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LMZ741 Expires:202405042130;;609438 Fzus53 Klot 041421 Nshlot
nearshore marine forecast national weather service chicago/romeoville il 921 am cdt Sat may 4 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>742-042130- winthrop harbor to wilmette harbor il- wilmette harbor to northerly island il- northerly island to calumet harbor il- 921 am cdt Sat may 4 2024
Rest of today - Southeast winds 5 to 10 kt increasing to 10 to 15 kt this afternoon. Mostly Sunny. Waves 1 to 2 ft.Tonight - South winds 10 to 15 kt becoming northwest overnight. Scattered showers and Thunderstorms. Some Thunderstorms may produce gusty winds. Waves 1 to 2 ft. Sunday - North winds 15 to 20 kt becoming northeast 10 to 15 kt. Decreasing cloudiness. Waves 1 to 3 ft building to 2 to 4 ft. Sunday night - Northeast winds 5 to 10 kt. Partly cloudy. Waves 2 to 4 ft subsiding to 1 to 2 ft.

LMZ700
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FXUS63 KLOT 041421 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 921 AM CDT Sat May 4 2024

KEY MESSAGES

- A line of scattered thunderstorms is expected to cross the area late this afternoon into mid-evening, with locally strong to severe wind gusts to 60 mph possible.

- Periodic shower and thunderstorm chances are expected next week. There is a potential for strong to severe storms along with a locally heavy rainfall threat on Tuesday.

UPDATE
Issued at 921 AM CDT Sat May 4 2024

A recent hand surface analysis reveals a 1009mb surface low pressure system centered in central Iowa ahead of a cold front arcing southward across far eastern Kansas and central Oklahoma.
An MCV is evident ahead of the front just to the northeast of Kansas City along the nose of a developing instability plume arcing northward along the backside of a surface high pressure system centered over southern Ontario. A narrow line of showers and storms approaching the Mississippi River continues to outpace the developing instability gradient and cold front to the west, and accordingly, should continue to decay. With that said, we can't rule out a shower survives as far east as I-39 by early afternoon.

The forecast for this afternoon and evening appears to be on track. Pockets of clearing ahead of the cold front and northward-progressing instability plume will allow for steepening low-level lapse rates and eroding inhibition. As a result, showers and thunderstorms appear poised to erupt along and ahead of the front this afternoon near the Mississippi River and spread eastward into northern Illinois and eventually northern Indiana this evening (reaching I-39 in the 5 to 7 pm timeframe, Chicago metro in the 6 to 8 pm timeframe, and northern Indiana in the 7-9 pm timeframe). With the upper-level jet displaced to our north, thunderstorms will be limited to "pulse" like single-cell behavior. However, somewhat linear forcing ahead of the cold front, as well as relatively dry low- level moisture profiles, will afford efficient amalgamation of surface outflow/cold pools and support "upscale growth" of initial single-cell thunderstorms into gusty multicell clusters.

While the environment doesn't appear supportive of widespread severe weather, prospects for a wall of 45-55 mph wind gusts along the leading edge of any consolidated thunderstorm outflows appears to be increasing this evening. Where thunderstorm pulses are most intense, locally damaging wind gusts of 60-65 mph may occur, as well. We'll have to keep a close eye on the integrity of the aforementioned MCV as it tracks toward central Illinois this afternoon, as it may provide just the oomph needed for a more focused threat area for damaging winds.

Updated products have been sent.

Borchardt

DISCUSSION
Issued at 314 AM CDT Sat May 4 2024

Through Sunday:

A retreating surface ridge extending southwestward across the western Great Lakes will give way to ongoing lee cyclogenesis ahead of a mid-level trough entering the central Great Plains early this morning. Robust convection from northern OK into western IA has likely enhanced the trough, with multiple MCVs evident along this corridor. An associated cold front just behind this convection will become the main focus for storm potential into our area later this afternoon into this evening.

Surface dew point trends today will be an important factor on the coverage and intensity of storms into the forecast area.
Friday's weak cold front has since stalled across central Illinois and will jump northward in response to diurnal warming and moistening today. However, the magnitude of moistening primarily from evapotranspiration remains less clear this afternoon. Current thought is daytime mixing into a very dry layer above the growing PBL will with offset rising dew points or at least reduce the depth of moisture sufficient for development and maintenance of convection. Have opted to use only scattered thunderstorm wording even as some CAMs depict higher coverage as a result of higher modeled dew points.

While it is expected that a more prominent MCV currently near Omaha will lift NE within the broader mid-level steering flow this morning, the extreme southern influence of this MCV may clip the northwest CWA late this morning. Capping should remain far too high to support convection this far southeast, but isolated attempts at convection may reach as close as far northwest Illinois.

Overall expectations are for a line of broken showers and storms to develop/intensify roughly along and around the Mississippi River by mid-afternoon within a pre-frontal ribbon of higher theta-e air. Coverage of thunderstorms should then begin a slow downward trend across the forecast area late afternoon through mid-evening as the drier pre-storm environment becomes less favorable with time. However, given an increasing reservoir of DCAPE with eastward extent across the CWA, the potential for strong to locally severe wind gusts may actually maximize across the west half of the CWA up to and through sunset. The antecedent dry air ahead of these storms combined with diurnal mixing of low-level moisture speaks to this wind risk, with the strongest winds likely occurring with any existing loosely organized multi-cell cluster or collapsing cores as storm intensity wanes with time.

