King County climate

King County Frost Dates and Growing Season

NOAA 1991-2020 normals for Guthrie, 7.6 km from King County, elevation 1760 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Guthrie. 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 April 4 and the first fall freeze around November 3, a growing season of about 213 days.

Median at 36°F
last April 16, first October 26
Median at 32°F
last April 4, first November 3
Median at 28°F
last March 19, first November 12

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°FApr 30Apr 26Apr 22Apr 19Apr 16Apr 13Apr 9Apr 4Mar 30
32°FApr 22Apr 18Apr 13Apr 8Apr 4Apr 1Mar 28Mar 22Mar 15
28°FApr 9Apr 2Mar 29Mar 24Mar 19Mar 14Mar 11Mar 6Mar 1
24°FMar 24Mar 17Mar 12Mar 8Mar 4Mar 2Feb 26Feb 21Feb 16
20°FMar 13Mar 6Mar 2Feb 27Feb 22Feb 17Feb 12Feb 6Jan 29
16°FMar 2Feb 25Feb 17Feb 10Feb 5Feb 1Jan 27Jan 16Jan 2

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°FOct 13Oct 18Oct 21Oct 24Oct 26Oct 29Nov 1Nov 3Nov 8
32°FOct 23Oct 27Oct 30Nov 1Nov 3Nov 6Nov 9Nov 13Nov 19
28°FOct 29Nov 2Nov 6Nov 10Nov 12Nov 15Nov 19Nov 24Dec 1
24°FNov 5Nov 11Nov 15Nov 19Nov 22Nov 26Dec 1Dec 4Dec 10
20°FNov 15Nov 22Nov 27Dec 1Dec 4Dec 8Dec 14Dec 19Jan 1
16°FNov 27Dec 4Dec 10Dec 16Dec 21Dec 28Jan 4Jan 14Jan 30

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°F213205200196192189185180174
32°F238230223218213209204198192
28°F263254248243237232227220211
24°F287279273268263258252246237
20°F318306299293287282276268260
16°F359344334327319312305297285

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
94% 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
3,026
Cooling degree days, base 65°F
2,391
Growing degree days, base 40°F
8,752
Growing degree days, base 45°F
7,185
Growing degree days, base 50°F
5,759
Growing degree days, base 55°F
4,482
Growing degree days, base 60°F
3,359

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January713032
February549264
March36119200
April15072386
May33251684
June1467916
July05941,059
August05661,031
September13318755
October12993441
November40010155
December678037

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 7.6 km from King County. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

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Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USC00413828 (Guthrie). Public domain.