Friday, May 11, 2007

Update on the Return

Shift work obligations and data issues preclude a complete discussion. I will try to be more comprehensive Monday-Wednesday next week.

There is no change from my thinking as discussed on 5/8. Global tropical SSTs remain similar and any development of La-Nina is still a monitoring issue. There has been a rapid coherent eastward shift of tropical convective forcing across the Eastern Hemisphere which does project onto a convectively coupled Kelvin wave. The centroid of this forcing is ~0/140E having recent 3-day OLRA less than minus 70 W/m**2. In fact, a surface westerly wind burst is accompanying this convection and may adversely impact development toward a cold event. This recent eastward shift has some similarities to the behavior of the tropical convective forcing seen during late March-early April. Tropical forcing also remains active across the Indian Ocean and even the South Pacific.

I think we are headed for a transient GSDM Stage 2 response for the early-mid part of next week (also seen during the first part of April), and most models have captured this evolution. As both the NWS/CPC and SPC forecasts show, probable weather impacts include heavy rainfall and severe storms focusing on the central-northern Plains and Upper Mississippi Valley.

However, I also think there will remain the issue of two regions of Eastern Hemisphere tropical convective forcing, the Indian Ocean and the west central Pacific (along with other matters). This favors a GSDM Stage 4-1 response meaning more western USA troughs interacting with above average subtropical jets. Hence I disagree with many week-2 ensemble means from various operational centers and recent official forecasts derived from them suggesting ridge conditions across the USA for that period. Shifted northwest, I am holding the line of resumption of exceptionally active weather for the Rockies and Plains perhaps starting as early as next weekend and continuing into week 3 (~19 May – June 1). Ramifications include more severe storms and flooding rainfall for the central and northern Plains, summertime heat across the Southeast and cool conditions with heavy snowfall for the Rockies. Recent ensemble numerical models have been starting to trend toward this prediction.


An experimental phase space plot of the GSDM (depicting a global wind oscillation) utilizing normalized relative AAM tendency (Y-axis) and normalized relative AAM (X-axis) can be found at

These are probabilistic statements, and work is ongoing to quantify in future posts. The WB (2007) paper on the GSDM has been published in the February issue of MWR. I will try to write an update sometime Monday-Wednesday next week.

Ed Berry

No comments: