Lewis and Clark County climate

Lewis and Clark County Frost Dates and Growing Season

NOAA 1991-2020 normals for Rogers Pass 9 Nne, 10.7 km from Lewis and Clark County, elevation 4200 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Rogers Pass 9 Nne. 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 21 and the first fall freeze around September 15, a growing season of about 115 days.

Median at 32°F
last May 21, first September 15
Median at 28°F
last May 7, first September 28

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 8Jun 1May 29May 25May 21May 17May 14May 11May 7
28°FMay 23May 16May 12May 10May 7May 5May 2Apr 30Apr 26
24°FMay 11May 7May 4May 1Apr 29Apr 26Apr 23Apr 18Apr 12
20°FMay 4Apr 30Apr 26Apr 23Apr 19Apr 15Apr 11Apr 7Apr 2
16°FApr 27Apr 21Apr 16Apr 12Apr 9Apr 6Apr 2Mar 28Mar 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°FAug 30Sep 3Sep 8Sep 11Sep 15Sep 18Sep 21Sep 25Sep 29
28°FSep 13Sep 19Sep 22Sep 25Sep 28Oct 1Oct 3Oct 7Oct 13
24°FSep 23Sep 28Oct 1Oct 4Oct 7Oct 11Oct 15Oct 20Oct 26
20°FOct 1Oct 5Oct 11Oct 15Oct 19Oct 22Oct 26Nov 1Nov 5
16°FOct 4Oct 14Oct 18Oct 23Oct 28Nov 1Nov 3Nov 8Nov 15

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°F1049891868176696152
32°F13612812411911511110610090
28°F163155151147143139135129122
24°F189180174169164160155149142
20°F210201193188183178173168158
16°F229219212206200195190184173

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
7,894
Cooling degree days, base 65°F
218
Growing degree days, base 40°F
3,417
Growing degree days, base 45°F
2,425
Growing degree days, base 50°F
1,619
Growing degree days, base 55°F
988
Growing degree days, base 60°F
522

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,19400
February1,07800
March95605
April704021
May4573111
June22516254
July69100497
August9084463
September30314210
October660151
November95007
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 10.7 km from Lewis and Clark 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 USC00247159 (Rogers Pass 9 Nne). Public domain.