Sunrise 5:24 AM   Sunset 8:11 PM Moonrise 3:28 PM   Moonset 3:09 AM

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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- Northerly Island To Calumet Harbor Il- Calumet Harbor Il To Gary In- 334 Am Cdt Sat May 18 2024
Today - South winds 5 to 10 kt becoming southeast 10 to 15 kt. Mostly Sunny. Waves 1 ft or less building to 1 to 2 ft. Tonight - South winds 10 to 15 kt becoming southwest 5 to 10 kt overnight. Slight chance of showers and Thunderstorms overnight. Waves 1 to 2 ft. Sunday - North winds 10 to 15 kt becoming northeast in the afternoon. Mostly Sunny. Waves around 1 ft. Sunday night - Northeast winds 5 to 10 kt becoming southeast overnight. Slight chance of showers and Thunderstorms overnight. Waves around 1 ft.

LMZ700
No data

FXUS63 KLOT 181105 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 605 AM CDT Sat May 18 2024

KEY MESSAGES

- Non-zero shower potential this afternoon/evening as a weak cold front moves across the area

- A few scattered shower and thunderstorms possible Sunday PM, mainly around and south of I-80

- Unseasonably warm weather expected through Tuesday night, interrupted only by lake cooling Sunday; then during any showers/storms Monday and Tuesday.

- A couple/few bouts of showers and storms expected Sunday night through Tuesday night, with many dry hours in between.

- Locally heavy rainfall possible Monday-Tuesday night, with some potential for severe weather Tuesday, mainly Tuesday night

DISCUSSION
Issued at 222 AM CDT Sat May 18 2024

Through Sunday Night:

As an upper level ridge grows to the northeast over Illinois on Saturday, warmer and drier conditions are expected to develop.
With both O'Hare and Midway remaining in the low 60s around 2 AM today and the knowledge that temps over performed on Friday, and clearer skies expected through the day to allow for maximum sunshine, afternoon high temperatures today were knocked up a couple degrees above guidance.

Meanwhile, a surface low in eastern South Dakota will slowly travel northeastward into Canada through the day. A cold front extending southward from the low will move over Iowa and eventually into northwestern Illinois Saturday evening. The consensus of high res guidance is suggesting that as the sun sets, the front will slowly weaken as it approaches Lee and Ogle counties with the showers slowly fizzling out. However, there is a non-zero chance that the front holds together a little bit better and can be just enough forcing along with some isentropic upglide to provide the occasional isolated shower.
But with the lack of confidence and better forcing closer to the low up north, PoPs were kept below the mentionable 15 percent as drier conditions are favored, with the exception being over the lake where there was slightly better instability favored in model soundings.

That front will continue to move through the forecast area on Sunday morning. As it moves east, it is projected to slow down and set up a boundary extending from southwestern Michigan to northern Missouri, pretty much along I-80. To the north of area will be slightly mode stable air and northeasterly winds driven off the lake to help add a little bit for cooler relief to the warm temperatures expected. However, along and south of I-80 will be better instability and dew points in the low 60s. The overall set up does not arouse too much excitement given that mid level lapse rates do not look overly impressive and deep layer shear is limiting; however, for areas south of I-80, there remains a chance for a few scattered showers and thunderstorms Sunday afternoon and into the evening.

DK

Monday through Friday:

Large scale pattern through early next week will feature western trough/eastern ridging. A weak front is expected to stall out over central IL/IN Sunday evening. It will then lift north as a warm front Sunday night in response to a lead shortwave trough ejecting out of the western trough and toward the Midwest.
Stronger forcing associated with the shortwave looks to arrive after the front is progged to have lifted north into WI, so better precip chances look to largely stay north/west of our CWA Sunday night.

Getting a pretty consistent signal in the various models that this lead shortwave will become convectively enhanced as convection across the central Plains congeals into an MCS and moves into the Corn Belt Sunday night. Chances for convection Monday will hinge on the ultimate evolution of this lead (likely convectively augmented) short wave trough that is expected to move into the western Great Lakes Monday. Some decaying remnant activity from this MCS could spill into our CWA Monday morning.

