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LMZ741 Expires:202404280915;;499806 Fzus53 Klot 280309 Nshlot
nearshore marine forecast national weather service chicago/romeoville il 1009 pm cdt Sat apr 27 2024
for waters within five nautical miles of shore on lake michigan
waves are provided as a range of significant wave heights, which is the average of the highest 1/3 of the waves, along with the average height of the highest 10 percent of the waves which will occasionally be encountered.
lmz740>742-280915- winthrop harbor to wilmette harbor il- wilmette harbor to northerly island il- northerly island to calumet harbor il- 1009 pm cdt Sat apr 27 2024
Rest of tonight - South winds 15 to 25 kt diminishing to 10 to 15 kt toward morning. Slight chance of showers and Thunderstorms late in the evening, then numerous showers and Thunderstorms after midnight. 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 280328 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1028 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.

MESOSCALE
Issued at 1028 PM CDT Sat Apr 27 2024

Portion of linear MCS moving out of eastern IA has weakened considerably over the past couple of hours, though convection across southern WI ahead of an MCV continues to exhibit strong/severe characteristics. Activity farther south appears to have struggled with development of diurnal CINH and veered low- level flow not supporting vigorous new updraft development on the outflow boundary moving into our western cwa at this time.
While non-zero, the severe thunderstorm threat does appear to have decreased across northern IL at this time, though will continue to monitor trends with the potential for organized strong convection remains.

Convective activity across central IL is likely slightly elevated, though may maintain the threat for isolated hail as it lifts into our southern counties. The west-east orientation of this activity, along with the approach of an MCV over northeast MO, may allow for some increase in intensity after midnight.
Locally heavy rainfall is looking more likely as well, from the I-80 corridor southward.

Ratzer

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 /00Z TAFS THROUGH 06Z MONDAY/
Issued at 650 PM CDT Sat Apr 27 2024

Key Aviation Messages: - Area of TS shifting across the Chicago terminals overnight.
- NE wind shift from TS outflow sometime mid-morning to early afternoon Sunday - Potential retreat of the outflow boundary and return toward S/SSE winds Sunday evening.
- Another round of possible TS late Sunday evening and overnight.

An enhanced mid-level disturbance near Kansas City will lift northeast toward northern Illinois overnight and interact with currently developing convection across eastern Iowa and southern Wisconsin. This should shift a somewhat organized area of TSRA eastward over the terminals generally in the 06-10Z window, with some lingering TS possible through sunrise Sunday. Outflow from this convection will then drift south into northern Illinois, turning winds NE at ORD and possibly MDW for much of Sunday afternoon. The onset time as well as the longevity of NE winds will depend heavily on the amount of convection over/near the region overnight. The current forecast depicts a NE wind shift by around noon, followed by a quickly veering SSE direction Sunday evening as a warm front surges back northward. If convection become rather robust overnight, ORD and possibly MDW may shift NE Sunday morning and remain NE or E through the remainder of the period. Additionally, MVFR ceilings north of the boundary/wind shift may lower in IFR levels at time.

The next round of potential convection will arrive late Sunday evening and especially after midnight Sunday night. A PROB30 for TS was included for 03-06Z (end of TAF period) to account for the potential for an earlier arrival of TS or scattered TS developing ahead of the main TS period.

Kluber

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM...None.

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