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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- Northerly Island To Calumet Harbor Il- 251 Pm Cdt Sat Apr 27 2024
.small craft advisory in effect until 10 pm cdt this evening - .
Late this afternoon - Southwest winds to 30 kt. Partly cloudy. Waves 2 to 4 ft. Tonight - Southwest winds 15 to 20 kt becoming south 10 to 15 kt. Showers and Thunderstorms. Waves 1 to 3 ft. Sunday - Southeast winds 10 to 20 kt turning east in the evening. Chance of showers and Thunderstorms. Waves 1 to 3 ft building to 2 to 4 ft.Sunday night - East winds 10 to 15 kt becoming south overnight. Scattered showers and Thunderstorms. Waves 2 to 4 ft. Monday - South winds 15 to 20 kt becoming southwest. Chance of showers and slight chance of Thunderstorms. Waves 2 to 4 ft.

LMZ700
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FXUS63 KLOT 271951 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 251 PM CDT Sat Apr 27 2024

KEY MESSAGES

- Several waves of showers and thunderstorms are expected over the next 48 hours. A few storms may produce localized flooding and severe weather (mainly tonight).

- After a break on Tuesday, a stormy pattern will return toward the middle of next week.

- Above average temperatures will prevail through at least the middle of next week.

DISCUSSION
Issued at 251 PM CDT Sat Apr 27 2024

Through Sunday Night:

A stout upper-level shortwave continues to dig into the Southern Plains in the wake of a departing shortwave now lifting into Canada.
The resulting broad region of southwesterly flow across much of the Mississippi River Valley has led to a summer-like day with partly cloudy skies, temperatures in the mid to upper 70s, dew points in the lower 60s, and breezy southwest winds gusting over 35 mph.

We are watching two areas for the development of thunderstorms this evening. The first area is along a zone of low-level confluence extending from near Topeka, KS to Milwaukee, WI, ahead of locally pooling low-level moisture across northern Illinois.
Early attempts at the initiation of sustained convection have thus far failed across Iowa and far northwestern Illinois, likely owing to a strong cap sampled by an 18Z DVN RAOB based near 850mb. While a storm or two may develop within the confluence axis across eastern Iowa, northwestern Illinois, and southern Wisconsin over the next few hours, a rapid expansion of ongoing convection across northern Kansas into northern Missouri and southern Iowa toward sunset appears to be more likely as the aforementioned upper-level shortwave currently in the Southern Plains lifts northeastward and lifts/cools the cap.

The second area we are watching for thunderstorm development is across central Illinois, that is, south of US-24, within a gradually destabilizing open warm sector. With nebulous forcing (e.g. offset from the low-level confluence axis), the development of thunderstorms in the second area will depend entirely the erosion of a cap via warming and moistening of the boundary layer. Based on AMDAR soundings from aircrafts ascending/descending from ORD, temperatures around 78-79F and dew points near 65-66F are needed for free convection to take place. With that in mind, it may be several hours yet before sustained convective attempts can take place, or at least until the leading influence of the upper-level shortwave arrives.
Given impressively steep mid-level lapse rates across the warm sector (>8 K/km from the base of the cap to around 500mb as sampled by the 18Z DVN RAOB), convection will likely intensify rapidly once the cap is breached in either area and pose a threat for locally damaging wind gusts and hail. Note it's possible much of our area is completely dry through at least sunset if the cap holds tight.

This evening, the expectation is for convection across northern Missouri and southern Iowa to grow upscale and "meet" developing free convection across the open warm sector across central Illinois in tandem with the intensifying low-level jet. While the pattern does not resemble more typical regimes for warm season flash flooding, the prolonged forcing of the 925-850mb jet, availability of PWATs of 1.25-1.5", and weak upper-level jet diffluence may support a band of training convection across central Illinois through the overnight hours. If convection can repeatedly train over the same area tonight, 6-hour rainfall amounts of 2-4" may fall supporting a threat for flash flooding of low lying areas such as ditches, farm fields, and low-lying roadways. The past two runs of the HRRR and most recent available HRRR/RAP/RRFS guidance continues to suggest the threat zone for training convection will be near US-24, though operational experience suggests it may end up just to the north or (more likely) south of the advertised axis.

Now, for the fly in the ointment. Assuming the forthcoming MO/IA convection does meet developing convection across central Illinois this evening, any convection developing along the Wisconsin state line would become "choked" off from the low- level jet. However, if the central Illinois convection fails to develop and meet the MO/IA convection, the confluence axis across far northern Illinois would be the "hotpot" for storms this evening and overnight given the replenishing low-level jet will be able to lift much further northward. In such a scenario, an axis of 2-4 inches of rain would be displaced closer to the Wisconsin state line. Moreover, any upscale growth out of MO/IA would support at least decaying complex of gusty thunderstorms to move across most of our area after dark. At this point, it's not clear which scenario (axis across central IL or near the Wisconsin state line), is most likely.

Toward daybreak Sunday, the MUCAPE reservoir feeding the overnight convection (wherever it is) will become exhausted, allowing the low- level jet (e.g. the "advective" component offsetting any southward propagation via consolidated outflows) to push activity northeastward away from our area. Meanwhile, the remnants of what will likely become an expansive squall line across the Southern Plains will northeastward into northern Illinois after daybreak in the form of thicker cloud cover or decaying showers. Tomorrow will accordingly start out wet and cloudy. Ongoing isentropic ascent may support isolated to scattered showers throughout the day, but overall, there should be dry hours tomorrow. A few breaks in clouds should nevertheless allow for highs to reach the low to mid 70s, especially southeast of I-55.

The next wave of showers and storms will arrive tomorrow night as the well-advertised upper-level shortwave trough finally lifts into the Upper Mississippi River Valley. Compared to tonight, instability tomorrow night looks far more meager in spite of favorable kinematics for severe weather. For this reason, we favor a broken band of showers and storms (perhaps with gusty winds) to move through the region during the overnight hours.

Borchardt

Monday through Saturday:

The upper trough currently ejecting into the southern Plains is forecast to be lifting into the upper Midwest as an upper low in conjunction with a surface low on Monday which will force a cold front through northeast IL and northwest IN. Thus, shower and thunderstorm chances are expected to persist through Monday afternoon. While instability continues to look rather modest, the trajectory of the upper low into Wisconsin will position a stout 500 mb jet streak over the area generating around 30 to 35 kts of shear. Therefore, if the warm-moist advection ahead of the front can destabilize the atmosphere I would not be surprised to see some of the thunderstorms try to become better organized Monday afternoon, especially east of I-55. However, the progressive nature of the front does look to limit the window for ample instability and shear to overlap in our area and therefore confidence on any severe threat occurring is low at this time. Regardless, showers and storms are expected to end from west to east Monday evening as the front exits.

Heading into Tuesday, modest upper-level height rises are expected in the wake of the aforementioned front which will promote a period of dry weather for Tuesday. Though, the reprieve looks to be short lived as a series of shortwave troughs are forecast to develop and traverse across the north- central CONUS. This will result in periods of showers and thunderstorms beginning Tuesday night and persisting through at least the end of next week. While details on exact timing and intensity of storms remains fluid, the area is expected to remain under in an unseasonably warm pattern with afternoon highs forecast to be in the mid to upper 70s through Thursday.

Yack

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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