First thing’s first, let’s recap last week.
There were more lightning strikes in Southern California to Central California last Wednesday than our August 2020 lighting siege event.
The map below is pretty incredible. If not for the slower movement to these storms versus the August 2020 event, plus quite a bit more rain with these storms in Southern California offset fire starts for now, but those may still yet be smoldering, and once the wind and heat get going into July we may still see fires pop up from this origin later.
So we have a slightly similar issue to watch in the Wednesday through Friday timeframe with a few notable differences.
Tropical storm Celia is weakening and that moisture will likely surge toward Southern California, but winds aloft are more west-southwest – which tends to keep the action east of us, versus the south-southeast wind we saw last week that brought the storms nearly up to the Santa Cruz Mountains and near Hollister.
So as of now, aside from a few high clouds, we aren’t expecting lightning issues from this. But cooling changes are coming – highs that have been in the 80s to 90s in the inland valley should drop to the 70s by Saturday through Monday.
Which on face value – cooler temps, onshore wind, higher humidity, should be good for fire danger, right?
But in a nutshell, cool temps, higher relative humidity percentage, while better than hot temps and low relative humidity percentage, increasing wind from any direction now is a concern due to the late summer, early fall fuel moisture conditions.
We’ve reached that breaking point and I think you’ve noticed the uptick in dozens of spot grass and brush fires.
Looking at the “ready to burn” and how fast a fire can grow due to weather and fuels’ ERC index we have the following:
Essentially – highest fire danger relative to time of year is actually in the inner Bay Area, followed by the Diablo Range and East Bay hills and Santa Cruz Mountains, then North Bay.
All above and near max dry values for late June.
These conditions now put us in an area where any gusty wind, sea breeze driven or its dry and warm counterpart offshore winds are a major fire concern.
As we saw in Orange County with the Coastal Fire, it was the afternoon sea breeze that powered that fire up Aliso Canyon to the homes on the ridge. We’re in that scenario now.
Note July climatology: Don’t expect much help. However, pinning a giant “stay tuned” to 4th of July for possible showers into interior Northern California.
Now, the showers are welcome, but the potential windy weather over the mountains that could come with it may not be as much … as we like to say, stay tuned!
Below average valley temps with cool and breezy conditions near the coast and inner bay should continue through the weekend and 4th of July Monday.
If the chilly coastal temps and mild valley highs are making you think summer has left the Bay Area, don’t worry the long range outlook brings the heat back in about another week!
Source: NBC Bay Area