And since the average temperature have been rising for several decades, being a certain amount above/below average means the actual temperature is higher than it would have been for the same amount above/below average in the past.http://www.wunderground.com/blog/JeffMasters/comment.html?entrynum=2071
By Dr. Jeff Masters
Published: 2:57 PM GMT on April 13, 2012
A dangerous tornado outbreak is expected on Saturday over the Plains, says NOAA's Storm Prediction Center (SPC). Warm, moist air flowing northwards from a Gulf of Mexico that has record-warm waters will collide with cold air funneling down from Canada, creating a highly unstable air mass capable of creating strong thunderstorm updrafts. An impressive upper-level low, accompanied by a jet stream with 105 - 125 mph winds at middle levels of the atmosphere, will create plenty of wind shear, giving the air the spin it needs to form tornadoes. SPC has has issued their highest level of alert, a "High Risk," for portions of Oklahoma and Kansas for Saturday. Included in the "High Risk" area are the cities of Wichita and Oklahoma City. This is the second time SPC has issued a "High Risk" forecast this year; the first "High Risk" forecast came for the March 2 tornado outbreak, which killed 41 people and did $1.5 - $2 billion in damage. It is very unusual for SPC to issue a "High Risk" forecast more than a day in advance of a suspected tornado outbreak; according to Wikipedia, today's "High Risk" forecast is only the second time SPC has ever done this. The other instance was for the April 6 - 8, 2006 tornado outbreak focused over Tennessee, which generated 73 tornadoes that killed 13 and did $1.5 billion in damage. People living in the threatened area need to pay close attention on Saturday to this dangerous severe weather situation.
Temperatures in the states bordering the Gulf of Mexico were the 3rd - 7th warmest on record during the first three months of 2012, according to NOAA's National Climatic Data Center. This allowed Sea Surface Temperatures (SSTs) in the Gulf of Mexico (25 - 30°N, 85 - 90°W) to climb to 1.4°C (2.5°F) above average during March 2012, according to a wunderground analysis of the Hadley Centre SST data set. This is the warmest March value on record for the Gulf of Mexico, going back over a century of record keeping. During the first two weeks of April, Gulf of Mexico waters remained about 1.5°C above average, putting April on pace to have the warmest April water temperatures on record. Only one year in the past century has had April water temperatures in the Gulf of Mexico more than 1.1°C above average; that year was 2002 (1.4°C above average.) All that record-warm water is capable of putting record amounts of water vapor into the air, since evaporation increases when water is warmer. Because moist air is less dense than dry air, this warm, moist air flowing northwards from the Gulf of Mexico into the developing storm system over the Plains will be highly unstable once it encounters cold, dry air aloft. The record-warm waters of the Gulf of Mexico are a key reason for the high risk of severe weather over the Plains this weekend.