Mount Vernon climate

Mount Vernon Frost Dates and Growing Season

NOAA 1991-2020 normals for John Day, 12.3 km from Mount Vernon, elevation 3063 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for John Day. 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 28 and the first fall freeze around September 18, a growing season of about 113 days.

Median at 36°F
last June 19, first September 2
Median at 32°F
last May 28, first September 18
Median at 28°F
last May 8, first October 1

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 8Jun 29Jun 26Jun 22Jun 19Jun 15Jun 11Jun 6May 31
32°FJun 15Jun 8Jun 4May 31May 28May 24May 20May 16May 12
28°FMay 24May 18May 13May 11May 8May 4May 2Apr 30Apr 25
24°FMay 7May 2Apr 30Apr 27Apr 23Apr 19Apr 15Apr 11Apr 3
20°FApr 22Apr 13Apr 8Apr 4Mar 31Mar 27Mar 21Mar 15Mar 8
16°FMar 30Mar 20Mar 14Mar 9Mar 4Feb 28Feb 24Feb 18Feb 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 16Aug 23Aug 28Aug 31Sep 2Sep 5Sep 9Sep 13Sep 20
32°FSep 2Sep 7Sep 11Sep 15Sep 18Sep 21Sep 24Sep 29Oct 4
28°FSep 13Sep 19Sep 23Sep 28Oct 1Oct 3Oct 6Oct 10Oct 16
24°FSep 27Oct 3Oct 6Oct 10Oct 13Oct 17Oct 20Oct 26Nov 2
20°FOct 11Oct 17Oct 21Oct 26Oct 29Nov 1Nov 8Nov 14Nov 21
16°FOct 22Oct 29Nov 2Nov 9Nov 14Nov 19Nov 23Nov 28Dec 4

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°F1019386807670655847
32°F1361271231181131091049889
28°F165160155150146141135129121
24°F202193186181175171166160151
20°F244233225217212206201195183
16°F286276268261254249241232222

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
6,866
Cooling degree days, base 65°F
249
Growing degree days, base 40°F
3,642
Growing degree days, base 45°F
2,598
Growing degree days, base 50°F
1,760
Growing degree days, base 55°F
1,092
Growing degree days, base 60°F
587

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,06200
February87600
March80804
April636024
May4143125
June21523271
July53115527
August6191495
September23118256
October569054
November85404
December1,08800

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 12.3 km from Mount Vernon. 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 USC00354291 (John Day). Public domain.