Flathead County climate

Flathead County Frost Dates and Growing Season

NOAA 1991-2020 normals for Hungry Horse Dam, 5.7 km from Flathead County, elevation 3160 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Hungry Horse Dam. 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 9 and the first fall freeze around October 1, a growing season of about 141 days.

Median at 36°F
last May 28, first September 19
Median at 32°F
last May 9, first October 1
Median at 28°F
last April 27, first October 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°FJun 19Jun 11Jun 5May 31May 28May 23May 20May 14May 10
32°FMay 30May 23May 17May 13May 9May 6May 3Apr 30Apr 25
28°FMay 14May 7May 4Apr 30Apr 27Apr 24Apr 20Apr 15Apr 8
24°FMay 5Apr 29Apr 23Apr 18Apr 14Apr 10Apr 5Apr 1Mar 23
20°FApr 23Apr 14Apr 9Apr 4Mar 31Mar 26Mar 21Mar 14Mar 7
16°FApr 8Apr 1Mar 27Mar 22Mar 17Mar 12Mar 7Mar 3Feb 27

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 31Sep 6Sep 11Sep 15Sep 19Sep 21Sep 25Sep 28Oct 2
32°FSep 13Sep 21Sep 25Sep 29Oct 1Oct 4Oct 6Oct 10Oct 17
28°FSep 30Oct 4Oct 7Oct 11Oct 14Oct 18Oct 21Oct 26Oct 31
24°FOct 7Oct 14Oct 18Oct 23Oct 28Nov 1Nov 4Nov 9Nov 15
20°FOct 18Oct 24Oct 30Nov 2Nov 7Nov 12Nov 17Nov 22Nov 27
16°FOct 27Nov 2Nov 8Nov 14Nov 18Nov 23Nov 26Dec 1Dec 8

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°F135127121116110105999080
32°F165156151147141136130123111
28°F195187181175170164158152141
24°F224214208201196190184177167
20°F252242234227221214208203191
16°F274264257251244237232223212

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
7,786
Cooling degree days, base 65°F
293
Growing degree days, base 40°F
3,452
Growing degree days, base 45°F
2,509
Growing degree days, base 50°F
1,735
Growing degree days, base 55°F
1,104
Growing degree days, base 60°F
626

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,23400
February1,06500
March95300
April680015
May4044130
June20727280
July54134546
August65114516
September27214219
October667028
November97201
December1,21500

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 5.7 km from Flathead County. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

More

Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USC00244328 (Hungry Horse Dam). Public domain.