Saturday, December 31, 2005

Throw the RED Flag

The current circulation state globally, particularly across the Asian to North American sector, has been most difficult to understand, monitor, predict and, frankly, live with, for at least the past 30 days. In spite of the most significant tropical convective forcing remaining around 120E, we have seen the polar jet stream with wind speed anomalies in excess of 30 m/s extend from, at times, north of India to the west coast of the USA.

The start of this westerly flow regime can be traced back to a Rossby wave energy dispersion linked to a western Pacific convective flare-up during early-mid November. That resulted in a blocking structure around Kamchatka. Subsequent interactions with wavetrains and the eastern hemisphere tropical convection allowed blocking at the higher latitudes to expand and, at times, cover the entire Arctic during December. While westerly flow continued to be added to the mid-latitudes from the Indian Ocean/Indonesian tropical convective forcing (and other regions, as well), dynamic feedback mechanisms from the Arctic blocking also seemingly added westerly flow to the midlatitude belt. What these feedback mechanisms were and the details of the importance and timing of individual events is unknown at this time (requires further study).

Continuing on the notion of these unclear feedbacks, there was WEAK evidence that just before Christmas a wave energy dispersion from Asia into the Pacific was forcing tropical convection across the western hemisphere, particularly in the region of the Amazon rain forest over northern South America. I then went so far to think this response may be coherent enough to propagate back into the eastern hemisphere as a "MJO like" signal (see December 31 Blog posting). Upon further review, while the idea of a weak signal coming from the extratropics into the tropics of the western hemisphere is plausible, the "MJO like" signal thinking cannot be defended for this particular case. Hence the red flag (like a NFL game!). The wave energy disturbance mentioned above has, in fact, propagated back around into the western hemisphere at this time.

With all the above stated, I do not see any real strong physical mechanism to change what is in effect SDM Stage 3 in terms of the circulation (extended north Pacific jet) at least for the next 7-10 days. There is evidence that the anomalous westerlies across the midlatitudes are shifting poleward (both hemispheres), along with increasing cyclonic flow at the polar latitudes. For the USA, this would suggest that while the Pacific northwest states gets significant precipitation, generally westerly flow with embedded disturbances should prevail across the central part of the country, much like what most models are showing, for the next 7-10 days. I refuse to go beyond day 10 since, to me, there cannot be any hope of a scientifically sound prediction in this regime.

For southwest Kansas, temperatures are likely to stay well above normal with little or no precipitation for at least the next 7-10 days (on average). A Pacific storm system may bring some sprinkles early next week. High fire danger is already a concern, and is likely to remain so at least through the end of next week.

Ed Berry

No comments: