With March in the rearview window and May grey on the horizon, let’s take another look at how SoCal fared this winter with rainfall. The short answer is… not good.
March brought a modest storm to Los Angeles, finally bumping us over 4” of rain for the season. With 4.60”, water year (WY) 2017-18 now stands as the third driest year recorded in LA since 1877. A storm forecast for this weekend might, at the high end, get us to 5” and fourth driest. Either way, barring a truly unusual storm this spring or tropical storm remnants this summer, this year will fall around the 2nd percentile of water years, becoming just the 8th year on record to record less than 6” of rain.
Looking at multi-year trends, the last 7 water years just crushed the record low for a 7 year period, with 62.46” to the previous low of 68.75” (1958-65). The record low 6 year period of 1958-64 with 53.25” holds the record by a hair, with the current 6 year period at 53.76”.
Despite the severity of recent drought, it’s important not to read too much about long-term trends into this data. Looking at 10, 20, 30, and 50 year periods shows that recent decades have been on the dry side, but not unknown in LA’s past.
One additional metric I think might be interesting is “streakiness” of precipitation in LA – that is, do we tend to get a lot of dry years clustered together and a lot of wet years clustered together? Here’s a plot of the “streak” for each water year, with positive values representing above average and negative below average. So, a value of 2 means that year is year 2 in a streak of above-average precipitation; a value of -3 means that year is year 3 in a streak of below-average precipitation.
There are more below average years than above average years, a fact also represented in the histogram above that shows rainfall distribution is skewed by a small number of very wet years. There was one streak of 7 below average years, and a further three streaks of 5 below average years. The longest above average streak was in the mid-1960s, with 5 above average years. Four other periods have had 3 above average years. Again, LA rainfall is highly variable, and you cannot use this historic data to predict that we are “due” for a wet year or anything like that.
Looking at SoCal a little more broadly, the Central Coast has fared a little better than LA, but other places have done worse. Parts of coastal Orange County have had less than 25% of normal precipitation and it wouldn’t be surprising if some stations there record all-time lows.
The good news, such as it is, is that northern California received some beneficial storms in March that moved the year from “disaster” like we have in SoCal to just “bad”. The northern and central Sierra are at 65% of average for the whole water year and about 75% for year to date. The south Sierra is at about 52% and 62% respectively. An unusually strong April storm is poised to bring more rain and snow to the Sierra as well.
Finally, thanks to last year, major state reservoirs are still riding high with plenty of water for the summer.
Unfortunately, there isn’t really any hope left for SoCal this year. Let’s just write this year off, move on, and hope for a better winter next year.