Quiet weather this week; El Nino ramping up

The weather this week will be uneventful across the entire state, with only a few chances of light precipitation in NorCal around midweek.  Far more interesting is the current state of the equatorial Eastern Pacific. El Nino–which had, until the beginning of October, been appearing to falter–has recently gotten a huge boost from a major Kelvin wave propagating across the Pacific basin. SST anomalies have very nearly skyrocketed for the past 3-4 weeks, and all Nino regions are now well above average in terms of both SST and overall heat content. Though it is hard to say how long this trend will last, especially once the Kelvin wave reaches South America and the deep source of warm water eases temporarily, we are already in the “moderate” range of SST anomalies. I think there are roughly equal chances at this point that we remain in the moderate category or progress to a strong El Nino this winter. This prognosis is much more positive than one that would have been made a month or two ago. As we trend towards upper-end moderate or a strong event, the impacts on CA become more pronounced, and so for the upcoming winter there may be a pretty strong climatological signal (favoring above normal precipitation, and perhaps generally milder than average conditions). I will update the Seasonal Outlook this weekend, but this is the crux of the forecast: an above average winter looks to be twice as likely as either an average or a below average one. Things will become more certain by the end of November with regard to the likely peak strength of El Nino…