Stage 0.5 Update

There is no change to the discussion I issued on 4/20. Upper tropospheric zonal mean easterly wind flow anomalies are increasing throughout the tropical and subtropical atmospheres with much of that coming from the Eastern Hemisphere. In fact, anomalous twin subtropical anticyclones are becoming quite robust from Africa into Indonesia. Global relative atmospheric angular momentum (AAM) tendency remains at ~minus 20 Hadleys as of April 21st per reanalyis data (R1) with actual AAM about 1 standard deviation below the 1968-1997 R1 climatology.

We expect the tropical convective forcing to further respond to these wind signals starting this week and especially week 2. Anomalous rainfall should become noticeable from the equatorial Indian Ocean to Indonesia from ~80-120E while the South Pacific forcing weakens. Full-disk satellite imagery supports this trend. Whether or not this convection shifts east as a MJO afterwards is unclear. However, the multivariate Wheeler phase space plot suggests a MJO projection may be developing across the Indian Ocean. It is possible this convection may just persist in that region of the Eastern Hemisphere.

Our interpretation is GSDM Stage 0.5 best describes the global weather-climate situation (see link below). We think the global circulation will be in ~Stage 1.5 by the end of week 2. Signals are mixed whether or not a La-Nina is developing, and May is a critical transition month.

As discussed on 4/20, equatorial westerlies currently across the East Pacific will contribute to another North Pacific jet extension by early next week (which many ensembles do show) and then slam the western USA with more strong troughs starting later week 2. Thus after the current active period for much of the USA ends, there should be a respite. However, exceptionally active May weather looks probable later week 2 into possibly week 4 (shifted slightly north from April). Most readers should know the ramifications of strong mobile western states troughs and a southwest flow storm track on the Plains during May. For instance, several rounds of severe local storms on the Plains, late season snowstorms across the Rockies and summertime heat across the Deep South are probable.

Appendix

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

http://www.cdc.noaa.gov/map/images/gcm/gsdm_95d.jpg

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 by Thursday or Friday this week. I am on lots of travel and have to cover operational shifts.

Ed Berry