Palisades Dam and Powerplant Historic District · Public domainWinter nights in Irwin are colder than in 93% of U.S. towns
Expect July afternoons of about 83°. Precipitation is light here, less than falls on 91% of U.S. towns. Since 1901–2000, the year has warmed by 1.2°, most of all in March. Keep the boiler serviced for the months from October to April, which average under 50°.
A year in Irwin at a glance
Each month’s typical high and low, and what it brings.
What stands out about Irwin’s climate
Compared with towns across the U.S., with Irwin’s place in Idaho beside it.
What a year in Irwin feels like
Typical highs and lows by season and by month, from NOAA’s 30-year normals.
A freeze is possible in every month
About 198 nights a year fall to 32° or below, and 16 to 0° or below. Frost and day counts come from NOAA’s station at Swan Valley, ID, 2.5 miles from the town’s center, at about the same height.
The 2023 USDA map puts Irwin’s center in zone 5a: the coldest night of a typical winter, averaged over 1991–2020, reaches about −15°. Plant labels use the zone; frost dates tell you when to plant.
Freezing nights by month
Average nights at or below 32°
90° days by month
Average days at 90° or more
About 18 inches of rain a year
May is the wettest month and July the driest.
Rain + melted snow by month
Inches of water
Heating and cooling a home in Irwin
98% of the work is heating, so the furnace matters far more than the air conditioner. Months averaging under 55° run from October through May.
Degree days add up how far each day’s average falls below (heating) or rises above (cooling) 65° over a year. NOAA normals at Swan Valley, ID.
Living with Irwin’s climate
What the numbers mean for a garden, a free afternoon and a bad-weather day.
Zone 5a, and frost can come in any month. The longest frost-free stretch in half of years is about 69 days.
Months with highs of 62–84° and lows of 40° or more: June, July and August. 1.9 miles of trails are waiting.
Parks & trails →Of the yearly building losses FEMA expects in Irwin from natural hazards, avalanches are the largest share (40%); heat waves, cold waves, winter storms and ice storms together are 8%. Normals are averages; the hazards page covers the extremes.
Natural hazards →Irwin and similar climates
TownRatings has no climate map of the towns around Irwin yet.
Temperatures and precipitation come from NOAA’s 5 km grid; snow and day counts from the stations named on this page.
Warmer than it used to be
The last 30 years in Irwin averaged 1.2° warmer than the 1900s. The biggest change is in March (+3.2°).
A smaller shift than in 66% of U.S. towns.
Change by month
The two periods overlap (1991–2000 is in both), which makes the gap a little smaller than it would be between separate periods.
How Irwin compares
Against 12,954 U.S. towns of 2,500 or more (one per 5 km climate cell), with Irwin’s place among Idaho’s.
Irwin weather by month
Highs, lows and precipitation from NOAA’s grid at Irwin’s center. 1991–2020 normals.
| Month | High | Low | Rain + melt | 30-yr range |
|---|---|---|---|---|
| January | 29° | 10° | 1.8" | 12° – 26° |
| February | 34° | 13° | 1.3" | 18° – 30° |
| March | 44° | 21° | 1.4" | 26° – 38° |
| April | 53° | 28° | 1.7" | 35° – 46° |
| May | 64° | 36° | 2.1" | 44° – 55° |
| June | 73° | 42° | 1.7" | 52° – 62° |
| July | 83° | 48° | 0.9" | 56° – 72° |
| August | 82° | 46° | 1.1" | 58° – 69° |
| September | 73° | 38° | 1.4" | 52° – 60° |
| October | 58° | 29° | 1.6" | 35° – 49° |
| November | 42° | 20° | 1.5" | 22° – 39° |
| December | 30° | 12° | 1.7" | 16° – 26° |
| Year | 55° | 29° | 18.1" |
How much a month can swing
A normal is an average. November swings the most: from 1991 to 2020 its average ranged from 22° to 39°. September is the most reliable, staying within 8°.
Where Irwin’s numbers come from
Normals are 30-year averages, not a forecast. How TownRatings reads climate data →
- Temperatures and precipitation: NOAA’s gridded 1991–2020 normals for the 5 km cell at Irwin’s center.
- Frost, freezing nights, 90° and 100° days, heating and cooling: Swan Valley, ID, 2.5 miles from the town’s center, at about the same height; the nearest station that passes our checks for distance, height and agreement with the grid.
- Growing zone: USDA Plant Hardiness Zone Map, 2023. USDA Agricultural Research Service and PRISM Group, Oregon State University, prism.oregonstate.edu/phzm, accessed 8 Oct 2026.
- Then and now: NOAA’s gridded normals for 1901–2000 at the same cell.
- Storms and extremes: FEMA National Risk Index (expected annual losses to buildings).
Common questions about Irwin’s climate
What is the weather like in Irwin?
A typical July afternoon reaches 83°, a January night drops to 10°. Colder January nights than 93% of U.S. towns; cooler July afternoons than 72% of U.S. towns.
How cold does it get in Irwin in winter?
January nights average 10°. About 198 nights a year reach 32° or below and 16 reach 0° or below. In the coldest January of the last 30 years the month averaged 12°.
How hot does it get in Irwin?
July, with a typical high of 83° and a low of 48°. About 14 days a year reach 90° (Swan Valley, ID).
When is the first and last frost in Irwin?
There is no reliable frost-free season: at Swan Valley, ID, every month has nights at 32° or below, about 198 a year. In half of years the longest stretch without a freeze is about 69 days.
What growing zone is Irwin in?
Zone 5a on the 2023 USDA Plant Hardiness Zone Map: the coldest night of a typical winter reaches about −15°.
What is the best time of year to visit Irwin?
By one simple rule (highs of 62–84° and lows of 40° or more), June, July and August. The rule is ours, not NOAA’s; the month table has the numbers to judge by.
Is Irwin getting warmer?
Yes: 1991–2020 averaged 1.2° warmer than 1901–2000 on NOAA’s grid. A smaller shift than 66% of U.S. towns.
Neighbors talk weather
Gardens, snow, and the best days of the year in Irwin.
Source: NOAA NCEI U.S. Climate Normals, 1991–2020 and 1901–2000; USDA/PRISM 2023 hardiness zones; FEMA National Risk Index. Today’s forecast: National Weather Service.