternoon, around 19-20z, to track southwest impacting GYY, MDW, and ORD. Still some uncertainty as to time specifics, but within an hour of the 19-21z timeline. With the easterly flow and general diurnal heating, forecast calls for development of FEW to pockets of SCT during the afternoon into the evening. Ceilings should clear out after sunset, with winds becoming more southeasterly Thursday night.

Baker

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