Sunrise 6:02 AM   Sunset 7:37 PM Moonrise 2:34 PM   Moonset 4:07 AM

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LMZ741 Expires:202404190330;;972358 Fzus53 Klot 181943 Nshlot
nearshore marine forecast national weather service chicago/romeoville il 243 pm cdt Thu apr 18 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>742-190330- winthrop harbor to wilmette harbor il- wilmette harbor to northerly island il- northerly island to calumet harbor il- 243 pm cdt Thu apr 18 2024
.small craft advisory in effect from Friday morning through Friday evening - .
Tonight - Northeast winds 10 to 20 kt becoming northwest 15 to 25 kt. Showers in the evening, then chance of showers after midnight. Waves 1 to 3 ft. Friday - Northwest winds 15 to 25 kt becoming west 20 to 25 kt by late morning. Mostly Sunny. Waves 1 to 3 ft. Friday night - West winds 10 to 20 kt. Partly cloudy. Waves 1 to 2 ft. Saturday - West winds 10 to 20 kt becoming northwest. Mostly Sunny in the morning, then becoming mostly cloudy. Waves 1 to 3 ft.

LMZ700
No data

FXUS63 KLOT 181940 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 240 PM CDT Thu Apr 18 2024

KEY MESSAGES

- Showers will spread across the area this afternoon and last through around midnight.

- Thunderstorms will spread from central Illinois toward or just north of US-24 this evening (between 4 and 10 PM) with a threat for damaging winds

- Much cooler temperatures are expected over the weekend with a threat for frost or freezes particularly on Saturday night.

- The next opportunity for rain will arrive Monday night into Tuesday.

MESOSCALE DISCUSSION
Issued at 233 PM CDT Thu Apr 18 2024

A cluster of thunderstorms recently developed in Hancock, Mcdonough, and Adams county, Illinois, and is rapidly lifting northeastward at about 50 mph. Radar data from KILX indicate a developing RIJ, and a wind gust of 40 mph was reported along the southern end of the cluster in spite of being located well north of the warm front located further to the south in southern Illinois. If the cluster holds together, it will move into Livingston, Ford, and Iroquois counties in approximately 2 to 3 hours. However, with the cluster so far removed from the instability axis and present much earlier previously advertised in model guidance, confidence in longevity is lower than average.

DISCUSSION
Issued at 147 PM CDT Thu Apr 18 2024

Through Friday:

Regional radar, water vapor, and satellite imagery augmented by a hand surface analysis depicts a surface low pressure system centered in central Missouri along a warm frontal boundary extending eastward across far southern Illinois and Indiana and ahead of a strong cold front arcing from central Wisconsin through eastern Oklahoma.
The arrival of a DCVA ahead of a subtle upper-level shortwave within upper-level southwesterly flow atop the cold front and surface low has recently caused thunderstorms to erupt across Missouri.

Over the next 12 hours, the surface low will lift east-northeastward across central Illinois and eventually northern Indiana as the cold front surges southeastward into northwestern Illinois. As the surface low and cold front approach in the next few hours, the local surface pressure gradient across northeastern Illinois will "kink" and allow for a lake breeze to surge inland and merge with the cold front. The net result will be a stark temperature gradient across the region by late afternoon, with readings in the mid 50s expected near the Wisconsin state line and Lake Michigan shoreline to the mid to upper 70s along US-24.

In addition, increasing linear low-level convergence along combining frontal boundaries as well as ahead of the approaching surface low will lead to increasing coverage of thunderstorms across northeastern Missouri and central Illinois over the next few hours. In fact, the southern edge of a 500mb jet streak across the central Great Lakes will afford some 45-55kt of convective-layer shear by early evening, allowing for thunderstorms to quickly evolve into clusters or just one continuous squall line as they lift northeastward. Even as thunderstorms lift into relatively cooler and more stable air, a dry low-level airmass ("onion" sounding) and tight packing of the surface pressure gradient along the immediate northwestern edge of the surface low will allow for convective severe surface wind production with the most organized clusters. With that said, the northern extent of any eventual thunderstorm clusters remains challenging to pinpoint, and will depend on the eventual track of the surface low down to the county scale. Based on how convection is already evolving across northern Missouri, our gut feel is the northern edge of any eventual cluster may be near or just north of US-24, between 4 PM and 10 PM this evening. However, a reasonable worst case scenario is the convection lifts as far north as I-80, particularly once it reaches northwestern Indiana. Northwest of the most active thunderstorms, persistent showers and a rouge lightning strike or two will prevail this evening within increasingly breezy north to northwesterly winds (gust of 20-30 mph).

