Significant Storm Impacting California Monday/Tuesday

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After a fairly warm and dry weekend, California will be greeted by the second week of January by a fairly substantial Pacific storm that is anticipated to bring widespread moderate to heavy precipitation in addition to strong winds and potentially thunderstorms in some places of the state. As mentioned in the previous update, there is a cutoff low around a thousand miles off the southern California coast churning up plenty of subtropical convection in association with a potent (100 knot) subtropical jet; additionally, there’s a trough diving out of the Gulf of Alaska right behind this closed low. Over the weekend, these two separate systems will phase (merge) together, and by Monday become a single, yet fairly potent mid-latitude cyclone.

The trough digging off the west coast will become backed by a powerful Pacific jet that extends well out to Japan, with upper-level winds in this jet exceeding 150 knots at 300mb. This jet will wrap around the trough, and as this happens the closed low to the south will get phased in with the growing Gulf of Alaskan upper-low. Late Sunday into Monday, as the cutoff low and trough become completely phased, a surface cyclone begins developing (aka, cyclogenesis initiates). The starting pressure will be around 1010 mb Sunday, give or take a couple bars. By late Monday, the pressure will have dropped to somewhere around 995 mb, if model solutions as of Saturday are near accurate.

While a 15 mb drop in 24 hours doesn’t quality as a “bomb”, it is certainly a notable drop during development and intensification and strengthening pressure gradient will definitely help the low-level jet get amped up to the 50 – 60 knot range at 900 mb (~2000 feet AGL). There are still discrepancies in regards to the position and exact track of the low as it develops and strengthens off the northern/central California coast, but in most solutions that have been presented the impacts are fairly similar in regard to wind and precipitation.

As the low strengthens offshore Monday, the deep plume of subtropical moisture with precipitable water values in excess of 1.5 inches will become completely fused with the baroclinic zone and the large precipitation shield associated with it. This large shield of precipitation will begin working its way over the state through Monday morning, with northern California seeing the brunt of it. By Monday night, it appears the precipitation shield will have passed east and the cold frontal precipitation band itself begins to approach shore. High resolution models suggest a possible NCFR developing in this second band of heavier precipitation associated with the cold front, meaning there could be some thunderstorms embedded.

Wind & NCFR Potential

A narrow band of weak instability along the cold front and very strong low-level shear could potentially create a setup conducive of coastal wind damage and waterspouts from the Bay Area all the way to coastal San Diego county. With the 50 – 60 knot low-level jet associated with the front, any convection/thunderstorms along this band could easily transport these strong winds a couple thousand feet up down to the surface, although winds could gust to 40 – 45 mph well ahead of this without any help from thunderstorms throughout the coast, LA basin, bay area, central valley, and sierra throughout Monday, especially overnight. Thunderstorms could just increase those gusts to potentially severe levels (generally viewed as wind in excess of 60 mph).

Given precipitation will be falling across portions of the state for nearly 24 hours straight (~8am Monday through ~8am Tuesday), all of which associated with a moisture plume with 1″+ precipitable water values, this system could be classified as an atmospheric river, a term that gets tossed around a little too much. In order to be classified as an AR, most meteorologists would likely lean toward the >24 hour duration and a notable plume of moisture.

Rainfall

Precipitation will begin to subside Tuesday as the low starts to fill and weaken off the coast and work its way into southern California with another shot of precipitation when it does, with some showers here and there through the day elsewhere. When we’re all set and done, essentially the entire state (outside of rain shadowed areas and of course the deserts) an inch or more of rain. In the central valley, anywhere from 1 – 2.5 inches looks likely with higher totals on the east side of the valley. Coastal areas will pick up 1 – 3″, 1 – 2″ for the LA basin, and anywhere from 3 – 6″ in mountainous terrain including the west slope of the sierra and its foothills, as well as coastal hills south of Monterey, and mountains of Santa Barbara, Ventura, and Los Angeles county. These amounts falling in about a 24  – 36 hour period could cause local flooding, and perhaps more of a concern: mudslides and debris flows near burn scars and other steep terrain.

Note: given my lack of southern California experience, I chose to not include the entire state in my personal QPF forecast for this system and continue with the northern California-centered map.

Snow levels will be around 7500 – 8500 feet in the sierra for most of Monday with accumulation, but finally drop a bit Monday night and Tuesday to allow for 6 – 10″ above 6000 feet. Upwards of 2 – 4 feet could fall above 8000 – 9000 feet in the central sierra, however.

Things dry out Wednesday through the weekend, but models have been consistently suggesting that next week could be potentially stormy, with the first storm arriving as soon as early next week once again. This is still far to distant in the future when it comes to accurate forecasting, so check back in here at the end of this week to hear more if this potential becomes reality. Stay tuned.

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