Will the Northwest Have A Water Problem This Fall?

Today, the city of Seattle announced a water advisory and asked folks to be careful with their water usage.  The reason is clear:   the city's reservoir levels are well below normal and dropping rapidly (see graphic).  

The origin of the plummeting water levels is no mystery:

1. record-low snowpack, resulting in less streamflow during the late spring and summer
2. dry conditions during the same period
3. MUCH warmer than normal temperatures the last few months, resulting in enhanced evaporation from the surface and greatly increased water usage.

Fortunately, the city has some back-up sources of water, such as wells near Seattle-Tacoma Airport, and extra pumps at Chester Morse reservoir for increasing "low-water" capacity.

But the assumption is that rains will come back in the fall, preventing a serious water shortage.  But what if the fall is unusually dry? We don't have the multi-year storage like California.  Could we have a problem?

I starting thinking about such questions as I looked at the latest seasonal forecasts, particularly since the latest numerical predictions are for an extremely dry fall.  

Let's start with the latest forecast from the National Weather Service's Climate Forecast System (CFS) model. I will show you the precipitation anomalies (difference from normal) for Sept to November and December through February.

As you can see for yourself, the forecasts are for the fall to be dry and winter to be VERY dry.

 Winter temperatures? Warmer than normal (see graphic)

OK, that is just one U.S. seasonal forecast system.  How about the North American seasonal forecasting system (NMME), averaging more American and Canadian seasonal forecasts?  Here are the precipitation anomalies for November through January from this system: much drier than normal.

And there is an international seasonal prediction systems as well (IMME).  Here is the precipitation anomaly forecast for the same months.  Same story.....dry.

You will note that all of these forecasts are not only dry around here, but far wetter than normal in California.   This is a typical El Nino pattern, especially for the strong events.  And it looks like we are going to have a very strong El Nino in place this fall and winter.  So strong that we are already seeing the effects of El Nino on our weather this summer (southern CA has been unusually wet the past few months and we have been unusually dry).  This situation will save California, providing real relief for their multi-year drought.

Keep in mind that drier than normal for us does not mean no rain.  Clouds and rain will return, but conditions should be perceptibly drier and warmer than normal this fall.  Our reservoirs will get very low before the serious rains return.   And I worry about next summer.  Drier and warmer than normal conditions will result in another low snowpack year, although probably not as extreme as this year.  Why do I say this?   The warmth next year will probably not be as crazy hot as last year.

Seattle water managers are watching the situation carefully and I suspect they will soon ratchet things down another notch, restricting the amount of water used for irrigating lawns. Time to let our grass go brown.  Next winter, regional water agencies, like Seattle's SPU will have to be very careful to save as much water as possible for use next summer.

One final thing.   Forty years ago, our ability to make seasonal forecasts was basically non-existent and we didn't understand the implications of El Nino.  Today, we have useful guidance.   We have come a long way.