After the storms and cold front clear the forecast area by early Sunday morning, dry and cooler conditions are expected through the remainder of the period.

Kluber

Sunday Night through Friday:

Quiet weather starts the period Sunday night, as surface high pressure is progged to be drifting east across the Great Lakes region. Farther south, guidance continues to depict mid-level short wave approaching IL late, though it is deamplifying as it moves into the short wave upper level ridge axis. Forecast soundings depict very dry low and mid-levels initially in place across the forecast area, though persistent isentropic ascent and associated warm/moist advection does eventually produce saturation and precipitation development into central IL/IN and the LOT/ILX border region after midnight. Mid-level lapse rates are unimpressive in forecast sounding guidance, suggesting little/no thunder threat. Showers are expected to persist into Monday across the southern parts of the cwa as the mid-level wave continues its transit of the area. NBM blended pop guidance appears to spread too far north in this scenario, and note that much of the EPS/GEPS/GFS ensembles are dry along/north of the I-88 corridor through the day across northern IL.

Farther west, guidance continues to depict a deep upper level trough across the western CONUS, within which a strong, negative-tilt short wave develops a closed upper low over the northern high plains by Monday night. Along with a series of short waves propagating around the southeast periphery of the upper circulation and upper level diffluence ahead of a strong cyclonically-curved upper jet, this sets the stage for southerly low level warm/moist advection from the western Gulf into the mid-Mississippi Valley and upper Midwest Monday night, with a northward lifting warm front spreading into the area into Tuesday morning. Thus shower and thunderstorm chances look to ramp-up into the forecast area from the west especially after midnight and into Tuesday morning. While the primary upper/surface lows will be well to our northwest Tuesday, guidance continues to depict a strong mid-level jet is progged to spread east across the area during the day, with a surface wave developing along an occluding cold front approaching the Mississippi Valley later in the day. Combined with a warm, moist and conditionally unstable low- level air mass, additional thunderstorms (potentially strong to severe) are likely within the region Tuesday afternoon/evening. In addition to severe weather, high precipitable water values around 1.75 inches will support locally heavy rainfall will be possible especially if storms train over a particular area. SPC and WPC outlooks for Tuesday for both severe and heavy rainfall potential continue to appear appropriate at this distance.

The upper trough is forecast to drift east across the upper Midwest and northern Great Lakes region during the mid and late week period, maintaining an active weather pattern across our forecast area. While the primary low-level theta-E axis will shift southeast of the forecast area Wednesday through Friday, a series of short waves rotating through the southern periphery of the upper Midwest upper low will likely produce additional periods of showers and thunderstorms, especially during the diurnally more favorable afternoon and evening hours.

Temperatures will remain above average through most of the period, with daytime highs in the 70s to low 80s (Tuesday and Wednesday have the potential to be the warmest days). Cooler, more average temperatures are expected by Friday, with highs in the mid-60s.

Ratzer

AVIATION /12Z TAFS THROUGH 18Z SUNDAY/
Issued at 607 AM CDT Sat May 4 2024

Aviation Key Messages:

- IFR stratus may approach KGYY from the east this morning, but should lift/scatter as it does.

- Scattered gusty thunderstorms this afternoon/early evening.

- MVFR ceilings likely behind a cold front late tonight into Sunday morning.

- Winds become southeast this morning, then south-southwest around midday. Winds may become somewhat variable behind storms this evening, before shifting north-northeast behind the cold front tonight, and eventually northeast Sunday morning.

Weak surface low pressure was over south central IA this morning, with a cold front trailing into the southern Plains.
This low and cold front will move east-northeast to the western Great Lakes by early this evening, with high pressure building eastward into the region Sunday morning.

Early this morning, an area of LIFR stratus/fog had developed across MI and far northern IN, and was moving west-northwest.
With the sun already up and mixing commencing, expect that this will lift and scatter especially over northwest IN, though will approach KGYY as it does so. Can't completely rule out a brief period of IFR conditions there, but think the stratus deck will scatter prior to reaching the airport. Otherwise, southeast winds will eventually turn south-southwest across the terminals by midday, under thickening VFR high clouds. Earlier high-res model runs had moved the lake breeze inland through KORD and KMDW later this afternoon, but have since backed off of that solution and it now appears the boundary will remain east of those airports.

Farther west, an area of showers and thunderstorms was noted across southern MN/IA and MO. Expectation is that these storms will weaken, with dissipation across southern IA/MO this morning. Scattered storms are then expected to redevelop this afternoon in the vicinity of the MS river, then push east across northern IL. Storms are expected to reach KRFD around 21Z, and the metro terminals 23-00Z, albeit in a gradually weakening state. Storms may produce wind gusts in excess of 30 kts. Winds may become a bit variable behind the storms later this evening, though the gradient would support a westerly direction. The cold front will then bring a wind shift to the north- northwest overnight, with winds eventually turning northeast Sunday morning. An extensive area of MVFR ceilings will likely follow the cold front, lingering into Sunday morning.

Ratzer

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

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