Lewistown climate

Lewistown Frost Dates and Growing Season

NOAA 1991-2020 normals for Lewistown Muni AP, 2.7 km from Lewistown, elevation 4145 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Lewistown Muni 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 May 23 and the first fall freeze around September 21, a growing season of about 119 days.

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

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 24Aug 30Sep 2Sep 5Sep 8Sep 10Sep 13Sep 17Sep 21
32°FSep 8Sep 12Sep 17Sep 19Sep 21Sep 23Sep 26Sep 29Oct 3
28°FSep 18Sep 23Sep 27Sep 29Oct 1Oct 4Oct 7Oct 11Oct 15
24°FSep 26Sep 30Oct 3Oct 6Oct 10Oct 13Oct 16Oct 20Oct 25
20°FOct 2Oct 7Oct 12Oct 15Oct 18Oct 22Oct 26Oct 30Nov 3
16°FOct 10Oct 15Oct 19Oct 24Oct 28Nov 1Nov 4Nov 7Nov 12

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°F10810196918682767060
32°F13813112612311911511010698
28°F162156152148145141137132125
24°F187180174170165162157152146
20°F209201196191186181176170162
16°F227219213208203198193187178

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
8,177
Cooling degree days, base 65°F
229
Growing degree days, base 40°F
3,388
Growing degree days, base 45°F
2,424
Growing degree days, base 50°F
1,632
Growing degree days, base 55°F
1,005
Growing degree days, base 60°F
537

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,28200
February1,13101
March99704
April723019
May4661100
June22617253
July65101501
August8191479
September28918219
October671150
November98607
December1,26200

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 2.7 km from Lewistown. 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 USW00024036 (Lewistown Muni AP). Public domain.