Clark Fork climate

Clark Fork Frost Dates and Growing Season

NOAA 1991-2020 normals for Heron 2 Nw, 14.8 km from Clark Fork, elevation 2240 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Heron 2 Nw. 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 20 and the first fall freeze around September 20, a growing season of about 119 days.

Median at 32°F
last May 20, first September 20
Median at 28°F
last May 2, first October 7

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°F–––––––––
32°FJun 5May 31May 26May 22May 20May 16May 12May 8May 4
28°FMay 16May 11May 8May 5May 2Apr 30Apr 28Apr 23Apr 18
24°FMay 2Apr 27Apr 24Apr 19Apr 14Apr 9Apr 4Mar 30Mar 22
20°FApr 18Apr 8Apr 2Mar 28Mar 23Mar 18Mar 14Mar 8Mar 3
16°FMar 27Mar 18Mar 13Mar 10Mar 7Mar 4Mar 1Feb 25Feb 17

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°F–––––––––
32°FSep 1Sep 6Sep 10Sep 15Sep 20Sep 23Sep 26Oct 1Oct 5
28°FSep 22Sep 27Oct 1Oct 3Oct 7Oct 11Oct 16Oct 21Oct 27
24°FOct 3Oct 10Oct 17Oct 22Oct 27Oct 31Nov 3Nov 9Nov 18
20°FOct 18Oct 26Nov 1Nov 4Nov 9Nov 14Nov 20Nov 26Dec 4
16°FOct 30Nov 7Nov 13Nov 20Nov 25Nov 30Dec 4Dec 9Dec 18

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°F1019385787265585142
32°F14513613012511911410910393
28°F183174168163157153147142133
24°F225213206199193187182174163
20°F266254245238232224217209198
16°F295284275269262257250241228

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
7,364
Cooling degree days, base 65°F
130
Growing degree days, base 40°F
3,279
Growing degree days, base 45°F
2,296
Growing degree days, base 50°F
1,491
Growing degree days, base 55°F
849
Growing degree days, base 60°F
391

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,14500
February96700
March87700
April629014
May3782130
June20212263
July7661450
August8953430
September2894185
October646019
November90801
December1,15800

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 14.8 km from Clark Fork. 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 USC00244084 (Heron 2 Nw). Public domain.