Each year, the Los Angeles Department of Water & Power (DWP) issues a preliminary runoff forecast in February, March, and April, based on snow surveys on the first of the month. This month’s snow surveys were 25% of average in the Mono Basin, and so it is not too surprising that the April–September runoff forecast is also 25%. The April to July forecast of 23% is looking somewhat worse than runoff during that period for the rest of the state.
The forecast for the 2014 runoff year (April 1, 2014–March 31, 2015) is 33% of average. That is the number we watch on April 1st and after the final May 1st runoff forecast, because it determines the flow requirements in the streams for the remainder of the runoff year. To get out of the “Dry” year category, the forecast needs to exceed 68.5% of average runoff.
The February 1st forecast is a very early indication of how much runoff to expect, and by April 1st things can be very different. For example, last year the February 1st forecast was 92% of average, with a reasonable minimum of 70%, but by April 1st DWP was only forecasting 66% of average runoff. The April 1st forecast for just the months April–September 2013 was 62%. The forecasts assume median precipitation will fall, and 2013 was so dry, April–September runoff turned out to be only 55% of average. After March 31st, we will be able to tally the runoff for the entire 2013 runoff year, but it will probably end up around 53%. 53% was last April’s forecast reasonable minimum, or what to expect if precipitation had been less than 9 out of 10 years.
The range around this February’s 33% forecast for the runoff year is 20–46%. This means it could be as dry as 20% of average if we get only the precipitation we see in the driest 1 out of 10 years—or as wet as 46% of average if we get more than the precipitation than we see in 9 out of 10 years. Either outcome is unlikely. There is an 80% chance of ending up somewhere in between. The top end of that range is about where 1976, 1977, and 2007 runoff ended up—our three driest years on record. It is unlikely to be even this wet.
I repeat: 2014 is unlikely to be as wet as our driest recorded runoff year. We should expect to set a new record.
So it appears we are likely to add a new data point to our Mono Basin experience, somewhere between 20% of average and the driest year we’ve seen so far. Most likely around 33% of average. And for April–September, the expected range is even worse: 13–37%. We will get an update in March, and again in April, with the final forecast in May. Let’s hope that like last year, we beat the odds on the February 1st forecast—but this time in the wet direction.