Full disk satellite imagery and other monitoring tools suggest the recent eastward shift of the tropical convective forcing from the Indian Ocean into Indonesia has stalled. The centroid of the forcing is currently at ~0/140E while a faster component has excited convection downstream at around 10S, 180E, enhancing an eastward shifted South Pacific Convergence Zone. The overall convective pattern is losing its MJO characteristics. Previous thinking was for the MJO to intensify the convection in the region of the South Pacific leading to the “classic warm ENSO” global circulation response by about the middle of this month. We now believe this is not likely. Instead, coupling may occur west of the date line near 10S/160-170E while the Indian Ocean remains convectively active, roughly every 30 days. In fact, the South Indian Ocean may see intense rainfall in about 10-14 days since SSTs remain above average there. Convective forcing over the Indian and west Pacific Oceans may consolidate around Indonesia during weeks 3-4.
Zonal mean easterly wind anomalies have propagated into the subtropics and been replaced by equatorial westerly flow anomalies. The latter have been most robust across the East Pacific. Some of this anomalous westerly flow is coming back into the Eastern Hemisphere, and should lead to respectable intensification of the subtropical westerly jet during the next couple of weeks. However, considering the tropical forcing and other dynamical processes, this intensification does not appear to be enough to allow an extended combined North Pacific jet to reach the USA west coast. Instead, as most ensembles are showing and as is already happening, ridge amplification will occur across the north central Pacific into Alaska with a downstream western USA trough for week 1 and then shift slightly east week 2 (GSDM Stages 1-2). For weeks 3-4, the most probable scenario would a retrogression of the ridge-trough pattern while a subtropical jet “undercuts” the ridge and heads into California. This scenario is not consistent with El-Nino composites, possibly due to strong convective forcing from other regions such as the Indian Ocean.
The USA is in for a huge weather change. As Arctic initially plunges into western states, significant baroclinic development on the Plains is likely especially Sunday-Tuesday next week. The models are playing catch up. Impacts will include another blizzard from the Rockies into the Upper Mississippi Valley while heavy rain and severe thunderstorms occur across the Deep South and Ohio Valley. While week 2 should be generally cold and dry for much of the country, weeks 3-4 may once again become stormy with continued cold. Locations such as the California coast may not see decent chances for precipitation until after the 20 January.
We will try to do another posting this Friday. Please see past discussions for URLs.
Ed Berry and Klaus Weickmann