Sunrise 5:26 AM   Sunset 8:08 PM Moonrise 1:26 PM   Moonset 2:32 AM

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LMZ740 Winthrop Harbor To Wilmette Harbor Il- Wilmette Harbor To Northerly Island Il- Northerly Island To Calumet Harbor Il- 908 Am Cdt Thu May 16 2024
Rest of today - Southeast winds 5 to 10 kt. Scattered showers and Thunderstorms in the afternoon. Waves 1 to 2 ft. Tonight - South winds 5 to 10 kt. Patchy fog. Isolated showers. Waves 1 to 2 ft. Friday - Variable winds less than 5 kt becoming northeast 5 to 10 kt in the afternoon. Patchy fog in the morning. Waves around 1 ft.Friday night - East winds 5 to 10 kt. Partly cloudy. Waves around 1 ft.

LMZ700
No data

FXUS63 KLOT 161758 AFDLOT

Area Forecast Discussion National Weather Service Chicago/Romeoville, IL 1258 PM CDT Thu May 16 2024

KEY MESSAGES

- Scattered showers and thunderstorms are expected this afternoon. A stray strong storm cannot be ruled out near the Wisconsin border this evening (5% or lower chance).

- Patchy to areas of dense fog may develop overnight, especially northwest of I-55.

- Friday will be pleasant with highs around 80 and nearly calm winds. A lake breeze will surge inland during the afternoon, and a stray storm cannot be ruled out south of Interstate 80 (20-30% chance).

- Aside from isolated storm chances Saturday, mostly dry conditions are expected this weekend.

- An active pattern is likely to develop next week bringing several opportunities for showers and storms.

UPDATE
Issued at 1153 AM CDT Thu May 16 2024

Only update to the going forecast this morning was to add some better time resolution to the shower and thunderstorm chances through this afternoon.

In spite of the recent increase in mid and high-level cloud cover, the low-level airmass continues to destabilize late this morning as ~60 degree dewpoints continue to work northward into the area in advance of low pressure shifting into the Upper Great Lakes. With capping expected to be weak to non-existent this afternoon, widely scattered showers and thunderstorms are expected for much of the area this afternoon. Recent satellite and radar trends even suggest that things are already getting underway, and see no reason why this will not continue.

Modest mid-level lapse rates and deep layer shear on the order of 25 to 30 kt suggests that pulse type storms and/or loosely organized clusters will be the primary type. Accordingly, the overall severe threat will remain on the lower side today. That being said, we cannot rule out a couple instances of wind and hail with some of the strongest storms.

Most of the activity is expected to wane with sunset this evening. The only exception being a cluster of storms with a cold front potentially persisting with a cold front for a couple hours after sunset this evening, mainly across northwestern parts of the area.

KJB

DISCUSSION
Issued at 304 AM CDT Wed May 16 2024

Through Friday...

An early-morning hand surface analysis revealed a surface low pressure system in central Minnesota along a cold front arcing southwestward through western Iowa, southeastern Nebraska, and far northwestern Kansas. A few thunderstorms were noted ahead of the front in western Wisconsin and central Iowa. A second area of low pressure (really a mesoscale convective vortex, or MCV) was noted along the Oklahoma and Kansas Border along the backside of a relative disorganized mesoscale convective system approaching western Missouri. Ahead of the MCV, separate areas of showers and thunderstorms, and cold front, an expansive area of low-level moisture with surface dew points in the low to mid 60s and PWATs ranging from 1.0 to 1.3" (re: 00Z TOP and SGF RAOBs). The northern edge of the low-level moisture remains stark and near the Illinois and Missouri border, thanks to continued, albeit light, northeasterly flow on the backside of a departing high pressure system moving into southern Ontario. Finally, a baggy upper-level trough is evident across the Upper Mississippi River Valley water vapor imagery, including a subtle embedded shortwave propagating into northwestern Iowa at press time.

The forecast for today remains on track. Pressure falls across the northern Great Lakes will cause surface winds to turn southwesterly this morning, allowing the reservoir of moisture to our southwest to creep northeastward. Forecast soundings depict minimal capping by early afternoon which should allow for widely scattered showers and a few thunderstorms to develop in somewhat random fashion. However, heavy cloud cover should stunt MLCAPE from growing to more than 750 thereby limiting the intensity of any thunderstorm that develops.
With that said, any clearing that can take place late this afternoon particularly near the Wisconsin border may allow for locally higher instability and steeper low-level lapse rates ahead of the approaching cold front. As a result, we can't rule out a locally strong thunderstorm from developing along the cold front this evening between 4 and 8 PM, again, near the Wisconsin state line (generally a 5% chance or lower). Overall, the concern for severe weather today is pretty darn low.

Tonight, the cold front will decay into a surface ridge directly overhead, leading to nearly calm winds. Thinning cloud cover, especially northwest of I-55, will allow for efficient radiational cooling within the still-moist airmass, supporting areas of fog.
Conceptually, it's easy to envision pockets of dense fog (with visibility less than one quarter of a mile) developing by daybreak, particularly in areas where wet vegetation remains.

