There are two factors we have to watch out for in California. First, during La Nina events, the subtropical branch of the jet stream is normally very weak due to the colder Eastern Pacific Waters. This can be a mixed blessing. One one hand, it means a high ridge over the eastern pacific is more likely to form even during the winter, leading to dry and warm days. Yet, the other side of the coin is that this leaves our coast much more subsceptible to artic blasts as a result of the wims of the northen branch of the jet stream.

	La Nina? • NWS Prediction for October • Synopsis of 1998 Freeze • More NWS Seasonal Outlooks • Our Conclusion Be on the lookout this winter. Hard freeze events are predicted well in advance by the National Weather Service. During the months of December and January, make sure to consult their Forecast Page on a regular basis and pay attention to the weather forecast during the months of December and January.

Last Winter, this is precicely what happened around December '98. The jet stream sent Canadian cold air straight over land in a northeast to southwest pattern. Prior to the arrival of the series of cold short wave impulses, the weather was dry and warm. A lack of moisture meant that radiative cooling was very intense. With daytime temperatures already in the low to mid-40's and crystal clear skies, it didn't take but a couple of hours of darkness to send the temperature plummeting into the 20's throughout central and northern California. For more details on last winter's freeze, click here.

But will there be a repeat this year? Already, we have had a very dry fall, and even on the coast, the humidity levels have been very low. No one really knows. Some long term computer models seem to indicate that the winter will warmer than average, which has lead meterologists to simply state that the winter will be near normal. This is based on the reasoning that the warm models cancel out the historical probability of colder conditions typically associated with the La Nina phase of ENSO events.

The second factor which will affect the coast is that already cooler waters make for a cooler starting point for overnight temperature drops. In a way, the ocean always serves as a temperature baseline, and if this baseline is already a few degrees cooler, then this means temeperatures will cool off even more as a result of radiative losses. This translates into more frequent light frosts even without the help of the jetstream. More frequent harder freezes can be expected for subsceptible areas such as the bottoms of canyons and valleys, especially in the Santa Cruz mountains.

While it may turn out to be a beautiful and abnormally warm winter, the best advice is still to be on the lookout. Hard freeze events are predicted well in advance by the National Weather Service. Therefore during the months of December and January, make sure to consult the NWS Forecast Page on a regular basis. Another way to get even more lead time is to browse the discussion boards frequented by warm climate plant hobbyists up north. The best boards we know of are the Hardy palm and Subtropical Board and the Northwest Palms Board. A freeze in the Pacific Northwest doesn't necessarily imply an impending freeze in our area. But if one will make it this far south, it is due to hit our neighbors up north first.

The Cloudforest Gardener

Visit the Weather Cockpit