Daniel Swain Profile picture
Jan 2 17 tweets 10 min read
Okay, folks. Starting to look like it's going to be a rough 10+ days from a flood risk perspective in Northern California, with a series of very wet & high-impact storms. Brief thread now; blog post late this PM; YouTube live Q&A Tue. [1/n] #CAwx #CAwater
The next inbound storm looks like it will be quite strong. A rapidly deepening surface low (i.e., meteorological "bomb cyclone") will remain well offshore, but the associated warm and cold fronts will bring widespread heavy rain and strong winds to NorCal later Wed. #CAwx [2/n] Map showing ECMWF model representation of "bomb cyclone
This storm would be fairly notable in its own right, as it will be associated with an unusually well-defined warm *and* cold frontal passages and an exceptionally moist and relatively warm #AtmosphericRiver. #CAwater #CAwx [3/n] Map depicting ECMWF representation of Wednesday storm, inclu
However, Wed storm impacts will be further elevated by fact that soils are now saturated throughout most of NorCal, with some rivers running high (& active major flooding/levee breaks along Cosumnes River in Sac County). Runoff will begin almost immediately. #CAwater [4/n]
Additionally, I am expecting a pretty intense cold frontal passage Wednesday night. So high rainfall *rates* (i.e., 1-6 hrly amounts) will be a greater concern than total precip volume. I would not be surprised to see a NCFR develop, and/or embedded thunderstorms. #CAwx [5/n]
So there could be some significant flash flood/mudslide risk, with impacts greater than "nuisance" roadway/urban flooding--especially along creeks, streams, and smaller rivers. SF Bay Area and central Sierra foothills perhaps at most risk. [6/n] #CAwx #CAwater
Mainstem rivers will rise sharply, and a few may approach/reach minor flood stage, but main concern with this system are smaller watersheds (plus Cosumnes, given ongoing events there). [7/n] #CAwx #CAwater
One possible silver lining: it actually appears there may be a slight gap in the heaviest precip totals exactly along the axis of heaviest precip from the last storm--so that may help dampen impacts there slightly. But other areas will see higher impacts. [8/n] #CAwx #CAwater
In addition to heavy rain/flood risk, strong winds are likely (even down to lower elevations) across most of NorCal. Widespread gusts of 50-60 mph are likely, with some higher. Given saturated soils, fairly widespread power outages and downed trees possible. [9/n] #CAwx #CAwater
The @NWSBayArea is using strong language to describe the potential impacts of the Wednesday storm. I think this is appropriate; this certainly has the potential to be one of the higher impact storms in some years, at least in certain locations. [10/n] #CAwx #CAwater Text in image reads: "To put it simply, this will likel
By late Thursday, NorCal will get a bit of a breather (though light to moderate precipitation will still be possible at times through the weekend). But then...things get potentially even hairier by early next week... [11/n] #CAwx #CAwater
Given what will by then be extremely wet antecedent conditions, I am increasingly concerned regarding what ensembles are suggesting *could* occur next wk. They depict not only continuation of wet pattern, but *potential* for warm, long-duration #AtmosphericRiver. #CAWx #CAwater
It's still very early, and the details are guaranteed to change. The ensembles are suggesting a wide range in possible impacts from "moderate" to "extreme." However, given antecedent hydrologic conditions, this bears close watching. [13/n] #CAwx #CAwater
For example: here is a visual representation of possible precipitation accumulations for a point near Santa Rosa, CA over the next ~16 days from the ECMWF ensemble. The mean ~16 day accumulation is nearly 15 inches, with a range from ~8 to ~25 (!) inches. [14/n] #CAwx #CAwater Graphical representation of precipitation accumulations over
At this time, ensemble average portends a high risk of significant flooding across much of NorCal. Driest ensemble members would produce at least minor flooding; the wettest would likely produce severe flooding. But range of possible outcomes is still wide. #CAwx #CAwater [15/n]
All in all, this has the potential to be a period of very high impact weather and flood risk across Northern California. Southern California will also see some significant precipitation at times, but not nearly as much as NorCal--so my focus is going to be up there. #CAwx [16/n]
I will be writing a blog post this PM (edit: now evening!), & I'll have live YouTube office hours tomorrow morning at 9am Pacific Time (see first Tweet). To journalists: I strongly encourage you to attend the YouTube event, where I will be answering live questions. #CAwx [17/end]

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More from @Weather_West

Dec 29, 2022
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
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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…
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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, 2022
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...
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Oct 7, 2022
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
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Sep 4, 2022
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