Sunrise 5:23 AM   Sunset 8:13 PM Moonrise 6:38 PM   Moonset 4:07 AM

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LMZ741 Expires:202405212115;;609341 Fzus53 Klot 211707 Nshlot
nearshore marine forecast national weather service chicago/romeoville il 1207 pm cdt Tue may 21 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.
lmz740-741-212115- winthrop harbor to wilmette harbor il- wilmette harbor to northerly island il- 1207 pm cdt Tue may 21 2024
.small craft advisory in effect from 3 pm cdt this afternoon through late tonight - .
Rest of today - Southeast winds 10 to 15 kt becoming south and increasing to 20 to 25 kt. Partly cloudy. Waves around 1 ft building to 1 to 3 ft.Tonight - South winds to 30 kt with occasional gale force gusts to 35 kt becoming southwest after midnight, then diminishing to 15 to 25 kt toward Sunrise. Showers and Thunderstorms in the evening. Some Thunderstorms may be severe with wind gusts over 50 kt. Waves 3 to 5 ft.Wednesday - Southwest winds 10 to 20 kt becoming west. Partly cloudy in the morning then clearing. Waves 1 to 3 ft. Wednesday night - West winds 10 to 15 kt. Mostly clear. Waves around 1 ft.

LMZ700
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FXUS63 KLOT 211742 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1242 PM CDT Tue May 21 2024

KEY MESSAGES

- Severe thunderstorms likely this evening, with potential for widespread, and potentially significant, damaging winds and tornadoes

- Non-thunderstorm winds could gusts over 40 mph from the south this evening, then from the southwest in the wake of the storms later tonight

- There will be dry breaks at times, but an active pattern for showers and storms returns Friday through early next week

UPDATE
Issued at 1142 AM CDT Tue May 21 2024

We continue to closely assess both observational and model trends with the potential for significant severe convection in the region this evening. The effective warm front has already cleared all but far northeast Illinois while temps across much of the remainder of the CWA are making a quick push at reaching 90F. Meanwhile, slight mid-level height rises and strong low- level WAA will cap nearly the entire CWA through the late afternoon. Cannot completely rule out that convection in northeast Iowa clips northwest Ogle and Winnebago counties, but trends support the entire CWA remaining dry through at least 6pm.

Convection has developed across south-central Nebraska just ahead of the surface low, with the associated cold front and negatively tilted trough becoming the primary focus for growing convection across Iowa and northern Missouri this afternoon.
Early trends along with recent CAM guidance support a more northward shift in the core of significant convection this evening, with a notable focus now into southern Wisconsin. This ties into a northwest trend for a more favorable kinematic field, especially given the substantial displacement of the surface low (expected to be over southeast MN) from our area.
This ultimately results in guidance producing a split of more robust convection across a substantial swath of the CWA this evening as dynamics are unable to overcome the remaining low- level capping. However, as is often seen with highly dynamic systems, guidance tends to decouple too quickly after sunset and correspondingly dissipates convection faster than is typically observed. We therefore expect that upscale growth of convection will survive well into the forecast area this evening. Of important note, decaying convection will still support a significant damaging wind risk owing to substantial low-level shear profiles and a pre-convective pool of DCAPE up to around 1500J/kg.

As a whole, we will stay the course with the current forecast and overall messaging, but begin to more closely highlight areas roughly north of a La Salle to Cook County line (and especially closer to the WI line) for higher coverage of significant severe potential. Otherwise, details on specific hazards remain valid from the early morning forecast discussion.

Kluber

DISCUSSION
Issued at 303 AM CDT Tue May 21 2024

Through Wednesday:

Early this morning our CWA is bisected by a warm front located roughly between I-80 and I-88/290. There's been a bit of isolated convection popping up briefly to the north of this front, but brunt of the convective activity is over Iowa and Nebraska, where warm air advection/isentropic ascent is maximized at the terminus of broad 35-55kt nocturnal low level wind maxima. Low level jet is progged to remain west of the Mississippi River through the morning hours, so while a few more isolated showers or even a t-storm cannot be ruled out over our northern CWA, the prospects for widespread organized convection appear low through the remainder of the overnight into the morning hours. We will continue to keep an eye on convection over IA to make sure it doesn't do anything funny, but all indications are that this activity should move northeastward into WI this morning.

