Weather West: Active Weather Pattern Through End of Year

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The following is an excerpt from a recent article on the Weather West Blog

By Daniel Swain of Weather West

After a lull, active pattern (gradually) picks up steam again this week

With this blog post, I’m just going to focus on what’s coming over the next 1-2 weeks (since there is much to discuss! I’ll talk about the rest of the season to come next time.). Most of California has been experiencing a relatively active weather pattern for the past few weeks, with very heavy Sierra Nevada snowfall allowing statewide snowpack to recover from near zero to as high as 95% of average as of today! This wet and relatively cold pattern has brought significant short-term drought relief, especially to NorCal, although recent rains in SoCal have helped as well.

After a brief several day lull, which is ending this evening in northwestern CA, an active pattern is once again expected to envelop the state by midweek. The initial couple of systems will actually not be very impressive in most of the state–mainly bringing light to moderate precipitation to only portions of the state at a time. Some individual model runs were suggesting this initial event could potentially be very wet, but ensemble solutions have consistently suggested a more moderate precip event this week and now the operational model runs are largely in agreement. The one caveat is that there is quite a bit of subtropical moisture lurking out there over the eastern Pacific just southwest of California. Some of that moisture will move over SoCal later this week, bringing some light (to locally moderate) rain. Right now, it does not appear that this subtropical moisture will phase with the storm energy coming in from the north/northwest, likely precluding the occurrence of a major storm event this week in California. There is a very slight chance this could change, but for now I’d count on mostly light to moderate precipitation nearly statewide, with some modest Sierra snowfall as well through about Thursday. In fact, I’d expect the heaviest rainfall with the initial wave to occur across SoCal (vs NorCal), but totals will likely be limited to 1-2 inches in the wettest spots (with less than that in most places). But thereafter things get much more interesting…

Widespread low elevation rain; heavy high elevation snow into the holiday weekend

The pattern this entire season (since early autumn) has (for the most part) been characterized by a strong, persistent ridge of high pressure over the northeastern Pacific. This is a classic moderate-to-strong La Nina pattern, and one that had been well-predicted by the seasonal models. However, in recent weeks, this ridge has shifted a bit westward (into the Gulf of Alaska, rather than along or just west of the West Coast). While this shift is quite modest in the large-scale scheme of things, it is hugely consequential for weather conditions along the West Coast and California. Instead of completely blocking the storm track and sending Pacific weather systems directly into BC or AK, this new ridge alignment and retrogression is allowing strong meridional flow to develop over the far eastern Pacific–causing modified Arctic air to move southward off the west coast of BC and the PacNW. This pattern will amplify further in the coming days–bringing even colder and unsettled conditions across BC/the PacNW and probably, to at least some extent, California as well.

In practical terms, this could yield some very interesting West Coast weather between Christmas and the New Year. At the moment, multiple models and their ensembles are suggesting a fairly high potential for a modified but still very cold Arctic airmass to overspread the PacNW during this period, with widespread sub-freezing temperatures and accumulating snowfall all the way down to sea level. This does include cities like Vancouver, Seattle, and Portland, and indeed I would expect travel along the entire I-5 corridor from about Redding northward to become extremely difficult, if impossible.

In California, this shift to a much colder pattern will be heralded first by additional (heavy) mountain snowfall, which will likely be measured in feet in the Sierra Nevada. Elsewhere, I would expect occasional bouts of low-topped convective precipitation–which may be semi-organized and come in waves that include heavy downpours, isolated thunderstorms, and possibly locally accumulating small hail at lower elevations. I don’t currently foresee any big individual storms (from a widespread heavy rain/wind perspective), but there will likely still be travel challenges given heavy and low elevation snowfall. But by early next week, things could get even more interesting, especially in NorCal…

Very cold & unstable airmass could bring low-elevation snow early next week

If the atmosphere plays its cards right, there is a potential for some very low snow levels in NorCal early next week (in addition to the possibility of widespread, disruptive sea level snow in the Pacific Northwest).


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