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LMZ741 Expires:202404241200;;254019 Fzus53 Klot 232001 Nshlot
nearshore marine forecast national weather service chicago/romeoville il issued by national weather service lincoln il 301 pm cdt Tue apr 23 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>744-241200- winthrop harbor to wilmette harbor il- wilmette harbor to northerly island il- northerly island to calumet harbor il- calumet harbor il to gary in-gary to burns harbor in- 301 pm cdt Tue apr 23 2024
.small craft advisory in effect through Wednesday evening - .
Tonight - Northwest winds 15 to 25 kt becoming north to 30 kt. Chance of showers and slight chance of Thunderstorms in the evening. Some Thunderstorms may produce gusty winds in the evening. Waves 1 to 2 ft building to 6 to 9 ft occasionally to 12 ft. Wednesday - North winds 20 to 25 kt. Mostly cloudy in the morning then clearing. Waves 7 to 10 ft occasionally to 13 ft subsiding to 5 to 8 ft occasionally to 10 ft. Wednesday night - Northeast winds 10 to 15 kt diminishing to 5 to 10 kt. Mostly clear in the evening, then becoming partly cloudy. Waves 4 to 6 ft occasionally to 8 ft subsiding to 2 to 4 ft. Thursday - Southeast winds 5 to 10 kt becoming east 10 to 15 kt in the afternoon. Mostly Sunny. Waves 1 to 3 ft.

LMZ700
No data

FXUS63 KLOT 231853 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL Issued by National Weather Service Lincoln IL 153 PM CDT Tue Apr 23 2024

KEY MESSAGES

- There is a level 1 of 5 (marginal) risk that scattered thunderstorms through around 8 pm this evening turn severe roughly along and north of I-80. Strong wind gusts and hail are the main concerns.

- There is a 50-70% chance for a freeze Wednesday night into Thursday morning outside the Metro, with the highest chances generally west of I-57. Tender vegetation may be damaged if protective actions are not taken.

- Multiple rounds of thunderstorms late Friday through the weekend will pose a risk for severe weather and heavy rain.
Damaging wind gusts, large hail, localized flooding, and a tornado are all concerns, though confidence is low in when and where chances will be highest at this time. Have multiple ways to receive warnings, and a plan for where you'll seek shelter.

DISCUSSION
Issued at 149 PM CDT Tue Apr 23 2024

Through Wednesday Night:

At 145pm, current radar imagery shows showers with a few embedded thunderstorms roughly south of an El Paso to Valparaiso line. Over the next couple hours, the pockets of sunshine behind this wave of precip will result in steepening low level lapse rates for some weak instability favoring isolated showers and thunderstorms ahead of the cold front slated to reach I-55 around 6-7pm. The stamps from the 12z HRRR are looking a little less aggressive than they did in the 00z iteration, though still around 50-60% of HRRR membership generates 500 J/kg or more of SBCAPE at some point between 3 and 8 pm along a narrow corridor extending from roughly Chicago- Waukegan southwest to Galesburg. Given low LCLs and 30-40 kt of 0-3 km wind shear along the front, there's a nonzero (though very small) chance for a landspout tornado with this activity.
However, the steep lapse rates and dry mid level air would seem to favor more of a momentum transfer wind concern with the stronger cells; the HRRR shows this in its wind gust field, with pockets of 40+mph gusts under showers this evening. In other news, it's turning quite warm behind the front given the drier airmass fostering efficient radiational warming with the anticipated sunshine; the last few iterations of the HRRR suggest a few locations will top out around 70 degF for highs before the evening's over. Short term guidance suggests northerly winds will stay 5-10 mph tonight, except quite a bit higher near the Lakeshore, which should in theory prevent frost from developing and simultaneously offset radiational cooling to diminish chances of a freeze (NBM gives 10-20% probabilities north of a Georgetown to Waukegan line) as well.

The cool advection in the wake of the front will suppress tomorrow's highs, most notably near Lake Michigan where the Lake-chilled boundary layer airmass will be advected inland throughout the afternoon to keep temps in the 40s in many locations, including the Chicago Metro. Tomorrow night, there is a higher chance of widespread frost as high pressure building into the region leads to light/calm winds allowing for more optimal radiational cooling. The lowest temps will occur away from the Lake and west of I-57; further east, the weak northerly/northeasterly flow will advect southward a shallow marine layer and attendant stratus which should offset nocturnal cooling. The NBM suggests west of the I-57 corridor there will be a 50-75% chance for a freeze, with a 15-30% chance for a hard freeze (temps < 28 degF) north of Rockford, so frost and/or freeze headlines will likely become necessary.

