Frost dates / Wyoming / Casper

Frost dates in Casper, WY

The station on this page is CASPER WWTP, WY US (USC00481569), 2 miles from the city point. Population in the Census 2024 estimate is 58,823.

Zone 5aUSDA average annual extreme minimum -20°F to -15°F, from ZIP 82644 (2.7 miles from the city point used on this page).

Median last 32°F is May 10. Median first 32°F is September 29. The 50% growing-season length is 138 days. The green band runs from the median last frost to the median first frost. Thin ticks mark the 10%, 50%, and 90% dates.

Outside the median season Growing seasonTicks: 10%, 50%, and 90%

Last 32°F in spring

NCEI’s probability is the chance the last spring minimum of 32°F or lower occurs on or after the date.

Probability	Date	How to read it
90%	April 30	Earlier date. A last 32°F on or after this day is common.
50%	May 10	Median.
10%	May 24	Later date. A last 32°F on or after this day is uncommon.

First 32°F in fall

NCEI’s probability is the chance the first autumn minimum of 32°F or lower occurs on or before the date.

Probability	Date	How to read it
10%	September 14	Earlier date. A first 32°F on or before this day is uncommon.
50%	September 29	Median.
90%	October 12	Later date. A first 32°F on or before this day is common.

32°F growing-season length

Days between the last spring 32°F and the first autumn 32°F. The figure is a length the season meets or exceeds at that probability, from NCEI, not a subtraction done on this site.

Probability the season is at least this long	Days
10%	155
50%	138
90%	118

28°F threshold

	10%	50%	90%
Last, on or after	May 14	May 1	April 17
First, on or before	September 21	October 6	October 23
Season at least	178 days	156 days	136 days

36°F threshold

	10%	50%	90%
Last, on or after	June 6	May 23	May 11
First, on or before	September 6	September 19	October 2
Season at least	134 days	116 days	96 days

Station CASPER WWTP, WY US (USC00481569). Completeness flag R, 18 years used for the 32°F dates. NCEI’s annual probability of at least one day at or below 32°F is 100%. Freeze dates are conditional on that threshold occurring. Elevation 1553.6 m. The same probability language applies to the 28°F and 36°F rows when those dates passed the same quality checks.

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Casper, WY

Zone 5aAverage annual extreme minimum -20°F to -15°F

Outside the median season Growing seasonTicks: 10%, 50%, and 90%

Wyoming. Nearest qualified station: CASPER WWTP, WY US (USC00481569), 2 miles. Median last 32°F May 10. Median first 32°F September 29. The 50% growing-season length is 138 days.

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32°F probabilities at this station

	10%	50%	90%
Last 32°F on or after	May 24	May 10	April 30
First 32°F on or before	September 14	September 29	October 12

Column order matches the NCEI file: for the last freeze, 10% is the later date and 90% is the earlier date. How to read the probabilities.

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Planting calendar

Frost dates for Wyoming are above. Planting windows appear for states with a cited extension guide. The fall and seed tools use the numbers you enter and the station dates.

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