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Daniel Swain Profile picture
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13 Oct, 5 tweets, 4 min read
California, on statewide basis, is now experiencing its worst drought in observational record going back to late 1800s--narrowly beating out peak of last drought in 2014-15 (as measured by PDSI, a metric that takes into account both precip & temperature). #CAwx #CAfire #CAwater
There is a clear trend toward increasing aridity in California--and yet little trend in mean precipitation. How can this be? A very strong warming trend due to #ClimateChange means same the amount of water falling from sky just doesn't go as far as it used to. #CAwx #CAwater
We explored this phenomena in research published in 2015 (finding that rising temps are the main factor behind increasing CA drought severity):
pnas.org/content/112/13…

and 2018 (increasing "precip whiplash" despite little change in mean):
nature.com/articles/s4155…
#CAwater #CAwx
*phenomenon.
Another even clearer way to look at this with slightly less noisy data: the 1-month September 2021 PDSI value was far lower than any previous value since at least the late 1800s. #CAdrought #CAwater #CAwx

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More from @Weather_West

Daniel Swain Profile picture
16 Jul
Dry lightning event of *some* magnitude is now looking increasingly likely across *some portions* of CA on Sun/Mon. Details to come. But first, some thoughts on potential wildfire risks posed if this comes to fruition. (Thread) #CAwx #CAfire
Usually, fewer than 1 in 10 lightning strikes actually ignites a wildland fire. These numbers can be higher if lightning is not accompanied by precip, or if lightning occurs under unusually dry conditions in dense vegetation. But in general, lightning ignition *rate* is low-ish.
A big problem thus arises when there are a very large number of dry lightning strikes. This occurred during the extraordinary and historic Aug 2020 event in Northern CA, where 10,000+ strikes were observed (and subsequently *hundreds* of fires were ignited).
Read 12 tweets
Daniel Swain Profile picture
18 Jun
Posting as heads-up to weather modeling world (not as a realistic prediction of future conditions): the (new) GFS has occasionally been spitting out completely absurd surface temperatures for CA's Central Valley. 18z run today shows 128-130F. Anyone know what's up? Image
An intense and very possibly record-breaking heatwave is indeed possible in that interval, but not "hottest temperature in modern history" hot. I'm assuming it's the surface scheme, but then again, the 850mb temps are absurdly high also (though maybe feedback from sfc?). Image
Seems as if the ECMWF is now doing something similar. Either both models share some kind of similar surface scheme bias/error that involves sporadic positive/self-reinforcing surface temp feedback during extreme events (still by far most likely), or...hmm.
Read 4 tweets
Daniel Swain Profile picture
7 Jun
Correction to give more accurate context: vegetation dryness & flammability metrics (1000hr fuel moisture & ERC, respectively) are indeed exceeding record levels for *calendar* date over most of Sierra Nevada, but *not* records for *any date.* (Phew!) (1/4) #CAwx #CAfire
For those interested, the confusion apparently arose due to a differing period of record for NorCal vs. SoCal data via the NorthOps/SouthOps GACCs. In SoCal, period of record is only ~10 years, so "any date" records less meaningful. So: data not wrong, but context is missing.
This really doesn't change the overall picture: there's still an exceptionally severe drought across all of Northern California and vegetation is still exceptionally dry--even relative to extreme values of recent years. But it's important to get the details right! (3/4)
Read 4 tweets
Daniel Swain Profile picture
27 May
@MichaelWWara @jtemple As I've been emphasizing recently, the *predictable* aspects of Fire Season 2021 (soil moisture/vegetation dryness) are as bad or worse as any observed historically. The level of landscape flammability--especially in denser brush & forests--is genuinely scary. BUT... (1/2)
@MichaelWWara @jtemple ...A big part of what makes certain fire seasons exceptionally severe (from lives lost, homes burned, & ecosystems damaged perspective) has substantial random component. Do ignitions primarily occur during extreme fire wx? Do we see many heatwaves/wind/dry lightning events? (2/3)
@MichaelWWara @jtemple Latter aspects are largely not predictable in advance (except perhaps frequency of heatwaves--which are indeed expected to be elevated). In 2020, we were very unlucky w/all the unpredictable elements. I hope we won't be in 2021...but that's not a good management plan! (3/3)
Read 4 tweets
Daniel Swain Profile picture
2 Mar
My perspective piece, "A shorter, sharper rainy season amplifies California wildfire risk," is now out in GRL. I discuss recent findings pointing toward shortening & sharpening wet season, & implications for ecology/wildfire. (1/17) #CAwx #CAfire #CAwater agupubs.onlinelibrary.wiley.com/doi/abs/10.102…
This perspective is in response to a recent analysis led by Jelena Luković showing that seasonal onset of CA precipitation has become progressively delayed (by ~1 month) in recent decades, w/ shorter but sharper rainy season. Underlying paper: agupubs.onlinelibrary.wiley.com/doi/full/10.10… (2)
Record heat, plus late arrival of seasonal rains, have played a key role in CA's extremely severe wildfire seasons in recent years. Autumn 2020 exemplified this trend: vegetation conditions were, by a wide margin, the most flammable on record.#CAfire agupubs.onlinelibrary.wiley.com/doi/abs/10.102… (3)
Read 18 tweets
Daniel Swain Profile picture
1 Mar
March 1st update: 2020-2021 "wet season" in California remains dismally dry in most places. In fact, wide swaths of both NorCal & SoCal are well under 50% of average precipitation. It has also been a warmer than average winter overall, despite some cold interludes. #CAwx #CAwater
Statewide average snowpack has quickly fallen from late January highs (around 70-75% of average for the date) to around 61% of average for the date as of Mar 1. #CAwx #CAwater
Outside of a brief period of possible showers across coastal SoCal on Wednesday, the next ~5 days still look very dry across most of CA. #CAwx
Read 5 tweets

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