The Wicked Ridge of the West Coast

A large area of tropical convection covers much of the eastern hemisphere from about 60-150E, generally along the equator from roughly 10S-10N. Within this envelope, two separate intense regions exist, one across the eastern Indian Ocean (IO) and the other along 140-150E. This situation of 2 areas of tropical convective forcing across the eastern hemisphere is nothing new. We have seen this behavior from time to time since about 2001, and feel that warm SST anomalies in those regions are a contributor.

Large-scale extratropical baroclinic cyclone development is currently in progress west of the date line (see satellite pictures), as evidenced by a southward surge of cold air deep into the tropics from Asia. That, along with other complex feedback issues, is allowing influences from the west Pacific convection to dominate. This means yet another jet extension and rapid amplification of the downstream ridge to a position along the North American west coast into Alaska by this weekend. Even though the models did struggle with the temporary discontinous retrogression, they did capture the above scenario very well, and we would favor the more amplified solutions such as offered by the single run of the ECMWF and CDC ensemble (all very similar). The latter includes a trough digging southward into the central Rockies this weekend. This latest ridge is not good news for west coast precipitation and the already low snow pack regions across the northwest mountains.

We feel this situation will go through a life cycle of ~10 days, and may lead to a large blocking anticyclone across Alaska by early next week. Afterwards, perhaps during the time frame of 23-30 December, the westerlies will hopefully undercut the ridge and while the IO convection becomes dominate. If that is true, this would suggest opportunites of precipitation initially over the Pacific Northwest, then spreading south, as a trough develops across the western states. Again, this is just speculation.

For the DDC CWA, initially the source for cold air is not that impressive. However, true Arctic air coming into southwest Kansas is a real possibility early next week (~ 19-21 December). Chances for decent precipitation do not look good; however, a light snowfall is a plausible concern for this weekend (similar to what we saw around 7 December).

Ed Berry