Sunrise 5:22 AM   Sunset 8:07 PM Moonrise 5:29 PM   Moonset 3:43 AM

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LMZ744 Expires:202405202115;;544980 Fzus53 Klot 201453 Nshlot
nearshore marine forecast national weather service chicago/romeoville il 953 am cdt Mon may 20 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.
lmz742>745-202115- northerly island to calumet harbor il- calumet harbor il to gary in-gary to burns harbor in- burns harbor to michigan city in- 953 am cdt Mon may 20 2024
Rest of today - South winds 10 to 15 kt becoming southwest 15 to 20 kt. Chance of showers and Thunderstorms in the afternoon. Waves 1 to 2 ft. Tonight - Southwest winds 10 to 20 kt becoming south and diminishing to 5 to 10 kt overnight. Chance of showers and Thunderstorms in the evening, then slight chance of showers and Thunderstorms overnight. Waves 1 to 2 ft. Tuesday - South winds 5 to 10 kt increasing to 15 to 25 kt. Chance of showers and Thunderstorms. Waves around 1 ft building to 1 to 3 ft. Tuesday night - South winds to 30 kt becoming southwest 20 to 25 kt. Showers and Thunderstorms likely. Waves 2 to 4 ft.
winds and waves will be higher in and near storms.

LMZ700
No data

FXUS63 KLOT 201552 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1052 AM CDT Mon May 20 2024

KEY MESSAGES

- Scattered t-storms likely today, a few severe storms with large hail and damaging winds possible this afternoon.

- Another round of thunderstorms is expected Tuesday evening into the nighttime hours. Severe thunderstorms with damaging winds and brief QLCS type tornadoes are possible with this round.

- Another chance for showers Friday into Saturday

MESOSCALE
Issued at 1052 AM CDT Mon May 20 2024

The warm front has now moved/mixed northward to near the I-88 corridor and across Chicago (12F temp spread at ORD/MDW).
Recent mesoanalysis indicates that areas south of the warm front have generally become uncapped with MLCAPE values nearing 2000 J/kg. Meanwhile, effective shear profiles are becoming more favorable ahead of the MCV across the western CWA The first area of interest noted earlier has spurred a renewed axis of mainly elevated convection, though some of the southern cells are attempting to become surface based near the warm front.
While this axis appears to continue to outrun the better environment, the proximity of the warm front and increasing potential for surface-based convection supports isolated severe storms with a non-zero brief tornado risk over the next hour or two into far northeast Illinois.

As for the second area to the southwest, we continue to keep a close eye on the potential for convection to erupt in and around the Peoria area over the next hour or so. So while convection is slower to materialize than previously expected in this area, it remains a favored area of a growing cluster of severe convection into the Chicago metro 2-4pm.

Kluber

Issued at 806 AM CDT Mon May 20 2024

We continue to assess the impacts from a compact MCV over southern Iowa (originating from a complex of convection over Kansas last evening). While the low-level environment remains stable this morning with the warm front still south of I-80 as of 8am, the shallow nature of the front combined with 25-30 knot winds above the frontal surface and abundant sunshine will allow the front to mix/jump north across most of northern Illinois over the next 2-3 hours. This will result in a quick destabilization ahead of two areas of interest, one with a broken line of showers and storms from around the Quad Cities northward, and another with an area of agitated cloud cover crossing the Mississippi River from around Keokuk to Quincy.
While the first will likely clip the western CWA through mid- morning, destabilization may not occur early enough for surface- based convection. However, we are honing in on the second area for elevated convective initiation over the next hour or two. This convection should eventually become surface- based while nearing/over the far southwest CWA by around 11am.
With a locally enhanced mid-level speed max associated with the MCV, the resultant effective shear combined with an increasingly favorable thermodynamic environment may support a cluster of severe convection through the central CWA into the Chicago metro between 11am-2pm. Primary hazards will be damaging wind and large hail, but a brief tornado cannot be ruled out with any boundary interactions and marginally favorable 0-1km wind profiles.

