Daniel Swain Profile picture
Dec 29 8 tweets 6 min read
Some thoughts on impacts from ongoing and likely prolonged very wet spell in NorCal. (Short thread). TL;DR: widespread heavy precipitation is likely, with mostly minor flood issues becoming more widespread this weekend & *possibly* more significant next week. [1/n] #CAwx #CAwater
Lighter precipitation continues today, but the next significant storm will be this weekend. This will come in the form of a moderately strong low pressure system coupled with a robust, warm, and relatively slow-moving atmospheric river. #CAwx #CAwater [2/n] Image
This New Year's storm will bring heavy rainfall to essentially all of NorCal, and widespread precipitation to SoCal as well. This storm will be quite warm, too, with very high freezing levels initially bringing rain even at 7,000 feet. #CAwx #CAwater [3/n] Precipitation map from ECMW...
Flood Watches are in effect across much of NorCal for this system. At this time, it still appears that any flooding from weekend storm would be mostly minor and of the urban/small stream variety, but will likely be more widespread than with the last storm. #CAwx [4/n]
However, the New Year's storm won't be the last in the sequence. In fact, a veritable parade of Pacific storms may continue to affect most or all of California for the next 10+ days. Multi-model ensembles agree on potential for large 10-14 day precip accumulations. #CAwx [5/n] Map showing ECMWF ensemble ...
For example: here is a visual depiction of presently projected range of precipitation trajectories over the next 16 days for Sacramento. Mean is ~9.5 inches (quite a lot for Sac!), but range is ~6 to ~18 (!) inches over that period (a three-fold difference)! #CAwx #CAwater [6/n] Image
This is why assessing impacts over the next couple of weeks is tricky. Flood impacts from the low end of ensemble range would be minor/unremarkable; impacts from the upper end would be quite high. The median? Probably widespread minor to locally moderate flooding. #CAwx #CAwater
This is a great pattern for drought relief. It's likely being caused by some unanticipated strong sub-seasonal scale forcing (certainly isn't a La Nina pattern!) that will likely dissipate by mid-Jan. But we will see a whole lot of precip between now & then!#CAwx #CAwater [8/end]

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More from @Weather_West

Dec 20
Still not seeing many images of damage, but based on anecdotes from folks in quake zone it does sound like there was damage to some structures & especially to infrastructure. I suspect that ongoing widespread regional power outages are reason we haven't heard more yet.#earthquake
This points to an emerging paradox in media reporting on disasters that's become really apparent in recent years: the more severe the damage, the less news initially makes it out (because power is out and telecoms are down) and the less coverage they receive.
In our hyper-connected information era, it often feels like there's a presumption that if something was a big deal, it must be prominent on social media (& if it's not, it's not a big deal). But that assumption breaks down if people can't get online in the worst-affected areas...
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Dec 20
To @JFrancisClimate's point, there has been some interesting recent work suggesting that amplified Arctic warming may be causing the *stratospheric* polar vortex to become disrupted more often in winter. But this is still the topic of very active research. science.org/doi/10.1126/sc…
And there's further evidence that Arctic warming may be associated w/jet stream patterns favorable for warm season weather extremes. So my view is that there is something going on! It just doesn't appear to be manifesting as colder winter extremes. science.org/doi/10.1126/sc…
BTW, my personal/professional view, essentially informed speculation, is actually that we're going to eventually confirm that Arctic amplification/sea ice/snow cover/high lat ocean temperature forcings are quite important in certain regions/seasons (including California!).
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Dec 19
The upcoming Arctic outbreak across Central U.S. is going to be the "real deal". Temperatures will likely rival anything seen in 30 years (or even longer!) across portions of the Central Plains. In a warming climate, that's saying quite a lot. #COwx #NEwx #WYwx #KSwx (1/6) Map depicting predicted tem...
Temperatures will fall, in incredibly dramatic fashion ( of the course of just a few hours) by 40 or more degrees in some areas as a powerful Arctic front pushes through. Temperatures will plummet well below zero (-20 to -30F) across a broad region, w/even lower windchills. (2/6) Map depicting predicted dep...
This level of cold will be potentially dangerous even in places accustomed to pretty low temperatures in winter, since even those spots haven't seen temperatures this low in 30 or more years. This will be especially true across portions of Wyoming and Colorado. #WYwx #COwx (3/6) Map depicting the predicted...
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Oct 7
Latest seasonal predictions from NMME/IMME ensembles out today. Overall North Pacific pattern still looks very #LaNina-like: unusually wet PacNW and BC; unusually dry in SoCal & Lower Colorado Basin due to persistent NE Pacific ridging. HOWEVER... #WAwx #CAwx #AZwx (1/4) Maps of seasonal prediction...Image
...However, exact position of N. Pac. ridge is key. Too far east, & CA stays dry, but far enough west & Sierra benefits from cold storms diving south. High confidence in winter ridge, but CA will be on razor's edge--exact position will dictate dry vs wet overall. (2/4)#CAwx
This is why seasonal prediction is hard. It's quite likely models are correct about strong, anomalous North Pacific ridge signal (mainly due to #LaNina). But that doesn't directly translate to CA precip--it only offers a modest tilt in odds toward dry winter. #CAwx #CAwater (3/4)
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Sep 4
The coming week is shaping up to be a pretty crazy weather week in California, TBH--even more so than previously thought. Ongoing heatwave expected to be longer & peak even higher, & now we have potential influence of a soon-to develop hurricane to consider. (1/n) #CAwx #CAfire Map of predicted 500mb geopotential heights over the Western
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Interior Northern California is now heading for a truly dangerous, searing heatwave. All-time September records are now all but guaranteed in the Central Valley (on multiple consecutive days!), and all-time (any month) records now appear well within reach. #CAwx #CAfire
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Aug 12
New work co-led by @xingyhuang and me on the rising risk of a California #megaflood due to #ClimateChange is out today in @ScienceAdvances! This paper also describes the new #ARkStorm2 scenarios in detail, & will be the basis for ongoing work. (Thread:1/n) science.org/doi/10.1126/sc…
For context: the work and findings presented here represent the first phase of the broader #ARkStorm2 project, a multi-year, cross-institutional effort involving multiple @UofCalifornia campuses, @DRIScience, @USGS, @CA_DWR, & @NCAR_Science. (2/n) science.org/doi/10.1126/sc…
In this work, we develop a pair of plausible extreme, month-long winter storm sequences in California--one from the recent historical climate, and one from a much warmer future climate. These sequences involve a multi-week series of successive #AtmosphericRiver storms. (3/n)
Read 21 tweets

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