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Dr. Daniel Swain Profile picture
@Weather_West
Aug 24, 2020 • 3 tweets • 3 min read • Read on X
Finally some good news to report: last night's dry lightning event was less widespread & intense than earlier feared. Nearly all lightning spared Bay Area, though there were strikes in Central Valley & western Sierra foothills that may have sparked new fires. (1/3) #CAwx #CAfire Image
What happened? Well, there was *plenty* of elevated convection just above everywhere, but it wasn't *quite* deep enough to generate much lightning. Models were slightly off with timing, and may have underestimated smoke effect, and that made all the difference.#CAwx #CAFire (2/3)
Also: we still aren't totally out of the woods in NorCal. Dry thunderstorms are still expected today, mainly in northern 1/3 of state but possibly clipping North Bay. These could yet spark new fires. But all in all, a better than expected outcome. #CAwx #CAfire (3/3) Image

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More from @Weather_West

Dr. Daniel Swain Profile picture
Feb 21
A period of very warm mid-winter conditions appear likely for much of California and portions of the interior West in late Feb. A strong ridge will form during this period, with large-scale subsidence & warm air advection allowing for a genuine winter "heat wave" next week. #CAwx Snapshot from ECMWF ensemble for next week, which depicts a narrow-ish but strong ridge centered along the West Coast that will bring a period of mild weather/anomalous winter warmth.
Temps in 80s will be widespread in SoCal, & even 90s are possible. T-shirt and shorts weather will also extend into NorCal and beyond (well into 70s), as well as desert SW (where late spring-like temps in 90s are likely). Late Feb records may be broken in some places. #CAwx #AZwx Temperatures will be far above average, and in some cases record warm for late February, across portions of the West Coast next week. This ECMWF ensemble snapshot of temperature anomalies captures that, highlighting southern CA and the Desert SW as particularly anomalous hotspots.
Looking ahead to the end of winter and spring, the California/U.S. West precipitation dipole (i.e., unusually wet in the north and unusually dry in the south) appears likely to continue. The dryness may expand to include more of CA/West Coast later in spring, too. #CAwx #CAwater Spring precipitation prediction for the United States from the C3S superensemble. It suggests a relatively high likelihood of drier than usual conditions across the interior SW (lower Colorado River basin) as well as southern California, with an increased chance of above-average precipitation in the Pacific Northwest.
Read 5 tweets
Dr. Daniel Swain Profile picture
Feb 7
Out today in @GlobalChangeBio is our (brief!) rapid-response piece on the broader context surrounding the January 2025 Southern California wildfires & relevance of wet-to-dry hydroclimate whiplash to fire both locally and globally on a warming Earth. [1/8] onlinelibrary.wiley.com/doi/full/10.11…
@GlobalChangeBio Coastal Southern California experienced an exceptional wet-to-dry "hydroclimate whiplash event" between May/June 2024 (following a second anomalously wet winter) and Dec/Jan 2024/5 (characterized by the driest start to season on record in many parts of the region). [2/8] Figure panel from Swain e al. 2025. The relevant portion of the caption reads: Map depicting the geographic scope and magnitude of the June 2024–December 2024 wet-to-dry hydroclimate whiplash event in southern California as measured using 6-month Standardized Precipitation and Evaporation Index (SPEI6) differences. The black rectangle region encompasses the region that experienced the most extreme fire weather conditions on January 7-8 2025, and the white open star depicts the approximate location of Los Angeles, CA.
@GlobalChangeBio This whiplash event caused a pronounced vegetation accumulation-then-desiccation cycle, adding more fuel for (potential) fires to come then drying it out to record levels for time of year--setting stage for destructive fires when strong winds arrived.[3/8]
onlinelibrary.wiley.com/doi/full/10.11…
Read 8 tweets
Dr. Daniel Swain Profile picture
Jan 17
After becoming increasingly enmeshed in the wildfire world, you start to notice things about the way we've systematically altered our relationship with the natural environment in a way that has increased the risk of destructive fires. And then you stop being able to unsee them.
I've always lived in the American West, in different parts of California and on the Colorado Front Range. I've traveled throughout the West extensively; even as a child, fire was always there in the distance--a companion on family roadtrips, a backdrop to summer afternoons.
One of my earliest memories (fuzzy though it is; I was very young) was ash falling from the sky while out at a San Francisco park. The date? October 20, 1991--the day the devastating Oakland Hills fire tore through around 3,000 structures and killed 25 in the Berkeley Hills.
Read 23 tweets
Dr. Daniel Swain Profile picture
Jan 11
Our paper on "Hydroclimate volatility on a Warming Earth" is out in Nature Reviews Earth & Environment. We assess existing scientific literature & conduct new analysis--concluding that "#HydroclimateWhiplash" is increasing due to #ClimateChange. [Thread] nature.com/articles/s4301…
"Hydroclimate volatility" refers to rapid transitions between unusually wet & dry conditions relative to local baseline. For quantitative analysis, we define a new "hydroclimate whiplash" metric using Standardized Precipitation & Evaporation Index (SPEI). nature.com/articles/s4301…Figure 1 from Swain et al. 2025. Caption: Location, date and impacts of select hydroclimate whiplash events from 2016 to 2023, and the corresponding magnitude of changes in the Standardized Precipitation Evapotranspiration Index (SPEI; shading). The brown shades represent wet-to-dry events and green shades dry-to-wet events. The events do not represent a comprehensive catalogue of all whiplash events but are illustrative of the breadth and diversity of geographies subject to such rapid transitions. Societally and ecologically consequential hydroclimate whiplash events can occur in virtually...
One key aspect of the framework for understanding such volatility is that it encompasses both "supply" & "demand" side of water balance--that is, not only precipitation (or lack thereof), but also evaporation (or evaporative demand). Precipitation doesn't tell the whole story!
Read 16 tweets
Dr. Daniel Swain Profile picture
Jan 9
Is there a link between #ClimateChange & increasing risk/severity of #wildfire in California--including the still-unfolding disaster? Yes. Is climate change the only factor at play? No, of course not. So what's really going on? [Thread] #CAfire #CAwx #LAfires iopscience.iop.org/article/10.108…
There is rarely, if ever, a singular cause of complex disasters--so don't be fooled by disingenuous "straw man" binaries! For wildfire, encroachment into high risk areas, historical land management, & climate all matter (to contextually varying degrees). theguardian.com/environment/20…
What is the primary link between #ClimateChange and wildfire, both in CA specifically and more broadly globally? Vegetation moisture, and therefore its flammability (which includes propensity to ignition, combustion intensity, rates of spread, and etc.). nature.com/articles/s4155…
Read 23 tweets
Dr. Daniel Swain Profile picture
Jan 1
Apropos of recent, ah, speculation...a few thoughts about #fog.

Fog is, simply, a shallow cloud that extends all the way to the ground. It's usually composed of condensed water droplets--though "ice fog," composed of ice crystals, can occur if it's cold enough. [Thread: 1/10]
#Fog usually forms when air cools to its dew point (i.e., when temperature decreases & relative humidity reaches 100%, causing air to become saturated,) and the air can no longer "hold" additional water vapor, which condenses out as (visible!) liquid droplets. [Thread: 2/10]
There are several processes that can cool air to its dew point & create #fog, including warm, moist air moving over a cool surface ("advection fog"), cold air moving over warm water ("steam fog"), and rapid cooling of the Earth's surface at night ("radiation fog"). [Thread: 3/10]
Read 10 tweets

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