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Will it freeze in Ottawa in the next 7 nights?
The earliest overnight low at or below 32°F in the available NWS frost forecast is Fri, Nov 6: 30°F (Fri, Nov 6, 6:00 PM CST to Sat, Nov 7, 6:00 AM CST).
Tonight: forecast low 39°F. Above 36°F.
NWS forecast period: Thu, Nov 5, 6:00 PM CST to Fri, Nov 6, 6:00 AM CST.
3 of the next 7 nights are forecast at 36°F or colder, cold enough for frost on exposed plants.
- Tonight 39°F Above 36°F · Areas Of Drizzle then Mostly Cloudy
- Fri, Nov 6 30°F Freeze · Mostly Clear
- Sat, Nov 7 34°F Frost possible · Mostly Clear
- Sun, Nov 8 35°F Frost possible · Mostly Clear
- Mon, Nov 9 41°F Above 36°F · Mostly Clear
- Tue, Nov 10 39°F Above 36°F · Partly Cloudy
- Wed, Nov 11 39°F Above 36°F · Mostly Clear
Overnight lows from the National Weather Service forecast for this location. This covers the available forecast window, not the first freeze of the whole season. Frost can form above 32°F on clear, calm nights.
Frost and freeze probabilities
Freeze dates shift from year to year. Spring percentages describe the chance of a freeze after the stated date; fall percentages describe the chance of a freeze by that date. The 10% spring date is the cautious one: a later freeze happens about 1 year in 10.
These dates describe historical probabilities, not this year’s forecast or the earliest and latest possible freezes. Different weather stations or probability thresholds can produce different published dates for the same city.
Average first hard freeze in Ottawa
The median first fall hard-freeze date (28°F) is October 27, from NOAA’s 1991–2020 Climate Normals at Marseilles Lock. This is the 50% probability date, not a forecast for this year.
| Temperature | Last spring date 90% · 50% · 10% chance of a later freeze | First fall date 10% · 50% · 90% chance of an earlier freeze | Season |
|---|---|---|---|
| 36°F frost possible | Apr 22 · May 7 · May 22 | Sep 24 · Oct 7 · Oct 20 | 152 days |
| 32°F freeze | Apr 12 · Apr 27 · May 13 | Oct 3 · Oct 17 · Oct 29 | 171 days |
| 28°F hard freeze | Apr 2 · Apr 15 · May 1 | Oct 14 · Oct 27 · Nov 9 | 194 days |
Average high temperature by month
1991–2020 normals at Marseilles Lock
Average low temperature by month
Months with lows near or below freezing carry frost risk
Where these frost dates come from
The frost dates for Ottawa, IL come from the nearest weather station with freeze statistics in NOAA’s 1991–2020 Climate Normals: Marseilles Lock (station USC00115372, elevation 490 ft), 4.0 mi from Ottawa. Local conditions (valleys, hilltops, distance to water, city heat) can shift frost dates by days or weeks. The hardiness zone is from the 2023 USDA Plant Hardiness Zone Map (USDA ARS and Oregon State University PRISM). See our methodology.
Frost dates near Ottawa
| City | Distance | Last spring frost | First fall frost |
|---|---|---|---|
| LaSalle, IL | 14 mi | Apr 24 | Oct 18 |
| Streator, IL | 16 mi | Apr 12 | Oct 28 |
| Peru, IL | 16 mi | Apr 24 | Oct 18 |
| Spring Valley, IL | 19 mi | Apr 24 | Oct 18 |
| Mendota, IL | 20 mi | Apr 27 | Oct 15 |
| Morris, IL | 21 mi | Apr 20 | Oct 23 |
| Sandwich, IL | 23 mi | Apr 28 | Oct 13 |
| Plano, IL | 27 mi | Apr 28 | Oct 13 |
| Coal City, IL | 29 mi | Apr 16 | Oct 26 |
| Minooka, IL | 29 mi | Apr 16 | Oct 26 |
Frequently asked questions
When is the last frost in Ottawa?
On average, the last spring frost (32°F) in Ottawa is around April 27. In about 1 year in 10 it comes after May 13, and in 9 years in 10 it comes after April 12.
When is the first frost in Ottawa?
The first fall frost (32°F) usually arrives around October 17. There’s a 10% chance of a freeze by October 3 and a 90% chance by October 29.
Can it frost in Ottawa after the last frost date?
Yes. Frost can form on clear, calm nights even when the air stays above 32°F. Temperatures of 36°F or lower, cold enough for frost on exposed plants, have a 50% chance of occurring after May 7.
What growing zone is Ottawa?
Ottawa is in USDA Plant Hardiness Zone 5b (average annual extreme minimum temperature −15 to −10°F), based on the 2023 USDA map for ZIP code 61350. See growing zones in Illinois.
More: all Illinois frost dates · Illinois growing zones · frost vs. freeze