Frost dates / Illinois / Mount Prospect
Frost dates in Mount Prospect, IL
The station on this page is CHICAGO PALWAUKEE AP, IL US (USW00004838), 4.3 miles from the city point. Population in the Census 2024 estimate is 55,460.
Zone 6aUSDA average annual extreme minimum -10°F to -5°F, from ZIP 60056 (0.2 miles from the city point used on this page).
Outside the median season Growing seasonTicks: 10%, 50%, and 90%
Last 32°F in spring
NCEI’s probability is the chance the last spring minimum of 32°F or lower occurs on or after the date.
| Probability | Date | How to read it |
|---|---|---|
| 90% | April 4 | Earlier date. A last 32°F on or after this day is common. |
| 50% | April 19 | Median. |
| 10% | May 3 | Later date. A last 32°F on or after this day is uncommon. |
First 32°F in fall
NCEI’s probability is the chance the first autumn minimum of 32°F or lower occurs on or before the date.
| Probability | Date | How to read it |
|---|---|---|
| 10% | October 13 | Earlier date. A first 32°F on or before this day is uncommon. |
| 50% | October 26 | Median. |
| 90% | November 9 | Later date. A first 32°F on or before this day is common. |
32°F growing-season length
Days between the last spring 32°F and the first autumn 32°F. The figure is a length the season meets or exceeds at that probability, from NCEI, not a subtraction done on this site.
| Probability the season is at least this long | Days |
|---|---|
| 10% | 207 |
| 50% | 190 |
| 90% | 170 |
28°F threshold
| 10% | 50% | 90% | |
|---|---|---|---|
| Last, on or after | April 20 | April 5 | March 22 |
| First, on or before | October 26 | November 7 | November 21 |
| Season at least | 234 days | 212 days | 194 days |
36°F threshold
| 10% | 50% | 90% | |
|---|---|---|---|
| Last, on or after | May 15 | April 30 | April 16 |
| First, on or before | October 1 | October 16 | October 27 |
| Season at least | 186 days | 167 days | 148 days |
Station CHICAGO PALWAUKEE AP, IL US (USW00004838). Completeness flag R, 24 years used for the 32°F dates. NCEI’s annual probability of at least one day at or below 32°F is 100%. Freeze dates are conditional on that threshold occurring. Elevation 193.9 m. The same probability language applies to the 28°F and 36°F rows when those dates passed the same quality checks.
Planting calendar for Mount Prospect · Zone 6a
Check a crop against this station
Mount Prospect, IL
Zone 6aAverage annual extreme minimum -10°F to -5°F
Outside the median season Growing seasonTicks: 10%, 50%, and 90%
Illinois. Nearest qualified station: CHICAGO PALWAUKEE AP, IL US (USW00004838), 4.3 miles. Median last 32°F April 19. Median first 32°F October 26. The 50% growing-season length is 190 days.
Open the ZIP page · Crop guides for Illinois
32°F probabilities at this station
| 10% | 50% | 90% | |
|---|---|---|---|
| Last 32°F on or after | May 3 | April 19 | April 4 |
| First 32°F on or before | October 13 | October 26 | November 9 |
Column order matches the NCEI file: for the last freeze, 10% is the later date and 90% is the earlier date. How to read the probabilities.
Planting calendar
Windows below are from University of Illinois Extension, When to Plant, retrieved 2026-10-10. The planting chart is divided into Northern Illinois, Central Illinois, and Southern Illinois. Median dates for the last 32-degree freeze on the same page: Northern Illinois April 8-29; Central Illinois April 8-15; Southern Illinois April 1-8. The page also says southern Illinois can generally start crops two weeks earlier than central Illinois and northern Illinois about two weeks later; the crop rows below are the chart dates, not a recalculation of that sentence.
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