Thread by @Weather_West

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]

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]

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

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

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]