Despite the fact that it’s already late February, which is relatively late in the context of “meteorological winter”, the pattern which appears to be setting up for the remainder of the week across the West Coast is an extremely cold one. A very strong, soon-to-be 1045 mb high over the Gulf of Alaska lies at the northern extent of a huge and rather spectacular-looking blocking ridge over the Eastern Pacific. A very long fetch of northerly/northeasterly flow on the eastern side of the block–originating in the Canadian Yukon–will terminate off the West Coast of the continental United States by midweek.
By Wednesday, an area of low pressure is expected to develop within this northerly flow regime off the coast of British Columbia and Washington state, progressing slowly southward through the rest of the week and into next weekend. There is high confidence in this overall pattern evolution, as virtually all forecast models agree on this basic flow pattern. The potential sensible weather impacts for California, however, depend tremendously upon the details. Almost certain at this juncture is that some extremely cold continental polar air will move directly over Washington state, bringing Seattle possible record cold temperatures and probably widespread/possibly heavy snow. Similar, if slightly more moderate, conditions are expected south into central Oregon, including the Portland area. Over the past several days, the numerical models have been struggling with the exact orientation of the upper low off the coast, including how far it will retrograde offshore, how elongated it will become, and how meridional the flow over California will be by Thursday and Friday. Given the magnitude of the cold air available and the extremely favorable synoptic-scale pattern, a very impressive event is possible over California if we can maintain even a moderate degree of meridional flow ahead of the low. Most models are currently indicating that this will be the case. The ECMWF was the first model to indicate this potential pattern evolution 4 or so days ago, switching off with the GFS in the meantime. At present, the ECMWF, Canadian, and the NAM are most aggressive with the snow potential over California, though the most recent GFS (including GFS ensembles) would still indicate some potential for lowland snowfall.
As a descriptive average of recent model output, 500 mb temperatures may fall to nearly -40 C over NorCal, 850 mb temperatures as low as -8 to -10 C, and 1000-500 mb thickness may potentially drop below 520 dm. If these numbers are indeed in the ballpark, I would expect widespread snow to near (or even at) sea level on Friday and Saturday. The models do generate convective precipitation associated with the low, so confidence that local heavy precip could drag snow levels to the surface is even higher. This expectation, however, is contingent on temperatures to be as cold as presently indicated by the models. As we are still a few days out, there is plenty of room for error, especially given that the pattern’s geographic origins are in such a meteorologically data sparse region. What does seem more certain, though, is that snow levels will be extremely low, and that areas 1000 feet or above are highly likely to see significant snow accumulations. The potential is certainly there for at least some sea level snowfall later this week and into the coming weekend in NorCal. Prospects for lowland snowfall are even less certain for SoCal, and I would not expect snow to fall all the way to sea level that far south in any case, but it is conceivable that the coastal hills under 1500 feet could see some white stuff, even in the Los Angeles area.
Additionally, the ECMWF and to a lesser extent the GFS indicate the potential for the pattern to “reload” by early next week, with another extremely cold low pressure system dropping down the Pacific Coast. That is still some ways out, and would probably affect the Pacific Northwest more substantially than California, but it too bears watching.
In my opinion, this pattern brings the greatest potential for widespread sea level or near-sea level snowfall in quite a few years in Northern California. I feel that chances for snow in the Sacramento Valley are notably higher for this event than in December 2009 (an event which brought high expectations but failed to produce measurable snow on the Valley floor). There is still much uncertainty regarding the details, and these will continue to be hashed out over the next couple of days. Snow at sea level is by no means guaranteed to occur this week, but the possibility is more than speculative at this point. The screaming message is that winter remains well entrenched across the West Coast, and California will be subject to a period of very anomalous and active weather in the coming 1-2 weeks. Stay tuned.
© 2011 WEATHER WEST