Even though rainfall in the Los Angeles area is way below average this rain season, some plants along our local trails — such as this poison oak — are flourishing.
The growth of plants is dependent on a mind-boggling mix of interrelated factors. Maybe it was the rain in January in combination with the unseasonably warm weather in January and February. Or maybe there was some carryover in vitality from last year’s wet rain season. Or maybe it was something else. Whatever the case, poison oak in the Santa Monica Mountains seems to be doing very well this year!
The title photo is of poison oak along the Rogers Road segment of the Backbone Trail. It was taken March 10, 2018. Watch for overhanging branches!
Once again Southern California is facing another very dry rain year. Since July 1, Downtown Los Angeles (USC) has recorded only 1.97 inches of rain. This is more than 8.5 inches below normal.
At this point it appears likely the rainfall recorded at Los Angeles from July 1 – February 28 will be the second driest for that period on record. If we don’t see some significant rain in March, we could be contending with 2006-2007 for the driest rain year on record.
Several weather models have been advertising a change to a wetter weather pattern for the West Coast and Southern California. At one point the ECMWF was forecasting several inches of rain in the Los Angeles area around March 1-2. This morning’s ECMWF run was far more stingy with the wet stuff, and precipitation completely disappeared from the GFS forecast for that period.
Never fear, these forecasts will likely change again. Model skill more than a few days out is very poor. Next week we should have a better idea if the pattern change is real, or just more model hype.
Update March 1, 2018. With only 1.99 inch of rain from July 1 to February 28, Downtown Los Angeles (USC) did end February with the second driest rain year to date. Depending on whether 1911-1912 is included in the ranking, the water year to-date, beginning October 1, is either the second or third most dry on record. Now all eyes turn to the storm that is forecast to move into the Los Angeles area this evening. This morning the CNRFC 72 hr. QPF for the Los Angeles area ranges from around 0.75 – 1.0 inch in the basin and valleys to around 1.5 – 1.75 inches in the mountains. Higher totals are forecast in the Ventura and Santa Barbara areas. Check with the NWS Forecast Office Los Angeles for the latest weather forecasts, advisories and warnings.
The valley oak pictured above — one of the larger oaks along Rocky Peak fire road — toppled over in the summer of 2016 following five years of drought.
Fire and drought are a natural part of the valley oak’s habitat and the trees have evolved to withstand ordinary variations in their environment. However, severe fires or extended droughts, or fire in combination with drought can overcome the tree’s defenses.
The drought may have been the culminating factor in the felling of this oak, but fire and other factors may have also played a role.
According to the Fire Effects Information System (FEIS), the heart-rot fungus Armillaria mellea is usually present in valley oaks and larger oaks tend to be hollow or rotten in the center. The toppled oak was hollow near its base and its interior appears to have been blackened by fire. The FEIS describes instances where the decaying wood in the interior of older valley oaks could ignite in a fire, but leave the exterior bark uncharred.
Assuming Downtown Los Angeles (USC) doesn’t get more than 0.02 inch of rain before the end of the year, the first 6 months of the 2017-18 Rain Year will be the second driest since recordkeeping began in July 1877. Los Angeles has recorded a paltry 0.19 inch of rain since July 1. Only 1962 recorded less rainfall over the six month period. November and December have been particularly dry, with only 0.01 inch being recorded at Los Angeles during each of these months.
What has happened in the past when there has been such a slow start to the July 1 – June 30 Rain Year?
If we look at the 10 driest July-Decembers in Los Angeles, the average Rain Year rainfall for those years is only 9.4 inches, or about 63% of normal. And in all 10 years, the Rain Year rainfall turned out to be below normal. Even if we take the 20 driest July-Decembers the Rain Year average rainfall is about the same — 9.6 inches — and only 2 of the 20 years had above average rainfall.
So, historically, when the first six months of the Rain Year have been very dry, the amount of rain for the entire Rain Year has almost always been below average. We’ll see if that’s the case this time!
The groundwater in upper Las Virgenes Canyon appears to have been replenished by the above normal rainfall last rain season.
The little spring pictured above has persisted through the dry season and farther up the canyon a tiny stream has trickled defiantly through the Summer. The mainstem creek in upper Las Virgenes Canyon isn’t flowing as it was during the Winter, but the sand at the crossing near the Cheeseboro connector trail remains damp.
It shouldn’t take a huge amount of rain to get the creek flowing again. We’ll see!