MJO-related pattern change on the way…

Filed in Uncategorized by on October 24, 2008 14 Comments

Very warm–even locally record-breaking–temperatures are occuring throughout California today and will continue for the next few days. Fire danger will be high as occasional dry north winds develop in favored reass. All of this warm and dry weather, though, will probably be a memory by this time next week.

In the interm, a fairly weak low will drift over NorCal this weekend and early next week. Although not likely, mid-level cirulations over the ocean, especially this time of year, do have a rather disconcerting propensity to unexpectedly develop elevated convection in the form of bands of light showers and isolated thunderstorms. As such, it would be prudent to keep an eye on this low, as there is some decent instability associated with it.

As has been noted in recent days, the Madden-Julian Oscillation has been very active in the West Pacific over the past 2 weeks. The strong MJO-related signal is beginning to propagate across the Pacific, and the mid-latitude forcing associated with the MJO-induced convection near the equator will begin to impact our weather by day 5. This is a bit early for a strong MJO-induced stormy period, but the models are beginning to come into agreement that there will be some significant rainfall in CA (most of the state, actually) next week. Operational numerical forecast models do not really incorporate the MJO-related signals very well, and so tend to be very volatile when active phases in the MJO pop up. Much will likely change in the forecast between now and next Thursday, so don’t look to closely at any patricular model output just yet. I do think there exists the possibility of some heavy precipitation and strong winds at some point, but this remains to be seen. Again, the entire state would stand to see some significant rainfall if this pattern change does indeed occur, especially if the last of the cold fronts is able to get a subtropical moisture tap as the latest GFS is indicating.

 An active MJO bodes well for a winter that is not catastrophically dry–it may still be below average, but there may at least be some brief periods of substantial rainfall associated with these MJO events. I may update the Seasonal Outlook accordingly if this keeps up…