Kluber

DISCUSSION
Issued at 325 AM CDT Mon May 20 2024

Today and tonight:

A pair of MCVs will likely be the significant players in our weather today. MCV #1 located over northeast IA as of 07z will continue to move northeastward and generally away from the area this morning. A north-south line of convection extending south of this vort across eastern IA should begin a weakening phase in the next couple of hours as it progresses east-northeast into a more stable air mass. Nonetheless, some showers and perhaps a t-storm could affect our western and northwestern CWA early this morning in association with this feature before dissipating and/or moving north into WI by mid morning. Severe wx with this activity is unlikely.

MCV #2 should be the more problematic feature today. Currently over northwest Missouri, hi-res model guidance matches well with simple extrapolation, both have this features moving into southwest WI early this afternoon. A formidable mid level jet accompanies this feature with the latest VWP from KEAX showing 50-55kt winds from 700-500mb. While this MCV will probably weaken some this morning given the waning convective activity, the RAP (which seems to have initialized this feature pretty well when compared with observational data) maintains a 40-50kt 700mb jetlet and 35-45kt at 500mb into the early/mid afternoon on the southeastern flanks of this MCV across far northern IL into southern WI.

This stronger flow with the vort max results in some respectable low-mid level hodographs for a time late this morning through early-mid afternoon. While 0-6km bulk shear is only progged to be around 30kt, almost all of the shear will be focused in the 0-3km layer where hodographs are somewhat elongated and a bit curved. Ongoing convection with MCV #2 is expected to weaken/dissipate this morning and likely allow for at least some mixed sunshine to heat the moistening boundary layer as warm front lifts north across the CWA this morning.
Assuming convective debris isn't more extensive than expected, then MLCAPE of around 2000 J/kg could be realized by early afternoon with minimal inhibition left. Assuming convection develops over northern IL this afternoon, and that looks pretty likely, the next question will be how soon. If robust convection can develop by early afternoon, favorable shear profiles will be in place to support an storm scale organization and including maybe some messy supercell structures. Given the shear maximized in the low levels with largely streamwise vorticity in the 0-1km layer, there could even be a couple hour window with some low end tornado threat. This would particularly be near the northward retreating warm front and assuming robust convection develops early enough to reap the benefits of the stronger mid- level flow associated with the MCV.

Heading into the mid and especially late afternoon, shear profiles will quickly weaken and hodographs collapse in on themselves. This means our severe threat should become relegated to mainly a pulse type severe threat with accompanying brief marginal hail and localized microburst where any updrafts find some untapped CAPE to feed on. Thunderstorms should dissipate and then end this evening as the MCV lifts north out of the area and the boundary stabilizes. Maintained some slight chance pops late this evening and overnight as a nod to the RAP which has another MCV and associated convective complex rolling across the area. Not seeing any evidence of this feature upstream now and this scenario seems unlikely, with most likely scenario being mostly dry conditions tonight.

Tuesday into Tuesday night:

Guidance varies with where the warm front will be to start the day, likely due to models' attempts to resolve convective influences. If the warm front starts out in our area in the morning, a strong and deepening cyclone to our west should drive it quickly north into Wisconsin. Another MCS and associated MCV should organize over the central Plains tonight with this system likely moving toward the MS River late tonight into early Tuesday morning. Can't rule out some weakening form of this MCS bringing some showers and storms to our NW CWA in the morning, with the warm front and synoptic scale ascent shifting north into WI, would tend to think the synoptics will overwhelm this system and lead to it decaying Tuesday morning.

Barring any unexpected surprises from any meandering MCVs, Tuesday afternoon looks to be windy, largely rain free, and unseasonably warm to hot. Expecting high temps to reach at least into the upper 80s with lower 90s a distinct possibility given decent amount of sunshine during the afternoon.

Potentially explosive severe thunderstorms are expected Tuesday afternoon across western or central IA south into northern Missouri. This activity is expected to eventually congeal into a QLCS that will march east and northeastward toward the MS River by early to mid evening Tuesday. Gusty southerly winds should keep the boundary layer somewhat mixed and temps warm Tues evening, likely slowing the nocturnal stabilization of the boundary layer. Even with that, still expect convection to be encountering much weaker and weakening instability and growing boundary layer convective inhibition as it moves into our CWA mid-late evening Tuesday. Assuming the convection doesn't outrun the strong synoptic ascent, a still somewhat formidable, but weakening QLCS could march across much of the CWA Tuesday night.

