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Looking more promising

Tropical convective forcing is getting better organized with the centroid ~0/120E while extending from the central Indian Ocean into the South Pacific. SST forcing and interactions involving the tropics and extratropics have been assisting the slow development of this region of enhanced tropical rainfall.

Other faster time scales of tropical forcing are also occurring. Several weak-moderate flare-ups of convection occurred across the warm SSTs of the west central-South Pacific during the past couple of weeks. A response was for zonal mean anomalous westerly flow (~5 m/s) across the northern hemisphere subtropical atmosphere (~25N). Wave energy dispersing through the Eastern Hemisphere subtropics interacting with this added westerly flow has led to a local intensification of the of the jet coming off of east Asia, leading to the split flow pattern across the North Pacific. This was the type of west Pacific jet intensification we thought was probable a couple of weeks ago, and felt the models would not capture this until that signal was represented in their initial conditions.

We think several different time scales of forcing involving complex tropical-extratropical interactions have a possibility of phasing together during the next couple of weeks to allow a MJO to develop in the region of the Indian Ocean to Indonesia. The Wheeler phase plot already supports this notion. Zonal mean anomalous easterly flow remains quite strong throughout the tropical and subtropical atmospheres contributing to a low relative atmospheric angular momentum regime. We think our loosely GSDM Stage 1 situation will mature during the next couple of weeks. For ARB, this means the troughs currently moving off of east Asia will be forced onto the west coast as ridge amplification occurs ~140-160W. The synoptic details are unclear; however, we think the odds do favor a "few" strong troughs to impact ARB during at weeks 2-3. Many ensembles are trending toward this solution.

Ed Berry and Klaus Weickmann
NOAA NWS and NOAA/ESRL/PSD

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