Based on consensus of the latest guidance suite, the strongest forcing with this MCV looks to remain north of our area Monday.
However, assuming convective debris doesn't stifle destabilization, strong heating of moderately moist air mass could allow for 1-2k J/kg of MLCAPE to be realized Monday afternoon with minimal inhibition. Given the expected unstable/uncapped air mass over the area, the glancing blow from the vort passing to our north and/or remnant outflow boundary (or even outflow enhanced synoptic boundary) could be enough to spark renewed or reinvigorated convection Monday afternoon.
Weakening mid-level flow is expected as mid-level speed max exits the region, so weak shear profiles Monday afternoon should limit severe threat to just an isolated pulse severe storm or weakly organized multi-cells.

Another round of potentially severe convection is expected Monday afternoon across the central Plains as a significant portion of the western trough breaks off and moves northeast into the Plains. This Plains convection Monday afternoon and evening could once again congeal into an MCS and track across the Corn Belt Monday night and eventually into the western Great Lakes Tuesday morning. This far out, it is impossible to say with any degree of confidence where this convection could end up Tuesday morning (assuming it hold together).

By Tuesday afternoon, strong, negatively upper trough is expected to track across the northern Plains into the upper Mississippi Valley. Strong surface cyclone is expected to support potentially widespread severe thunderstorm development Tuesday afternoon. Guidance has continued to slow (a very common theme with medium range guidance with a big system like this)
suggesting the better threat of afternoon severe weather Tuesday could end up well west of our CWA across Iowa, with out CWA potentially dry (and VERY warm, but more on that in a bit)
Tuesday afternoon. Eventually this convection would likely move east across the MS River and into our CWA later Tuesday or overnight Tuesday night, probably in, or soon to begin, a weakening phase by the time it arrives in our CWA Given the progged strength of the synoptic system, we will need to monitor the potential for severe wx Tues (mainly at night) closely the next few days.

Unseasonably warm temperatures (well into the 80s) are expected Monday, assuming left over convection (or convective debris)
doesn't survive into our area and impede warming or alter wind fields resulting in a lake breeze. Tuesday has potential to be even warmer, again with the same convective caveats as Monday.
In fact, with slower solution suggesting a better chance of dry or mostly dry conditions Tuesday afternoon, temps could really get out of hand. Progged 925mb temps at least solidly into the lower 20s (Celsius), if not mid 20s, means with any amount of sunshine, some areas could see highs eclipsing 90 degrees Tuesday. Didn't stray from current NBM mid- upper 80s for now given uncertainties, but if trends continue then some areas will probable see their first 90 of the year Tuesday.

Forecast confidence Wednesday onward decreases. Chance pops offered up by NBM for Wednesday are contingent on the system slowing and front stalling out in our CWA Conceptually, the far more likely scenario is the front will be pushed safely south and east of our CWA with dry conditions Wednesday, but didn't adjust with lowering NBM pops for now given the uncertainties and propensity for these systems to slow in later run. Thursday (at this point) will probably be dry in between systems, but large degree of model spread from Thursday onward supports maintains slight chance to chance pops each day (which are close to climo for this time of year). Temperatures should return to more seasonable levels (70s) behind the Tuesday night cold front midweek and probably through the rest of the upcoming work week.

-Izzi

AVIATION /12Z TAFS THROUGH 12Z SUNDAY/
Issued at 605 AM CDT Sat May 18 2024

VFR conditions expected through the TAF cycle. After light south to southwest winds through this evening, a weak front will move across the terminal causing winds to shift to light north to northwest during the predawn hours Sunday. At ORD and MDW, winds should veer around to northeast within a couple hours after sunrise Sunday.

- Izzi

LOT WATCHES/WARNINGS/ADVISORIES
IL...None.
IN...Air Quality Alert until midnight CDT tonight for INZ001-INZ002- INZ010-INZ011.

LM...None.

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