Some sort of pattern change in the offing–flip a coin or three?

Filed in Uncategorized by on November 21, 2008 93 Comments

Well, in terms of the sensible weather, very little has changed since my last update. Varying degrees of offshore flow have persisted across CA for the past few weeks, and for the most part (and outside of the far north coast), there has been no rainfall whatsoever since the fairly significant NorCal deluge during the first few days of the month. Expect the current dry pattern to continue through  the weekend and probably the earliest part of next week. After that, though, all bets are off.

The global forecast models have lately been struggling even more than they usually do during the fall transition period, and given the miserable performance of numerical models during the fall and spring seasons during the past couple of years, this is truly saying something.¬† Huge run-to-run and day-to-day variations have occured (in fact, the only thing that has been consistent is the lack of continuity), and in the absense of strong MJO or ENSO signals from the tropical Pacific, this is fairly surprising. Regardless of recent performance, a sort of “consensus” is beginning to develop amongst the models that a fairly momentous change in the mean flow pattern over the Eastern Pacific and North America will occur over the next 4-7 days. There is some vague agreement that a cutoff low will be generated by the partially split flow early next week, and that this low will drift off the coast of CA. Certainly, the low will be deep enough to generate precip somewhere in CA, but the location, duration, and intensity, as well as the type of precipitation, is largely up in the air at this point. If the low stays far enough offshore early in the week, it may be able to pick up a rather juicy plume of subtropical moisture on its east side and produce widespread showers and possible thunderstorms in an extremely moist atmosphere with faitly mild temps and strong dynamic forcing. Thecore of the low would then move inland, bringing more showers and isolated thunderstorms (but of a more popcorn-style variety), with lower snow levels (perhaps 4000 feet or so). The models also seem to believe that a second system–this time a deep and jet-forced low from the Gulf of Alaska–will plunge south into CA on the heels of the first, dynamically weaker system. Depending on the exact trajectory of this system, there could be a rather dizzying array of outcomes, from mostly dry and cold with a dangerously strong offshore wind event in SoCal (easterly “inside slider” track) to quite cold, wet, and unstable over the whole state (offshore track) with relatively low snow levels (below 3000 feet; certainly the lowest of the season thus far). We may not know for a few more days which scenario is more likely.

And then, we arrive at the long range forecast. The GFS has actually had more consistency over the past 48 hours or so for the day 8-16 period than over the next week. This gives slighty more confidence to the long range than I would otherwise have, especially given recent model performance. A very active jet stream is indicated over North America, with an extremely large area of very high integrated kinetic energy well above the boundary layer (jet stream above 150 kts for most of the country to a long period of time). This will lead to a highly-amplified flow pattern. In such highly-amplified flow patterns, the West Coast can be susceptible to Arctic air outbreaks from the north/northeast. OR and CA’s coldest weather on record has occured as a result of extremely amplified flow patterns, and although it seems a bit early in the season, many of NorCal’s all-time low temperature records were actually set in early-mid December. I am certainly not predicting record-breaking cold over CA by any means, but any time a pattern such as this shows up on the models it is wise to watch very closely. Stay tuned…but don’t get too excited unless this pattern expectation persists into the “7-day-or-less” timeframe.