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LMZ740 Winthrop Harbor To Wilmette Harbor Il- 156 Pm Cdt Sat May 4 2024
.dense fog advisory in effect until 7 pm cdt this evening - .
Late this afternoon - Southeast winds 10 to 15 kt. Areas of dense fog. Waves around 1 ft. Tonight - Southeast winds 10 to 15 kt becoming north after midnight. Patchy fog and gusty showers and Thunderstorms before midnight. Waves 1 to 2 ft.Sunday - North winds 15 to 20 kt diminishing to 10 to 15 kt. Decreasing cloudiness. Waves 1 to 2 ft. Sunday night - North winds 5 to 10 kt. Mostly clear. Waves 1 to 2 ft. Monday - Northeast winds 5 to 10 kt becoming east 10 to 15 kt. Mostly Sunny. Waves 1 to 2 ft building to 2 to 4 ft.

LMZ700
No data

FXUS63 KLOT 041840 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 140 PM CDT Sat May 4 2024

KEY MESSAGES

- Gusty showers and storms will sweep across the area this afternoon and evening. Stay weather aware if you have outdoor plans!

- Sunday will be noticeably cooler with onshore flow and highs in the upper 50s (lakeside) to upper 60s (inland).

- There is a potential for strong to severe storms along with a locally heavy rainfall Tuesday and Wednesday

- Periodic showers are expected Thursday through Saturday

DISCUSSION
Issued at 114 PM CDT Sat May 4 2024

Through Sunday Night:

A surface low continues to lift northeastward into central Wisconsin ahead of an eastward-moving cold front currently in eastern Iowa and central Missouri. Ahead of the cold front, the airmass continues to destabilize with mostly sunny skies and southwesterly low-level moisture advection supporting the development of 1000-1500 J/kg of MLCAPE along and east of the Mississippi River. The low-level moisture plume advecting into the region is relatively shallow, leading to a reservoir of DCAPE in excess of 1000 J/kg along and ahead of the cold front. Meanwhile, well ahead of the cold front, shallow cumulus clouds dot the sky and temperatures are in the mid to upper 70s.

Recent 1-min GOES imagery depicts the very first attempts to initiate convection are taking place in far northeastern Missouri along the cold front and ahead of a decaying MCV. With observed AMDAR and forecast soundings still depicting a shallow cap from 850 to 800 mb, it may take another hour or so of heating and moistening ahead of the front before sustained convective cells can take hold. With that in mind, thunderstorms are expected erupt near or just east of the Mississippi River by 3 PM and progress eastward into northern Illinois thereafter (reaching I-39 in the 5 to 7 pm timeframe, Chicago metro in the 6 to 8 pm timeframe, and northern Indiana in the 7-9 pm timeframe, give or take an hour).

With our area removed from the best upper-level flow and hence deep- layer shear across the northern Great Lakes, convective cells will exhibit pulse-like single-cell behavior before the high DCAPE environment encourages consolidating outflows and quick upscale growth into several multi-cell clusters. As a result, our expectation remains for gusty thunderstorm clusters (gusts of 45-55 mph) to sweep across our area this evening. Wherever initial thunderstorms are most intense (e.g. initial "push" of outflow), locally damaging winds (55 to 65 mph) are a distinct threat, as well. The current SPC threat level 2/5 area near and west of I-39 highlights where the threat for damaging wind gusts is locally higher, though in reality, most (all?) of our area should see convective gusty winds at some point this evening. In fact, with virtually no low-level shear to resist the outward-spreading outflows, gusty to locally damaging winds may occur well before rain and lightning arrives at any given location. As a result, for those with outdoor plans this warm and summer-like evening, stay weather aware and heed any statements or warnings issued for your area.

The cold front will sweep across the state along or just behind the gusty thunderstorm clusters after sunset, causing winds to turn northwesterly. Shallow clouds in the wake of the front may encourage a few sprinkles through the overnight hours, particularly near Lake Michigan where low-level convergence will be maximized.

Tomorrow (Sunday) will be noticeably cooler thanks to onshore winds and filtered sunshine. Highs will top off in the upper 50s lakeside to upper 60s inland. Tomorrow night will be cool with overnight lows in the low to mid 40s.

Borchardt

Monday through Saturday:

A short wave trough will be passing over the area Monday morning and move east of the area by Monday afternoon.
Persistent southerly flow in the midlevels will help keep moisture available, but with minor ridging taking place through much of the day allowing for a brief window of height rises and very limited instability during the peak heating hours, opted to cap chances for rain at 20 to 30 percent south of I-88.

