*Very stormy period developing for all of California; potential for major flooding*

Anyone who ever said that winter weather in California was boring has never experienced a pattern like the one we’re headed for over the next several weeks. After a very prolonged period of benign and mostly dry weather across the entire state brought about by an anomalously and almost bizarrely persistent high-latitude block for much of the winter season thus far, a major shift in the hemispheric flow pattern will being a prolonged period of heavy rainfall and strong winds to the entire state of California. Though there have been a few large storms California over the last few years, the first decade of the new millennium  was generally not known for its active weather patterns in our state. The new decade appears to be trying to make up for some lost time in that regard. For the first time in at least several years, a powerful southern stream branch of the jet stream over the Pacific is expected to roar across CA for at least the next two weeks, potentially bringing a tremendous amount of precipitation and frequent strong wind events.

First things first, however. A decent rainmaker moved through Northern California overnight, bringing mostly moderate but locally heavy precipitation. Looking out the window this afternoon, I see the first direct sunlight in weeks here in the fog-plagued Central Valley, but this break will be short-lived as there is a rather potent-looking convective system several hundred miles off the coast. This system will lift northeast and move inland overnight, bringing periods of rain with embedded thunderstorms to much of NorCal. Precipitation could be locally heavy once again due to convective elements. SoCal will stay dry from this system except for perhaps a few light showers in the north. Showers and thunderstorms wane by tomorrow afternoon in the north and the Central Valley will likely see a couple more days of dense Tule fog as a brief quiescent period occurs between Thursday and early Saturday. Later on Saturday, however, the first of a great number of strong to powerful storms will begin approach the coast.

16-day GFS precipitation accumulation. Note that the scale is much larger than on typical precipitation maps.

All this excitement revolves around the southern branch of the jet stream over the Pacific, which is known to be energized by El Nino. Currently, the strong El Nino is reaching its peak in the Eastern Pacific, and now finally appears to be exerting an influence on our weather. The strong jet has been apparent for quite some time out over the open water, but the persistent block had prevented it from reaching the coast. Now that the block has dissolved completely, a 200+ kt jet is barreling towards us. Multiple large and powerful storm systems are expected to slam into CA from the west and northwest over the coming two weeks, all riding this extremely powerful jet stream directly into the state. The jet will itself provide tremendous dynamic lift, in addition to directing numerous disturbances right at the state and supplying them with an ample oceanic moisture source. The jet will be at quite a low latitude over much of the Pacific, so these storms will be quite cold, at least initially. Very heavy rainfall and strong to potentially very strong winds will impact the lower elevations beginning late Sunday and continuing through at least the following Sunday. This will be the case for the entire state, from (and south of) the Mexican border all the way up to Oregon.  Above 3000-4000 feet, precipitation will be all snow, and since temperatures will be unusually cold for a precipitation event of this magnitude, a truly prodigious amount of snowfall is likely to occur in the mountains, possibly measured in the tens of feet in the Sierra after it’s all said and done. But there’s a big and rather threatening caveat to that (discussed below).Individual storm events are going to be hard to time for at least few more days, since this jet is just about as powerful as they come (on this planet, anyway). Between this Sunday and the following Sunday, I expect categorical statewide rainfall totals in excess of 3-4 inches. That is likely to be a huge underestimate for most areas. Much of NorCal is likely to see 5-10 inches in the lowlands, with 10-20 inches in orographically-favored areas. Most of SoCal will see 3-6 inches at lower elevations, with perhaps triple that amount in favored areas.

This is where things get even more interesting, though. The models are virtually unanimous in “reloading” the powerful jet stream and forming an additional persistent kink 2000-3000 miles to our southwest after next Sunday. This is a truly ominous pattern, because it implies the potential for a strong Pineapple-type connection to develop. Indeed, the 12z GFS now shows copious warm rains falling between days 12 and 16 across the entire state. Normally, such as scenario out beyond day seven would be dubious at best. Since the models are in such truly remarkable agreement, however, and because of the extremely high potential impact of such an event, it’s worth mentioning now. Since there will be a massive volume of freshly-fallen snow (even at relatively low elevations between 3000-5000 feet), even a moderately warm storm event would cause very serious flooding. This situation will have to monitored closely. Even if the tropical connection does not develop, expected rains in the coming 7-10 days will likely be sufficient to cause flooding in and of themselves (even in spite of dry antecedent conditions).

An ominous “Pineapple Express” connection, GFS model

In addition to very heavy precipitation, powerful winds may result from very steep pressure gradients associated with the large and deep low pressure centers expect ed to begin approaching the coast by early next week. Though it’s not clear at the moment just how powerful these winds may be, there is certainly the potential for a widespread damaging wind event at some point, and the high Sierra peaks are likely to see gusts in the 100-200 mph range (since the 200kt jet at 200-300 mb will essentially run directly into the mountains at some point). The details of this will have to be hashed out as the event(s) draw closer.

In short, the next 2-3 weeks (at least) are likely to be more active across California than any other 2-3 week period in recent memory. The potential exists for a dangerous flood scenario to arise at some point during this interval, especially with the possibility of a heavy rain-on-snow event during late week 2.  In some parts of Southern California, a whole season’s worth of rain could fall over the course of 5-10 days. This is likely to be a rather memorable event. Stay tuned.