The 2011-2012 winter season has been very active, but not because of winter weather. The season did begin on a cold note as the last week in November and the first two weeks in December were marked by mostly below average temperatures. During this cold snap, there were two winter precipitation events which required the issuance of winter weather advisories. There have been a few short-lived cold outbreaks since the middle of December, but overall temperatures have been very mild. The above average temperatures have placed the main climate stations in the top ten warmest winters.
Average daily temperatures for November 2011 thru February 2012.
Birmingham: 51.6 degrees (9th warmest winter)
Tuscaloosa: 52.0 degrees (4th warmest winter)
Anniston: 50.8 degrees (6th warmest winter)
Montgomery: 54.4 degrees (5th warmest winter)
Most locations have received below normal rainfall this winter. The southeast portions of central Alabama are still under an extreme drought due to a long term rainfall deficit. Some areas in western Alabama received 7 to 8 inches of rainfall during the month of January, which is 2 to 3 inches above normal. Most of the central Alabama had a rainfall deficit in February, the exception being areas south of I-20 and west of I-65.
Rainfall totals and departures from normal for November 2011 thru February 2012.
Birmingham: 20.47 inches (+1.80)
Tuscaloosa: 16.96 inches (-3.41)
Anniston: 14.66 inches (-3.63)
Montgomery: 17.23 inches (-2.17)
The mild winter has also included several significant severe weather events. There were four tornadoes across central Alabama on November 16th, six tornadoes on December 22nd, eleven tornadoes on January 23rd and four tornadoes on March 2nd. While it is not unusual to have tornadoes in November, which is our secondary severe weather season, the high number of tornadoes in December and January is not normal, especially an EF-3 tornado.
On a regional scale the high number of tornadoes could be attributed to several factors. Warmer temperatures give more potential energy for thunderstorms. There has also been a lack of cold air intrusions into the Gulf of Mexico this winter, which means warmer than normal water temperatures. The warmer water in turn produces higher surface dew point temperatures, which is a significant ingredient for severe thunderstorms.
There have also been large scale atmospheric circulations that have contributed to the mild weather. One of the methods used to track and forecast atmospheric flow patterns across North America is the Arctic Oscillation (AO) index. A positive AO index is indicative of higher pressure over the middle latitudes which keeps colder air poleward. This translates to warmer than normal temperatures east of the Rocky Mountains. The AO index has been positive for most of the winter. The AO index became negative in late January and early February which brought some very cold air into Alabama the second Week in February. The AO index has been positive since the middle of February and temperatures have been much above normal since then.
ENSO stands for El Nino/Southern Oscillation. The ENSO cycle refers to the coherent and sometimes very strong year-to-year variations in sea-surface temperatures, convective rainfall, surface air pressure and atmospheric circulation that occur across the equatorial Pacific Ocean. El Nino and La Nina represent opposite extremes in the ENSO cycle. El Nino refers to the above-average sea-surface temperatures that periodically develop across the equatorial pacific. It represents the warm phase of the ENSO cycle and is sometimes referred to as a pacific warm episode. La Nina refers to the periodic cooling of sea-surface temperatures across the equatorial Pacific. It represents the cold phase of the Enso cycle and is sometimes referred to as a pacific cold episode.
The equatorial Pacific has been in a La Nina pattern since the summer of 2011. La Nina conditions typically produce warmer and drier conditions across the southern United States. La Nina conditions are weakening and are forecast to become ENSO neutral this spring.
Central Alabama Climate Outlooks (temperature/rainfall)
Temperatures: 50-60 percent chance of above normal temperatures
Rainfall: equal chances of above average/average/below average values
March 2012 thru May 2012
Temperatures: 40% chance of above normal temperatures
Rainfall: equal chances of above average/average/below average north to 30-40% below average chances south