Daniel SwainThe following is an excerpt with permission from the
By Daniel Swain of Weather West
Wet antecedent conditions set stage for future flood risk
There’s so much going on at the moment this section is going to be shorter than recent events would usually justify, but here goes.
A strong storm on New Year’s eve brought very heavy 24-hr precipitation accumulations to a relatively narrow but highly populated swath of NorCal, from around San Francisco in the central Bay Area eastward to the Central Sierra foothills. Here, some locations actually came close to (or even exceeded) all-time 24 hour precipitation records. This was pretty unexpected–while heavy rain was predicted, the combination of modest stalling of the initial atmospheric river, and then later unusually strong frontogenesis in combination with the unexpectedly slow movement of a surface low directly onto the San Mateo County coast produce a much longer and more intense precipitation event across these areas than had been in the forecast. (This also produced stronger-than-expected winds as the low departed the southern Sacramento Valley, producing substantial damage.) These factors resulted in widespread urban/small stream flooding in these areas–quite significant, in some locations–as well as unanticipated major flooding on the Cosumnes River in/near southern Sacramento County. The Cosumnes flooding has apparently resulted from multiple (as many as 5!) separate levee breaches, and the subsequent inundation has closed Highway 99, caused significant property damage, and unfortunately cost at least one life.
(As an aside: this is just speculation on my part for now, but I suspect the very heavy rainfall in the Sierra foothills that form the Cosumnes watershed may have been further amplified by the aftereffects of the massive Caldor Fire from 2021. It’s well known at this point that runoff can be greatly amplified following large/intense fires in steep terrain. It will be important to understand whether a portion of the unexpectedly severe flooding on the Cosumnes can be attributed to changes in the post-fire hydrologic regime).
Although these unexpectedly severe impacts affected a relatively modest swath of California–much of NorCal experienced a relatively run-of-the mill storm–I would say this represents one of the most conspicuous recent “forecast busts” that I can remember. It was caused by a fairly unique confluence of events that I don’t think will be repeated anytime soon. Nonetheless, this event set the hydrologic stage for what is to come: soils are now saturated, rivers are generally running high, and further heavy precipitation will result in rapid (nearly instantaneous runoff)–increasing the risk of widespread flooding later this week and beyond.
Strong, high impact storm across Northern California Wed-Thu
I’ll cut to the chase: the next major storm in the series, due in later Wednesday into early Thursday, will be a high-impact storm in Northern California, with widespread heavy rain and strong winds. It’ll bring some pretty respectable precipitation to much of Southern California, as well, but impacts will be dramatically lower down south so I’m going to focus most of the rest of this discussion on NorCal.
The parent low pressure system will likely become a “bomb cyclone” (i.e., one that deepens very rapidly)–but in this case the low will stay far away from the California coast. Nonetheless: in the process of rapidly spinning up, this low will generate a strong cold front (associated with another very moist atmospheric river). In fact, this storm will also have an unusually well-defined warm front (at least by California standards): the NWP output looks like a literal textbook example of a mid-latitude cyclone.
My main concern with this system is with the 2-6 hours leading up to cold frontal passage itself. This will be a fairly intense front–likely characterized by high hourly rainfall rates, strong wind gusts, and maybe even embedded lightning. Thus, the peak flood and mudslide risk will be less from overall accumulations (which may even be lower than the NYE storm in many places), but more from rapid rain accumulations. Given completely saturated soils, flash flooding of creeks/streams/urban areas is likely, along with mudslides. Smaller rivers will also rise rapidly, with some reaching at minor to perhaps moderate flood stage. Depending on the details, flooding could be locally significant–especially within faster-responding watersheds.
Substantial wind-related hazards are also likely. Winds will likely be higher and more widespread than the NYE storm (except for perhaps in the Southern Sac Valley, where winds will be comparable). Widespread gusts of 50-60 mph, with some higher in the usual spots, will likely occur. With saturated soils, these winds could bring down a lot of drought-weakened trees and cause fairly widespread power outages. So this will likely be a major rain AND wind event in NorCal…with widespread disruption plausible.
SoCal will also experience a significant storm from this sequence, including potential 1 in/hr hour rainfall rates (for a brief period) and strong winds. But at the moment, I just don’t have enough time to characterize the SoCal impacts in a comprehensive way (since they won’t be nearly as high as in NorCal)–just too much going on at the moment.
Medium range ensembles ringing alarm bells re: further escalating flood risk
And here’s where things get even more…interesting.
The ECMWF ensemble suggests that the North Pacific storm parade will continue until further notice–potentially bringing widespread flooding to Northern California.
As recently as a couple of days ago, the multi-model ensembles had been suggesting an end to the extremely wet pattern in Northern California by mid-January. But they have substantially changed their tune–now suggesting a continuation, and perhaps even an escalation, of the storm train by next week. Recent operational runs (i.e., single ensemble members) of both GFS and ECMWF have been spitting out eyeball-popping double digit rainfall totals across most of NorCal, but the ensembles (although less biblical) are starting to hit pretty concerning territory.
The next storm of high concern might occur around next Mon/Tue. Right now, the GFS and ECMWF are both hinting at the potential for this one to arrive in the form of a possibly very prolonged (multi-day) atmospheric river with a strong subtropical moisture tap–very much resembling a classic “atmospheric river family” setup. And, even further out, there may even be the potential for a second similar storm sequence later in the week, with little break in between.
*If* this scenario were to play out exactly as depicted in current NWP solutions, there would be the potential for widespread major flooding across much of NorCal sometime in the 7-10 day period. *HOWEVER*…7-10 days in the future is still a long way out in weather model land, especially given the high complexity of the pattern that would be required for that to occur. The forecast will likely evolve considerably between now of then–and it is far too early to discuss specifics. But given how remarkably wet the ensembles are overall during this period–irrespective of whether this specific storm sequence occurs–it is probably time to start preparing for a potentially very significant storm sequence/flood event next week (even if its occurrence is far from guaranteed at this early juncture).
Also, I would strongly advise folks to focus on the weather model *ensembles,* rather than individual operational model runs, during this period (well, generally, too, but especially now when the stakes are rising).
Overall, this period is looking comparable magnitude to the peak of the extremely wet 2016-2017 winter in Northern California. Impacts, however, could well greater (or less) than those during 16-17, as individual storm sequencing will strongly dictate the magnitude of flooding that will result. There is little doubt at this point that there will be widespread flooding across most of the northern half of California over the next 10 days or so; the only question is whether it remains minor to moderate, or becomes severe and historically significant. Right now, both possibilities appear to be on the table–but recent trends in the ensembles have significantly elevated my personal level of concern regarding a major flood event. Stay tuned.
For day to day updates, you can follow me (as always) on Twitter.
Tune into YouTube live “office hours” at 9am PT on Tue for Q & A
I’ll be answering any/all questions related to the ongoing/upcoming storm onslaught on YouTube Live tomorrow morning! You can ask questions via chat if you join live, but can also view the recording at any time after the event concludes.
