The long stretch of dry, boring weather is nearing its end – at least temporarily, as our ridge continues sliding southeastward into the southwestern U.S., allowing a large set of lows in the eastern Pacific to nudge eastward toward the west coast. A long fetch of westerlies across the Pacific has, and will continue to tap into a significant amount of subtropical moisture east of Hawaii, transporting it to the western U.S. coast by Wednesday night in the Pacific northwest as one of many deep lows in the eastern Pacific/Gulf of Alaska begin rotating into British Columbia. By later Thursday, the long fetch of moisture ahead of the cold front begins to slide into northern California, with moderate to heavy rain spreading inland through the day into the night.
It currently appears that the stream of moisture will remain aimed at northern California through at least early Saturday as potential lows develop along the jet transporting this moisture into the west coast; possibly through much of Saturday if some model solutions are correct. Each low/wave that develops along the jet will aid lift along the front and continuing to support moderate to heavy precipitation over the 24 – 48 hour period across northern California, in addition to a SW to NE oriented 130kt+ upper-level jet… and perhaps more importantly, a 40 – 60kt mid/low-level jet blasting south out of the south into the foothills and mountains. These strong southerlies will create a perfect orographic enhancement across south and southwest facing slopes across the foothills and mountains, as well as help precipitation bank up against the higher elevations and boost totals along the east side of the valley, too.
12z GFS’s forecast 300mb winds and heights, valid at 4pm Friday, with a long belt of strong westerlies streaming into the West...
The strong southerly mid-level jet will support a period of gusty winds across northern California during peak precipitation intensity, which should occur Friday afternoon into the evening. 850mb winds increase during this period into the 45 to 65kt range, and believe there will be enough groundward force from heavy rain to drive some 35 to 45mph gusts to the surface across much of the region. In the higher elevations (sierra/parts of the northern mountains), gusts in excess of 50 to 60mph will be possible. Given these winds (in both the higher and lower elevations) combined with periods of moderate to heavy rainfall, some scattered down trees are likely, and thus probably some power outages here and there.
A second and fairly deep cyclone will traverse out of the central/eastern Pacific toward the west coast late Sunday into Monday – sending another cold front into northern California Sunday night into Monday morning with another, but much quicker wave of moderate precipitation to add on top of Thursday – Saturday’s event. There’s still some fairly notable differences with this system between models, with the GFS for instance remaining the wetter model, bringing the low further south, and the ECMWF taking the low further north, limiting precipitation.
Rainfall amounts through late Monday still vary by model, but a blend between major models generates 1.5 – 3.5″ across much of the Sacramento and northern San Joaquin valley, 3 – 6″ across much of the foothills, mountains, coast, and coastal mountains, with 6 – 12″ in orographically favored slopes, including much of the higher elevations of Butte, Plumas, Shasta, Sierra, Yuba, and Sierra counties. I would like to point out that the amounts I’ve mentioned are fairly low compared to what some model output suggests… for instance, the GFS suggests 4″+ for the east side of the valley from Sacramento northward, and 15″+ in the highest elevations of Butte, Plumas, and western Shasta counties… quite a bit more than I’ve personally went with. But, this is a good base to begin with for now, with room to increase or decrease a bit if needed in the next 24 – 36 hours before the event begins.
850mb temperatures along the belt of moisture look like they’ll be in the 3 – 8c range, which will generally support snow levels well up around 6000 – 7000ft, possibly a bit higher in the south and on the lower end of that range in the north. Thus, while snow levels will be high and this event won’t greatly contribute to the snowpack in the more frequented parts of the sierra, the high sierra (>7500 – 8000ft) should pick up a couple feet of snow, at least, through Monday.
Things dry out by Tuesday, and warm up rapidly through the week as the pattern completely flips once again to an extraordinary warm pattern as the flow switches offshore. For now, lets just focus on the fun part of the forecast: Thursday through the weekend, before we get to the more boring pattern we don’t need.
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