Major pattern change on the way

Filed in Uncategorized by on January 4, 2007 0 Comments

Today’s “storm” was quite pathetic–we got 0.05 in. in the San Rafael hills with no significant winds. Cold air has moved in behind the front and temperatures are dropping rapidly. Warm afternoons and cold nights will be the rule for the next few days, but the sensible weather really will be quite pleasant for all of CA. Another strong and cold Santa Ana event is ongoing in SoCal, but that will die down pretty quickly.

The discussion becomes interesting in the 6/7 day period. The very-long advertised pattern change in the Eastern Pacific is still mostly on track. There is still a great deal of uncertainty regarding the evolution of this potentially extremely cold pattern along the West Coast. Here are the latest developments:

The exact location of and extent of the meridonal elongation of the block expected to develop in the E. Pac. will be critical in the advection of extremely cold polar air into the Western states. Today’s 12z GFS run was the most impressive yet in terms of cold air in CA/OR/WA and the potential for precip. to occur during the periods of coldest air aloft. The 18z (and most recent) GFS was somewhat less emphatic with the cold air advection and associated moisture in the Arctic trough. The main reason: the GFS has been trending towards a more easterly development of the trough downstream of the blocking ridge, which would limit to some extent the magnitude of the cold air and even more so the amount of moisture contained in this trough. Unlike model solutions a few days ago, the tough is not expected to become dramatically elongated in a SW/NE fashion. This limits overwater trajectory and cuts down on the chance for precipitation. That being said, however, the 18z run is the first model run to indicate this and is currently the outlier among the other models: the ECMWF, which had not been indicating anywhere near as deep and cold a trough as the GFS had been 24 hours ago, is now indicating a pattern much more favorable for cold weather and precipitation. I don’t expect that we’ll really know exactly what will happen until a few days before the event. Even if the 18z GFS were to pan out, however, snow flurries would still be possible along virtually the entire Pacific Coast north of Santa Cruz. And I do fully expect the 00z run to come in wetter and colder once again with a slightly more overwater trajectory for the trough. What may be complicating this pattern…and certainly the models’ interpretation of it…is the developing strong MJO over the Western Pacific. This may be the reason why the GFS has trended towards a less persistent, weaker, and more easterly block and associated trough. This could be a valid trend; however, the models tend to do very poorly with such forecasts when a developing MJO is involved. The potential certainly still exists for the southern stream to break under the block and bring warm tropical rains to CA (after day 14 or so) even if the models don’t indicate it explicitly right now. So, in short, the forecast will likely change a lot over the next few days, so stay tuned.

A note: if the pattern developed exactly as the 12z GFS indicated it would, the associated snow levels and accumulations in CA would rival the storm of March 2005. In that storm, patchy accumulations of snow and hail caused numerous problems in the San Francisco Bay Area. 850 mb temps then were about -4 to -5 C over the Bay Area. 12z GFS had 850 mb temps to -11 C over the North Bay and      -13.5 C over Eureka on next Saturday.