Daniel Swain
The following is an excerpt published with permission from Weather West
Explosively developing low pressure system will bring rain/wind starting this evening
This will be a short blog post since I was not initially planning on writing another one focused on the present storm sequence–but on rare occasions I’ll write a short-fused Weather West piece if the severity of an upcoming event justifies it. After checking the latest modeling this morning when I woke up, I think this one probably qualifies. So without any additional exposition, here’s what’s going on.
Unlike most winter storms that affect California, which more typically develop well to the west out over the open Pacific and often make landfall as they either maintain their strength or are already beginning to weaken, the storm slated to slam California starting later today will do just the opposite. This system did not really exist in coherent form out over the remote North Pacific, but is instead now rapidly developing “in situ” only 400-500 miles off the coast of central and southern California. This system will take the form of an explosively deepening surface low pressure center (or possibly a pair of similarly deep centers orbiting around a common center) just west of the Central California coast by late tonight. Several models are suggesting that this storm will meet the formal (latitude-adjusted) meteorological criteria for a “bomb cyclone” (i.e., a low pressure system that undergoes a rapid rate of deepening (at California’s latitude, around 17 mBb in 24 hours). Regardless of whether it meets the technical threshold, the main point is that this storm will be become rapidly stronger precisely at the moment it makes its closest approach to the Central Coast; this temporal evolution, plus its proximity to coast as it does so, will greatly elevate impacts relative to more ordinary California winter storms.
Why is this storm going to be so unusual? There are a range of factors that have to line up for rapid cyclogenesis to occur close to the CA coast, and they don’t align often. One is robust “baroclinic instability”–a kind of atmospheric instability that derives from the potential energy contained by the atmospheric flow in an environment of strong horizontal air temperature differentials (this is the kind of potential energy that essentially all winter storms draw upon, but the California coast is not generally a favorable region for this to occur to a substantial degree). Additionally, the region will be in a relatively conductive position relative to a strong Pacific jet streak aligned over Central CA. This configuration, including the presence of upper divergence, allows for “ventilation” at upper levels of the atmosphere right above the low pressure center–which facilitates deepening/evacuation of air from the low itself and enhancing its cyclonic spin. This powerful low is also tapping into some deep subtropical moisture, and the circulation around this system will aim that plume squarely at central and southern California. Due to these additional factors, this event may end up punching well above its weight on the 5-category atmospheric river scaling system.
Major, damaging windstorm likely for Central Coast and possibly parts of interior NorCal
With an explosively developing surface low expected to be located just a couple hundred miles southwest of the SF Bay Area by late tonight, a major and possibly widespread damaging windstorm will be likely along much of the Central Coast, portions of the SF Bay Area, and possibly also farther north and east (especially across the northern San Joaquin and southern/central Sacramento Valley). Models still differ as to exactly how much this low pressure center will deepen, but a ~985mb low is possible and very strong surface pressure gradients/very strong potential wind gusts will likely result along the coast from western Santa Barbara County northward toward about San Mateo County. Recent model runs suggest that very high gust potential may expand inland and northward more than previously expected (depending on the exact track of the surface low, which remains difficult to pinpoint with detail at this point), possibly affecting the northern SF Bay Area and much of the Sacramento/norther San Joaquin valleys as well. Gusty winds will be possible almost everywhere in CA with this event, but the areas highlighted above are most likely to see highly disruptive and possibly substantially damaging winds at times between this evening and Monday morning.
How high might wind gusts get in these areas? Again, there is still a substantial range–but since multiple models are now indicating gusts of 70-85 mph out over open water west of Monterey County, it’s highly plausible that gusts at least that strong could mix down to the surface near the coast and over ridgelines, and locally elsewhere in pretty widespread fashion. (A few classically windy places on more remote higher ridgelines that often see high winds could potentially see 90+ mph gusts, though gusts that high are very unlikely where people actually live.) More widespread wind gusts of 55-70 mph are likely across much of the Central Coast, SF & Monterey Bay Areas, and near the I-80 corridor in the Central Valley (including Sacramento).
Importantly, and unlike some windstorms that bring very strong winds to the coast/hills but only modest breezes elsewhere, this storm specifically has a high potential to bring widespread strong/damaging wind gusts to lower elevations. This is partly due to the enhanced convective instability associated with the low (see more below), and partly because there are indications of a possible “sting jet”-like feature potentially developing on the backside of the circulation (a narrow corridor of strong winds at higher altitudes that descends close to the surface amid subsiding air in the cool sector of a rapidly developing cyclone). Either or both of these features will have above-average potential to mix strong winds aloft closer to (or even all the way down to) the surface, especially in bands of intense rainfall.
Widespread tree and power line damage is likely in these areas, so major power outages and travel disruptions are likely. As noted: the Central Coast is almost guaranteed to experience these winds at this point, but there is more uncertainty regarding the northward and eastward extent (though most of the SF Bay Area and Sacramento area are increasingly in the crosshairs).
Substantial risk of major urban/flash flooding across much of SoCal (and maybe Central CA too?)