Tomorrow, any morning fog will erode and give way to partly cloudy skies nearly calm winds and highs near 80 will make for a spectacular and warm May day. The exception, of course, will be along the Lake Michigan shoreline as a lake breeze surges inland during the afternoon hours. Meanwhile, south of I-80, a few showers or thunderstorms cannot be ruled out as a weak wave passes overhead. Perhaps the best chances, generally between 20 and 30%, will be along any intersection of the southward-moving lake breeze and any northward-moving outflow from thunderstorms across central Illinois and Indiana.

Friday Night and Beyond...

The upper trough progged to bring us rain chances on Friday is expected to be traversing along the southern Ohio River Valley as we head into Saturday. Guidance has come into notably better agreement that the main core of the trough should be well east of our area by Saturday morning allowing mid-level height rises (higher pressure) to build into our area. However, forecast soundings do show some modest instability developing across the southeastern corner of our forecast area (i.e. areas south of the Kankakee River and east of I-57) which in conjunction to the closer proximity to the trough may allow for a low chance (<20%) of an isolated shower and/or thunderstorm Saturday afternoon. Additionally, guidance continues to depict a northern stream shortwave moving across the Upper Midwest toward the northern Great Lakes which is expected to push a cold front through northern IL Saturday night. Since the anticipated arrival time of the front is expected to be after sunset in our area, guidance continues to favor any showers/storms along the front dissipating prior to reaching the I-39 corridor. Thus, I saw no need to change the dry POPs offered by the NBM but still caution that a stray shower or rumble of thunder near I-39 and the IL-WI line Saturday evening is not completely out of the question just yet.

Aside from these minor rain chances, dry conditions are largely expected for the weekend as the aforementioned height rises pivot overhead on Sunday. Despite the passage of the cold front Saturday night, temperatures for both Saturday and Sunday are expected to be in the seasonably warm category with highs in the lower to mid-80s and overnight lows in the 60s. Though, the light surface winds will allow daily lake breezes to develop and surge inland keeping high temperatures cooler (in the upper 60s to lower 70s) for locales near the lake.

Heading into Monday and the beginning of next week, guidance continues to be in strong agreement that a more active pattern is likely to develop across the central CONUS. While there are some subtle differences in timing and intensity, the general idea is as follows. The northern jet stream is forecast to dig into the Pacific Northwest and phase with a developing sub- tropical jet across the southwestern US on Sunday. This phasing is expected to develop a broad trough across the western CONUS on Monday allowing the aforementioned sub-tropical jet to intensify and nose into the southern Plains and mid-Mississippi Valley. Not only will the sub-tropical jet aid in enhancing the southwest low-level flow over the Plains and Midwest supporting seasonably warm and humid conditions, but it will also allow any shortwaves that break off to eject towards the Midwest and southern Great Lakes. This combination of synoptic forcing, warm-humid air, and potential wind shear should promote the necessary ingredients needed for period of rather stormy weather in the region next week including the threat for severe weather.

As stated above, there are some subtle timing issues as to when any shortwaves will break off and where exactly they will track. However, the 00z guidance suite has started to highlight a couple periods of potential interest for our area. The first of these periods is within the Monday through Wednesday timeframe with the second favored toward the end of next week into the following weekend. Obviously the coverage and severity of any potential storms in our area remains uncertain given we are still 5-7+ days out, but this will be a period to monitor closely. Therefore, we continue to recommend keeping an eye on subsequent forecasts over the coming days for any changes. It may also be wise to review your severe weather plans just in case our area does become favored.

Yack

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/
Issued at 1240 PM CDT Thu May 16 2024

Key aviation messages:

- Isolated to scattered TSRA this afternoon into early evening - Low stratus and/or fog early Friday AM, greatest fog potential at RFD - Lake breeze develops late Friday AM

Widely scattered showers have developed already this afternoon which have produced a few lightning strikes. Accordingly, opted to move up the TSRA timing to the start of the TAF to account for this first "batch" as they move through. Expect shower/storm potential to decrease by this evening, with the exception being along the cold front near RFD which may see a few storms persist after 00Z. Winds could briefly gust up to 35kt beneath any storms that move directly overhead.

There continues to be a signal for low ceilings and/or fog development late tonight into early Friday morning.
Confidence in whether this remains mainly stratus versus fog is lower for the Chicago area terminals with this update. Opted to account for it with a TEMPO for MVFR vis and cigs. IFR or lower cigs can't be ruled out. Confidence in seeing fog is higher toward RFD where a TEMPO for IFR vsby has been introduced.
Expect any lingering low stratus/fog to lift and scatter out after daybreak Friday.

Steady southwest winds this afternoon turn light and variable overnight (under 5 kt) as they gradually veer to northwest overnight. A lake breeze is expected to develop by mid morning and push inland turning winds northeast to east in the afternoon.

Petr

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

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