Monday, February 05, 2007

It's Officially a Drought

The National Drought Mitigation Center has declared a moderate to severe drought over much of southern California. Click here to see the U.S. Drought Monitor map for the southwest.

Much ado has been made about how dry this El Nino winter has been. But, El Nino only loads the dice for a wetter than average year. Warm offshore water doesn't bring rain if the jet stream (a fast-moving current of air several miles above the earth's surface) doesn't cooperate.

The jet stream brings unsettled, stormy weather below it. (Think Bernoulli's principle; fast-moving air has lower pressure.) Often, in February, the jet stream splits into two with the southern branch residing over southern California. Some years, the jet stream splits a little bit earlier or later (or never). Anyway, that's why February is historically the wettest month in LA.

The California Regional Weather Server has an excellent jet stream page. I like to look at the eastern Pacific and western north America plots. Notice how, lately, the jet stream tends to just miss us, either to the north or to the south?

Compare this with the record setting rainy winter of 2005.

Is it really an El Nino?
Take a look at the National Weather Service's tropical Pacific sea surface temperature (SST) and anomaly animation. (The anomaly is the difference between the actual conditions and the climatological average.) Last fall, it looked like we were headed into an El Nino situation with the SSTs in the eastern Pacific warmer than average. But it doesn't look like that is true any longer. By January, the El Nino near the equator had all but disappeared. If anything, the SSTs off the southern California coast had become colder than average.

Slightly off-topic
I like the SST analysis and anomaly plots from the Navy's Fleet Numerical Meteorology and Oceanography Center better. Unfortunately, they are in the midst of a computer system upgrade and have no recent data available. However, take a look at the SSTs during Hurricane Katrina. Type "30 C to F" into Google if you think in Fahrenheit instead of Celsius. Can you spot the Gulf Stream?

One of my coworkers has plotted LA rainfall statistics in a meaningful way. Go take a look.
Read more about Los Angeles Rainfall statistics (skewness!) in an earlier post. That's all folks.


  1. From you, "in a meaningful way" is high praise indeed.