Wednesday, February 26, 2014

Finally some rain for California

As most people are probably aware by now, California is in the midst of a drought of historical significance.  The past several years have had below-normal rainfall, and combined with a particularly dry winter so far, water levels are extremely low.  Much of the west has been suffering from below normal precipitation. Here's the estimate of the percentage of normal snowpack from mountain sites across the west from the National Weather and Climate Center at the beginning of February:

Several storms have moved through the Pacific Northwest in February, bringing the snowpack up here near Seattle to near normal.  Much of the Rockies is also doing rather well.  But from Oregon down through California the snowpack remains at 25-50% of normal.  It's also pretty low in southern Utah and on the high mountains of Arizona and New Mexico.  For California this is critical---communities depend on the mountain snowpack for their water throughout the year.  Here's a summary of the reservoir levels currently throughout California.  Most are running only about half or less of their average values for this time of year.Without snowmelt to replenish them, it's going to be a rough year...

To illustrate how far below normal the precipitation this year has been, we can look at precipitation since September for several cities in the California central valley.  Here's Sacramento:
The top panel is the daily temperature range and the bottom panel is the total precipitation so far.  The upper curve is the normal precipitation and the brighter green below is the actual precipitation.  Normally by this point in the winter Sacramento has seen almost 15" of rain.  So far this year, only about 5".  It's a similar story further south in Fresno:
They already don't receive much rain---only 6" on average by this point---but they've only gotten 1.36" by the beginning of February (not sure why the graph hasn't been updated since then).  One thing you may note on these graphs is that there can be precipitation events that bring a lot of rain.  For instance, in early February Sacramento's precipitation was only at around 2 inches---they gained another three over the course of just a day or two in early February.  So large systems can bring lots of rain to California, and it looks like that's what we're about to see.

Starting tonight, two low pressure centers are forecast to move toward the California coast through the end of the week.  Here's the ECMWF forecast of surface pressure and 3-hour accumulated precipitation for late this evening (from Weather Underground):
Note the first low bringing substantial rain to the central California coast and more precipitation to the Sierras.  The snowpack will be growing...  You can see the next low spinning further off the coast.  This is forecast to move in on Friday:
This low looks to bring even more precipitation.  These are the 12-hour precipitation totals (I believe...Weather Underground isn't exactly clear about this) and you can see some heavy precipitation throughout the coast.  Here's the precipitation totals forecast overnight Friday and into Saturday:
You'll notice the incredible enhancement of the precipitation along the coastal mountain ranges---forecasts of up to 2"+ along some of the crests throughout the Big Sur region.  I don't know how helpful that will be, exactly, as most of the water retention in California comes off of the Sierras and not the coastal mountains.  I'd also be concerned with flash flooding with these extraordinary amounts of rain.  However, the Sierras also look to get a fair amount of precipitation as well, so the snowpack will continue to deepen.

Even with this rain, California will still need a very wet spring to recover from the dryness of the winter to date and the cumulative effects of previous dry years.  I suspect if the water supplies remain low and more cuts are made that this may turn into the next hot button "is this evidence of climate change?" issue.  We still cannot separate out the effects of climate change on any single event, even a multi-year event, as this is still within the limits of natural variability.  So people will ask the question, but we really have no answer yet...

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