Potentially very wet pattern on the way for California

Filed in Uncategorized by on December 12, 2010 93 Comments

Much of November and the early part of December has been punctuated by near-normal precipitation but rather turbulent temperature fluctuations. Thanksgiving week saw an unseasonably cold outbreak of Arctic air overspread much of the state, while much of Southern California currently basks in record-high temperatures in the 70s and 80s.  No notable major storms, however, have really impacted the state during the first few weeks of the traditional wet season. This may change rather abruptly later this week, as a powerful Pacific jet aims a series of significant storm systems right at Central California.

GFS 200 mb winds (NCEP)

Global models have been indicating the potential for a significant shift in the longwave pattern over North America over the next few days. The West Coast ridge, which has contrasted strongly against the very deep and cold trough over the eastern half of the country in recent days, will be replaced by a deep and broad mean trough over the far eastern Pacific. Once this occurs, a 150+ kt zonal jet stream will arrive in California at the base of the trough.  This westerly/southwesterly flow is expected to pick up a substantial amount of subtropical moisture as it moves towards the state, and so the storms from late this week into early next week will be generally warm and wet. The models are already keying in at least 2 or 3 significant surface lows/discrete storm events during the next 10 days, the brunt of which will probably impact NorCal between Friday Tuesday. These storms will likely be pretty strong, and will certainly have the potential to produce heavy rain/snow and some strong winds at times. It’s not presently clear how far south the strongest storms will reach, so although I do expect SoCal to get at least some rainfall, confidence in a major storm for SoCal is not nearly as high as for places further north. Given the amount subtropical moisture that could potentially be involved and the very favorable orientation of the jet overhead, parts of NorCal could have some hydrological issues by early next week. Antecedent conditions are rather dry, however, so it will take at least a storm or two’s worth of heavy precipitation to get regional waterways flowing high. The GFS does indicate that the potential for wet or very wet weather could continue for quite some time, which could present some problems down the road. Stay tuned…

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