At a critical juncture, uncertainty abounds

The water situation in the state of California continues to become more serious with each passing dry day. This January warm and dry spell has been remarkable, and I do expect a couple more days of record highs in some areas before the pattern does begin to change. The nature of this pattern change, though, is anything but clear. The medium-range models agree that the persistent ridge that has persisted near or directly over CA for several weeks will finally begin to be eroded from the west by a weak undercutting southerly split of the jet stream. A weak cutoff low will ride into CA on this split flow around days 5-6. The models have been inconsistent as to the exact strength of the low and the precipitation that it will bring, but generally the models have become weaker and drier with time, and given the antecedent very dry weather I would expect that the models will probably experience a categorical wet bias, especially with the first system. Depending on the track, this low may be able to generate isolated to scattered showers, but precip will be light at beast and very possibly nonexistent in many areas. A second system will slowly follow this initial cut-off low by days 8-9, per the GFS, and this one may be deeper/stronger. Strength, in this context, is a very relative thing, and even this second low currently looks unlikely to produce much more than showers. I am extremely pessimistic about precip chances for these two systems because they’re the type of “non-storms” that show up in the models after a long dry spell and tend to evaporate before reaching CA. This split flow setup is also highyl dubious, and I really don’t think that a jet that weak will support a long-lived cut-off as the GFS indicates. The 00z GFS this evening was wetter than previous runs, but I still don’t think it looks very promising for any “drought-busting rains.” In fact, even the wettest rusn of the GFS would bring below-normal precip over the next 16 days to the entire state at a time when we absolutely need much heavier than average precipitation to fill our reservoirs and build our snowpack. The lack of January rain is also taking its toll on local water districts, even those which fared the last two dry years fairly well. Rainfall has been less than half of what would normally be expected by this time of year in the watersheds that comprise the Marin Municipal Water District, and the rainy season is now half over. This means that some places are already missing more than 25% of their total season precip, and the skies continue to be clear. I will likely be updating the California Climate section at some point in the next few days to discuss the ongoing drought. Rain dances might be in order.

Saturday addendum: The MJO does appear to be entering an active and progressive phase, and numerical models to predict an increase in intensity and an eastward propagation across the Pacific. If this is the case, we do stand a chance at a major pattern change bringing significant precipitation, but not for another 2-3 weeks…


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