As part of normal procedure, the National Weather Service in Birmingham issues frost advisories and freeze watches and warnings to coincide with the end of the growing season in the fall and the beginning of the next growing season in the spring. On Thursday, November 25th, Thanksgiving Day, we will end the issuance of frost advisories, as well as freeze watches and warnings for the fall season. These products will resume on or after March 15th.
Hard freeze watches and warnings will still be issued when temperatures are expected to be 15 degrees or colder anywhere in Central Alabama. These watches and warnings will be issued with the normal heading of NPWBMX.
With the fact that Central Alabama has only experienced one or two locations with freezing temperatures this year, we thought we'd take a minute to review some statistics about when we normally see the first widespread freeze.
The map below is a depiction of the median date of the first fall freeze during the 30-year climatological normals period of 1971-2000. The median is defined as the middle value of an ordered set of values. In other words, 50% of the time freezing temperatures occur before the given date and 50% of the time freezing temperatures occur after the given date. The map is divided by color into 10-day increments. Two facts to point out: notice the further south you travel down the state, the later the first freeze date. Also, notice the areas shaded in blue in the northern most counties of the state. These areas are part of the Tennessee River valley, which illustrates how bodies of water will hold enough heat to keep nearby land from freezing.
Taking a look at some actual numbers for 2010, as of November 4th, Birmingham has only dropped to 39 degrees this fall, on October 5th. The median average date for the first freeze at Birmingham is November 10th. By November 19th, Birmingham has had a freeze 90% of the time. By November 30th, Birmingham has had a first freeze 99% of the time.
As of November 4th, Montgomery has only dropped to 39 degrees this fall, on October 30th. The median average date for the first freeze at Montgomery is November 13th. By November 17th, Montgomery has had a freeze 75% of the time and by November 29th, Montgomery has had a freeze 90% of the time. By December 11th, Montgomery has had a first freeze 99% of the time.
The last time the Birmingham and Montgomery areas did not experience a fall freeze was in the Fall of 1931. The first freeze that cool season was not until January 10, 1932 in Birmingham and January 31, 1932 in Montgomery. Montgomery also did not have a fall freeze in 1923. This is based on observations from 1900 to 2009.