Potent Phased Storm Sunday into Monday

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Over the past couple of weeks, a stubborn cutoff low has been swirling to the east of Hawaii, supporting loads of subtropical/tropical convection blooms throughout this piece of the Pacific. During the day Saturday, a trough diving out of the Gulf of Alaska catches the cutoff low from the back end and sweeps up its energy. The cutoff low will be blasting northeastward a strong subtropical jet reaching Washington state from the eastern Pacific between California and Hawaii transporting deep subtropical moisture from continuing subtropical convection in the tropical Pacific near Hawaii. This moisture plume along with the cutoff low and cold Gulf of Alaskan trough phase into a single large-scale storm within the scope of 24 hours (of Saturday).

That said, large-scale storms aren’t made of one piece. The first piece, the original subtropical cutoff low, gets engulfed by the Pacific jet wrapped around the Gulf of Alaskan trough through the day Saturday and is quickly transported northeastward. By Saturday night, it’ll have impacted the British Columbia coast with plenty of heavy precipitation and strong winds — as the surface cyclone will have deepened from 1004mb several hundred miles off the southern California coast to around 988mb Saturday evening within a hundred miles of the BC coast just prior to impact.

A secondary surface cyclone develops from a new piece of energy dropping out of the Gulf of Alaska Saturday night, and gets directed toward northern California through the overnight hours as it begins to approach the leftover subtropical moisture plume the first original low left behind sitting overhead of our region. By Sunday, the Pacific & subtropical jet will have completely merged and appear as a single Pacific jet — albeit, a strong jet at that. An upper-level jet streak in excess of 155 knots will wrap the base of the trough and blast northern California to Washington Sunday.

18z GFS’s forecast 300mb wind and 500mb height, valid at 11am Sunday. Note the potent jet streak raging over northern California, Oregon, and Washington – with blacks and greys suggesting winds at this height in excess of 135 – 150 knots.

As the new low grabs a hold of the moisture plume, its cold front quickly becomes more well-defined and capable of heavy precipitation given the already available strong upper-level support in relation to the strong mid & upper-level jet overhead, as well as the weak but held-together surface cyclone off the California coast. Preciptition begins to amp up ahead of the front along the coastal mountains north of the bay area, north coast, northern mountains, and northern Sacramento valley by early Sunday morning (pre-dawn hours), and increases in intensity as the jet and moisture plume overlap better after sunrise. The front sags south by mid to late-morning — reaching the bay area and the southern/central Sacramento valley, foothills, and sierra by 9 – 11am in the bay and central valley, and 11am – 1pm in the southern Sacramento valley and sierra.

The front looks to get hung up in the southern Sacramento valley/northern San Joaquin valley and adjacent foothills and mountains into the evening as the low drops south along the coast instead of inland, although the moisture plume will drop south — meaning precipitation will become more showery and less widespread/intense. Still, this means precip will continue throughout the mountains and parts of the valley through the evening, boosting totals. Because the moisture plume is based out of the subtropics, you can imagine temperatures several thousand feet into the atmosphere will be warm until it’s shunted south. This means snow levels through the day Sunday will be based well up around 9,000 – 11,000 feet. Some convection along the cold front itself could lower snow levels a little for a short time, but it shouldn’t cause significant effects in itself. Snow levels will drop after the front passes Sunday evening, and by Monday morning should be around 5000 – 6000 feet with light accumulations down to that level.

0z NAM’s forecast 850mb heights, wind vectors (direction), and temperature (fill) valid at 11am Sunday. Any temperature that isn’t cyan is above freezing at this level (approximately 5000 feet) — with the yellow and orange suggesting temperatures still in excess of 50 degrees F at this height.

For those of you who have been reading norcalweather.net’s blogs for the past few years, you probably know convection is a big point of talk here and are waiting for more on that little mention in the snow level talk above. High resolution models have been suggesting an NCFR (narrow cold frontal rain band) could develop along the front, and end up being convective in nature. This has to do (at least somewhat) with temperatures warming into the low 60s prior to the front dumping rain over the central and eastern side of the central & southern Sacramento valley in the morning. Further warming in the northern & central San Joaquin valley is expected, and the light southerly flow could potentially allow for some warm air to be transported northward, converging with the front upon its arrival.

All of that said, I wouldn’t rule out some thunderstorm activity or at least convection in the cold frontal rain band as it works its way through the Sacramento & northern San Joaquin valley Sunday late morning/afternoon as well as into the foothills and mountains. A strong southwesterly low-level jet along the cold front of 40 – 45 knots will also aid orographic enhancement in the foothills and mountains that face southwest, in addition to potentially be driven close to the surface in stronger embedded showers or thunderstorms with some gusts to 35 mph possible with the front through the valley.

Total precipitation from Sunday through Monday should bring about half an inch to an inch of rain throughout much of the valley with local amounts higher, especially on the east side of the valley and up near Redding where some training post-frontal showers and thunderstorms could continually develop Sunday night. In the foothills and western slope of the sierra, 1 – 3″ are expected from Sonora northward — with the >2″ region most likely being from Placerville to Paradise. Along the coast, 1 – 3″ are expected in the northwest corner, with 1/2 an inch to an inch elsewhere, and 1 – 2″ in the coastal mountains above 3000 feet. A quick 6 – 12″ of snow looks likely above 6000 – 7000 feet Sunday night and Monday morning, and could cause some Monday morning travel impacts. Sunday looks okay aside from rain & wind through most passes until after 8pm, which in itself could be risky.

Models differ with where the core of the surface low will go Sunday night/early Monday, but the general consensus is it’ll be guided inland somewhere into central or north-central California, continuing showers into the night to early Monday morning. The trough begins to depart Monday afternoon, leaving us clear through at least the middle of next week. Models diverge after that and there’s a chance you’ll hear more from norcalweather.net within the next 5 – 8 days.

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