Sunrise 5:50 AM   Sunset 7:49 PM Moonrise 12:00 AM   Moonset 7:43 AM

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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- 357 Am Cdt Sat Apr 27 2024
.small craft advisory in effect through this evening - .
Today - South winds 20 to 25 kt becoming southwest to 30 kt with a few gales to 35 kt. Isolated Thunderstorms early in the morning then a slight chance of showers and Thunderstorms in the afternoon. Waves 3 to 6 ft occasionally to 8 ft early in the morning then subsiding to 2 to 4 ft.Tonight - South winds 20 to 25 kt diminishing to 10 to 15 kt. Chance of showers and Thunderstorms in the evening, then showers and Thunderstorms likely overnight. Waves 2 to 4 ft subsiding to 1 to 3 ft. Sunday - South winds 10 to 15 kt increasing to 15 to 20 kt. Showers and a chance of Thunderstorms in the morning, then chance of showers and slight chance of Thunderstorms in the afternoon. Waves 1 to 3 ft. Sunday night - South winds 10 to 20 kt. Chance of showers and slight chance of Thunderstorms in the evening, then showers and a chance of Thunderstorms overnight. Waves 1 to 3 ft.

LMZ700
No data

FXUS63 KLOT 270852 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 352 AM CDT Sat Apr 27 2024

KEY MESSAGES

- Summer-like conditions today with high temperatures in the upper 70s to low 80s and increased humidity levels.

- Breezy today with wind gusts as high as 40-45 mph possible from mid-morning through the early afternoon.

- Isolated showers and thunderstorms expected to develop this afternoon with coverage increasing markedly tonight. There are lower-end chances for damaging wind gusts with this evening's storms and for localized flooding tonight.

- After a break Tuesday, an active weather pattern returns midweek with multiple periods of showers/storms expected through Friday.

DISCUSSION
Issued at 352 AM CDT Sat Apr 27 2024

Through Sunday:

After any lingering overnight convection clears the area this morning, the already breezy southerly winds will get even breezier by mid-morning as we begin to tap into 40 kt flow in the lower part of an overhead low-level jet. This will likely translate to 40-45 mph gusts being observed during a brief window of time between the mid-morning and early afternoon before the magnitude of the gusts lessens a bit as the low-level jet continues to gradually wane going into this afternoon. These southerly/south-southwesterly winds will also serve to advect in an unseasonably warm and moist air mass, characterized by summer-like dew points in the low to mid 60s and temperatures that will likely climb into the upper 70s to around 80F with the assistance of some filtered sunshine.

Height rises, large-scale subsidence on the backside of the departing upper-level trough, and the development of a capping inversion should ensure that most of our forecast area will remain dry through the daytime hours. However, by about the mid-afternoon, the capping inversion will likely be eroded as a result of surface heating, and some combination of broad isentropic ascent and low- level confluence should help isolated to scattered convection to sprout. Confidence is highest in this occurring in our southern counties, where high resolution models continue to suggest that there may be a zone of more focused ascent, but can't entirely rule out a stray shower/storm or two popping up elsewhere, so continue to carry slight chance PoPs across the entire CWA during the mid-late afternoon. There will be a plentiful amount of instability around come this afternoon (MLCAPE of up to around 2500 J/kg), so would not be surprised if a couple of storms managed to grow deep enough to support a threat for downbursts and/or hail, but the latest thinking is that the relatively broad and modest forcing for ascent should preclude the development of any overly rambunctious convection capable of making full use of the available instability.

Meanwhile, to our west and southwest, a frontal zone is expected to light up with convection during the afternoon and evening as large scale forcing for ascent increases in advance of another upper-level trough ejecting into the Great Plains. Convection that originates in Iowa and Missouri is likely to grow upscale with time and track in our general direction, with most of the latest CAM runs envisioning that at least a semi-organized multicell storm cluster will be knocking on the doorstep of our western CWA by mid-evening. While these storms are expected to lose steam with time and eastward extent, forecast soundings indicate that the degree of available buoyancy here this evening could still be adequate to support the development of new, deep updrafts along the storm cluster that would carry a threat to produce strong to severe downburst winds, particularly across our northwestern CWA This threat largely hinges upon this storm cluster developing a robust consolidated cold pool that would be able to overcome increasing nocturnal boundary layer stability, which does not appear to be a given at this time.

