Alabama's Mild Winter of 2011 - 2012
The 2011-2012 Winter Season has been active, but not because of winter weather. Here are some highlights so far this season:

The season did begin with a prolonged cold snap. 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. Since the middle of December there have been a few cold snaps, but those lasted only a few days.

Each month has been progressively warmer with respect to departures from normal. Here is a look at Birmingham’s average monthly temperature and departure from normal.

November 55.8 (+1.4)      December 49.5 (+3.4)       January 49.7 (+5.9)

January’s 49.7 degree average temperature at Birmingham does not place it in the top 10 warmest Januarys, but it is the second warmest January in the past 25 years, only behind the 50.7 degree average in 2006.

The above average monthly temperatures are more noticeable when you separate out the average maximum temperatures. By looking at the average maximum temperature for the three month period (November 2011 – January 2012), all of our four main central Alabama climate stations are in the top 10 warmest since records began at each station.

Birmingham: 8th Warmest

Tuscaloosa: 4th Warmest

Anniston: 5th Warmest

Montgomery: 3rd Warmest

Most locations received near normal or above normal rainfall this winter; the exception being the southern one-third of central Alabama where a long term rainfall deficit continues. 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.

The winter has been marked by several significant severe weather events. There were four tornadoes across central Alabama on November 16th, six tornadoes reported on December 22nd, and eleven tornadoes on January 23rd. 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. The high number of tornadoes could be related to the low latitude surface low tracks and the fact the Gulf of Mexico water temperatures are still relatively warm, due to the lack of cold air intrusions. In January, there have only been ten EF-3 or higher rated tornadoes in Alabama since 1950.

What could be leading to this type of weather pattern?
 
Large Scale Pressure and Jet Stream Influences
Hemispheric jet stream flow, high zonal versus a low zonal, pattern drives the hemispheric weather patterns. During the month of January 2012 an average high zonal flow occurred across North America. A broader west to east direct jet stream across Canada and the northern tier United States maintained the coldest temperatures well north of Birmingham Alabama. A pattern change to low zonal flow (greater north to south transport) in early February 2012 preliminarily indicated a return to normal seasonal temperatures.

One of the most prominent teleconnection patterns in all seasons is the North Atlantic Oscillation (NAO). The NAO is a large-scale fluctuation in atmospheric pressure between the subtropical high pressure system located near the Azores in the Atlantic Ocean and the sub-polar low pressure system near Iceland and is quantified in the NAO Index.

Strong positive phases of the NAO tend to be associated with above-average temperatures in the eastern United States. The NOA index has been in a positive phase since October 2011.

ENSO stands for El Niño/ 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 Niño and La Niña represent opposite extremes in the ENSO cycle. El Niño refers to the above-average sea-surface temperatures that periodically develop across the east-central equatorial Pacific. It represents the warm phase of the ENSO cycle, and is sometimes referred to as a Pacific warm episode. La Niña refers to the periodic cooling of sea-surface temperatures across the east-central 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 for the past year and it is forecast to continue into the Spring of 2012.

La Nina conditions typically produce warmer and drier conditions across the southern United States.

Here is the latest ENSO Discussion.


Additionally, the Atlantic Oscillation (AO) could be the overriding factor in our temperature extremes along with the active southern stream La Nina. In the AO, in the positive phase, frigid winter air does not extend as far into the middle of North America as it would during the negative phase of the oscillation. This keeps much of the United States east of the Rocky Mountains warmer than normal. We have been in a positive phase of the AO through late Jan and now the pattern has switched to a negative phase which shouldn't be a coincidence that we are now receiving a blast of cooler air through the mid US and into the southeast. You can view the AO trends and forecast here:

Atlantic Oscillation

Also a strong AO can negate the affects of a strong La Nina. Last year was a good example of this when we had one of the stronger La Ninas on record but also a strong AO. As a result last years winter was much colder regardless of the La Nina phase.
 
Temperature and Precipitation Statistics for Central Alabama
Location - Birmingham
November 2011
December 2011
January 2012
Average Max T (Departure)
66.1 (+1.7) 59.6  (+3.9) 60.7  (+6.6)
Average Min T (Departure)
45.6  (+2.8) 39.5  (+2.9) 38.8  (+3.6)
Average T (Departure)
55.8  (+1.4) 49.5 (+3.4) 49.7  (+5.9)
Precipitation (Departure)
6.37 (+1.52) 5.24  (+0.79) 5.90  (+1.06)
 
 
Location - Tuscaloosa
November 2011
December 2011
January 2012
Average Max T (Departure)
67.9  (+1.7) 60.7 (+3.3) 63.0  (+8.0)
Average Min T (Departure)
43.5  (+2.2) 38.5  (+2.9) 38.6  (+4.7)
Average T (Departure)
55.7  (+0.8) 49.6  (+2.7) 50.8  (+6.1)
Precipitation (Departure)
3.32  (-1.76) 4.46  ( -0.16) 5.28 ( -0.07)
 
Location - Anniston
November 2011
December 2011
January 2012
Average Max T (Departure)
66.4  (+1.4) 60.5  (+4.2) 60.7  (+6.7)
Average Min T (Departure)
43.4  (+3.0) 37.9  (+3.6) 36.3  (+3.4)
Average T (Departure)
54.9  ( +1.4) 49.2  (+3.7) 48.5  (+5.1)
Precipitation (Departure)
4.97  (+0.39)