-30 C at 500 hPa over Oakland CA this morning (15 May 2011) is a rather unusually cold temperature for that location. I went back and checked May over the past 12 years back to 1999 and found only one other occurrence of -30 C in Oakland during May. It was last year on 23 May 2010 during an unusually cold May in the Bay Area. This late May cold spell produced the latest freeze in the historical record at Napa CA with 31 deg F at the Napa Airport on 23 May 2010. Previous to 2010 the latest freeze had been 8 May 1913.
Even colder than -25 C is unusual in Oakland CA in May. I found 7 days colder than -25 C at 500 hPa over the past 12 years:
11 May 2000 -> -26 C 26 May 2000 -> -26 20 May 2002 -> -28 21 May 2002 -> -26 23 May 2010 -> -30 27 May 2010 -> -26 15 May 2011 -> -30
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