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Warm, moist, even humid conditions expected to persist into Friday as a long fetch of sub-tropical moisture (an atmmospheric river) extending to Hawaii pounds far northwestern California into the Pacific Northwest with heavy rains. While this has, and will continue to bring showers to the northern sierra, northern mountains, and quite a bit in the northwestern corner of the state, the valley should remain mostly dry minus a few showers here and there.
A more elongated trough and associated deep low in the Gulf of Alaska will sufficiently stretch out vertically to allow a cold front to slide through northern California through the day Saturday in the far northern reach of the state, and by evening in the Sacramento valley and adjacent regions. 1.1″ precipitable water band slides down the coast with the front, with .9 – 1″ values working their way inland along and ahead of the frontal precipitation band. Vertical motion with the frontal band are looking pretty intense, however somewhat narrow. Nonetheless, I expect the front to slide with with a period of 1 – 3 hours of precipitation for most locales across the lower elevations, with a 3 – 6 hour period in the foothills and mountains north of Tuolumne county, as the front pretty rapidly decays south of the northern San Joaquin valley due to the front loosing lift with increasing distance from the parent low.
Convective wise, if the front were to come in a bit faster, during the late afternoon for instance Saturday or within a couple hours of sunset, some pre-frontal instability may promote some convection ahead of the front. Current timing in the early to mid-evening doesn’t look very favorable for pre-frontal convective action… but sometimes fronts such as these move a bit faster than modeled. If the system was to come in much later, during the early morning hours of Sunday for instance, it would prove much more favorable for convection during the morning and afternoon Sunday in the cold pool. However that’s a little more far-fetched than the faster timing idea. I’ll monitor timing compared to model data as the system nears during the day Saturday in case any changes occur for the better convective wise.
Snow levels will start out around 7000 – 8000ft Saturday afternoon/early evening, then fall off rapidly as colder air mixes out the lingering mid and low-level warmth, allowing snow levels to dip to around 3500 – 4000ft by mid to late-evening with the tail end of the front and any post-frontal showers overnight.
Precipitation amounts from this system won’t be nearly as significant as the last system, however in the mountains widespread amounts of .5 – 1″ are possible in the mountains/foothills/sierra north of Tuolumne county. Orographically favored slopes of Butte, Plumas, Shasta, and Sierra counties could see around 1 – 2″ locally. In the valley, a generally .1 – .3″ are possible from the center of the valley east, with local amounts of up to a half inch along the eastern side where lift is maximized.
4km NAM’s forecast QPF through the weekend. Note some of the precipitation from Butte county north falls from the Friday system, not just the weekend front.
These QPFs will equate to 6 – 12″ of snow in the sierra, with locally higher amounts up to 16″ possible along favored slopes.
4km NAM’s forecast snowfall through the weekend.
850mb winds increase to 30 – 45kts in the central and northern Sacramento valley ahead of the front… some of which could be mixed down to the surface with pre-frontal dynamics/precipitation, perhaps allowing some 35 – 45mph gusts north of Sacramento. From the sierra crest east into western Nevada, 700mb winds of 60 – 70kts could be drove down by strong downward forcing along the east slope of the sierra, where 50 – 70mph winds are possible Saturday afternoon into the evening.
4km NAM’s forecast surface wind gusts for the later afternoon hours of Saturday.
Cooler post-frontal airmass slides overhead Sunday, cooling things down compared to what we’ve experienced most of the week into the upper 50s to low-60s in the valley.
Things remain dry into early next week while the Gulf of Alaskan low reloads, sending another weak, but cold trough south into the western U.S. At the moment, models aren’t in very good agreement with the system, with the ECMWF being a bit wetter than the U.S.’s GFS., however the ECMWF has been trending further inland over the last couple runs, while the GFS has been pretty consistent with trajectory… and the GFS’s trajectory is actually a bit colder than the ECMWF’s. The GEM keeps the trough further north, not generating much precip for us in northern California.
I shall wait for better agreement for next week’s system, and once agreement has been met, as long as the GEM isn’t closest to correct, I’ll have an update out by early next week regarding the trough, as potential for low elevation snow exists.
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