Chelan County climate

Chelan County Frost Dates and Growing Season

NOAA 1991-2020 normals for Plain, 9.5 km from Chelan County, elevation 1936 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Plain. A median date has an even chance of a later freeze in spring, or an earlier one in fall.

The last spring freeze typically comes around May 22 and the first fall freeze around September 22, a growing season of about 121 days.

Median at 36°F
last June 13, first September 8
Median at 32°F
last May 22, first September 22
Median at 28°F
last May 3, first October 4

Last freeze in spring

The chance of a later freeze than the date shown. At 10 percent, one spring in ten still freezes after the date; planting after the 10 percent date for 32°F is the cautious choice.

Threshold10%20%30%40%50%60%70%80%90%
36°FJul 1Jun 26Jun 21Jun 17Jun 13Jun 8Jun 4May 30May 24
32°FJun 13Jun 5May 31May 26May 22May 18May 14May 9May 2
28°FMay 20May 14May 9May 6May 3Apr 30Apr 27Apr 24Apr 19
24°FMay 3Apr 26Apr 22Apr 17Apr 13Apr 6Apr 2Mar 29Mar 21
20°FApr 7Mar 31Mar 24Mar 20Mar 16Mar 12Mar 9Mar 6Mar 1
16°FMar 16Mar 11Mar 7Mar 4Mar 1Feb 27Feb 22Feb 17Feb 11

First freeze in fall

The chance of an earlier freeze than the date shown. At 10 percent, one fall in ten freezes before the date.

Threshold10%20%30%40%50%60%70%80%90%
36°FAug 24Aug 30Sep 2Sep 5Sep 8Sep 10Sep 13Sep 17Sep 21
32°FSep 7Sep 12Sep 16Sep 19Sep 22Sep 25Sep 27Sep 30Oct 4
28°FSep 22Sep 27Oct 1Oct 2Oct 4Oct 8Oct 11Oct 15Oct 20
24°FOct 4Oct 10Oct 15Oct 19Oct 23Oct 26Oct 30Nov 2Nov 9
20°FOct 19Oct 27Oct 31Nov 4Nov 8Nov 13Nov 18Nov 22Nov 29
16°FOct 31Nov 7Nov 13Nov 19Nov 24Nov 28Dec 3Dec 7Dec 16

Growing season length

Days between the last spring and first fall freeze at each threshold, with the chance of a season at least that long.

Threshold10%20%30%40%50%60%70%80%90%
36°F10810095908581777162
32°F14413613012512111611110497
28°F177170163159153149144138128
24°F220211204199192188182175166
20°F262254248242235230224217207
16°F299288281274269263257249239

How often it gets that cold

The share of years with at least one night at or below each temperature.

36°F or colder
100% of years
32°F or colder
100% of years
28°F or colder
100% of years
24°F or colder
100% of years
20°F or colder
100% of years
16°F or colder
100% of years

Degree days

A degree day is one degree of the daily mean temperature beyond a base, for one day. Heating degree days count cold (the fuel a house burns), cooling degree days heat, growing degree days the warmth a crop needs; each crop has its own base.

Heating degree days, base 65°F
7,206
Cooling degree days, base 65°F
219
Growing degree days, base 40°F
3,513
Growing degree days, base 45°F
2,517
Growing degree days, base 50°F
1,691
Growing degree days, base 55°F
1,027
Growing degree days, base 60°F
534

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,16300
February93900
March85401
April615019
May3763136
June20422273
July6295498
August6584484
September23115245
October598036
November91700
December1,18100

What this means

NOAA computes these from the daily lows at the station over 1991-2020, fitting the dates of each year's last and first freeze to a distribution, so a date can be stated at any level of risk. 36°F is a frost risk for tender plants on a clear, calm night, since the ground cools below the air above it; 32°F is a freeze; 28°F is a hard freeze that kills most annuals.

The station is 9.5 km from Chelan County. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

More

Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USC00456534 (Plain). Public domain.