LA Rain: Time to Push the Panic Button?

The Thomas Fire is still burning in Ventura and Santa Barbara Counties, while the Creek Fire, Skirball Fire, and Rye Fire recently contained in LA County. There has been almost no rain this fall in southern California, and indeed in what feels like a troubling case of déjà vu, the whole southwest has had a below average water year so far.

WY precip

Like many Californians staring at week after week of dry weather forecasts, I’m worried about the state slipping back into drought, so I decided to look at the history of dry fall weather in LA. The overall picture is not particularly inspiring, but there is some good news.

Through December 17, LA has 0.11” of rain for this water year (October 2017 to September 2018). In LA, since 1877, there have been two water years with zero rainfall for the October-December period: 1929-1930 and 1903-1904. One other year, 1958-1959, had only 0.06”. All of these years ended up being below average, though 1929-1930 ended with 11.21”, about the 32nd percentile in the distribution. 1958-1959 ended with 5.13”, the 4th driest year on record for LA and drier than any of the recent drought years.

This year looks dry through the fairly reliable 7-day forecast period leading up to Christmas. After that the longer range forecasts bring some rain to SoCal, so maybe we’ll end October-December a little bit better off than we are right now.

GFS-10dayCanadian-10day

Fall precipitation is an important part of rainfall for southern California, but dry periods are not unknown. Of the 140 water years on record, 41 have had less than 2” of rain in October-December. Of those 41 water years, 4 still finished the year above the long term average of 14.77” and 8 finished the year above the long term median of 12.94”. So based on straight historical probabilities, there’s still about a 20% chance of finishing at median or above. The distribution of fall precipitation is even more extreme than the overall distribution of LA precipitation, itself one of the world’s most variable.

OD dist

The good news, such as it is, is that historically fall precipitation has literally zero bearing on what happens in the rest of the water year. If you plot October-December precipitation against the remainder of the water year, there’s no correlation; a linear trendline yields an R-squared value about as close to zero as can be.

OD vs RWY

However, for the yearly total, missing out on fall rain puts SoCal in a hole it’s just difficult to get out of.

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