Fall tidings: early showers and an intensifying La Nina

Even as far as summers go in this part of the world, this one has been pretty quiescent as far as the weather is concerned. Though there were a couple of fairly intense heat events, they were very short-lived, and did not produce widespread record-breaking heat. Much of the summer has, in fact, been well below average temperature-wise for most of California. The monsoon, despite being very impressive over Arizona and Sonora, never made it very far west over California and warm season convective storms have been uncommon over mountain and high-desert regions. Fire danger, accordingly, has been relatively low (especially when compared to recent years), and strong offshore wind events have been virtually non-existent thus far. It probably isn’t fair to blame this summer’s unusually boring weather on La Nina, at not entirely, since the atmospheric circulation has only recently responded to the cooling of the Eastern Pacific. The upshot of our cool and quiet summer, however, may well be an early and active fall.

An unseasonably deep trough of low pressure is digging southward just off the coast this evening. This system is similar to the one that moved over the state about a week ago, except this time there’s a bit more moisture associated with it and the trough and the jet stream is digging a little further south. Much of NorCal will actually see a threat of precipitation tomorrow, with mountain areas threatened with isolated thunderstorms and even coastal and valley areas potentially seeing some afternoon showers.

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Precipitation should be exclusively convective in nature, so coverage will be patchy, but some downpours could be locally heavy. The trough will move to the east by Thursday, bringing drier and warmer weather to the state. We should then see about a week of very seasonable, comfortable conditions across the entire state.

By later next week, though, an interesting scenario has begun to pop up on the numerical model output. Another deep trough over the Pacific Northwest is depicted by the GFS and ECMWF to slide down the West Coast by next Wednesday, possibly bringing yet another threat of earl-season precipitation to portions of the state. The GFS, however, indicates that the base of the trough may pinch off into a cutoff low and retrogress westward over the Pacific. With a nearly stationary but deep low off the coast, California would be in a very favorable position to recieve advected subtropical or tropical moisture from decaying Eastern Pacific storms. In addition, California would under a large area of divergence aloft, adding to the potential for some interesting weather.

This is all, of course, a ways off, and things are likely to change. But September is a month traditionally associated with cutoffs and synoptic-scale thunderstorm events over California, so it’s certainly something to keep an eye on.

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I’ll also give brief mention to the evolving La Nina event over the Eastern Pacific. In recent days, an intense low-level easterly wind anomaly has developed off of the South American coast, and a deep pool of anomalously cold waters have begun to surface in this region.

Since we’re already nearing sea surface temperature anomalies characteristic of a strong La Nina event, I now expect that there’s a pretty good chance that we’ll officially swing into the “strong” category within the next two months. The dynamical models and statistical models are unanimous in keeping the present La Nina around for our wet season, so I think we may have some pretty strong teleconnective indications for the upcoming winter. I’m going to hold off on being more specific, though, for at least a few more weeks. Right now,  it does appear that an early start to West Coast troughiness has already begun, and the potential for active fall weather is much higher than usual across California.

Stay tuned.

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