Re-developing La Nina and negative PNA: bad news for CA?

Filed in Uncategorized by on December 30, 2008 112 Comments

After a period of cold and unsettled weather across CA, the weather has become rather stagnant. Some heavy rain did fall over extreme NorCal–near the Oregon border, mountain slopes saw more than 7 inches over a 48 hours period. Some light showers persist over the far north and will continue through the rest of the week; south of about Mendocino, though, the weather will be dry (if cool and foggy in valley locations). Some inland areas away from the fog could bask in near-70 degree weather–not totally unheard of this time of year, but certainly a change from the prevailing conditions as of late.

The GFS is currently depicting a fairly weak cold trough dropping south into CA from the north on Friday and Saturday. A fairly weak cold front will pass through NorCal, bringing a period of light to moderate showers. Behind the front, air aloft will become somewhat unstable, but moisture and dynamics will be severely lacking and I would expect any convective precip to be isolated at best. After the weekend “event,” conditions will become even less active–and by day 10, it does appear that a powerful ridge will set up and deflect even light precipitation well north into Canada for the forseeable future. It’s important to remember that mid-winter dry spells are very common in CA, but the problem here is that we haven’t had sufficient precip so far this season to comfortably enter a prolonged dry spell that looks like it will last at least 2-3 weeks. The first snow survey of the season today indicated that, even after this series of cold storms, snow water content is still somewhat below average for this time of year. I expect next month’s survey to be significantly further behind normal values, given the lack of precip and relatively warm temps expected over the next 1-3+ weeks.

For the longer term, several factors are going to be in play. The Climate Prediction Center has just confirmed what a quick glance at SST anomaly maps has been showing for some time: La Nina has redeveloped, despite the fact that nearly all numerical models over the summerhad indicated otherwise. The prevailing pattern so far this winter has been similar to one that might be expected from a weak to moderate La Nina event. Unfortunately, this dones not bode particularly well for above-average precipitation for the rest of the wet season (which is what we would need to avoid water rationing next summer). This is not to say that the rest of the year will be catastrophically dry, but given the prevailing patterns in conjunction with the current negative state of the PNA teleconnection, I’m not particularly optimistic about rain prospects, at least for the next 2-5 weeks. It would be rather nice, actually, if I had to eat my words…