Active Pattern Returns to California

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After a very dry December ridden with historic wildland fires and damaging statewide offshore wind events, a wetter but still rather warm pattern is swapping out spots with the pesky ridge that has plagued the western U.S. for weeks. This pattern is being broken down via a long-standing cutoff low that has been spiraling around in the eastern Pacific between California and Hawaii since right after Christmas, churning up loads of subtropical convection given it’s been thriving in the moist subtropical latitudes. Now as of Wednesday (1/3), the low has finally approached the coast and will send in a few bands of showers through the night and into Thursday.

This storm won’t be very significant, but will sustain itself long enough before moving ashore to drop anywhere from a few hundredths to a few tenths of an inch of rain to much of the lower elevations of northern and north-central California, especially in mountainous terrain the further west you are. This storm will drop little snow in populated regions given its subtropical decent and thus very high snow levels.

A piece of the long-time cutoff low will split off and get caught up in a larger scale system originating from the Gulf of Alaska Thursday and help a new, somewhat stronger shortwave just in time for Thursday night and Friday. This shortwave will likely bring more precipitation than the first ‘foot in the door’ system seen Wednesday/Thursday given a far more organized upper-level jet and even a split mid-level jet as a weak Pacific and subtropical jet get entangled in the eastern Pacific in this rather complex and messy combination of airmasses and large quantity of atmospheric moisture available.

0z GFS’s forecast 300mb height (fill), 500mb heights (brown contours), and MSLP (blue contours), valid at 5pm Saturday as the subtropical cutoff low pushes further north and the westerlies begin to near the cutoff low to begin the merger.

Simultaneously to the second shortwaves arrival, a new low forms in the subtropics well southwest of California’s coast, once again essentially between California and Hawaii. This low kicks off another large mass of convection in extraordinarily moist environment the low helps draw further and further northeast with its progression. By Saturday afternoon, the precipitation from the second shortwave will have cleared and we’ll (California) be under some ridging, which will last through the rest of the weekend. However, over the weekend and well offshore, that cutoff low will have started its merger with a piece of energy dropping out of the Gulf of Alaska.

This merger has been a struggle for forecast models to get a handle on, but in the past day they’ve started to really refine their solutions. The ECMWF has been the most stable in its output, suggesting the low send inland its cold front Tuesday, and the low itself take until Wednesday to finally collapse and die off the southern California coast after hanging out off the Bay Area’s coast for much of its life. The GFS on the other hand, the more finicky model, suggests a Tuesday night frontal impact and Wednesday night low impact.

0z GFS’s forecast MSLP (blue contours, 500mb heights (brown contours), and 500mb wind (color fill) as of 1/4, valid at 5am Tuesday 1/9.)

The GFS has the low bottom out further offshore than the ECMWF, thus meaning it rains itself out more offshore — therefore, meaning the ECMWF dumps heavier precipitation over northern California, while the GFS dumps heavier precipitation on southern/central California. However, either solution dumps a solid inch or so of rain across much of the state, with many inches of liquid precipitation falling across mountainous terrain. One of the more problematic impacts that stand out in these solutions is potential for mud and debris flows over burn scars in southern California, as current output generates 1 – 3″ of rain or more in the mountains of Santa Barbara, Ventura, and Los Angles counties. Similar totals are progged for northern California mountains as well with plenty of summer burn scars to go around.

0z GFS’s forecast 24 hour* precipitation totals, valid next Wednesday at 5am and 10am in southern California’s map. Note: totals have changed significantly over the last 48 hours, and these amounts will continue to change.

There will likely be further changes to the forecast for next week and the potential major storm looming, as models tend to have a tougher time with these (potentially) very deep cyclones over weaker lows and troughs.

This is a bare-bones update for now as I’m on vacation. Upon return this weekend I’ll get a more thorough update out, but for now I wanted to get at least something out there to get my foot in the door with these systems now that they have theirs in door. As always, stay tuned.

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