WeatherWhys Blog

A new weather pattern arrives -- finally

A new weather pattern arrives -- finally

After the record setting onslaught of wet storms that began in October, it now appears that a stretch of dry weather is upon us. But will it last?

Four months ago I wrote that this upcoming rainy season would be characterized as a sandwich. The “sandwich season” would have a wetter than normal start and finish, but see a dearth of rain in the middle. Although the first slice of bread on this sandwich has been a lot more impressive than I expected (it must have been the heel), we have now snapped into a typical La Nina pattern. Finally! As the strong polar jet stream retreats to its normal position near Seattle, fog has descended on the central valley of California. That, too, is normal. But it is worth noting how dramatic the extreme rains have been. Bakersfield has received enough rainfall that, if it were possible to carry over rain to the next water year, no more precipitation would be necessary until November 21st of this year. Records were broken in December (as detailed in the blog previous to this one). Only one record remains yet to be broken and that is the all-time water year record of 14.73” set in 1997-98. As of today we are sitting at 7.41”, which is slightly over 50% of the way there with the bulk of the rainy season still ahead. However, I feel fairly confident that this record will hold. While December ended 766% of normal for the month, average precipitation for February and March is much higher. Such stratospheric records are not expected. Plus, we are headed into the meaty (and dry) portion of the sandwich.

La Nina is currently rated as moderate, with average sea surface temperatures running about 1.5 Celsius degrees cooler than normal. The official declaration of either an El Nino or La Nina comes upon 5 consecutive months of at least 0.5 degrees above or at least 0.5 degrees below normal ocean temperatures in the equatorial Pacific. Additionally, those months are evaluated as 90-day blended months, so that the August value is actually calculated as July through September. With the release of December’s ocean temperature anomalies it will finally be official that we are in a La Nina cycle. That data is expected any day now. But it hasn’t been a secret. Since last May the significant shift from El Nino to La Nina was quite apparent. And now after the unusual circumstances that were thrown into the mix (a blocking high over Canada and the tropical Madden-Julian Oscillation, explained in an earlier blog) we finally get to experience a La Nina winter in Kern County. The good news is that our southern Sierra currently has about 268% of normal snowpack, with equivalent melt water containment of about 26”.

If the jet stream doesn’t dip back south at all, we are still probably looking favorable for the summer ahead, water-wise. However, I would be surprised if no more rain at all falls. According to my sandwich theory, following a below normal January, February and most of March, rains will kick back in again for late March through the end of April. Fog and low clouds in the valley will be more common this winter. Hopefully cotton farmers can plant earlier than last year when El Nino rains persisted well into May. The wildfire season late this coming summer will likely be bad again due to the excessive rains from October through December which will bring excessive underbrush for the dry season. 2010 was very wet with an El Nino at the beginning, ending the calendar year at exactly 12.50” in Bakersfield. Of course, many spots around the south valley and in the mountains had much more rain than that. In all, the past year featured 5 warmer than normal months and 4 cooler. 3 months were near normal. We saw 6 wetter than normal months (including the wettest month on record in December) and 2 drier than normal months. 4 months were near normal. Perhaps the most interesting fact about 2010 is that despite all the extreme rainfall at the end and 4 heat waves in the middle, readings for the entire year calculated a mean temperature of 65.0- exactly normal, right to a tenth of a degree.

So perhaps the old adage is correct, “It all evens out in the end”. These major swings one way and other will all average out. If that is correct (and La Nina says it is), we should expect much drier weather for the bulk of the rainy season which begins next month.