The National Weather Service office in Bismarck will stop issuing frost advisories on Thursday, September 22...but will continue providing freeze watches and freeze warnings for western and central North Dakota through October 15...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.
The following graphics from the Midwest Regional Climate Center in Champaign, Illinois depict the median date of a 32°F freeze and a 28°F freeze. The median date for a 32°F freeze ranges from the middle of September over the northwest and parts of far southwest and north central North Dakota, to late September for the remainder of western and central North Dakota. A 28°F freeze or a hard freeze generally occurs in late September over much of the west and north central, to early October for the southwest and south central. These graphics are based on the newly released 30 year climatological normals (1981–2010) from the National Climatic Data Center.