I’ll cut to the chase: this event is likely to bring widespread heavy to very heavy rainfall across all of Southern California, including the coastal plain. 3-6 inches in a 36-48 hour period is likely in most lower elevation spots (locally more possible), with at least double and perhaps triple that (9-15+ inches) in the most orographically favored parts of the Transverse Ranges. There are several additional aggravating factors that might amplify the regional flood risk even more than might otherwise be expected with these kind of rain totals. First, soils are already saturated–flooding has occurred repeatedly across portions of SoCal with recent storms, including locally damaging and even deadly flash floods in urban settings (though in localized fashion so far). Second, this storm will be notable for its intense hourly and sub-hourly rainfall rates. So rainfall intensity will be potentially very high at times, with bands of 0.5-1.0+ in/hr likely even at lower elevations (and potentially even higher intensities in the mountains). This will be caused by a combination of very high atmospheric moisture content, decent frontal forcing, and additionally some robust convective instability that will lead to some training rainbands and possibly some stronger thunderstorms (again, see below).
As the NWS in Oxnard has been messaging strongly for days, this setup is conducive to one that could produce widespread, and quite possibly major, flash flooding and urban flooding throughout a highly populated part of Southern CA. The very highest risk still appears to be along and just below the Transverse Ranges from Santa Barbara County eastward into Los Angeles County, but later in the storm major flash flood risk will likely extend farther southeastward down to San Diego county and even south of the international border. Rapid inundations of fast-responding watersheds, significant urban and roadway flooding, and possibly also debris flows will be the main threats, but larger rivers also have the potential to flood in SoCal if rainbands are espsecially persistent.
Whether this is “merely” a notable or instead becomes a genuinely historic rain/flood event in SoCal will largely come down to two factors: 1) how long the initial frontal rainband and associated atmospheric river stall perpendicular to the Transverse Ranges , and 2) whether the secondary frontal wave/low pressure system, which will mainly affect areas from LA County southeastward Mon-Tue, becomes strong enough and stalls out long enough to produce a second round of heavy to very heavy precipitation. Right now, it is difficult to tell the degree to which either of these possibilities will play out. Either way, there’s going to be a ton of rain and a lot of flooding–but just how bad it gets will likely depend on these two still uncertain factors (and also whether any really intense localized thunderstorm downpours develop.
Substantial flood risk will also arise along the Central Coast from western Santa Barbara northward to the Santa Cruz Mountains. The Big Sur coast, in particular, will be hammered by this storm and debris flows/landslides of some magnitude are likely there. Here, the combination of a high-end windstorm with locally torrential downpours will likely yield considerable disruption and possibly significant damage that takes a while to recover from.
Northward into the SF Bay Area: this will likely not be a historic rainstorm for this part of the state. However, there is the potential for very intense hourly rainfall rates at the peak of the storm even this far north–possibly coinciding with damaging wind gusts and even lightning–so this has the potential to cause significant urban flooding and widespread creek/small stream flooding even into the Bay Area although larger rivers will likely stay within their banks. This will feel like a genuinely dramatic storm from the Central Coast up through much of the SF Bay region and Sacramento area due to the combination of high winds, very intense rainbands, and possibly some stronger thunderstorms mixed in.
Big-time Sierra snowfall (at last!) a mixed blessing, with major SWE gains but also highly disruptive accumulations
I won’t spend a whole lot of time on the specifics here–partly because there’s still considerable uncertainty in the details–but the big picture is clear: this is, finally, going to be a major snowstorm for the entire Sierra Nevada range and likely down to reasonably low elevations. Very heavy snowfall (multiple feet?) is likely at Lake Tahoe level and even below, after a winter of pretty dismal accumulations below 6,500 feet. This will be good news for snowpack and ski resorts, but will also lead to a travel nightmare and (as is the norm these days) I’d expect a lot of folks to be stuck either on the roads or at their destinations. Mountain travel Sunday and likely part of Monday will essentially be impossible due to very heavy snow and locally strong winds as well.
Severe thunderstorm risk, just to round things out?
Just in case all of the above were not enough: there is also a pretty good chance of thunderstorms with this dynamic storm, and some of them may even become strong to severe. There’s a pretty solid amount of CAPE out there (by CA standards–at least a few hundred J/kg) over the open water associated with both the cold front of the storm, its trailing cold pool, and also with the subtropical moisture plume). The strong winds aloft associated with the low itself will also offer some decent vertical wind shear, which (combined with robust upward vertical motion at synoptic scale from the broader circulation) may combine to generate some intense squall lines spiraling around the low later Saturday into Sunday. This could lead to corridors of torrential rainfall, very strong wind gusts, lightning (and perhaps even a brief waterspout/coastal tornadic spin-up from about the Monterey Bay Area southward). Not only will this activity add to the perceived “drama,” but it will also raise the odds that 1) very strong wind gusts mix down to the surface, even at lower elevations, and 2) flash flooding occurs along the Central Coast and SoCal, because these convective bands may be associated with very heavy hourly rain accumulations and could linger (“train”) over the same area for multiple hours.
In sum: be careful out there, since this looks like a major, dynamic, and possibly dangerous storm that will affect many or even most Californians in the next ~48 hours. I’ll be on social media, as usual, and already have scheduled a YouTube live session for 10am Sunday but now also plan to hold another “pop-up” live session this afternoon or evening at a TBD time (so either subscribe to the YouTube channel for reminders/announcements or keep an eye on my other social media accounts for timing!).
