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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- 943 Pm Cdt Thu Apr 25 2024
.small craft advisory in effect from 7 am cdt Friday through Saturday evening - .
Rest of tonight - East winds 10 to 15 kt becoming southeast 15 to 20 kt late. Mostly clear. Waves 1 to 3 ft building to 3 to 5 ft late. Friday - Southeast winds 15 to 25 kt. Showers likely in the afternoon. Waves 3 to 6 ft occasionally to 8 ft. Friday night - Southeast winds 20 to 25 kt becoming south overnight. Showers and a chance of Thunderstorms. Waves 4 to 6 ft occasionally to 8 ft. Saturday - South winds 20 to 25 kt increasing to 30 kt. Chance of showers and Thunderstorms, mainly in the morning. Waves 3 to 5 ft.

LMZ700
No data

FXUS63 KLOT 260912 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 412 AM CDT Fri Apr 26 2024

KEY MESSAGES

- Rain moves into the area late this morning and afternoon and continues into tonight. Isolated to scattered thunderstorms may be seen during this time as well, particularly this evening.

- The weekend looks warm and breezy with high temperatures in the upper 70s to lower 80s and overnight lows in the low to mid 60s.

- Multiple rounds of showers and thunderstorms over the weekend through Monday, some of which could be severe late Saturday and again Sunday night.

- After a brief break, shower/storm potential returns toward the middle of next week

DISCUSSION
Issued at 412 AM CDT Fri Apr 26 2024

Through Saturday:

GOES infrared satellite imagery has quite the colorful look to it early this morning across the central Plains with two groups of widespread deep convection evident. One of these is a large conglomerate of thunderstorms over eastern Nebraska and western Iowa, while the other is a newly blossoming swath of isentropically- induced convection over western Missouri. This second thunderstorm cluster is of most relevance to our weather forecast for today as, with time, these storms will propagate towards and eventually into our forecast area later this morning and afternoon.

As this cluster approaches the area, it is likely to outpace the instability reservoir that it currently resides in and, hence, should arrive in a weakened state, with most locales in northern Illinois and northwest likely to just see a plain rain today as a result. The best chances for any lightning in our area through this afternoon will thus likely remain confined to the southwestern half of our CWA The eastern periphery of the main instability plume will eventually creep into the area later tonight and lead to a slightly better chance for thunderstorms area-wide, though exact storm coverage will hinge largely upon how this afternoon and evening's severe thunderstorms in the central/lower Missouri River Valley evolve and whether they manage to survive their northeastward trek across Iowa/Missouri into Illinois. Suppose that there is an outside chance of a strong to marginally severe thunderstorm sneaking into our western counties late this evening -- especially if the cold pools from the upstream convection manage to consolidate and yield a more organized forward-propagating storm cluster. However, in all likelihood, the upstream thunderstorms should weaken below severe limits by the time they reach our forecast area as they encounter an increasingly stabilized boundary layer with eastward extent.

The 992-994 mb surface low passing to our northwest will be culpable for a tight surface pressure gradient across the region that will keep breezy southerly winds in place today through Saturday.
Depending on how deeply we are able to mix Saturday morning, a brief period of 40-45 mph gusts may be observed for a few hours before the overhead low-level jet weakens, resulting in gusts diminishing to closer to 30 mph Saturday afternoon. These winds will also serve to advect in an unseasonably warm and moist air mass with temperatures and dew points likely to be on the rise tonight into Saturday morning. Despite a good deal of cloud cover likely being around on Saturday, warm air advection on its own will push temperatures into mid to upper 70s, and with the expectation that there will still be some sunshine to be seen on Saturday, highs will likely end up near 80 degrees across our forecast area. When paired with dew points climbing into the 60s, Saturday will certainly have a summer-like feel to it.

With this relatively muggy air mass in place on Saturday, there will also be a summer-esque potential for isolated showers and storms to develop in the afternoon. The general thinking is that height rises and subsidence on the backside of the departing upper-level trough, relatively nebulous large scale forcing for ascent, and the presence of a capping inversion should favor most or all of the area remaining dry through most of the daytime hours once any lingering morning showers depart. However, by the late afternoon, forecast soundings show that the cap should have weakened or eroded entirely, and isolated attempts at convection may start to see some success wherever warm air advection and/or confluence in the low-level flow is maximized. Thus, have kept slight chance PoPs going during the afternoon to account for this possibility, with the highest probabilities currently advertised in our southeastern counties, where the convective signal is greatest in the 00Z/06Z CAM suite. With the heart of the aforementioned instability plume expected to be positioned over the area by Saturday afternoon, MLCAPE values may get as high as 2500-3000 J/kg during peak heating, which would be more than enough to support the development of thunderstorms that could grow deep enough to carry some threat of strong to severe downburst winds and/or hail. However, with lingering uncertainties about shear profile quality and the aforementioned dearth of forcing for ascent, whether an isolated threat for severe weather will materialize Saturday afternoon remains in question.

Ogorek

Saturday Night through Thursday:

By Saturday evening the lead short-wave trough and attendant surface low will continue to weaken while lifting northeast into Ontario. Showers and storms may persist along the trailing cold front near and north of the WI/IL stateline into Saturday evening. Meanwhile a secondary and more amplified upper level trough will eject off the Rockies into the PLains with an associated deepening surface low setting the stage for significant severe weather potential near the NE/KS/IA/MO stateline region. This activity is expected to lift northeast toward the area by Saturday evening/overnight. With a warm and moist airmass still in place into the evening (near record warm low temperatures in the low-mid 60s!), a cluster or line of strong to severe storms may manage to persist into portions of the area, currently favoring areas north of a Peru to McHenry, IL line.

The details become less clear heading into Sunday with the evolution of the broader system will be dependent on what transpires Saturday and Saturday night, though there is a signal that much of the area could be dry for much of the daytime hours on Sunday with temperatures warming back into the upper 70s to around 80. Showers and storms are anticipated to develop well to the west across portions of MO/IA which then push east Sunday night across the area. A strong to potentially severe storm cannot be ruled out with this activity though it will likely be on a weakening trend with eastward extent. Showers may linger into Monday as the system lifts out of the region and the trailing cold front gradually moves through.

Southwest flow quickly returns in the wake of the cold frontal passage with highs still in the mid-upper 70s Tuesday and Wednesday. Several mid-level disturbances moves within the broader quasi-zonal upper-jet which could bring additional rounds of showers/storms toward the middle of next week with temperatures turning more seasonable by the end of the week.

Petr

AVIATION /06Z TAFS THROUGH 12Z SATURDAY/
Issued at 1235 AM CDT Fri Apr 26 2024

Key aviation messages:

- Multiple waves of SHRA this afternoon through tonight - 30% chance of TS late this evening/overnight

An band of showers is still anticipated to spread across the area Friday afternoon. While a few embedded lightning strikes cannot be ruled out with this initial round, the potential remains too low to include with this update. Additional waves of showers likely continue into the evening and overnight hours with associated MVFR ceilings. There is a signal that TS may accompany these showers late this evening into the overnight hours and have accordingly introduced a PROB30 group for TS during the 2-7Z timeframe, earliest at RFD.

Winds will remain light E to ESE the rest of tonight then steadily increase after daybreak with SE gusts to 25kt expected by mid-late morning. Winds remain gusty into the evening and overnight hours, potentially nearing 30kt at times and gradually veer SSE then SSW into early Saturday morning.

Petr

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM...Small Craft Advisory from 7 AM this morning to 10 PM CDT Saturday for Winthrop Harbor to Northerly Is. IL.

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