WeatherWhys Blog

Sandwich Seasons

Sandwich Seasons

October has come and gone- with above normal temperatures and more rainfall than an average October would typically see. Is this the start of a wet winter?

The month just past brought .59” of rain in Bakersfield. That’s 197% of normal. We had 4 separate storm systems pass through the region, each delivering beneficial widespread rainfall. Hot weather at the beginning of October was punctuated by thunderstorms which sent flooding rains to areas just east of Bakersfield. Many part of the county received quite a bit more rainfall than fell at Meadows Field. Another round of rain came on the 6th and 7th, the 18th and 19th, the 24th and 25th, and finally last Saturday. It was very good and has led to a slight surplus for the 2010-11 water year. But all indications are that this wet October is only an apparition, bearing no resemblance to how the full winter rainfall will pan out.

Instead, this is what I have come to call a “Sandwich Season”. The start is encouragingly wet. In April the season ends on a high note. But most everything in between is dry. While there are certainly a few periods of rain, we are likely to see nearly all strong weather systems stay well to our north, affecting the Pacific northwest with flooding rainfall. It is part and parcel of the well know meteorological phenomenon known as “La Nina”. What a turnabout! This time last year it was all about El Nino, and we enjoyed above normal precipitation that ended a 3-year drought. But our current hydrological status is tenuous, especially given the computer model prediction for sea surface temperatures in the equatorial Pacific. Those temperatures are what define El Ninos and La Ninas. They trended sharply cooler this past summer and are now hovering about 2 Celsius degrees below average. Models are in good agreement that the current cool La Nina water temperature pattern will easily persist through the entire upcoming winter season. Hence, if past La Nina history is any gauge we will have relatively dry weather for the winter.

But back to the Sandwich Season concept. I have noted this dynamic many times in my decades as a meteorologist. Not only do they appear in La Nina years but they are also observed in El Nino years which start off dry, end that way too, but have several back-to-back wet months in the middle. There are other years in which no such pattern exists, random wet and dry months throughout the winter. However, it is my view we are at the beginning of another sandwich season. So, what might cause this?

I believe perennial cycles are constantly in motion around the world (and universe). Temperature variations in the Pacific can be seen through long term slow movements of warm or cool water from west to east, and back again. The same is true of slow movements in long wave atmospheric patterns. They are best seen in “time versus longitude” depictions (known as Hovmoller diagrams). In a very slow progression, that can only be realized if you step way back to analyze movement of a single large temperature wave in the ocean over weeks and months, these anomalies seem to interact with and against each other. It is like a small pendulum swinging on a clock’s large pendulum. Or like a Tilt-A-Whirl at the carnival sometimes spinning slowly then spinning faster depending upon whether its angular motion is meeting an upward climb or a downward fall. Jet stream patterns often illustrate peaks and troughs in activity. The “Sandwich” is therefore composed of two peaks (the bread) and a long trough (the filling) as a cool season occurs. The peaks are manifestations of long waves settling in to their destined spots as defined by La Nina. It is probably more complicated than that, but what we all see in the end is a Sandwich.

This winter, I am forecasting below normal precipitation following the wet October with another upsurge in late March and April. We are set to deal with two or three very cold periods from late November through February (7-10 freezing days) in an otherwise mild winter. Above average dense fog days are also foreseen due to plenty of upper level high pressure over California pushing those wet storms up into Seattle. This year we get back into our dry ways with the possibility of slipping back into a drought- thanks to a sandwich. Instead of last year’s French Dip, this year we’ve got peanut butter and jelly (mainly peanut butter).