Courtesy of Doctor Daniel Swain at weatherwest.com
As previously noted, this autumn looks like an especially warm one across the interior Western U.S. and likely extending increasingly toward the coast in CA (especially central and southern CA) Sep through Oct or Nov. The latest batch of seasonal model outlooks came out a few days ago, and they continue to paint a similar picture: very high odds of a broadly much warmer than average airmass over the Southwest for autumn, and also a modest tilt in the odds toward a drier-than-average autumn (especially Sep and Oct and especially in central and southern CA as well as the central/southern Rockies). Unless there is another “saved by the bell” moment (like last year’s tropical soaking from Hilary or a very early season atmospheric river up north), this likely portends a very active autumn fire season across much of CA and the interior West–potentially leading to a second peak in fire activity that could eclipse July’s wildfire surge despite conditions that will likely not be quite as hot.
Why is this the case? Well, vegetation in most of California and across much of the interior West (outside of the core monsoon zone in the deep SW) reaches peak seasonal dryness in September or October following the canonically long and dry summer (which, in many spots, has been record hot to date). That, combined with a spurt of grass and brush growth thanks to milder and wetter growing conditions the past 2 winters and springs, means there’s both more fuel for the (potential) fires at lower elevations as well as the potential for vegetation to be even drier than usual (perhaps approaching record dryness levels again) in precisely those areas most susceptible to autumn heatwaves and offshore (Santa Ana/Diablo) wind events (and therefore wind-driven fires). That could be a volatile combination this year, and updated seasonal outlooks from federal fire agencies mirror this concern.
We don’t have any ability to predict offshore wind events at seasonal scale, nor (beneficial) atypical rain events or (harmful) autumn lightning outbreaks. But we can, to a reasonable degree of certainly, predict how anomalously warm the autumn may be–and we also know with 100% certainty what came before (i.e., a wet winter/spring and record hot July). The stage is therefore set for a potentially fiery autumn–perhaps much more so than “usual”–but we could get lucky (or unlucky) with early rains, offshore wind events, or fall lightning events. That is to say: while the predictable aspects of autumn fire risk are ringing alarm bells, that isn’t always the whole story (based, essentially, on luck in one direction or another). But I do think it’s quite likely that this autumn will feature much more active fire conditions not only in California (where this is not uncommon) but also across portions of the interior West (where it is decidedly less common)–and almost certainly more active conditions than the past couple of (relatively quiet) years. So…enjoy the relative reprieve over the next 10 days or so, and stay tuned!