The warm front will quickly move north into Wisconsin this morning allowing unseasonably warm and relatively humid air to overspread the CWA today. As convection shifts north into Wisconsin this morning, blow off cirrus should decrease as well, with it looking probable that we'll see a considerable amount of sunshine this afternoon. Given progged 925mb temps climbing into the 23-25C range this afternoon, seems like 90F is within reach across most of the CWA this afternoon. The very warm temps and strong winds will promote deep mixing and will probably see dewpoints mix out a bit this afternoon. The slight dropping of afternoon dewpoints should help keep the warm sector capped with chances of convection in the warm sector during the day less than 15%.

Mid and upper level trough are progged to deepen quickly this afternoon, taking on an increasingly negative tilt while moving toward the upper Mississippi Valley. As this takes place, the associated sfc low will quickly deepen as it lifts north into MN this evening. All signs point toward a potentially significant severe weather outbreak this afternoon into this evening across the region. Impressive elevated mixed layer advecting northeastward into the Midwest, juxtaposed above moderately humid and very warm boundary layer will result in strong instability. Meanwhile, wind fields through the atmosphere will strengthen this afternoon into tonight in response to the deepening cyclone, providing for very favorable shear profiles.

Initial convective development this afternoon will probably be a mix of supercells and/or short line segments over western IA into northern MO. Over time, due to strong linear forcing anticipate this activity to congeal into a fast moving and potentially dangerous squall line as it tracks toward the Mississippi River early this evening. Nocturnal stabilization of the boundary layer over our CWA this evening will be somewhat stunted by the very strong southerly winds (potentially gusting over 40 mph at times). These strong winds should temper the decrease in MLCAPE and increase in MLCIN. Given the strong synoptic forcing and expected organization of the QLCS, it is reasonable to plan for and expect this dangerous squall line to only weaken slowly as it moves across our CWA

Synoptically enhanced southerly low level jet is expected to increase to 45-50kt early this evening in advance of the squall line. These powerful LLJ winds are progged to extend down to 500-1000m agl, creating extreme low level shear. So even with only modest CAPE (500-1000 J/kg), very large, curve low level hodographs and strong 0-3km shear will be favorable for mesovortex tornadoes embedded within this QLCS. Given the extreme nature of the shear, cannot rule out a strong (EF2) QLCS tornado or two, particularly over our western CWA where the line may encounter slightly stronger instability.

In addition to the non-trivial tornado threat, the extremely strong wind fields will provide an opportunity for this squall line to produce widespread damaging winds, potentially locally in excess of 75 mph. The thermodynamic environment this squall line encounters will be gradually becoming less favorable as it reaches our eastern CWA and northwestern Indiana. That should result in some weakening of the QLCS, however, it is not uncommon for these type of strongly forced squall lines to continue to pose a damaging wind threat well into more hostile thermodynamic environments, particularly when background kinematic fields are as strong as they will be this evening.

Behind this squall line, a cold front will sweep across the area overnight with strong and gusty southwest winds expected.
It is possible that there could be synoptic winds flirting with wind advisory criteria both in the warm sector this evening as the LLJ ramps up, and again for a couple/few hours behind the cold front. Given some uncertainties and in the interest of not clouding the main message (the severe wx threat), opted to hold off on a wind advisory at this time.

Drier, though still very mild, air mass will result in a pretty delightful day across the area Wednesday.

- Izzi

Wednesday Night through Monday:

Models are starting to key in on the clearer conditions allowing for better heating to develop Thursday and potentially Friday. Previous forecasts capped temperatures in the 70s through the weekend, but now it looks like there is a better chance for temps to return to the 80s for much of the area for Thursday and Friday as a warm front lifts northward from Central Illinois (more on this in a second). Unmentionable PoPs south of I-80 along and south of the warm front were kept in the forecast with only a few low end slight chances (15 to 20 percent) on Thursday afternoon. With little capping, perhaps a pulse shower pops up in southern Ford or Iroquois Counties, but confidence is low and kept the forecast mostly dry through Friday morning.

