Flagstaff climate

Flagstaff Frost Dates and Growing Season

NOAA 1991-2020 normals for Flagstaff Pulliam AP, 6.1 km from Flagstaff, elevation 7003 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Flagstaff Pulliam 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 June 4 and the first fall freeze around September 28, a growing season of about 115 days.

Median at 36°F
last June 16, first September 19
Median at 32°F
last June 4, first September 28
Median at 28°F
last May 20, first October 9

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 1Jun 26Jun 22Jun 19Jun 16Jun 13Jun 10Jun 7Jun 3
32°FJun 19Jun 14Jun 10Jun 6Jun 4Jun 1May 30May 26May 21
28°FJun 6Jun 1May 29May 24May 20May 17May 13May 7May 1
24°FMay 19May 13May 7May 4May 1Apr 28Apr 25Apr 21Apr 15
20°FMay 1Apr 25Apr 21Apr 17Apr 13Apr 9Apr 6Apr 2Mar 26
16°FApr 15Apr 8Apr 3Mar 30Mar 25Mar 19Mar 14Mar 10Mar 5

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 6Sep 11Sep 14Sep 16Sep 19Sep 21Sep 23Sep 25Sep 28
32°FSep 16Sep 20Sep 23Sep 26Sep 28Oct 1Oct 2Oct 5Oct 9
28°FSep 28Oct 1Oct 4Oct 6Oct 9Oct 11Oct 14Oct 17Oct 22
24°FOct 7Oct 11Oct 15Oct 17Oct 20Oct 23Oct 27Oct 30Nov 3
20°FOct 16Oct 21Oct 26Oct 29Nov 1Nov 4Nov 7Nov 12Nov 16
16°FOct 27Nov 2Nov 6Nov 9Nov 12Nov 16Nov 18Nov 21Nov 26

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°F109104100969289857973
32°F13312712211811511110810396
28°F162155149145139135130125117
24°F192184179175170166161156148
20°F224215210205201197192186177
16°F255246240234229224219212203

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
6,746
Cooling degree days, base 65°F
145
Growing degree days, base 40°F
3,638
Growing degree days, base 45°F
2,581
Growing degree days, base 50°F
1,723
Growing degree days, base 55°F
1,032
Growing degree days, base 60°F
495

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,07000
February90600
March83700
April639012
May4241104
June14822328
July2376518
August4443464
September2054255
October541042
November82401
December1,08700

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 6.1 km from Flagstaff. 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 USW00003103 (Flagstaff Pulliam AP). Public domain.