textproduct: Chicago

KEY MESSAGES

- Scattered showers and embedded thunderstorms this morning, mainly for areas along and northwest of a Peru, IL to Waukegan, IL line.

- Additional rounds of showers and thunderstorms Monday through Tuesday. Some of which could be severe.

- Summer-like warmth and breezy south-southwest winds will prevail through Tuesday before cooler temperatures return for the middle of the week.

DISCUSSION

Issued at 330 AM CDT Sun May 17 2026

A pair of MCVs are traversing southern IA and northern MO this morning ahead of a mid-level shortwave trough that is lifting towards the Upper Mississippi Valley. Given the trajectory of the shortwave, the two MCVs should get pushed northeast across eastern IA into southwest WI through the morning which in turn should keep the bulk of any associated shower and thunderstorm activity in northwest IL, eastern IA, and southern WI. However, with how broad the rain shield is in current radar imagery suspect that some scattered showers will impact portions of our CWA (mainly areas along and northwest of a Peru, IL to Waukegan, IL line) around daybreak through late morning. With instability looking rather limited in recent forecast soundings the threat for thunder in our area is low, but cannot be fully ruled out so have maintained a 20% chance for such in the forecast.

Once these showers/isolated storms lift into WI later this morning, dry conditions are expected for the rest of our Sunday. With skies expected to become partly to mostly sunny this afternoon and the persistent south-southwest flow, temperatures will top out in the lower to mid-80s. However, a weak front and outflow boundary from the aforementioned showers/storms is forecast to settle near the IL-WI line which will keep winds more southeasterly along the northern IL shore and thus keep temperatures cooler in the 70s in these locations. Additionally, winds today will be on the breezy side with gusts around 25-30 mph. In fact if deeper mixing can be achieved this afternoon, locally higher gusts upwards of 35+ mph may also be seen.

Heading into tonight, wind gusts will ease as the atmosphere attempts to decouple, though with the continued warm advection and the tight pressure gradient due to the developing surface low in the plains a few 20-25 mph gusts may linger through the night. Otherwise, expect a mostly dry night with mild temperatures in the mid to upper 60s (lower 70s closer to Chicago).

While we enjoy a generally quiet night, the broad upper trough over the Western CONUS will be ejecting eastward with a couple of shortwaves traversing the pattern in the Plains and Mid- Mississippi Valley. These shortwaves are expected to lead to the development of showers and thunderstorms across the central Plains and IA along a frontal boundary which will in turn move towards northern IL and northwest IN on Monday. As a result, the showers and storms that develop in the Plains tonight are forecast to move into our area early Monday morning (likely after daybreak). Given that these showers/storms will arrive during on non-diurnally favorable time, instability should be somewhat limited and therefore allow the showers/storms to enter in a decaying state. However, if the storms arrive in a more organized fashion the steep lapse rates (around 7-8 C/km) and 30-35 kts of shear could support a couple of stronger storms capable of gusty winds and hail.

Depending on how widespread and intense showers/storms are Monday morning will have an impact on whether or not additional storms develop in our area Monday afternoon. While guidance is in decent agreement on the morning showers/storms being in a decaying state (i.e. more scattered coverage), forecast soundings do show the atmosphere to be weakly capped Monday afternoon. Couple this with the fact that the better forcing is forecast to reside in the central Plains and western IA (closer to the upper trough) and most guidance is suggesting that northern IL and northwest IN could stay dry Monday afternoon. That said, if any thunderstorms do develop Monday afternoon the warm and weakly capped airmass in combination with 40-45 kts of shear will support a threat for severe thunderstorms. With wind profiles looking mostly unidirectional, the main threat with any severe storms should be damaging winds and large hail. Though, if storms develop and/or linger into the evening where the low-level jet increases the low-level shear then a tornado threat could materialize. Because of this potential the Storm Prediction Center has kept all of northern IL and northwest IN in a level 2 of 5 risk for severe weather on Monday.

