Wednesday, December 28, 2005

Clear as MUD

Since my last posting, perhaps the most significant change has been the tropical convective forcing shifting into the southern hemisphere with the seasonal cycle. At this time the centroid is centered around 5-10S/110-120E, and the Australian monsoon has started. Nevertheless, in the zonal mean anomalously strong westerly flow continues across the midlatitudes of both hemispheres, with values as high as ~20 meters/sec. That includes the extended polar jet stream across the north Pacific Ocean basin, with a deep low centered around the date line and generally split flow/ridge conditions across central North America. Several weather systems will continue to impact the west coast and progress across the CONUS for at least the next 1-2 weeks (details unknown after about day 3).

As I have stated several times, the northern midlatude circulation is not consistent with the location of the tropical convective forcing. Instead, strongly nonlinear feedbacks from extratropical dynamics including the polar latitude blocks have contributed. There is evidence the nonlinear forcing is weakening with the tropics exterting an increasing influence on the global circulation.

If there is to be a linear extratropical response, then a ridge should develop across the central and eastern Pacific with a western USA trough by on the order of days 10-14 into week 3 (SDM stage 1). Indeed, many models today suggest this possibility by showing a contraction of the east Asian jet by roughly the middle of next week. However, only additional monitoring of both the weather-climate situation and model runs is the best anyone could offer in terms of days 3-14 predictions at this point. Please remember models do not predict the character of any kind of coherent tropical convective forcing beyond about days 5-7 and the details of any possible pattern change is unclear. It should go without typing that confidence in any prediction particularly across the USA (anywhere) for days 3-14 is about as low as it gets (despite what might appear to be good model agreement) at this time.

For southwest Kansas, at least through this upcoming weekend continued warmer than normal and dry. There may be an opportunity for some light precipitation in the Monday-Wednesday time frame next week (much better east and north) while temperatures stay above normal. Week 2 is unclear.

Ed Berry

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