Of additional interest with tonight's thunderstorms is the potential for localized flash flooding. It does not have the look of an ideal flash flood setup by any means, but it does appear that storm orientation could somewhat align with storm motion vectors for a time and promote some potential for training convection. The nose of a strong low-level jet to our southwest will also pivot in our direction tonight and provide a continuous replenishment of moisture into the region, which will likely keep thunderstorms and heavy rainfall ongoing in at least part of our forecast area throughout the night. Thus, some potential for localized minor flooding does seem to exist in our forecast area tonight, and WPC's Marginal Risk in their Day 1 Excessive Rainfall Outlook continues to look appropriate for highlighting this possibility.

Shower and storm coverage is likely to diminish after daybreak on Sunday as the low-level jet diurnally weakens. However, with surface low pressure and an associated cold front still to our west, warm conveyor belt showers and storms will likely continue in some capacity in our forecast area during the day on Sunday, with the highest chances for precipitation remaining across our western counties through sunset.

Ogorek

Sunday Night through Friday:

Heading into Sunday evening upstream convection is likely across Iowa and Missouri and pushing east of the Mississippi River. This activity reaches the area by late evening. While instability will be on a weakening trend, the arrival of better forcing and a strengthening low-level jet would support a localized damaging wind and large hail threat, especially west of a McHenry to Kankakee, IL line.

A continued slower eastward progression of the surface cold front will likely allow for showers to persist into the day on Monday. Instability is much more muted compared to over the weekend, though a few thunderstorms can't be ruled out, potentially lingering into the afternoon and early evening hours east of I-57. Severe weather is not expected. Once showers exit to the east late Monday, skies clear out as weak mid-level ridging builds in allowing overnight lows Monday night to dip back into the upper 40s to lower 50s. Tuesday will be noticeably less humid as we remain between weather system. Ample sunshine and a return to southwest flow will still allow temperatures to warm well into the 70s again.

The break in the precipitation appears to be short lived as ensemble guidance continues to support a return to an active weather pattern midweek with multiple chances for showers and thunderstorms Tuesday night through Friday as multiple disturbances move within the broader northern stream quasi-zonal upper jet.

Petr

AVIATION /06Z TAFS THROUGH 12Z SUNDAY/
Issued at 1250 AM CDT Sat Apr 27 2024

Key Aviation Messages: - Scattered SHRA with isolated TS early this morning - SW wind gusts over 30kt expected after daybreak - Periods of MVFR ceilings through the period - SHRA/TSRA redevelops late evening through early Sunday AM

A narrow line of elevated convection will lift across the Chicago area terminals near the start of the TAF period and are accounted for with a TEMPO for TS from 6-7Z. Confidence in whether additional showers/storms develop overnight remains on the lower side, and accordingly opted to convert to a VCTS mention through ~10Z to account for the back edge of a cluster of storms west of Peoria that are lifting northeast toward the area. Any lingering showers will diminish in coverage toward daybreak.

Winds early this morning are breezy out of the southeast around 25kt with sporadic gusts near 30kt in the metro. Directions will gradually veer S to SW with time overnight. More persistent gusts in excess of 30kt are expected to develop after daybreak and continue into the afternoon before easing toward sunset.

Additional isolated showers can't be ruled out during the afternoon, though confidence remains too low for a formal TAF mention. Confidence does increase in showers developing by mid- late evening along with the potential for embedded TS. The timing for this period will continue to be refined with later updates.

Petr

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM...Small Craft Advisory until 10 PM CDT this evening for the IL and IN nearshore waters.

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