A weaker (relative to the short term forecast above) upper level trough will move out of the Plains and toward Ontario Friday night into Saturday. The associated surface low still gradually move northward through Iowa on Friday, lifting the previously mentioned warm front with it before a cold front sweeps across the area in the evening. Notably, models have sped of the progression of this system from a Saturday morning event to now more of a Friday event. Model soundings are starting suggest better mid level lapse rates with sufficient instability aloft. With additional forcing from the cold front, showers and thunderstorms have a chance to develop. One limiting factor is the projected wind shear values along the line do not look very impressive, limiting the severe potential. Additionally, the forecast kept likely PoPs for Friday afternoon for areas southwest of I-39, but only chance PoPs for the rest of the area because it is currently expected to move east over the area after sunset and therefore have weaker instability without the diurnal heating component. But given how models have changed, there is a lot of uncertainty in the exact timing of the frontal passage.

Other than lingering showers Saturday morning, brief ridging should allow for a slightly drier Saturday afternoon through the start of Sunday. Both Euro and GFS ensembles are suggesting two weak waves next week, one moving northward from St. Louis Sunday night, as well as a slightly deeper one over the northern Plains arriving over Illinois sometime between Monday and into Tuesday. While there will certainly be breaks, it is hard to argue with the NBM's minimal slight chance PoPs through next week. While PoPs are higher as models take their best guess at timing at this range of the forecast will probably need to be adjusted in the coming days. The main takeaway is that regardless of the magnitude, it looks like a fairly active pattern will remain in place through the end of the forecast period.

DK

AVIATION /12Z TAFS THROUGH 18Z WEDNESDAY/
Issued at 1240 PM CDT Tue May 21 2024

Key aviation messages:

- Strong S to SSW winds this afternoon with gusts to 30-40kt - Line of severe TSRA expected to move through late evening - Strong SW to W winds develop in wake of TSRA with gusts to 30-40kt

The surface warm front has lifted well north of the terminals early this afternoon with S to SSW winds prevailing. Wind speeds and gusts are expected to steadily increase over the next few hours with gusts in excess of 30kt becoming common by mid afternoon. Can't rule out gusts as high as 40kt at times. This will present crosswind challenges for east to west oriented runways. Also a brief return to SSE (~170 degrees) can't be ruled out ahead of the line of storms as it approaches.

Will continue monitoring upstream convection across east central IA, though the current trajectory suggests this activity will remain just north of RFD and accordingly have opted to keep the afternoon dry for all sites with VFR cumulus expected areawide beneath increasing coverage of cirrus.

No changes to TSRA timing were made with this update with a line of strong to severe thunderstorms expected to move across the area roughly between 01Z-06Z earliest at RFD and latest at GYY. There is a recent trend in guidance where the line of storms may begin to decrease in coverage as it moves across the area; however, even if this were to occur the threat for severe thunderstorms remains, some potentially significant (especially near RFD) where accordingly the gusts in the TEMPO group have been increased to 50kt. The primary hazard with these storms will be damaging winds, though the potential for hail and brief tornadoes also exists.

In the wake of the storms, guidance continues to suggest winds remain strong out of the SW to W overnight. There may be a period where gusts exceed 40kt for 1 hour period though confidence in this remains too low to include with this update and will likely be handled tactically based on upstream observations as storms begin to move into Illinois later this evening.

Other than patchy MVFR stratus potentially lingering overnight, especially out toward RFD, conditions quiet down heading into the daytime hours of Wednesday with steady WSW to W winds and gusts in the lower 20kt range expected through the afternoon.

Petr

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM...Dense Fog Advisory until 1 PM CDT this afternoon for Winthrop Harbor to Wilmette Harbor IL.

Small Craft Advisory from 3 PM this afternoon to 6 AM CDT Wednesday for Winthrop Harbor to Northerly Is. IL.

Small Craft Advisory until 6 AM CDT Wednesday for Northerly Is. IL to Michigan City IN.

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