Very Active Beginning to January Underway; Significant Precipitation Event Inbound

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Howdy everyone — it’s been a while since you’ve heard from norcalweather.net. I’ve had a busy past month and a half with family, the holidays, and other shenanigans, but weather doesn’t pay much attention to calendar days and has continued to be relatively active across California, especially the northern half. Now that we’re away from the chaos of the holidays, bits of free time are a little more accessible to get back into the swing of things. One of things include what we’ll begin this first week of January with: a cold upper-low diving into northern California from British Columbia, fetching some modest moisture from off the coast to help the system generate a fairly significant amount of snow in the mountains north of Highway-50 through Tuesday night. This first system’s cold origins and inland trajectory are typically non-conducive for significant precipitation, usually just spawning some scattered showers purely from a narrow column of strong lift and some weak instability to flush out any available moisture. In this case however, a potent subtropical jet exists transporting deep atmospheric moisture content from the tropical eastern Pacific.

While the brunt of this moisture plume remains offshore for the duration of this first, cold storm, enough will be tapped via the westerly flow aloft to generate copious upslope/orographic mountain precipitation through the day Monday, onward into Tuesday morning. 850mb temperatures being driven south with the Canadian-origin upper low will range from -3 to -5c Monday and Monday night, before beginning to modify Tuesday ahead of the next system (talked about below). This period will feature snow levels down to near or in the northern Sacramento valley in the Redding region/northern mountains, to 800 – 1200ft throughout the coastal mountains & western slope of the sierra. Precipitation will as mentioned generally remain north of Highway-50 with this system — and as much as 1 – 2 feet of snow look to be a good bet over 1000ft in the northern mountains, with 1 – 2ft in the northern sierra & coastal mountains over 2000 – 3000ft. Numbers aside, this means some foothill communities could receive a heavy layer of snow, most especially in Butte, Plumas, Sierra, and Shasta counties it seems at this time.

GFS’s forecast 850mb temperatures (fill) + 500mb winds & heights, valid at 1pm Monday while cold air is still being rotated in from the west-northwest, keeping snow levels low.

In the lower elevations (that being the valley), showers are expected through the day Monday & early Tuesday, a very cold rain overnight Monday along regions of the valley bordering the sierra and coastal mountains. If cloud breaks occur in the valley Monday, some thunderstorms are quite possible in the Sacramento valley from around Sacramento northward where the best moisture and lift line up.

Meanwhile, during the early part of the week, a barrage of storminess in the eastern Pacific gets going, thanks mainly to the subtropical jet rolling up right under a cold upper-level low hanging out on the western U.S. coast, setting up the perfect scenario for wave cyclones to takeoff in the rich moisture & temperature gradient created between the two. By Monday night, the first in a potential series of wave cyclones develops off the California coast, as a piece of a cutoff/trapped mid-latitude cyclone north of Hawaii splits off and travels along the fierce westerly jet adjacent to it. This piece of energy helps initialize the cyclone’s development, with a precipitation shield developing as soon as early late Monday night offshore — feeding into northern California by early Tuesday morning. Now, while this low isn’t expected to become a behemoth of a cyclone (likely to remain at or slightly above 1000mb, in fact), the amount of moisture it’s riding on will make this system a moderate to locally heavy precipitation producer as it moves ashore through Tuesday night into early Wednesday. This system will rapidly modify the lingering cold air that dropped in Sunday and Monday, raising snow levels to around 5000 – 6000ft by Tuesday.

Models are sketchy with the Thursday/Friday timeframe, as the ECMWF indicates ridging, meanwhile the GFS suggests a fire hose of deep subtropical moisture remains aimed at California through Wednesday and Thursday via the strong zonal flow keeping northern California wet. They both, however, agree that another system will impact on the weekend, as the low that was caught up near Hawaii gets lassoed by the parent low diving out of the Gulf of Alaska. This wave will surge northeast and begin to spread another wave of moderate to perhaps heavy precipitation throughout most of California by Friday night and Saturday. There is a chance this system could be the strongest of the week, given strong upper-level divergence as the polar jet whacks into the subtropical jet blasting central & southern California, strengthening the powerful cyclone continuing to drop out of the Gulf of Alaska, off the Pacific Northwest’s coast by Saturday. This system is expected to dump rain on much of California Saturday through Sunday or Monday (ECMWF drags it out into Monday), until the parent low floating around off the Pacific Northwest coast Saturday/Sunday retreats northward, back into the Gulf of Alaska.

Models have flopped from a dryer, colder pattern this week, to the now very wet pattern we’re anticipating within a matter of a few days — and model differences still exist in precipitation amounts and timing of some of these waves. However, there is strong agreement that this week will rank high on precipitation totals within a one week period. The GFS and ECMWF  are both in agreement that the lower elevations alone (valley, coast, and delta) could receive 3 – 6″ of rain through next Monday, with the mountains as a whole likely to receive double-digit precipitation amounts. How far those double digits go is a question still. The GFS has been aiming high, with upwards of 2 feet of liquid precipitation falling along the western slope while the ECMWF maxes out at 15″ in the same region. Something I’d like to point out is that the ECMWF is trending wetter, primarily with the “second” storm that falls on the weekend into next Monday.

GFS’s forecast 500mb winds and heights, valid at 5pm Saturday. You’ll notice a very well defined and strong upper-low off the British Columbia/Pacific Northwest coast that has dropped out of the Gulf of Alaska, driving northeastward a robust subtropical jet that extends to near Hawaii into California.

This isn’t to mention snowfall, either. While snow levels will rise after Wednesday, especially over the weekend, elevations over 6000ft will see snow primarily throughout the timeframe, and we could be talking anywhere from 6 to 9 feet of snow by the time next Monday night rolls around. Occasionally some melting may occur if snow levels fluctuate, which will need to be monitored for rapid spikes in runoff into the valley.

In any case, a very, very highly anomalous week of precipitation is ahead — with precipitation amounts that make up anywhere from ~25 – 35% of annual totals potentially falling in the time span of a single week across northern California. I’ll have another post out on Wednesday regarding this very stormy pattern in hopes by then models have converged on a single solution for the powerful weekend/early next week storm. Models suggest a stormy pattern through much of next week additionally, and if this is to pan out, waterways of all sizes could very well see problems. Stay tuned for more.

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