RAIN, TO SHOWERS, TO….WELL, YOU KNOW THE REST

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Among the hazards of long range weather forecasts are their inaccuracy. And, although the models may flip back to a wetter scenario, the current (new) signals are for mostly dry weather to continue through the middle of February.

This follows several weeks of positive atmospheric signs of a major pattern shift to wetter weather. The change has been subtle on a global scale, but enough to effectively cut off the flow of moisture to California and the Southwest through mid-February.

DETAILS OF THE CHANGE

For weeks we’ve been talking about high pressure shifting west toward Alaska at the end of January, to allow a low pressure trough to drop down into the West and bring stormy weather.

Now, models have come into agreement that the low trough will indeed drop south as predicted, but just to the east of California. Here is how the Euro and GFS models see the pressure setup at the end of the month, and the first few days of February. (Images 1 and 2) A subtle shift of the blues (low pressure) to the east, but enough to keep us mostly dry.

By the 5th of February, the Euro (and other models) shows high pressure right along the coast, and the low trough pushed even further east. (Image 3)

Through the 10th, the Euro ensemble and GFS ensemble agree….very little rain or snow for California. (Images 4 and 5)

WHAT’S GOING ON?

We are in a weak to moderate La Nina year. Here is the distribution of sea surface temperatures in November. (Image 6) Cool surface water along the equator in the eastern Pacific, classic La Nina pattern. Also cold water off the northwest coast of the U.S. and up toward Alaska.

Now look at sea surface temperatures as of January 25th. (Image 7) Warm surface water has pushed east, into the northeast Pacific. In the past this has been associated with blocking high pressure in the northeast Pacific, and this is what we’re seeing now.

This of course is one of many possible factors affecting our weather pattern. The Pacific North American pattern, which helps predict a ridge or trough along the West Coast, has gone neutral through mid February after a short blip into negative territory (troughing) at the start of the month. (Image 8)

And, the Madden Julian Oscillation, after showing signs of advancing into an active phase over the Indian Ocean, is stuck in a very inactive phase for West Coast storms. (Image 9) That could change (blue dots are hopeful forecasts), but not likely in the next couple of weeks.

WHERE THIS LEAVES US

Considering all of that, we are (still) in a fairly good position. Most regions of the state are still running above average for rain and snowfall. (Image 10)

Also, the typical La Nina year sees more rain and snow fall in February and March than an average year does. In fact, in years with a wet December and dry January like we’ve just experienced, February and March (and April) have been wetter than average in the Sierra. (Image 11)

So, keep the faith. As we’ve often said, although the models can be off in the dry direction in the long range, they can also miss in a wet direction. That is what we’re hoping for.

And, the Euro ensemble, after predicting a dry middle of February (Image 12), is hinting that normal rain may return late in the month. (Image 13)