An extremely powerful storm system has been raking the Pacific Northwest with powerful winds, intense rainfall and low elevation snowfall for the past 48 hours. Peak winds exceeded 120 mph in several locations near the coast, leading to widespread damage. 10-12 inch rainfall in less than 24 hours will lead to major flooding on local rivers. Far NorCal saw some strong winds (gusts 55-65 mph) and some significant rainfall (1-3 inches), but the majority of the state will see very little from this system beyond some light rainfall about as far south as the Bay Area. After a brief period of calm weather midweek, a significant system will begin to take shape in the far eastern Pacific that will impact CA by Thursday. A cutoff will form and by intensified by a jet streak on the convergent side of the low. The 12z GFS is a little less bullish than previous runs with the strength of the incoming system, but still indicates a fairly substantial event, and the NAM and ECMWF are still showing an impressive system. Since this will be a cold and dynamic system, convection will be a concern throughout the event. We could even see some prefrontal thunderstorms in the Sac Valley on Thursday if the lift is great enough on the east side of the low (though that will be more likely if yesterday’s scenario plays out). Some rather heavy rainfall will occur with the frontal band itself along with the threat of embedded convective elements (thunderstorms), and with a sub-995 mb low right off the coast there could actually be some strong winds as well. Once the low moves inland, snow levels drop and an unstable regime brings numerous showers and scattered thundestorms to the state for a good 24 hours (poss. some isolated severe weather in the Valley). Models have shied away a bit from the active pattern that they were indicating for days 7-15, but they may trend back. La Nina has likely peaked at this point, entering a mature stage within the past few weeks (La Vieja, by some climatologists) and may begin to transition to El Nino by the late winter. This La Nina, like the last El Nino, was odd–much of the Pacific Ocean actually has above average SSTs at the moment (except for the tongue of negative SST anomalies near the equator in the traditional ENSO zone). Normally, the MJO wouldn’t have much of an effect on Western weather during ENSO cold episodes. Now, however, a strong MJO signal is developing in the Wstern Pacific which may lead to some interesting weather later this month if it is able to propagate eastward (amplification of the mean ridge into AK and the Arctic w/downstream trough West U.S.–very, very cold pattern!). We’ll see what happens. In the meantime, enjoy the rain this week…
