Weather West: After Frustrating False Starts, Strong Indications of Major Pacific Jet Extension & Active Pattern by Late Dec

The following is an excerpt published with permission from Weather West

If you were in California this autumn, you probably already noticed: this fall was unusually warm and dry nearly everywhere in the state (and, indeed, across most of the Western U.S.!). Despite that, given unusual summer rainfall thanks to Tropical Storm Hilary in southern California and the legacy of an unusually wet and (by recent standards!) cold winter in California during the 2022-2023 season, no extreme wildfire activity was observed this autumn in the state.

The last month certainly hasn’t be completely dry in the northern half of the state–occasional rain and high elevation snow events have occurred, though they’ve been a bit hard pinpoint very far in advance (as sometimes occurs during transition seasons). But SoCal has remained decidedly dry through the period–and for this reason, the prospect of renewed Santa Ana winds over the next couple of days will bring with it the prospect of increased wildfire risk (though, again, not extreme risk thanks to fairly typical vegetation moisture conditions). Sierra snowpack, though it did finally receive a modest boost this week, remains behind the curve for early Dec (though it’s quite early in the season, and early season deficits can be erased by a single strong and cold-enough storm).

The Pacific Northwest had also been unusually warm and dry for much of the autumn–but that changed dramatically over the past week (well, the “dry” part did, at least). A series of strong to even “extreme” atmospheric river events have soaked the entire PacNW (and extended even down onto the far North Coast of CA at times!), bringing not only record rainfall and moderate to major river flooding and landslides, but also *all time* record warm temperatures to certain areas. In fact, there were quite a few spots that set new daily rainfall records and new daily (or Dec monthly!) high temperature and atmospheric moisture records *within the same 24-48 hr period.* That’s because these ARs have been classic “Pineapple Express”-type storms, with deep moisture transport corridors extending all the way south toward the subtropics. Essentially the entire mid-latitude North Pacific is much warmer than average at present, and this has likely contributed to the remarkable and sometimes record-breaking warmth and moisture within these atmospheric rivers across the Pacific Northwest in recent days.

False starts to an active California pattern in Nov and Dec: some context

As I’ve discussed at length in my recent YouTube virtual office hours, it certainly has been a frustrating month or so for “model riders” of Weather West (and elsewhere). 2 or 3 distinct “pattern changes” toward more active and wetter conditions have, largely, failed to materialize in CA (though the storms are out there–either over the PacNW, as noted above, or in weakened form mainly in NorCal). Although this is aggravating, it’s also worth noting that such “false starts” in the model world are not uncommon, and that’s especially true during the autumn and spring transition seasons before the hemispheric “base state” is more favorable for a faster and more strongly “zonal” (west-to-east-oriented) jet stream.

What to I mean by that? In general, the stronger and more linear the jet stream, the more predictable the large-scale pattern (i.e., the placements of low and high pressure systems, and thus troughs and ridges). The jet stream has a highly predictable seasonal cycle (i.e., essentially every year, it retracts poleward in summer and slides equatorward in winter, as well as becoming stronger in winter and weakening in summer) in synch with meridional temperature gradients at planetary scale. But is also has a high degree of semi-predictable variability: this essentially represents all the daily to weekly “wiggles” in the longwave pattern that dictates warm, dry weather under a ridge of cool, wet weather within a trough. That high frequency variability is essentially what meteorologists try to predict, days to weeks in advance, to understand variations in weather in specific years.

The seasonal cycle of the jet also affects its ability to interact with other “forcings,” or external influences that might alter its position and strength. Such forcings include things like ENSO (and the current El Nino) and the Madden-Julian Oscillation (MJO), though there are others. (BTW–patterns like the PNA are generally *not* considered forcings since they’re simply an index that describes the state of the atmosphere in a given moment, rather than an actual physical cause of variability. Thus, they are sometimes said to be “diagnostic” rather than “prognostic”). This is one of the key reasons why I’m not overly concerned either by this particular dry autumn nor these recent ensemble predictive “busts”: the autumn seasonal base state is very different than the winter base state over the Pacific Ocean, and the Jan-Mar period is much more favorable for “constructive interference” between ENSO, the MJO, and the large scale pattern than the Sep-Nov/Dec period is.

Remarkable Pacific jet extension looks increasingly likely for mid-Dec

I’ll start by reiterating: despite what some of the hate mail in my email inbox might suggest, I can’t controlthe future; I can only try to predict it. That’s my way of reminding folks there are never any guarantees when predicting the future–I can’t guarantee any particular outcome. But right now, there are some pretty striking things appearing in the long-range ensemble model suites that are worthy of close consideration. That’s especially true as we move from Dec toward Jan, which will be the most favorable period for coherent coupling between the strong El Nino event in the Pacific with the MJO and other more stochastic variations in the North Pacific atmosphere.

For these reasons, when I see images like the one below , I sit up and take notice. Fresh off the presses (the 12z GFS ensemble average from 12/8), it depicts a dramatic eastward extension of the jet stream beginning within a week near the coast of Japan and then eventually extending across almost the entire North Pacific basin toward the California coast (over 5,000 miles away!) by about 2 weeks from now. Keep in mind that this animation is the *average* across dozens of individual ECMWF model members–so there is quite a bit of spatial and temporal smoothing. To see such an extremely strong and straight jet extension extending zonally (longitudinally) for thousands of miles is very unusual, and is often a portent of a very active weather pattern to come shortly thereafter along the U.S. West Coast (and, especially in a year like this one, perhaps California).