Fergus County climate

Fergus County Frost Dates and Growing Season

NOAA 1991-2020 normals for Lewistown 11ESE, 28.6 km from Fergus County, elevation 5028 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for Lewistown 11ESE. 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 19 and the first fall freeze around September 25, a growing season of about 128 days.

Median at 36°F
last June 1, first September 16
Median at 32°F
last May 19, first September 25
Median at 28°F
last May 7, first October 5

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 16Jun 10Jun 7Jun 4Jun 1May 31May 28May 24May 20
32°FJun 1May 29May 25May 22May 19May 16May 14May 11May 7
28°FMay 22May 16May 12May 10May 7May 4May 2Apr 29Apr 23
24°FMay 10May 5May 1Apr 29Apr 26Apr 22Apr 17Apr 12Apr 5
20°FApr 30Apr 25Apr 20Apr 15Apr 11Apr 8Apr 4Mar 31Mar 24
16°FApr 19Apr 13Apr 8Apr 4Apr 1Mar 29Mar 25Mar 20Mar 13

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 2Sep 7Sep 10Sep 13Sep 16Sep 18Sep 21Sep 23Sep 27
32°FSep 11Sep 17Sep 20Sep 23Sep 25Sep 28Sep 30Oct 4Oct 9
28°FSep 22Sep 26Sep 29Oct 1Oct 5Oct 8Oct 12Oct 16Oct 21
24°FSep 29Oct 4Oct 9Oct 13Oct 16Oct 19Oct 22Oct 26Oct 31
20°FOct 8Oct 15Oct 19Oct 23Oct 26Oct 29Nov 1Nov 5Nov 11
16°FOct 18Oct 25Oct 28Nov 1Nov 3Nov 6Nov 9Nov 14Nov 19

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°F124117112109105101979385
32°F146141136132128124119115108
28°F172164158154150146142136129
24°F197188182177173168162156149
20°F219212207202196191185179171
16°F237229225219215209204198190

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
8,412
Cooling degree days, base 65°F
246
Growing degree days, base 40°F
3,269
Growing degree days, base 45°F
2,347
Growing degree days, base 50°F
1,598
Growing degree days, base 55°F
998
Growing degree days, base 60°F
549

Growing degree days by month, base 50°F

Heating and cooling by month

MonthHeating DDCooling DDGrowing DD 50
January1,27700
February1,14602
March1,04503
April776017
May519190
June26610218
July7893481
August91106485
September30931234
October673561
November1,00508
December1,22700

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 28.6 km from Fergus County. 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 USC00244979 (Lewistown 11ESE). Public domain.