Multnomah County climate

Multnomah County Frost Dates and Growing Season

NOAA 1991-2020 normals for Portland Troutdale AP, 0.7 km from Multnomah County, elevation 29 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Portland Troutdale 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 16 and the first fall freeze around November 20, a growing season of about 250 days.

Median at 36°F
last April 19, first October 30
Median at 32°F
last March 16, first November 20
Median at 28°F
last February 14, first December 15

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 4May 1Apr 28Apr 24Apr 19Apr 14Apr 9Apr 2Mar 27
32°FApr 12Apr 2Mar 27Mar 21Mar 16Mar 10Mar 3Feb 26Feb 18
28°FMar 10Mar 1Feb 25Feb 19Feb 14Feb 6Jan 31Jan 15Dec 27
24°FFeb 20Feb 12Feb 4Jan 31Jan 22Jan 9Dec 31Dec 20Dec 6
20°FFeb 18Feb 5Jan 31Jan 20Jan 10Dec 30Dec 19Dec 12Dec 5
16°F–––––––––

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 11Oct 18Oct 22Oct 27Oct 30Nov 3Nov 8Nov 14Nov 22
32°FOct 30Nov 4Nov 9Nov 15Nov 20Nov 26Dec 1Dec 7Dec 17
28°FNov 15Nov 25Dec 2Dec 7Dec 15Dec 25Jan 1Jan 16Feb 3
24°FNov 30Dec 6Dec 14Dec 22Jan 1Jan 11Jan 25Feb 1Feb 13
20°FDec 2Dec 10Dec 16Dec 22Jan 1Jan 15Jan 27Feb 2Feb 17
16°F–––––––––

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°F226216209204198192187180171
32°F288276265257250242235227216
28°F358340328319311304294282268
24°F365365365365365354341331317
20°F365365365365365365365365349
16°F365365365365365365365365365

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
93% of years
24°F or colder
51% of years
20°F or colder
18% of years
16°F or colder
3% 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
4,155
Cooling degree days, base 65°F
427
Growing degree days, base 40°F
5,521
Growing degree days, base 45°F
3,964
Growing degree days, base 50°F
2,697
Growing degree days, base 55°F
1,706
Growing degree days, base 60°F
951

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January71305
February57809
March516041
April3793105
May20819279
June9146405
July14144595
August11154608
September7557432
October3064180
November537033
December72506

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 0.7 km from Multnomah 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 USW00024242 (Portland Troutdale AP). Public domain.