The National Weather Service office in Bismarck will stop issuing
frost advisories on Wednesday September 22nd but will continue
providing freeze watches and freeze warnings for western and central
North Dakota through October 15th, which marks the end of the local
growing season. Or until a hard freeze has been observed across all
of western and central North Dakota, whichever occurs first. A hard
freeze is defined as temperatures at or below 28 degrees Fahrenheit
for a period of 3 or more hours.
A freeze watch will be considered appropriate when there is the
potential for temperatures to be 32 degrees Fahrenheit or below for
greater than one hour over a geographically significant area during
the next 12 to 48 hours.
A freeze warning will be issued when the temperature is expected to
be 32 degrees Fahrenheit or below for greater than one hour over a
geographically significant area during the next 24 hours.
Significant damage to vegetation is likely as a result of these conditions.
Below is a table displaying the 10%, 50% and 90% dates for: the first fall season frost (36°F or colder), the first freeze (32°F or colder) and the first hard freeze (28°F or colder). To interpret this, consider the 50% probability for a hard freeze at Amidon. The date listed for the first fall season hard freeze is October 1st. This means that the first hard freeze will occur on or before this date on 5 out of every 10 years. Likewise, the 90% probability means that 9 out of every 10 years will have a hard freeze on or before October 17th. This same technique can be used to interpret the first frost and freeze dates.