Look back at March 2012

As I noted in previous posts, March 2012 was incredibly warm across the central and eastern U.S.  Now that the dust has settled, we can take a look back at what might have caused this early spring heatwave and just how infrequent such an event is.  The NOAA Earth System Research Laboratory released a preliminary analysis of the episode.  Here are the highlights:

• Over 7000 daily record highs were set across the country
• Anomalously strong and persistent southerly flow transported warm air poleward from the Gulf of Mexico into central Canada
• Warming was intense throughout the troposphere, indicative of deep layer meridional transport
• Lack of snow cover was likely not a major factor, except perhaps the northern Upper Midwest
• Signals of the upcoming heat wave were detectable well in advance of the event
• Global warming associated with greenhouse gas (GHG) emissions contributed ~ 5 to 10% of the intensity of the heat wave (preliminary estimate)

This last bullet is perhaps most interesting.  Assessing the influence of global warming on individual weather events is quite difficult, and since global warming came to the forefront of the atmospheric sciences, I have seen few weather events described as, in part, enhanced by global warming.  As the NOAA analysis points out, it's highly unlikely that warming due to GHG emissions caused the heat wave, but it is quite likely that it enhanced the magnitude of the event.  Studies also show that warming associated with GHG emissions  does increase the chance that extreme weather events, such as heat waves, will occur, and perhaps enhance them when they do occur.  Climate researchers often use an analogy of "loading the dice" to describe this.  The take home message:  The March heat wave was extraordinarily rare, but these types of events may occur more frequently as uncontrolled GHG emissions continue.

Back here on the West Coast, we finally had a month with above-normal rainfall in San Francisco.  The downtown observation site reported 5.71" of rain in March, which was 2.45" above normal.  We're still way below normal for the season though.  The Northwest has been extremely wet this March, with Portland and Spokane recording their wettest March on record.  Coming up, we have a decent chance for at least light rain in San Francisco on Tuesday night, followed by a warm up into the weekend.