After yet another multi-week quiescent lull across most of California, a drastic change in the prevailing pattern is now literally on the horizon. A deep trough and associated low pressure area are digging very sharply south from the Gulf of Alaska, and a highly meridional jet stream will bring some reinforcing cold air from the Arctic all the way through the middle of next week. The Pacific Northwest appears to be facing a very early-season lowland snow event, with snow and sub-freezing temperatures likely in both metro Portland, Oregon and Seattle, Washington by Monday. The extremely cold air mass will be moderated to somewhat by its overwater trajectory, but very cold temperatures aloft (as low as -34 C at 500 mb and -4 C at 850 mb) will overspread much of Northern California as the weekend progresses.
Over the past day, the models have been starting to key in on a brief phasing of the subtropical and subpolar jet streams over California late Friday night into Saturday. Recent water vapor imagery does seem to indicate a plume of subtropical moisture beginning to interact with the southern end of the cold front out over the open Pacific, so I suspect that rainfall totals, especially over southern parts of the state, may be even a bit higher than previously estimated. As it is, dynamics will be quite strong over Northern California. Favorable jet setup will put NorCal under a region of strong divergence aloft, and with multiple frontal boundaries moving through coupled with cold mid-level temperatures, I expect to see some fairly decent precipitation totals. Much of the precipitation with this event will be convective in nature, so some locally very heavy downpours will be possible. The best chance of thunderstorms will probably be later in the day on Saturday in NorCal, but may linger through at least Sunday if the GFS is correct and a couple more spokes of energy drop down the backside of the trough early next week.
The big story with this major storm, though, will be the very low snow levels associated with it. Snow levels in far Northern California will start out around 4000 feet before dropping rapidly to 2000 feet late Saturday and 1000 feet by late Sunday. Early Monday morning, snow levels could even drop below 1000 feet in Northern California inland valleys. Snow is therefore expected to very low elevations in the Sierra Nevada foothills and in the Coastal Mountains, and will probably fall on many Bay Area peaks. Travel will be very adversely affected, as blizzard conditions are expected in the Sierras along with the potential for up to 4 feet of snow by Monday.
There is some uncertainty regarding what will happen after Monday, since conditions in CA will be highly dependent on the exact placement of the deep trough axis and whether meridional flow from very high latitudes can continue to bring frigid air and occasional impulses into California. The latest00z GFS does indicate the potential for another significant disturbance to drop down the backside of the trough on Tuesday/Wednesday, bringing more showers to Northern California as well as a reinforcing shot of modified Arctic air. Worth noting here is the small possibility of some sea-level snow showers during this time frame (early Monday morning through Wednesday morning). If there is some evening clearing on Monday or Tuesday night and precipitation moves in during the early morning hours after substantial radiational cooling has occurred, some snow flurries or brief snow showers could possibly occur on the NorCal valley floors. Again, this looks like a pretty small possibility at this point, but it’s worth keeping an eye on. More likely is the chance of a widespread frost or possibly a hard freeze event if skies do indeed clear out entirely. Stay tuned!
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