Series of Potent Storms to Slam California Through First Week of November

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The month of October has been fairly active across the western U.S., with ultra-strong cyclones exploding in the eastern Pacific, powerful jet streams fueling storm development, and immense amounts of available moisture from tropical and ex-tropical storms waiting to be entrained. Early this week we saw a ridge-breaking storm that has opened the door for additional, stronger storms to drop into the immediate eastern Pacific ocean off the west coast. The first of two primary storms is taking shape offshore as I write this (it’s currently Wednesday afternoon), as a deep trough roots itself into the eastern Pacific, parented by an upper-level low in the Gulf of Alaska extending southward well off the Pacific Northwest coast. A wave forming along the front will rapidly close off into a upper-level low of its own tonight into Thursday off the California coast as a strong jet streak helps fuel and enhance it.

Meanwhile in the southeastern Pacific, (soon to be ex-) major hurricane Seymour is rolling northward, and as it does so ahead of the developing low off our coast will firehose its excess moisture northward and entrain it into the low’s front/precipitation shield. The amount of total-atmosphere moisture that’ll be tapped into this system will be quite extreme, with precipitable water values approaching (and in some cases along the central coast, exceeding) 2 inches — about 4x standard deviation (or a 250% above “normal” anomaly). This amount of moisture, combined with strong lift ahead of the low and along it’s associated warm & cold fronts will produce a significant amount of precipitation on a large-scale, and for an extended period of time given this storm will be slower moving than normal due to it being cutoff from the primary Pacific jet. Precipitation will begin with this system along the coast early Thursday morning, spreading inland by mid-morning throughout much of northern & central California. Precipitation will likely wane north of a line from around Santa Rosa to Chico Thursday night as the warm front sags southward and stalls somewhere around the I-80 corridor, with precipitation remaining fairly steady south of the boundary.

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The low begins to lift northward Friday morning and slides inland through the day, impacting somewhere along the central California coast in the morning, with precipitation becoming more showery and scattered through the day and evening as the low departs eastward out-of-state. This first system will be a major precipitation producer especially for areas south of Yuba City, with as much as two inches of rain possible in parts of the San Joaquin valley between Fresno and Stanislaus counties with a taper to lower amounts (in the lower elevations) to the north and south of this band — where the cold front sits the longest and the low slowly ventures through. Precipitation will begin to shut off south of Merced by late Friday into early Saturday, while precipitation actually begins to increase to the north as a moist southwest flow is enhanced with lift ahead of the second, stronger system developing offshore kicks in. This will keep northern California showery through the day Saturday, although some breaks between bands of showers could promote enough instability for, you guessed it: balloons to stick to walls (without rubbing them on your shirt). Whoops, I meant thunderstorms — my bad.

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NAM’s forecast 300mb heights and 6 hour precipitation, valid at 8am Friday, suggesting the low impacting just south of the Bay Area along the central California coast.

By the time Saturday afternoon passes, we should have a well-formed mid-latitude cyclone off the California coast. This system will be cooler than the previous, and while it won’t have a moisture tap to any significant subtropical elements, the moisture it has combined with a strong 140-knot (upper-level) jet streak will make up for any difference in moisture. The jet will compact on the southeastern end of the low creating stronger upper-level divergence, signaling strong low-level convergence with the cold front. Frontal precipitation with this storm will begin to fill in across northern California late Saturday into early Sunday morning, continuing through the afternoon. Precipitation with the frontal band will be moderate to heavy at times, which when combined with some gusty winds (30 – 40mph on a widespread scale, upwards of 45mph in the sierra) on top of the already saturated soils could mean some fallen trees here and there. Snow levels with this storm will begin around 8000 – 9000ft late Saturday, but fall to perhaps 5000ft by Sunday afternoon, with potentially impactful sierra/mountain snow.

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Sunday afternoon could be extra interesting thunderstorm wise according to the ECMWF, with the GFS beginning to trend in the same direction, as some clearing could occur behind the cold front ahead of the low moving ashore in the evening. The cold air aloft promoting low snow levels on top of a potentially warmed surface environment from any sunshine could lead to strong thunderstorms, but given this is still a few days out we’ll have to wait until Friday or preferably Saturday to know more details on this threat.

Precipitation through Monday currently looks like this: 1 – 2″ across much of the valley & bay area, 2 – 3″ along the coast and coastal mountains north of the bay and from Santa Cruz south to around northern San Luis Obispo county. Along the eastern side of the Sacramento valley, precipitation banking against the foothills & western slope could lead to a few bands of totals near 3″, while moving into the foothills & western slope 3 – 6″ is a good bet — with the same totals in favorable slopes of Shasta & Siskiyou counties. In the sierra, a quick 1 – 2 feet of snow is a good bet above 6000ft, primarily from the Sunday storm, as snow from the previous system will have fallen above 8000 – 9000 feet.

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There’s potential for another system late Monday into Tuesday, but I haven’t factored that into these forecast totals as of now due to models having some disagreement with track and strength of it. The GFS suggests perhaps another storm or two throughout the week too, while the ECMWF holds onto a stronger ridge to keep the interior dry for the most part through the rest of the week. For now though, the focus is on this week and it’s two storms.

I’ll update this post later in the week if anything significant changes with Sunday’s storm, especially if the thunderstorm threat remains. Stay tuned!

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