Rainy week ahead for much of California

Filed in Uncategorized by on January 31, 2010 32 Comments

Now that all the excitement of mid-January has calmed down substantially, we seem to have entered into a more moderately active pattern over the West Coast. While a fairly strong Pacific jet will remain not too far off the coast for the next week or more, its orientation is simply not favorable for bringing major storms into California. That said, much or most of the state will see precipitation this week, and in some places it will probably be fairly substantial. The early-week systems will be quite weak and are unlikely to bring rain any further south than the Santa Barbara area, and even NorCal will see only light precipitation. As week progresses, and the jet edges closer to the coast, rainfall will probably increase in most places, and moderate accumulations are likely at that time. Precipitation of this magnitude will actually be rather beneficial to a state still trying to recover from a multi-year water defect. Soils are still at or near saturation over much of the state, so the late week systems will have to be watched for signs of strengthening beyond what is currently depicted by the models. Right now, though, it does appear that California will simply see those seasonal Percent of Normal totals creep towards (and even above) average.

GFS 126 hr plot depicting the late-week system

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Also worth noting is that the current amplitude of the MJO signal is rather strong. Over the past week or so, an area of enhanced convection has been rather stationary in the Western Pacific, and this has prevented the MJO signal from propagating eastward. This enhanced convection corresponds to what one might expect during a significant El Nino year, so it’s possible that the MJO and ENSO signals are presently “overlapping.” This does provide some evidence that the atmosphere is responding to the sea surface temperature anomalies in the Pacific. Implications for California, as usual, are not clear-cut, but the strong and rather zonal subtropical jet over nearly the entire Pacific may be a consequence. The medium-range models do keep a relatively active pattern over CA for the foreseeable future (though they don’t explicitly predict any major storms), and I see no reason to doubt this.

MJO propagation timeseries (CPC)

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