I'll continue on with yesterday's note on the developments of Chatann and Rammasun.  This isn one of the most interesting things I have seen since I have been following the charts on a daily basis.  As you all know the NWPAC summer monsoon trough has a northwest to southeast tilt to it.  See for example last night's 850 hPa chart, which I have put up on my webpage.

Over in the northcentral Pacific (10 -20N, 160 - 180E on the chart) we have the easterly trades. Over the Indian Ocean and the South China Sea we have the monsoon westerly flow.  Where they meet, we have the classical monsoon tough.  Looking at the chart, you can draw a line from about 30, 120 to 10, 160.  This is the climatological monsoon trough or monsoon shear line with easterly trades to the northeast and monsoon westerlies to the southwest.

Now, this is a climatological feature.  What was nice about the development of Chat and Ram was that over the preceding week a westerly surge propagated along the equator from the west.  When it reached the longitudes of the monsoon trough, it undercut it from the south; and for reasons as yet not understood the shear vorticity increased across the trough and the two cyclones spun up