The Northern Hemisphere has been experiencing a very unusual, highly-amplified flow pattern over the past two months. Record-breaking cold and unprecedented snow events have occurred over all of the Northern Hemisphere continents during this period, with particularly notable cold outbreaks over the eastern 2/3 of the continental United States, nearly all of Europe, and much of Southeast Asia. With this large volume of Arctic air screaming south into lower latitudes, there must be poleward return flow somewhere to compensate. This northward meridional warm advection has been extremely impressive over the Pacific and Atlantic oceans, with very large volumes of very very warm air moving to surprisingly high latitudes. In recent days, it has been colder in central Florida than either southern Alaska or Greenland (credit to Jeff Masters for pointing this out). This entire process is fascinating to watch, but it is exceedingly unusual for this kind of pattern to be this persistent or this great in magnitude. While the mean flow regime across the Northern Hemisphere is expected to moderate in the coming days, this extended period of “thermal equalization” will probably have an effect on the pattern for the rest of winter, or at least for the next month or so. This may be good news for rain prospects in California, as a powerful zonal jet already exists, stretched out over nearly the entire Pacific Ocean.
GFS 200 mb wind chart for 180 hrs., 00z 1/9/2009
As the East Asian Jet expands even closer to the West Coast as we head into Week 2, there exists a strong likelihood that a significant amount of energy will be directed into CA. There is plenty of moisture and energy out there, as has been stated numerous times since October: it is only a matter of time before the jet breaks through. With the impending breakdown of the anomalous high latitude pressure patterns (huge blocking ridges locked in place), a very notable pattern change does indeed seem to be in the works. The GFS has been explicitly indicating the potential for some very robust and moist storms for the entire state; in fact, there are some indications that the jet may dig far enough south to give SoCal and Baja a round of very wet weather at some point (which would be–gasp–a pattern one might expect during a strong El Nino event). This is still a long way out, and the models will undoubtedly struggle with the breakdown of an already rare flow regime. Stay tuned, as this is the kind of pattern that can potentially erase significant rainfall defects over the course of a couple of major storms. Only time will tell…