Tuesday, April 10, 2012

"It is only beginning..."
- From Disney's The Prince of Egypt 

April 3, 2012 Tornado outbreak in Dallas-Fort Worth
Photo credit: Christian Science Monitor

9:25 PM EDT 4/10/12 (Forecaster Mike N.) Did you enjoy the March warmth, or would you have preferred an average month? Regardless of whch way you lean on the weather spectrum, we finally have data in from March around the country, which demonstrates an astounding array of shattered records. The Dallas tornado outbreak, itself a record event, coupled with the warm trend of March, may foretell a dangerous pattern ahead going into Spring. 

While the country, especially east of the Rockies, has been very warm since last November, none of the previous months (Nov - Feb) even come close to the warmth in March! A shocking 25 states had their warmest March on record in 2012, while another seven saw their second warmest, and five saw their third warmest.

Nationwide, this past month was the warmest March since U.S. records have been officially kept, going back 118 years. April has started out warm for a large portion of the country, but nowhere close to the scale of March. A number of factors went into making March so warm, and a few have now changed for the time being going into April. Among those factors are what atmospheric scientists and climatologists have identified as "teleconnections." These are data-based measurements that help track and explain large scale interactions between various Earth systems such as the atmosphere, the polar regions and the oceans.

La Nina/ENSO* (Current NOAA advisory) While it seems like everything is blamed on La Nina or El Nino, it did definitely have an impact on the weather. In March, this was a primarily west-based La Nina, in which the strongest cold anomalies in the equatorial Pacific lie in the center or western Pacific, with the east Pacific being closer to neutral or even warm. The animation above shows the changes in Sea Surface Temperature anomalies last month in the Pacific. These changes favored a strong southeast ridge, and in March, we saw a very strong ridge just off the southeast coast of the United States pumping the country with warmth. *El Nino Southern Oscillation

North Atlantic Oscillation (NAO) (NC State Info website) This climate index, a measure of large scale air mass movement of the Azores (Atlantic High) and the Icelandic Low, as shown to the right, has been positive almost every day since the end of October 2011. In March, there was very little variation until the end of the month. The NAO was fairly constant around +1 until a drop began around St. Patrick's Day. North American weather typically responds to changes in the NAO about 1-2 weeks later, and sure enough, the massive two week ridge broke about a week later. Still, the new ridge axis over Texas continued much above average warmth for the plains through the end of the month. Now, the NAO has flipped into the negative phase for the first time since October (and for over a few days, the first time since August), which is helping to bring slightly cooler conditions in, especially to the east coast by causing the jet stream to buckle around over the east. 

Madden-Julian Oscillation (MJO) (1-page NOAA Fact Sheet) Less is known about this variation than the others, which makes it harder to prove a relationship, but NOAA still has shown a statistical correlation to the US temperatures. The MJO has 8 different phases, each with a number that it cycles through. Each phase is characterized by locations of a tropical disturbance propagating around the equator. 

The MJO also has variations of strength in each phase. When the wave is stronger, it has a greater impact on US climate. Throughout the month of March, the MJO was propagating through phases 4-7 at a relatively high strength. All of those phases promote extreme warmth for the eastern and central United States. It recently moved into phase 8, which promotes warmth for the Central US, but cooler conditions for the immediate east coast, interestingly similar to where we are now. 

There are of course more impacts to the weather this past month, but I pinpointed these teleconnections as among the ones which may have had the  greatest impact on our weather because of the observed anomalous strength and duration of such events as recorded in the data. 

In plain language, all of this is just our fancy way of saying, "it's all connected!" If March was any indication of where Spring and Summer patterns will take us, then it may indeed only be just beginning.  

(Lead writer: Forecaster Mike N., Editor: Mr. Foot)

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