Wind profiles will be plenty favorable for a continued threat of damaging winds and perhaps a QLCS tornado threat. These threats appear to be greatest over our western CWA, with the threat lessening with eastward extent. The earlier the storms arrive, the more mesoscale organization that they can attain, the more significant the severe threat will be in our CWA
Certainly, the severe threat should be highest west of our CWA across Iowa and there are scenarios where we dodge a bullet with little or no severe weather in our area. But given the strength of the synoptic system and associated kinematic fields, Tue evening warrants close watching, especially across our western CWA

Wednesday and Wednesday:

GFS is finally starting to cave and give up on the idea of extensive post frontal convection Wednesday afternoon and evening across our CWA The 00z NAM came in dry, in line with the very consistent ECMWF and GEM models which have been steadfast in their dry forecast for our area for days now. NBM pops are still being weighted down by the 12z runs and the GFS and its ensemble members. Have coordinated a significant reduction in pops with neighboring offices and removal of NBM QPF with WPC for Wed into Wed night.

- Izzi

Thursday through Sunday:

Brief upper level ridging should allow for better height rises on Thursday for much drier conditions setting up. Winds should be out of the west and light with the occasional afternoon burst to 15 mph. Afternoon temperatures should warm into the 70s, with slightly cooler conditions in Northern Indiana along the immediate shoreline.

The next upper level trough ejects out of the Rockies Thursday night and starts tracking toward southeastern Manitoba. Despite the system's track being farther to the northwest than earlier in the week. As the reflected surface low passes through the Dakotas and Minnesota, a warm front will slowly enter Illinois on Friday. Southerly winds will slowly advect precipitable water amounts over an inch back into the area. The best instability will be in the warm sector behind the front in a Rockford to Fowler, IN line, but even then forcing does not look impressive.
So PoPs for scattered showers were kept at the chance threshold.

Overnight and into Saturday, the low exits into Canada as a secondary surface low strengthens over the Southern Plains.
Friday's surface front will be come a NW/SE oriented cold front and move over the region on Saturday. With it comes better forcing and instability to allow widely scattered thunderstorms to develop through the day. This far out, timing can be adjusted closer to the event. At this juncture, model soundings have weaker mid level lapse rates and deep layer shear to keep the word severe out of the forecast. There may be isolated showers that stick around through Monday, but there should be breaks in the rain. However, confidence remains to low this far out for specifics at this time.

DK

AVIATION /12Z TAFS THROUGH 18Z TUESDAY/
Issued at 631 AM CDT Mon May 20 2024

Key Aviation Impacts for the current TAF period:

- Scattered and possibly strong thunderstorms expected today, arriving later this morning for RFD and around noon for the Chicago terminals

The first round of showers and thunderstorms are moving northeast toward Milwaukee and away from the area. However, the next round of thunderstorms is in Eastern Iowa and is moving toward the northeastern Illinois airspace. Winds are light and out of the southeast, but are expected to switch over to the southwest and increase with gusts over 20 knots. Showers and thunderstorms are expected to arrive at RFD as early as 14Z, 16Z at DPA and 17Z for ORD/MDW/GYY. As the storms move over the terminals, lightning, strong erratic wind gusts, and potentially some hail remain as the main threats. Lastly, MVFR cigs and lower vis can be expected during the heaviest rainfall as they pass over terminals.

The expected end time for the showers and thunderstorm chances remains at 22Z. Winds behind the front should lighten and return to the southwest. As the center of low pressure moves into Eastern Michigan, latest guidance is suggesting that the front stalls latitudinally south of the area and gradually lifts northward, increasing the chances for light showers just before midnight. There maybe some elevated instability for an isolated rumble or two of thunder, but with the chances still lower, it was added as a PROB30 group. As that boundary starts to disintegrate, there is a chance for some low cloud development.
Confidence remains low in MVFR/IFR conditions, so the chance was introduced as a SCT group, keeping the terminals at VFR and dry through Tuesday morning for now.

DK

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