Walla Walla County climate

Walla Walla County Frost Dates and Growing Season

NOAA 1991-2020 normals for Walla Walla Rgnl AP, 21 km from Walla Walla County, elevation 1166 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Walla Walla Rgnl AP. 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 March 27 and the first fall freeze around October 31, a growing season of about 219 days.

Median at 36°F
last April 21, first October 19
Median at 32°F
last March 27, first October 31
Median at 28°F
last March 4, first November 14

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°FMay 8May 3Apr 29Apr 25Apr 21Apr 17Apr 13Apr 9Apr 2
32°FApr 22Apr 13Apr 5Mar 31Mar 27Mar 22Mar 16Mar 10Mar 3
28°FMar 27Mar 19Mar 12Mar 7Mar 4Feb 28Feb 25Feb 19Feb 9
24°FMar 10Mar 3Feb 28Feb 23Feb 18Feb 12Feb 5Jan 25Jan 10
20°FMar 3Feb 26Feb 18Feb 10Feb 1Jan 24Jan 14Jan 5Dec 25
16°FFeb 28Feb 19Feb 9Jan 31Jan 23Jan 12Jan 6Dec 30Dec 13

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 4Oct 9Oct 13Oct 16Oct 19Oct 22Oct 25Oct 29Nov 2
32°FOct 15Oct 20Oct 24Oct 28Oct 31Nov 3Nov 6Nov 11Nov 17
28°FOct 25Nov 1Nov 5Nov 9Nov 14Nov 18Nov 23Nov 27Dec 4
24°FNov 3Nov 11Nov 18Nov 24Nov 28Dec 2Dec 6Dec 12Dec 21
20°FNov 12Nov 22Nov 28Dec 2Dec 8Dec 12Dec 20Jan 1Jan 12
16°FNov 24Dec 1Dec 8Dec 14Dec 21Dec 29Jan 2Jan 12Feb 1

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°F205197190185180176170163155
32°F246238230224219213208200189
28°F287276268261253247241235225
24°F322309301295287280272262250
20°F350338330320313306298289275
16°F365361349341334327317308296

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
99% of years
20°F or colder
96% of years
16°F or colder
81% 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
4,798
Cooling degree days, base 65°F
936
Growing degree days, base 40°F
5,824
Growing degree days, base 45°F
4,442
Growing degree days, base 50°F
3,267
Growing degree days, base 55°F
2,310
Growing degree days, base 60°F
1,538

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January89005
February70807
March564039
April3762112
May18237324
June54116512
July3352813
August5320779
September68104487
October3586159
November678024
December91107

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 21 km from Walla Walla 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 USW00024160 (Walla Walla Rgnl AP). Public domain.