Weather West: Drought Continues to Intensify, Marine Layer Offers Temporary Relief

The following is an excerpt from a recent article on the Weather West Blog. 

By Daniel Swain of Weather West

California descends deeper into drought; “Drought Emergency” declared in NorCal

Root zone soil moisture at or near record lows across a broad portion of California, especially across the north and interior. #CAwx #CAdrought pic.twitter.com/iYSLeRnWJs

— Daniel Swain (@Weather_West) May 4, 2021

Well, unfortunately, the overall drought trajectory has not changed since the last post: it’s still getting worse. And pretty rapidly worse, at that, across much of NorCal. There, virtually no precipitation at all has fallen in places that are usually still pretty wet in April/May, and unusually persistent/recurrent north/northeast “blow dryer” winds have resulted in even more evaporation and snowmelt across the landscape.

The statewide Sierra Nevada snowpack is now down to an absolutely abysmal 8% of average for the calendar date–from 15% 5 days ago–and at this rate could be completely gone in just another week or so. Additionally, because soils underneath the snowpack have been so parched, and the air above the snowpack so dry, the majority of the snowpack we *did* have this year has either soaked into the soil (without runoff) or even sublimated directly back into the atmosphere. This means that streamflows are even lower than would otherwise have been expected in this very low precipitation year, and reservoirs will see very little further recharge for the rest of spring and summer.

Update: six days later, statewide snowpack is down to 8% of avg for date. Rapid melting & sublimation continues amid an unusually early spate of critical fire weather conditions (driven by record-dry vegetation for the date, anomalous warmth, and offshore winds). #CAwx #CAfire https://t.co/e93foytZbe

— Daniel Swain (@Weather_West) May 10, 2021

A formal drought emergency (which is, as I’ve previously mentioned, is at least as much a political designation as a scientific one) was declared for Sonoma and Mendocino Counties a couple of weeks ago, and was apparently dramatically expanded across much of NorCal earlier today. Some smaller water districts are now starting to issue urban conservation mandates. Notably, the Marin Municipal Water District, which lacks broader inter-ties to statewide water systems, is currently discussing placing an emergency pipeline across the Richmond-San Rafael bridge (as last happened in 1976-1977, but not during the 2012-2016 drought) if conditions do not improve by next winter.

Warm with elevated fire weather for a few more days

Warm temperatures and near-critical fire weather conditions will continue for a couple more days in NorCal before tapering off.

Recent days have brought much warmer than average temperatures to most of CA, and a very early spate of critical fire weather conditions in some areas across the northern portion of the state. These conditions will continue for a few more days, but slowly taper off–and cooler temperatures will likely arrive by the coming weekend (especially near the coast, where a pretty respectable marine layer will return.

Some short-term relief: Cooler weather statewide & robust coastal marine layer

Much cooler temperatures–even *cooler than average* temperatures, near the coast!–will develop by mid-May and persist toward the end of the month.

The good news I do have to report at the moment is essentially all in the short term. Once temperatures cool significantly later this week, they will likely remain relatively low for a solid 7-10 day period thereafter as some late season “troughiness” occurs near the West Coast. A couple of weak cut-off lows may even make it as far west as the Sierra Nevada or the Central Valley. Given the time of year and that they will be very moisture-starved, I would not expect much precipitation with these weak lows. But some model solutions are hinting at the possibility of isolated/scattered rain showers/thunderstorms across the CA interior (mainly Sierra Nevada, but perhaps also a few spots in the Central Valley if there is any significant wrap-around convection). This could happen on a couple of occasions over the next ~2 weeks. Most places, however, will probably not see any rain at all–the primary effect will to bring much cooler (and even locally below average) temperatures.

A secondary effect, thanks to the induced persistent northwesterly flow along the coast, will be an increase in the strength and persistence of the marine layer. This means lots of coastal fog and low cloudiness over the next couple of weeks–perhaps occasionally accompanied by accumulating coastal drizzle in both NorCal and SoCal. It won’t help with the drought, but it will very likely (if temporarily) tamp down the very early season wildfire risk in the coastal forests.

Ocean temperatures remain pretty chilly along the immediate CA coast despite a very large blob of anomalous warm water across most of the North Pacific.