Tuesday, May 17, 2011

April Temperatures in Context

It has certainly been a cold spring in Utah, but how cold?  Has Hell frozen over?  Is this the end of global warming?

According to the National Climatic Data Center, Utah's average temperature in April was below average, but only the 35th coolest of the past 117 Aprils.  If you want really cold, in relative terms, head further north to the Pacific Northwest where Washington observed its 2nd coldest April of the instrumented period.
Source: NCDC
The statewide average, however, is a bit deceptive for those of us who live in northern Utah where the departure from average was the largest in the state.  In contrast, southern Utah's departure from average was smaller or even slightly positive in a few spots.
Source: NCDC
At KSLC, the mean temperature for April was 45.4 F, 4.6F below the average for 1971-2000.  Not exactly BBQ weather, but it did make for good skiing.

As can be seen in the above analysis, we were a country divided in April with much below average temperatures in the Pacific Northwest and above average temperatures across much of the southern and eastern United States.  The former was strongly related to anomalous troughing over the Pacific Northwest that was connected with a deep anomalous trough over northern Canada and Greenland.  

This anomalous circulation pattern resulted in the remarkably cool Spring we've had over Utah and is not at all reflective of the end of global warming.  According to NCDC, April's global-mean temperature was the 7th warmest since 1880, 1.06F above the 20th century average.   Although the Pacific Northwest was quite cool, it was remarkably warm across much of Eurasia.  The UK saw its warmest April on record and Germany its second warmest.  
All of this shows that weather, like politics, is local.  It has been a cold spring in Utah thanks to an anomalous large scale circulation, but the global-mean temperature remains well above the 20th century average.  

April shows the power that persistent, high amplitude, large-scale circulation anomalies have on local weather.  Mother Nature can still give us a cold winter or spring on a regional scale.  

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