Sunrise 5:37 AM   Sunset 7:58 PM Moonrise 4:41 AM   Moonset 6:32 PM

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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- Northerly Island To Calumet Harbor Il- 939 Am Cdt Mon May 6 2024
Rest of today - East winds 5 to 10 kt becoming northeast 10 to 15 kt. Partly cloudy late in the morning then clearing. Waves 1 to 2 ft. Tonight - East winds 10 to 15 kt becoming southeast overnight. Mostly clear in the evening, then becoming mostly cloudy. Waves 1 to 3 ft. Tuesday - Southeast winds 10 to 20 kt becoming south in the afternoon. Showers and Thunderstorms in the morning, then chance of showers and Thunderstorms in the afternoon. Waves 2 to 4 ft.Tuesday night - Southwest winds 10 to 15 kt. Mostly clear. Waves 1 to 3 ft subsiding to around 1 ft.

LMZ700
No data

FXUS63 KLOT 061741 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1241 PM CDT Mon May 6 2024

KEY MESSAGES

- Thunderstorms likely Tuesday morning, with at least scattered redevelopment in the afternoon especially across east and southeast portions of the forecast area. Severe weather threat will exist with afternoon storms.

- Potential of severe thunderstorms across our southern CWA into central Illinois Wednesday afternoon and evening.

DISCUSSION
Issued at 336 AM CDT Mon May 6 2024

Through Tuesday:

Mainly dry and quiet weather conditions are expected through this evening, as surface high pressure drifts slowly east across the Great Lakes region. The only exception to this will be across our far southern counties, where a few scattered shower may affect areas south of about a Pontiac to Rensselaer line in association with a mid-level short wave tracking across the lower Ohio Valley and downstate IL/IN. Weak mid-level lapse rates (5-5.5 C/km) should provide little or no lightning threat.
Otherwise, morning high clouds will move off to the east, making for mostly sunny skies across the north and partly to mostly cloudy conditions south later this morning and afternoon.
East-southeasterly winds 10-15 mph will maintain lake-cooling along the Illinois shore of Lake Michigan, where temperatures will only top out in the lower 60s, while areas farther west and south of the lake should moderate into the mid-70s in most locations.

Benign weather continues into this evening, before attention turns to increasing shower and thunderstorm chances which will ramp up from the west in the pre-dawn hours of early Tuesday morning. An organized extensive north-south area of thunderstorms is expected to be ongoing across the Plains this evening, along a cold front trailing from an area of deep surface low pressure over the northern Plains. This front is expected to push east toward the Mississippi River through Tuesday morning, as a stout mid-level short wave trough lifts northeast into the mid/upper Mississippi Valley. Various CAM guidance and HRRR ensembles depict the Plains linear thunderstorm complex spreading east ahead of the front, reaching the WFO LOT forecast area pre-dawn Tuesday morning and reaching the Chicago metro area just after sunrise. While severe weather is expected well west/southwest of the area overnight, indications (HRRR calibrated severe probs, forecast soundings indicated elevated inflow bases) are that the primary severe threat area will remain well west/southwest of the cwa through Tuesday morning. However, sub- severe hail and wind gusts will be possible with the stronger cores here. Current CAM/HRRR guidance suggests storms will be largely through our IL counties by mid-morning and our northwest IN area prior to noon, though some scattered redevelopment appears possible on the trailing flank of the complex due to a southwesterly 35-40 kt low level jet.

Convective redevelopment and coverage then becomes less clear during the midday and afternoon hours on Tuesday. Both the footprint of the morning cold pool and the eastward movement of the now-occluding cold/warm fronts across the area will likely have an impact on if/where redevelopment occurs through the remainder of the day. Current high-res guidance indicates that the better low-level moisture and instability will be increasingly focused across the far east/southeast parts of the forecast area (and into IWX, IND and ILX's areas) during the afternoon, with shallower boundary layer moisture and mid-level drying (and likely clearing) farther to the west. That said, the area will remain beneath a favorable diffluent exit region of the upper jet, with steepening mid-level lapse rates associated with cooling provided by the mid-level trough axis and strong mid (40-50 kts) and upper level (70+ kts) flow providing substantial, mainly unidirectional deep-layer shear. HRRR/HRRR develop 1000-1500+ J/kg MLCAPEs during the afternoon (highest generally from the south Chicago burbs south and southeast), potentially supporting robust updrafts and a severe weather threat. Forecast hodographs are generally long and straight given the unidirectional wind/shear profiles, with wind/hail the primary severe threats. However, there would also be a non-zero tornado threat where boundary interactions or bowing segments might alter the low-level shear profiles. SPC has adjusted their day 2 outlook to pull the slight risk a little farther into our southeast cwa with a marginal risk for the remainder, and this looks appropriate.

Ratzer

Tuesday Night through Sunday:

While deep-layer drying behind Tuesday's rounds of potential convection is expected to greatly limit or end precip potential by Tuesday evening, a trailing mid-level wave along with steep mid-level lapse rates could result in a couple widely isolated showers or storms across the northern CWA through mid evening.
Otherwise, dry conditions amid mid-level ridging is expected across the area Tuesday night into Wednesday morning.

A mid-level low digging across the central Great Plains on Wednesday in conjunction with a departing right entrance of an upper jet streak and associated enhanced low-level flow will support another period of convection across the Ohio Valley Wednesday afternoon and evening. Robust low-level moisture advection ahead of an elongated and partly occluded surface low from western Iowa into central Illinois will result in a warm front nearing or possibly shifting northward into the southern CWA by late afternoon. Around and south of the warm front, ample instability and impressive shear profiles support discrete supercell structures capable of all severe hazards. We will need to closely monitor trends in our southern CWA over the next couple days given the proximity of the warm front and triple point and diurnally favorable time of the day.

Beyond Wednesday evening, a mid-level trough extending southwest across Quebec and southern Ontario will begin to phase with Wednesday's low. While some differences in guidance remain with regards to the exact degree of phasing and timing of trough and low, scattered showers and perhaps some diurnally driven thunderstorms are favored on Thursday. After a quick- moving ridge yields dry and seasonable temps on Friday, another upper-level low is progged to cross the western Great Lakes on Saturday. Like Thursday, scattered diurnally-enhanced showers and a few storms appear likely on Saturday. Diverging solutions with regards to the residence time of the upper-low over the region next weekend may support a continuation of showers on Sunday.

Kluber

AVIATION /18Z TAFS THROUGH 00Z WEDNESDAY/
Issued at 1240 PM CDT Mon May 6 2024

Key Aviation Messages: - Line of TSRA with gusty winds early Tuesday AM - Scattered TSRA possible Tuesday afternoon (30% chance)

VFR conditions are forecast through this evening and early overnight hours with generally east to southeast winds around 10kt.

A line of thunderstorms is expected to cross the area early Tuesday morning, potentially as early as 10Z toward RFD and 12Z for ORD/MDW with associated vsby/cig reductions down to as low as IFR at times. This line of storms may be accompanied by a gusty westerly wind shift before settling back to a southeast direction.

Once the initial line of storms moves east of the area, conditions become favorable for renewed isolated to scattered storm development by Tuesday afternoon as winds turn south and then southwesterly. Confidence in storm coverage is lower for areas north of I-80 and accordingly opted to introduce this potential with a PROB30 from 18-22Z. If storms do develop they would have the potential to become strong to severe.

Petr

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

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