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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- 945 Am Cdt Sat Apr 27 2024
.small craft advisory in effect through this evening - .
Rest of today - South winds to 30 kt with a few gale force gusts to 35 kt this morning becoming southwest 20 to 25 kt this afternoon. Isolated showers and Thunderstorms in the afternoon. Waves 2 to 4 ft. Tonight - Southwest winds 10 to 20 kt becoming south. Scattered showers and Thunderstorms in the evening, then showers and Thunderstorms overnight. Waves 1 to 3 ft. Sunday - South winds 10 to 15 kt increasing to 15 to 20 kt. Showers and a chance of Thunderstorms throughout the day. Waves 1 to 3 ft.Sunday night - South winds 10 to 20 kt. Chance of showers and Thunderstorms. Waves 1 to 3 ft.

LMZ700
No data

FXUS63 KLOT 271736 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1236 PM 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 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.

UPDATE
Issued at 945 AM CDT Sat Apr 27 2024

A recent hand surface analysis shows a pair of low pressure systems, with the first near the Twin Cities and the second near the Texas panhandle. Between both lows, broad south to southwesterly flow is advecting moist and arguably humid air as well as unseasonably warm temperatures into our region with current temperatures already above the average for late April and in the upper 60s to around 70.

Earlier this morning, both the 12Z ILX and DVN RAOBs sampled a modest capping inversion based near 850mb. While continued low- level warm air advection and subtle height rises (between the parent upper-level shortwaves tied to each aforementioned surface low pressure system) should reinforce the capping throughout the day, aggressive low-level moisture advection in tandem with forecast high temperatures in the upper 70s to around 80 may nevertheless allow for free convection to develop by mid-afternoon. Indeed, modifying the 12Z DVN/ILX RAOBs with expected surface conditions this afternoon show little to no capping with some 1000-1500 J/kg of MLCAPE. For this reason, it appears prospects for at least isolated thunderstorms are increasing across our area this afternoon.

Now, just how many thunderstorms develop this afternoon will probably dictate the placement and coverage of thunderstorms overnight. In the scenario where the capping inversion holds, showers and storms would be poised to develop near a stationary front draped across eastern Iowa and southern Wisconsin this evening in tandem with the intensifying low-level jet. The result of upscale growth of a cluster or two would then be poised to move into northern Illinois and eventually northwestern Indiana after dark. Recent iterations of the NAM3 show this idea. However, if the cap erodes fully this afternoon, scattered to even widespread showers and storms may erupt near the axis of highest low-level moisture somewhere near US-24 as early as 3-4 pm, and continue more or less unabated overnight as the low-level jet intersects composite west-to-east oriented outflow. In this scenario, a localized threat for flash flooding would materialize owing to repeated rounds of thunderstorms over the same area. It's worth noting the 00Z (overnight) run of the HRRR depicted a narrow axis of 24-hour LPMM of 2-4" near US-24. While the 00Z WRF-ARW appeared to dominate the 00Z HRRR signal, the fresh 12Z HRRR and recent iterations of the experimental RRFS depict a strikingly similar evolution with training thunderstorms near or just south of US-24. Regardless of where thunderstorms do form, the strongest will be capable of producing locally damaging winds and damaging hail.

All things considered, opted to increase PoPs dramatically to the 50-60% range along and south of I-80 this afternoon to account for the threat that the cap breaks and coverage of showers and storms is much higher than originally anticipated.

Updated products have been sent.

Borchardt

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 /12Z TAFS THROUGH 18Z SUNDAY/
Issued at 1235 PM CDT Sat Apr 27 2024

Key Aviation Impacts for the current TAF period:

- Strong wind gusts out of the south to southwest between 30 to 35 knots, occasionally as high as 40 knots through the early evening

- Pockets of MVFR cigs through sunset, otherwise VFR

- Showers and thunderstorms late tonight through Sunday morning with IFR conditions during periods of heaviest rainfall

- Lingering showers Sunday morning and MVFR conditions expected, but there is low confidence on the exact timing on when rain slowly diminishes

With prevailing southwest winds, a steady stream of clouds are passing over the area. There are periods where cigs may lower to MVFR conditions, but with afternoon mixing, it is expected to be mostly VFR through much of the day. The bigger impact will be the strong wind gusts at area terminals. ORD already had a peak wind gusts of 38 knots earlier this morning. The expectation is for winds to gust between 30 to 35 knots through the afternoon, but having occasional gusts up to 40 knots is possible. Winds will remain out of the south tonight diminishing down to 20 to 25 knots. There may be short instances of winds that switch to the southeast, but confidence is too low to add it to the TAF.

After 00Z, higher moisture content and instability will advect into the region increasing the risk for showers and thunderstorms around area terminals as early as 02Z. Latest guidance has not wavered to much, with the expectation for most thunderstorms to pass over around and after midnight. Cigs are expected to be MVFR prevailing, but conditions may reduce down to IFR levels around midnight as stronger cells provide heavier rain rates and reduce visibility.

Showers will continue into the early morning hours, though coverage will slowly diminish. There is lower confidence in when the breaks will occur, so it was decided to leave -SHRA in the TAF from 14Z-19Z, and add VCSH through much of the afternoon, though it would not be surprising if that was pulled in a later TAF issuance when better guidance can inform. More impactful is the expectation for cigs to hang around terminals. Given how poor the models are depicting the current cloud cover trends, TAFs were written more aggressive with lower MVFR conditions Sunday morning, not lifting to 2000 feet until the afternoon with better mixing. Lastly, another low level jet will increase tomorrow afternoon. And while timing might be adjusted, there was enough of a signal to add wind gusts again into the TAF between 20-25 knots.

DK

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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