Those height rises turn to height falls early on Tuesday morning as a deepening upper level low, a strengthening upper level jet, and strong surface cyclone develops over the Northern Plains. Models are still disagreeing on exact timing and location, but model runs have projected another, slightly surface low developing to our northwest over the northern Mississippi river. As a stronger push of moisture rides the southerly winds aloft northward, the expectation is for over an inch of precipitable water to surge into the region on Tuesday (with around a 50% chance for PWATs to be greater than 1.5 inches, which would put it well above the 90th percentile for the ILX sounding climatology for May 8th (00z)). Both the Euro Ensemble and GEFS models have the majority of their members keeping the strongest instability around the Ohio River corridor, but but have between a 40 to 60 percent chance for, at minimum, 1000 J/kg of CAPE available in our forecast area.

Deterministic models are suggesting the first boundary that will stretch all the way back to North Dakota will provide the first opportunity for showers and potentially storms in the morning.
After that boundary passes, better instability aided by daytime heating may lead to redevelopment along a second boundary in the afternoon which could have better forcing to develop stronger and potentially severe storms over the area. Some cells may also develop higher rain rates to allow some localized flooding concerns, though with drier Sunday and Monday forecast perhaps it will allow some time for soils to recover to be able to be more efficient absorbing the rainfall. Model runs as the date approaches can hopefully inform if there are better chances for training storms to occur (as a few ensemble members suggest).

On Wednesday, the upper level low at 500 mb begins to break down and switch from negative to a positively tilted upper level trough. The reflected surface low starts to phase with the low over Iowa and Minnesota, slowly moving eastward. With a warm front likely to develop pseudo-latitudinal over the forecast area, it will likely lead to a sharp gradient in afternoon temperatures. Better lapse rates and moisture will likely be to the south of the front creating a renewed environment for showers and potentially strong to severe storm development. The main question will be where exactly does that front set up. With renewed cyclogenesis during the day (albeit weaker than on Tuesday, but relative to the day), storm development could occur right along the front itself. With strong moisture return from the south, this could lead to a better set up for training storms and flooding.

The upper level trough will broaden Thursday and Friday as it moves eastward, and not leave the western Great Lakes until Saturday afternoon. Even if it is not record setting, there will still be plenty of lingering moisture through the end of the week. While the better axis for instability moves east of the area (taking the stronger to severe threat with it), additional lobes of vorticity that pass through the upper level trough could provide periods of showers through Saturday morning. As a ridge builds into Saturday, better height rises and subsidence could lead to a better chance for a drier period over next weekend, but models this far out would struggle to resolve a small weak wave that passes that could keep the periods of showers hanging around.

Temperatures will remain above average through most of the period, with daytime highs in the 70s to low 80s (Tuesday and Wednesday have the potential to be the warmest days). Cooler, more average temperatures are expected by Friday, with highs in the mid-60s.

DK

AVIATION /18Z TAFS THROUGH 00Z MONDAY/
Issued at 101 PM CDT Sat May 4 2024

The primary aviation weather concerns through the 18Z TAF period include:

* A line of gusty showers and thunderstorms this evening

* MVFR cigs expected late this evening into Sunday morning

A line of showers and thunderstorms will move into northern IL later this afternoon and into the Chicago metro around early evening. It's expected to reach RFD mid-late afternoon and the Chicago sites closer to 00Z. The rain potential over any given area will last only a few hours or so clearing the Chicago sites sometime near 03Z. It's possible that vsbys could get pulled down to MVFR for a brief period or two underneath any heavier rainfall. Additionally, gusty westerly winds will likely accompany the rain and storms. Gusts into the 30- 35 kt range look attainable while the rain moves in and it's not out of the question that we could see an isolated higher gust or two.

Cigs are expected to remain VFR ahead and during the bulk of the rain event. However, MVFR cigs should take hold this evening as the rain begins to move away. MVFR will persist through the night and much of Sunday morning over the Chicagoland sites. A period of IFR is possible overnight, through best guess is that we'll remain MVFR. At RFD, the MVFR looks to lift much sooner, likely late this evening or early in the overnight. The MVFR will clear over the metro during the late morning and early afternoon.

Meanwhile, SSW winds will continue to occasionally gust into the teens to near 20 kt this afternoon. Winds are expected to veer to westerly with the arrival of the rain. A couple of forecast models suggest we may see a backing in the wind field to SSE for an hour or two as the system approaches, through a lack of confidence left this potential wind shift out of the TAF. Winds at the time will be moving at near 10 kt with gusts as high as 20 kt. Any of those notably higher gusts just ahead of the system that were discussed will be out of the west. NW winds just over 10 kt will follow the rain out of the area this evening before flopping over to NNE early- mid morning. The majority of Sunday will see NE winds below 10 kt.

Doom

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM...Dense Fog Advisory until 7 PM CDT this evening for Winthrop Harbor to Northerly Is. IL.

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