Crest Frost Dates and Growing Season

NOAA 1991-2020 normals for El Cajon, 5.7 km from Crest, elevation 495 ft

Frost and the growing season

NOAA's 1991-2020 freeze probabilities for El Cajon. 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 January 6 and the first fall freeze around December 21, a growing season of about 350 days.

Median at 36°F: last February 6, first December 6
Median at 32°F: last January 6, first December 21
Median at 28°F: last December 31, first December 30

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.

Threshold	10%	20%	30%	40%	50%	60%	70%	80%	90%
36°F	Mar 3	Feb 25	Feb 18	Feb 12	Feb 6	Jan 31	Jan 25	Jan 16	Jan 6
32°F	Feb 7	Jan 30	Jan 21	Jan 12	Jan 6	Dec 31	Dec 28	Dec 22	Dec 13
28°F	Jan 15	Jan 10	Jan 6	Jan 3	Dec 31	Dec 29	Dec 23	Dec 15	Dec 8
24°F	–	–	–	–	–	–	–	–	–
20°F	–	–	–	–	–	–	–	–	–
16°F	–	–	–	–	–	–	–	–	–

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.

Threshold	10%	20%	30%	40%	50%	60%	70%	80%	90%
36°F	Nov 18	Nov 25	Nov 29	Dec 2	Dec 6	Dec 10	Dec 15	Dec 20	Dec 29
32°F	Dec 1	Dec 7	Dec 12	Dec 17	Dec 21	Dec 25	Dec 30	Jan 4	Jan 15
28°F	Dec 7	Dec 14	Dec 20	Dec 27	Dec 30	Jan 1	Jan 6	Jan 9	Jan 14
24°F	–	–	–	–	–	–	–	–	–
20°F	–	–	–	–	–	–	–	–	–
16°F	–	–	–	–	–	–	–	–	–

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.

Threshold	10%	20%	30%	40%	50%	60%	70%	80%	90%
36°F	336	324	314	308	300	293	287	279	269
32°F	365	365	362	356	350	344	338	331	317
28°F	365	365	365	365	365	365	365	365	357
24°F	365	365	365	365	365	365	365	365	365
20°F	365	365	365	365	365	365	365	365	365
16°F	365	365	365	365	365	365	365	365	365

How often it gets that cold

The share of years with at least one night at or below each temperature.

36°F or colder: 99% of years
32°F or colder: 73% of years
28°F or colder: 16% of years
24°F or colder: 0% of years
20°F or colder: 0% of years
16°F or colder: 0% 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: 1,309
Cooling degree days, base 65°F: 1,377
Growing degree days, base 40°F: 9,194
Growing degree days, base 45°F: 7,370
Growing degree days, base 50°F: 5,564
Growing degree days, base 55°F: 3,877
Growing degree days, base 60°F: 2,454

Growing degree days by month, base 50°F

Heating and cooling by month

Month	Heating DD	Cooling DD	Growing DD 50
January	283	1	189
February	232	2	194
March	176	10	300
April	100	28	378
May	42	75	498
June	4	157	603
July	0	301	766
August	0	361	826
September	1	292	741
October	25	127	567
November	139	23	336
December	307	1	166

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 5.7 km from Crest. Low ground, valleys and open fields freeze earlier and later than a town center or a hillside.

Sources

NOAA NCEI U.S. Climate Normals, 1991-2020, annual and seasonal normals for station USC00042706 (El Cajon). Public domain.