Talkin’ ’bout the Weather

Here in California, we are in a drought. Not a drought drought, where, I dunno, native flora and fauna are dying, preferably on a grim but picturesque dried lake bed of cracked mud. Like this. No, what we mean by drought in California: not enough rain and snow are falling to keep our reservoirs full.

Here’s a screen grab from Weather.com, showing the radar images of precipitation earlier today in California:

California Rain

The blue stuff is snow; the green, orange and red are rain. This is sort of the classic California rain situation: Storms come in from the west, rain a bit on their way to the Sierra, then have all their moisture wrung out of them as rain and snow by a long range of 10,000′ high granite mountains. The rain and, eventually, the snow when it melts, feed a bunch of rivers that water that valley to the west of the mountains – the Central Valley. The waters are enriched with vegetable matter from all the pine forests in the Sierra, as well as minerals dissolved from the granite. This, coupled with 300+ days of sunshine and mostly mild temperatures, makes the Central Valley the most perfect place for farming on the planet. California’s largest industry is not high tech or Hollywood – it’s agriculture.

Also note that no rain gets past the Sierra from Lake Tahoe (that’s the little marker with the ‘3’) all the down to Bakersfield. Tahoe is just south of Donner Pass (1), a relatively low spot that sometimes permits rain to get through to Reno and northern Nevada. The mountains to the north of there aren’t as tall, generally, as those to the south, and so rain gets through more often in the north. The southern Sierra is dominated by Mount Whitney, at 14,505′ the highest point in the contiguous US. It’s part of a group of really tall and impressive granite peaks which cast the first of four rain shadows that keep Death Valley so dry.

In the north, near Redding, you see some rain. This is important, as Lake Shasta, the state’s biggest single reservoir, is just north of that town. The dam was built in the 30s and 40s because – you’ll be shocked to hear this – sometimes, it rains a lot in California, and sometimes it doesn’t. Floods and water shortages for agriculture take turns.

People love those dramatic pictures of Lake Shasta during ‘droughts’ – during periods where more water is drawn down for agriculture than added by rain and snow melt for a year or two. Four years of that – which is what we’ve just had – and the lake is pretty sad looking:

Lake Shasta, returned to the valley it was before the US Bureau of Reclamation made it into a lake – at least, in those years when there’s plenty of rain.

There’s nothing wrong, really. The lake fills up when it’s wet and gets empty when it’s not.  That’s kind of the point – if rainfall were orderly and predictable, we wouldn’t need huge reservoirs to moderate it. This year, it is filling rapidly and might actually be full by the end of the month.

Speaking of which, you will also notice in the screen grab that there’s a ton of rain falling in LA. LA is more or less boxed in by mountains on three sides – thus, the LA basin. The highest set are the three ranges to the north. When rain comes from the south, as it is now via the ‘Atmospheric River‘  known as the ‘Pineapple Express‘ LA can get seriously drenched as those mountains act to wring the precipitation out. The three rivers in LA – the San Gabriel, San Fernando and Los Angeles – are now mainly open concrete storm drains. Weird as it may seem, flooding was seen as the big problem back in the 1920s, so the authorities made darn sure that wouldn’t happen if they could help it. It’s interesting to think of LA as three big river valleys, not a thousand square miles of pavement.

Meanwhile, south of Bakersfield, the Sierra end and the rain has a chance to get east. That area  east and south of the mountains is the Mojave Desert, so you can see that it doesn’t happen very often. The land around Lancaster and Barstow is still fairly high, like 4,000’ or more, so not a lot of precipitation gets through most years. The exception, which happened this year, is a southern storm heading north that skirts the mountains north of LA and then hits the backside of a series of three ranges to the east of the Sierra and west of Death Valley – then, you can get a few inches of rain in the desert.

As you can see, the chief characteristic of California weather is how much it can vary from place to place based on geography and the way the wind is blowing. All those mountains and valleys make for lots of local variations. Here in the Bay Area, the weather on the east side of the Berkeley hills is often dramatically different from that on the west side. The hills don’t stop much of the rain, but the tend to stop 100% of the fog. It is both much warmer and much colder in Concord than it is 20 miles away in San Francisco and its suburbs.

We’ll wrap this digression up with a brief look at local rainfall. Here is a map of my neighborhood showing rain gages:

Rain Gage Map

The north and far west of Contra Costa County are on the Bay. (2)  All but the flats in the far east are hills and valleys, with one mountin – Mt. Diablo – close to the center. The rain gage labeled ‘DBL 22’ sits atop Mt. Diablo.

The crazy part, even if predictable, is how much the average annual rainfall varies from gage to gage. Richmond City Hall, on the Bay to the far left, gets 21.8″ on average. Bethel Island Fire Station, maybe 20 miles away, gets an average of 11″. Both these gages are around sea level and on the Bay. The top of Mt. Diablo gets 27.5″; St. Mary’s College, nestled in a little valley a few miles away, 28.15″, while the Concord Pavilion in the hills a few miles northeast gets 17″.

It gets worse when we look at the actual data for a given year: so far this year, for the 28 gages on the map, 25% have gotten 100% or more of their season average precipitation; another 36% have gotten between 90% and 100%. Yet 25% have gotten 75% or less of their season average, one even has a mere 60% – ‘drought’ level! And the locations seem hardly to matter – there’s no readily discernable pattern to which gages have gotten their annual fill and which have not. Valleys, hills, peaks; high and low averages, they each have some that are at 100% or better, and some that are way low.

What’s going on here? Two things, it seems to me: first, the sample size is small: most of these gages have only been in place less than 40 years. Those averages may not represent much of anything. After a 1,000 years, then you might have something you would be willing to hang your hat on. Maybe. The other factor is the weird geography and very varied ways storms blow in. We have hills, valleys, a mountain, some flats; storms that come east to west, northwest to southeast, and southwest to northeast, some blasting in on strong winds and some drifting in with hardly a breeze. Different storms find those valleys and hit those hills and mountain in different ways, filling some gages while mostly missing others.

At least we’re getting a ton of rain now. The rainy season is drawing to a close – we tend to get little rain after March – but the storms we’re having now and over the next few days should bring annual rain and snow totals close to average, whatever that may mean.

  1. What do they serve with drinks at a cannibal restaurant? Donner Party Mix! I slay me.
  2. The cartographer’s convention is to call only the southern part of the estuary San Francisco Bay, while calling the northern part San Pablo Bay, the part east of there Suisun Bay, and anything east of that the Sacramento River Delta. Pishaw – it’s all one big estuary with naught but whim to divvy it up. I herein call it ‘the Bay’.
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Author: Joseph Moore

Enough with the smarty-pants Dante quote. Just some opinionated blogger dude.

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