Colstrip climate

Colstrip Frost Dates and Growing Season

NOAA 1991-2020 normals for Colstrip, 1.4 km from Colstrip, elevation 3218 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Colstrip. 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 15 and the first fall freeze around September 30, a growing season of about 138 days.

Median at 36°F
last May 26, first September 20
Median at 32°F
last May 15, first September 30
Median at 28°F
last May 3, first October 10

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°FJun 8Jun 4Jun 1May 29May 26May 23May 20May 17May 13
32°FMay 28May 24May 20May 17May 15May 12May 9May 6May 2
28°FMay 16May 12May 8May 5May 3May 1Apr 28Apr 24Apr 18
24°FMay 5May 1Apr 28Apr 24Apr 21Apr 17Apr 14Apr 11Apr 6
20°FApr 27Apr 21Apr 17Apr 13Apr 10Apr 6Apr 3Mar 30Mar 21
16°FApr 16Apr 10Apr 5Apr 2Mar 30Mar 24Mar 19Mar 14Mar 6

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°FSep 4Sep 10Sep 14Sep 17Sep 20Sep 23Sep 25Sep 29Oct 3
32°FSep 15Sep 20Sep 24Sep 27Sep 30Oct 3Oct 5Oct 9Oct 14
28°FSep 23Sep 30Oct 3Oct 6Oct 10Oct 13Oct 16Oct 20Oct 24
24°FOct 1Oct 8Oct 12Oct 16Oct 19Oct 23Oct 26Oct 30Nov 3
20°FOct 9Oct 16Oct 20Oct 23Oct 28Oct 31Nov 3Nov 8Nov 14
16°FOct 17Oct 23Oct 28Nov 1Nov 4Nov 9Nov 13Nov 18Nov 23

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°F13512912512111711310910295
32°F157150146141138134130125118
28°F179172167163159155151146139
24°F200194188184179175171166158
20°F224215208202198193188181174
16°F246236229224217211206199189

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
7,369
Cooling degree days, base 65°F
510
Growing degree days, base 40°F
4,186
Growing degree days, base 45°F
3,147
Growing degree days, base 50°F
2,267
Growing degree days, base 55°F
1,539
Growing degree days, base 60°F
952

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,25100
February1,08901
March915010
April641039
May3688173
June12359390
July24213654
August40183609
September20744312
October595371
November90809
December1,21000

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 1.4 km from Colstrip. 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 USC00241905 (Colstrip). Public domain.