What a difference a year makes

I first composed the winter 2010-11 forecast in September 2010, stating that a "sandwich season" would occur. That meant heavy precipitation early on, followed by a long dry period, and finally finishing up with another anomalously wet period. Indeed, that was what happened. But the intensity of rainfall was so far over the top that no one could have expected it. In the end, Bakersfield recorded 5.82" of rainfall. That was 766% of a normal December and 90% of an entire water year tally. Perhaps the most amazing aspect was that this deluge took place during a La Nina event, typically dry. On two days alone (the 18th and 19th), nearly 3 inches fell. And extreme rainfall blanketed the region. Fresno picked up 5.92", Tehachapi 8.94" and 10.23" was recorded in Los Angeles. Even Randsburg in the dry desert of far eastern Kern County reported 5.61". Flooding was widespread in many parts of the county and the Kern River Canyon (highway 178) was closed for 2 weeks due to rock slides.

But this December is now closing on a totally different note. It has been 40 days and 40 nights without measurable rainfall in Bakersfield. The last time was had rain was November 20th. With only a trace of rain for the entire month we've gone from feast to famine. This will tie for the second driest December on record. Only two December's have ever been completely dry, without even a trace of rain (1930 and 1989). This December will be the sixth one registering only a trace of moisture. It is interesting that this year is also a La Nina winter, just like last year. So obviously there was a twist last year.

Some climatologists believe it was the Arctic Oscillation that proved to be the caveat; a wildcard thrown into the equation. This feature went into an extremely negative phase, meaning that stratospheric temperatures above the arctic warmed dramatically. And just as water temperatures in the equatorial Pacific can dictate global weather patterns, as seen with El Nino and La Nina, the Arctic Oscillation often has a strong influence on positioning of the North Pacific polar jet stream. In December 2010 that was the case, and it overrode La Nina. However as 2011 comes to a close, it is clear that this year we are back to a rather normal La Nina.

Besides being very dry we also saw a string of freezing mornings. A total of 19 days of lows at 32 or below were reported. The climatological average number of freezing days in December is 5, and for the entire year only 12. So, as last December was very wet and mild, this December has been very cold and dry. Wow! What a difference a year makes.