textproduct: Chicago

KEY MESSAGES

- Generally cool pattern to persist through next week with occasional chances for scattered showers.

- Patchy frost possible in northwest IL tonight, better coverage of frost Friday night into Saturday morning inland of Lake Michigan.

DISCUSSION

Issued at 248 PM CDT Thu Apr 30 2026

A broad upper low continues to swirl above the Great Lakes and adjacent Ontario with several shortwave disturbances noted pivoting around the low. One of these shortwaves is currently drifting across eastern IA and stating to move into northern IL along with its baggy surface low. Associated with these features is some light rain showers and sprinkles which will continue to spread across northern IL and eventually into northwest IN through the rest of the afternoon and evening. While this initial wave of showers is expected to exit by midnight, another shortwave (currently in the Dakotas) will pivot into the area overnight and may bring with it another chance (20-30%) for showers. Depending on how quickly this second round of showers exits and clouds scatter out there could be just enough time for temperatures to cool and lead to some patchy frost development. The greatest potential for frost at this point is in northwest IL, but if clouds clear faster that potential could ooze east. Given the lower confidence on frost coverage have opted to hold off on a Frost Advisory for now.

Friday will feature similar conditions to today with highs once again in the mid-50s inland and in the upper 40s near the lake. Though, cloud cover will be a bit more expansive as yet another shortwave will be diving through the area. While mostly dry conditions are expected for most, there is a non-zero chance (15- 20%) for some isolated showers Friday afternoon. As the shortwave disturbance exits Friday evening, skies should quickly clear as a weak surface high moves overhead allowing temperatures to dip into the lower to mid-30s. The combination of clearing skies and light winds will make for favorable conditions for widespread frost development Friday night into Saturday morning. Should these trends continue we may need a Frost Advisory for Friday night but will handle that with tomorrows forecast package if needed. Regardless, anyone with sensitive plants should take precautions to protect them.

The weak surface high and modest height rises are expected to persist through Saturday which will give us a dry start to the weekend. However, the continued northwest winds will keep highs only in the mid to upper 50s inland and in the upper 40s to lower 50s near the lake. With the ridging overhead it seems skies should favor a mostly to partly sunny condition which may once again lead to some frost development Saturday night into Sunday especially in northwest IN.

Heading into Sunday, the continued troughing over the Great Lakes is forecast to deepen as a more amplified shortwave trough ejects out of Canada into the region. At the same time, another trough/upper low is forecast to develop and be oozing inland across southern CA and the southwest. These features will help to generate stout southwesterly flow across northern IL and northwest IN on Sunday which will lead to a slight warming trend with highs in the mid to upper 60s forecast areawide. With the warmer temperatures will also come the chances for showers and possibly a few thunderstorms, but latest forecast trends do have showers/storms holding off until Sunday evening when the aforementioned Canadian trough arrives. If this trend continues then Sunday may too be another dry day for us, however, if the trough speeds up then we could see rain as early as Sunday afternoon. Outside of the rain potential, winds on Sunday will be breezy with gusts likely around 30-35 mph.

The better chances for rain from this system is expected to occur on Monday when the Canadian trough pushes a cold front through the region. Depending on when the front arrives, there is a chance that temperatures on Monday could be the warmest of the week with highs currently forecast to be in the 70s. Though if the front comes through earlier then highs may be closer to what we see on Sunday. Regardless, showers and thunderstorms are expected to develop along the front and move through the area into Monday night. While forecast soundings do show some decent deep layer shear (around 35- 40 kts) moisture looks to be a limiting factor which should keep the threat for any severe weather low.

The cold front is forecast to shift south of the area on Tuesday which should return the cooler temperatures to the region as broad troughing remains over the Great Lakes. Thus, highs in the 50s (locally cooler near Lake Michigan) should be expected for the middle and later half of next week. Additionally, periodic shortwave disturbances will continue to dive through the broader trough and result in occasional chances for showers especially towards the later half of next week.

Yack

AVIATION /18Z TAFS THROUGH 00Z SATURDAY/

Issued at 1221 PM CDT Thu Apr 30 2026

Key Messages:

- Easterly wind shift at the main Chicago terminals ongoing with lake breeze boundary.

- A few scattered showers and locally lower CIGs mid to late this afternoon into this evening.

- Northerly winds turn northeasterly again across northeastern IL and northwest IN Friday afternoon.

A lake breeze boundary is currently moving right across ORD and MDW, resulting in an easterly wind shift. In its wake, winds will remain easterly the remainder of the day. Thereafter, winds will ease through the evening, then turn back to the north- northwest overnight into Friday morning before another lake breeze shifts the winds back northeasterly at the Chicago terminals Friday afternoon.

Otherwise, expect lower VFR CIGs this afternoon, along with some scattered showers. The best shower timing will be from mid to late afternoon into this evening.

KJB

LOT WATCHES/WARNINGS/ADVISORIES

IL...None. IN...None. LM...None.