Regardless of how Monday afternoon goes, the aforementioned frontal boundary will begin to move into the area late Monday night into Tuesday which will bring additional periods of showers and thunderstorms. Conceptually speaking, the morning round of storms on Tuesday should push the effective frontal boundary more into our eastern CWA (namely areas east of I-55) and that should be where the bulk of any showers/storms that redevelop Tuesday afternoon reside. Nevertheless, any storms that develop on Tuesday will once again have the potential to become severe due to the 35-40 kts of shear and decent instability. The main threat with any severe storms on Tuesday looks to be mainly damaging winds and hail, however, as wind profiles should be unidirectional by that point.

Outside of the shower and storm chances Monday and Tuesday, both days will also feature summer-like warmth. Highs both days are expected to be in the lower to mid-80s, though the presence of storms could limit warmth in some locations. Winds both days will also be breezy with southwesterly gusts around 25-35 mph expected.

The frontal boundary will move through our area Tuesday night brining an end to the showers and storms as high pressure builds into the Great Lakes. Therefore, the middle part of the week looks to be rain free. Temperatures though will be notably cooler with highs only in the 60s for Wednesday and Thursday inland and in the 50s near the lake due to onshore winds. As we head towards Memorial Day weekend another broad upper trough is forecast to develop over the western CONUS. While this should allow more southwest flow to return and warm temperatures back towards more seasonal values, it will also bring chances for showers and possible thunderstorms late week into the weekend. Obviously confidence on coverage and timing of any showers/storms is low at this range so stay tuned.

Yack

AVIATION /12Z TAFS THROUGH 18Z MONDAY/

Issued at 647 AM CDT Sun May 17 2026

Aviation forecast concerns for the 12Z TAFs:

- Low chance (25-30%) for scattered SHRA mainly northwest of Chicago this morning.

- Light southeast winds become gusty (near 30 kts) this afternoon, and remain breezy with gusts 20-25 kts tonight.

- Strong low level jet develops overnight (45-50 kts) which may produce LLWS conditions if surface gusts were to subside.

- Weakening line of SHRA/TSRA should approach the terminals around/just after daybreak Monday morning.

A weak, lake-assisted cold front has pushed into northeast IL early this morning, but has slowed appreciably over the past 1-2 hours and is expected to wash out across the Chicago terminals this morning. While a brief light northeast or east winds is possible early especially at ORD, winds should generally remains south-southeasterly this morning. Winds are expected to become southerly and strengthen by midday, with gusts 25-30 kts this afternoon with a few higher gusts possible late. Warm and breezy conditions will persist tonight, with winds likely remaining gusty in the 20-25 kt range. A strong (45-50 kt) low-level jet is expected to develop overnight which may produce LLWS conditions, though with surface winds expected to remain elevated and gusty have not included a LLWS mention in TAFs at this time, but could need to be added with later updates if surface winds decrease more than expected.

An area of rain/embedded thunderstorms was over eastern IA early this morning, tracking to the north-northeast in association with a mid-level circulation (MCV). Various CAM guidance has generally been to generous with the forecast eastward extent of SHRA/TSRA early this morning, thus confidence in impacts to the terminals have decreased. RFD appears to have the greatest potential for seeing SHRA develop this morning and persist through midday. With some guidance continuing to depict some SHRA as far east as Chicago, have maintained PROB30 SHRA mentionsfor all but GYY at this time - though it's possible especially the Chicago terminals remain dry.

Late tonight, a line of thunderstorms is forecast to push east from IA, and approach the terminals toward/just after daybreak in a decaying mode. Some differences in strength and timing of this line exists with models, though did at a PROB30 for TSRA after 10Z at RFD and 12-15Z for ORD/MDW for now.

Ratzer

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...Small Craft Advisory from 1 PM this afternoon to 10 PM CDT this evening for Wilmette Harbor IL to Michigan City IN.