2009

After a couple of weeks of relatively benign (if hot) weather, a dramatic change is in store for much of the state. After a weekend of triple-digit heat over much of interior CA, highs even in far-inland parts of NorCal will struggle to reach 75 by Tuesday. The culprit behind this dramatic change in the prevailing pattern is a strong early-season trough in the eastern Pacific. This trough will be supported by a powerful 150 kt jet as it digs southward on Monday and Tuesday, and there is a considerable amount of cold air aloft associated with this system. One other item to consider is a somewhat diffuse mass of mid-level subtropical moisture currently floating around to the southwest of CA. Lightning is occurring within this moisture plume at this time. At least some of this moisture is expected to be picked up by the incoming trough, but right now it appears that most of it will be shunted far to the east before it can more far enough north to affect much of CA. However, since this system is progged to have 850 mb temperatures in the low single digits C, it is possible that even marginal moisture may be enough to support some convective activity, especially with the jet streak overhead. Convection will be most likely over far N. CA, with lesser chances extending southward into the Bay Area and Sacramento Valley. It is certainly worth keeping an eye on the convective potential of this system, however, as insolation at the surface is still relatively strong this time of year and can lead to very steep lapse rates over inland areas. In any case, the most likely outcome is much cooler temperatures and rather windy conditions in many areas. In the wake of the trough, a strong northerly gradient will develop, and this will lead to increasing northerly winds at the surface Wednesday into Thursday. Since northerly winds this time of year tend to have a downsloping and drying component and fuels are still extremely dry after our long summer and drought, fire weather conditions will be extreme across almost the entire state for much of the week.

Temperatures will warm again late in the week as high pressure temporarily builds back in from the south, but this will quickly be displaced by a new and even stronger area of low pressure. The models are currently indicating a very deep cutoff low to retrograde from the inland Pacific Northwest to a position over NorCal by late next weekend. Given the time of year, this type of pattern could lead to a multitude of different sensible weather impacts. It does appear that some very strong offshore/northerly winds are possible at some point towards the beginning of the event, and as the low hangs out over NorCal for several days, the possibility of convective activity does become a concern. Much to discuss this week.