Tonight, showers will end from west to east as the surface low pulls away. A quick shot of low-level cold air advection will keep northwesterly winds breezy and lead to surface temperatures dropping to the mid to upper 30s (northwest) to lower 40s (southeast). Breezy winds should curtail the threat for frost development tonight at least on a widespread basis.

Tomorrow, a 1028mb+ surface high pressure system currently centered along the northwestern US and southwestern Canadian border will expand southeastward into the Great Lakes. The tightening of the surface pressure gradient will cause westerly winds to become breezy with gusts to 30 mph during the afternoon hours, especially north of I-88. Finally, an initially sunny morning should turn partly cloudy by mid-afternoon as a pocket of seasonably cool low-level air overspreads the warm ground leading to shallow instability. No precipitation is expected tomorrow.

Borchardt

Friday Night through Thursday:

The upcoming weekend's weather will be mostly uneventful aside from much cooler conditions than we've seen recently. For Saturday morning, continued cold advection behind the departing storm system from today will allow for morning lows to drop into the 30s area-wide with lower to middle 30s likely north of I-80 and outside of the city. Such areas may wake up to some patchy instances of frost. Winds staying up a bit through the night, around 10 to 15 mph, will likely inhibit the development of widespread frost. During the day, expect highs in the 50s and a cool breeze out of the northwest. A large surface high approaching from the west will keep conditions calm and clear Saturday night. This makes Saturday night/Sunday morning look more favorable for widespread frost and perhaps freezing temperatures, especially north of I-80. The high will scoot to our southwest on Sunday and pump a little bit of milder air into the region pulling highs into the upper 50s to near 60 degrees.

This weekend, a fast-moving, low amplitude trough will move onshore in the Pacific Northwest and zoom across the northern Plains and into the upper Midwest early next week. The surface response will be a clipper system that is progged to bring us our next solid chance for rain and thunderstorms following today late Monday and Tuesday.
A big favorite among medium range deterministic and ensemble guidance is for the low to pass to our north across Wisconsin and the cold front to cut across northern Illinois. The deterministic Euro and a handful of ensemble members track it a bit farther south across northern portions of our CWA Regardless, models are not excited about a severe potential with this storm at the moment as they show a pretty meager thermodynamic environment, but it's one that could get a few thunderstorms going. Models are hinting that general thunder probs may be maximized on Tuesday around the storm's triple point near the low pressure center, which again is favored to pass just to our north. But of course, that is very susceptible to change over the next several days. Ridging behind the storm system looks to keep Wednesday quiet before an active pattern may be kicking off in the region late next week.

Doom

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/
Issued at 1236 PM CDT Thu Apr 18 2024

Key Aviation Impacts for the current TAF period:

- Showers and VFR clouds through mid afternoon at area terminals

- A line of storms moves in this evening with lower cigs and vis, gustier northeast winds, and steadier rain rates.
Isolated lightning is possible, but confidence was too low to add to the TAFs

- VFR and westerly winds after midnight

The first round of rain is moving over northern Illinois and Northwest Indiana at the time this discussion was posted. Cigs have remained above 6000 feet and all the lightning remains closer to St. Louis. There may be breaks in the rain through 22Z, so switched the TEMPO to prevailing -SHRA with the rain in eastern Iowa moving west.

By 22Z, a cold front will through the area. This front will provide an opportunity for steadier rain rates, gustier northeasterly winds and cigs/vis down to MVFR levels. There is a chance for IFR conditions to develop but confidence was not there to add it to the TAF currently. As the system transpires, if stronger cells develop, amends will be needed to reduce conditions. Lastly, there will be instability present such that isolated lightning strikes are possible as the front moves through. Strong consideration was given to adding thunder to KGYY's TAF, but kept it out for now. The stronger confidence in thunder remains south of area terminals where stronger instability will be located in Central Illinois. But if there is more consistent strikes developing, it will be added to TAFs tactically.

Recent model runs have sped up the front so that the strongest cells might be east of terminals by 03Z. -SHRA was added for the chance for lingering precipitation behind the front.
Additionally, as the front passes, winds will slowly switch from the northeast to the north west and weaken. Clouds will drift east. There is a chance clouds between 2000 to 3000 feet linger a through midnight, but there was moderate confidence in skies clearing out so prevailed VFR through the end of the TAF.
Quieter conditions are expected on Friday with stronger wind gusts around 25 knots in the afternoon.

DK

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM...Small Craft Advisory from 10 AM to 7 PM CDT Friday for the IL nearshore waters.

Small Craft Advisory from 4 AM to 7 PM CDT Friday for the IN nearshore waters.

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