Weather West: Much Warmer and Drier Second Half of May

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

By Daniel Swain of Weather West

Recent conditions: mercifully cool and damp in NorCal, but severe drought is intensifying once again

The past 30 days have brought a mix of above (North Coast) and below average (South Coast) precipitation to California, but many places have been near average. (climatetoolbox.org)

It has been a truly bizarre Water Year thus far in California. With all-time record wet conditions in October in Norcal, followed by very snowy conditions in the Sierra during December, then all-time record dry and warm conditions during the height of the erstwhile “wet season” from January though March throughout much of the state, just about everything has occurred out of seasonal order and in rather bewildering sequence. Most recently, NorCal actually witnessed a 30 day period with near to slightly above average precipitation (!), but perhaps more impressively a very anomalously cool and windy period as well. In fact, some places along the North Coast saw their coolest April and early May period in at least 40 years! We don’t see weather like that too often these days.

Perhaps most notably, the past 30 days have been substantially cooler than average across most of NorCal. In fact, this was the coolest such period on record for parts of the North Coast! Meanwhile, all-time record spring heat occurred throughout the interior Southwest–fueling severe wildfires. (climatetoolbox.org)

Despite the winds, this precipitation combined with cool temperatures helped to (temporarily) defuse what is otherwise looking like a severe fire season in northern California. And it did, briefly, also help to stem the dramatic snowpack losses that have been occurring since December. But that effect has not persisted, since today’s measurements suggest statewide snowpack is already down to only 17% of average (and only 7% of average in the southern Sierra). This is consistent with multiple overall indicators suggesting that despite the recent precipitation and coolness, California’s drought has actually intensified overall since December.

Persistently warmer and drier weather on the way for second half of May

On Thursday and Friday this week, an “inside slider” type disturbance will bring strong northerly winds and very dry air to NorCal–rapidly increasing fire risk.

The relative dampness and coolness of recent weeks does appear to be largely in the rear view window, especially across inland areas. Conditions this week will remain on the reasonably mild side before warming up considerably next week as a subtropical ridge builds across the northeastern Pacific, with the season’s first triple digits expected across inland valley areas (and some model members are suggesting the potential for a more intense heatwave, but that isn’t the most likely outcome at this time).

Before that pronounced warming trend next, though, an Inside Slider-type system will bring a ~48 hour period of strong & gusty north winds to NorCal, along with a very dry airmass. This will dry out drought-stressed vegetation in a hurry, and will lead to a fairly sudden and dramatic increase in wildfire risk.

At the moment, warmer than average conditions appear likely to persist through the end of May, and most places in CA will remain completely dry during that period. The low/zero precipitation is really not all that unusual for the time of year, though conditions will probably be anomalously warm.

Thoughts about the summer to come: another year of extreme inland heat but relatively mild coast?

Now that summer is rapidly approaching, I thought I’d offer a few thoughts. First of all, it is now very clear that La Niña conditions are going to persist across the eastern tropical Pacific Ocean through the summer, and most likely into the autumn. Historically, this would have increased hopes of a slightly cooler than average summer, but long-term warming has wiped out that previous relationship. There is, however, for this reason a stronger signal that Arizona and New Mexico might see a decent monsoon this year–which would perhaps offer some modest mid-summer drought relief there. Then there is the legacy of the expanding and intensifying multi-decadal “megadrought” over the continental interior of the American West–bringing extremely dry antecedent soil conditions and low mountain snowpacks to start the season.