Bumgardner

Thursday through Tuesday:

As the high pressure ridge axis reaches northern IL, winds will veer to southeast on Thursday, advecting into the region a warmer airmass that will bring afternoon highs into the low 60s inland, mid 50s Lakeshore. All signs indicate Friday will be even warmer, though it'll come with a price: The deterministic GFS and ECMWF each show a pronounced theta-E gradient lifting north into the area at some point Friday, and while they disagree on timing each model suggests precip riding along that front. Fortunately for us, the associated sfc low will be displaced well to our west - across the central Plains - though locally backed winds enhancing low level shear and low LCLs along and south of that front could prove sufficient for a couple severe storms, especially across our south and west CWA where the NAM suggests around 1000 J/kg MUCAPE materializes by 00z Sat (7pm Friday). We've still got several days before that system arrives, so a lot is subject to change, though the CSU MLP paints a 15%+ probability area across virtually the entire CWA (save northwest IN).

Convection will certainly have an effect on the warm front's movement, but at this point there's fairly good agreement in the extended range models and their respective ensembles that Saturday will be much warmer; the NBM, in fact, gives a 50-65% chance for highs over 80 degF. This, of course, could be a bust if there's more cloud cover than currently forecast or thunderstorms develop during the day. Instability will certainly be higher on Saturday compared to Friday, with 50-60+% chances for more than 1500 J/kg SBCAPE during the evening west of I-55. However, it's uncertain whether there will be any trigger for storm initiation with the warm front expected to be so far north; it might come down to outflow boundaries from nocturnal convection Friday night into Saturday morning. CSU again depicts a 15% contour for severe weather on Saturday, so we'll be watching carefully.

Another sfc low will spin up in the southern Plains Saturday night and lift into IA Sunday afternoon/evening, increasing wind shear across our area and potentially favoring another round of thunderstorms which, once again, could turn severe. The SPC has highlighted our area with a level 2 of 5 (slight) risk for severe weather on Sunday, with all hazards (damaging wind gusts, large hail, and tornadoes) seemingly in play; of course, this will be conditional on Saturday night-Sunday morning convection, afternoon cloud cover, the precise track of the sfc low, and the strength of the low - all of which are difficult, if not impossible, to pin down at this time step. We'll continue to monitor.

Bumgardner

AVIATION /18Z TAFS THROUGH 00Z THURSDAY/
Issued at 1228 PM CDT Tue Apr 23 2024

* Concern through 06Z: Timing of convection with the front, wind shift from southwest to northwest/north * Concern after 06Z: MVFR ceilings overspreading KORD/KMDW overnight through about mid morning Thursday

Pronounced dry layer below 3 km remains in place per most recent ACARS soundings from KORD, which has kept the rain and lower ceilings south of the terminals thus far. High-res models have been trending a bit later with convection developing along the incoming cold front, focused on the period from 21Z-01Z, with a few of these storms potentially on the stronger side.
Have maintained the TEMPO periods at KORD/KMDW during this time frame. Wind shift with the front will become more sharp as the front gets further into the Chicago metro and the marine flow begins to kick in. More of a sharp switch from about 330 degrees to 010-020 degrees looks to take place sometime around 03-04Z.

As the lake flow kicks in, concern shifts toward lower ceilings at KORD/KMDW. Latest HRRR guidance suggests this could happen as early as 05Z, but more focus is on 06-07Z for MVFR conditions.
By 09Z, probabilities of ceilings below 3,000 feet are up to 80-90% at both sites, with improvement not until 15-16Z.

Outside of these sites, a more transient period of MVFR ceilings is hinted at for KRFD around 05-09Z. With KGYY being on the south end of the lake, once MVFR ceilings take hold, they are more likely to remain in place until midday.

Geelhart

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...None.
LM...Small Craft Advisory until 10 PM CDT Wednesday for the IL and IN nearshore waters.

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