Lewiston climate

Lewiston Frost Dates and Growing Season

NOAA 1991-2020 normals for Trenton, 8.2 km from Lewiston, elevation 4455 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Trenton. 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 17 and the first fall freeze around September 18, a growing season of about 121 days.

Median at 36°F
last June 5, first September 6
Median at 32°F
last May 17, first September 18
Median at 28°F
last May 2, 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°FJun 21Jun 16Jun 12Jun 9Jun 5Jun 2May 31May 25May 21
32°FJun 6May 31May 25May 21May 17May 13May 10May 7May 3
28°FMay 17May 11May 7May 4May 2Apr 30Apr 27Apr 24Apr 19
24°FMay 3Apr 28Apr 23Apr 20Apr 17Apr 13Apr 8Apr 4Mar 28
20°FApr 18Apr 10Apr 6Apr 2Mar 30Mar 25Mar 18Mar 14Mar 9
16°FMar 31Mar 25Mar 18Mar 14Mar 11Mar 8Mar 6Mar 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 23Aug 29Sep 1Sep 4Sep 6Sep 9Sep 12Sep 16Sep 21
32°FSep 1Sep 8Sep 11Sep 15Sep 18Sep 21Sep 25Sep 29Oct 4
28°FSep 15Sep 20Sep 25Sep 28Oct 1Oct 4Oct 8Oct 11Oct 16
24°FSep 28Oct 3Oct 8Oct 12Oct 16Oct 19Oct 22Oct 25Oct 31
20°FOct 13Oct 19Oct 22Oct 26Oct 30Nov 1Nov 4Nov 8Nov 13
16°FOct 23Oct 29Nov 1Nov 6Nov 10Nov 13Nov 17Nov 22Nov 27

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°F11210499959187827667
32°F14213613112712111611210698
28°F171164159154149145141135127
24°F205196190185180175169164155
20°F237229223218213208202196186
16°F264255249244240234229224215

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
7,312
Cooling degree days, base 65°F
368
Growing degree days, base 40°F
3,990
Growing degree days, base 45°F
2,952
Growing degree days, base 50°F
2,082
Growing degree days, base 55°F
1,364
Growing degree days, base 60°F
786

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,33600
February1,06800
March84503
April588032
May3464165
June12544374
July19177623
August32128561
September21015273
October571051
November90501
December1,26800

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 8.2 km from Lewiston. 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 USC00428828 (Trenton). Public domain.