An unusually cool summer continues over California; long-term prospects interesting

Summer 2010 has been a very cool one over much of California, especially along the coast where marine stratus has been rather extraordinarily persistent. Monsoonal surges have been tough to come by, though some very moisture-rich air has been residing over northern Mexico and Arizona for much of July and August thus far. After a relatively warm day today and another expected tomorrow, temperatures may once again begin to cool as increased troughiness develops over the far Eastern Pacific.

Source: NCEP


Most everywhere else in the Northern Hemisphere, though, it’s been quite a hot summer indeed. Most of you have already heard about the Great Russian Heat Wave of 2010, which has shattered all high temperature records imaginable in Moscow and across much of the western half of the country. The past month saw the warmest July global land surface temperatures ever observed.

Source: NCDC

Readily apparent in the above image is the tremendous warm anomaly over much of Europe and the Middle East. Interestingly, an area of fairly large cold anomalies is present just east (and downstream) of the region of record-smashing warmth. This is probably due to the extremely amplified flow pattern (induced by very persistent blocking) that has dramatically increased the amplitude of meridional (north-south) flow over much of Russia. As a result, return flow from the Arctic on the east side of the blocking ridge brought unusually cold temperatures to a limited region. This unusually deep trough has also contributed to the devastating and unprecedented flooding in Pakistan over the past month. Also of interest is the large area of cool anomalies located over the entire Eastern Pacific and over western parts of South America. This is almost certainly a product of the presently-intensifying La Nina event, perhaps with some contribution from the negative phase of the Pacific Decadal Oscillation (PDO), especially in the North Pacific.

Numerical models are presently in good agreement that the highly amplified pattern over Russia (the persistent blocking ridge) will soon be ousted, and more usual westerly flow will resume. This may have some downstream implications for the Western United States in the 7-10 day period. It’s possible that the persistent troughiness near the West Coast may retrograde far enough offshore to allow for some more seasonably warm summer temperatures across CA and the West. Stay tuned for updates on that possibility.

Finally, I’ll return for a moment to the intensifying La Nina in the Eastern Pacific.

Source: NESDIS

Sea surface temperature anomalies in the Nino 3.4 region are already in excess of 1°C, and given the present rate of decrease I would expected maximum cold anomalies not to be reached for at least a couple more months. Since we’ve technically already crossed the threshold of moderate La Nina, I think there’s a pretty good chance of a full-fledged moderate to strong La Nina persisting for at least the first part of winter. Implications for California, as always, are mixed and not especially well defined. Traditional impacts might include below normal precipitation for the southern part of the state, and above normal precipitation in the far north. However, it’s important to keep in mind that even strong La Nina years exhibit large variability, and some pretty notable flood events have occurred during such years (even if season-averaged precipitation was below normal!). Anecdotally, at least, there also seems to be some correlation between stronger La Nina episodes and significant individual cold events in California. We’ll have to see about that, but given the negative phase of the PDO and an ongoing trend of enhanced meridionality in the Northern Hemisphere, I suspect we may have a colder than average winter this year. Once again, only time will tell. I’ll have an update on the state of La Nina in a few weeks.


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