The Madden-Julian Oscillation emerged over the Maritime Continent this week (Phase 4) and has rapidly strengthened over the past four days.
Latest forecasts of the strength and phase of the MJO show that is is expected to move into Phase 5 with a strong signal on 10th January. It is expected to stay strong as it moves across the Maritime Continent and into the Western Pacific (Phase 6) on or around 14th January. The signal is expected to stay fairly robust as it emerges across the Western Pacific but the GFS then sees the signal rapidly weaken towards the last week of this month (from the 24th onward).
The MJO over the Maritime Continent tends to enhance the monsoon rainfall over northern Australia at this time of year and will also likely enhance the current tropical disturbance over Northern Territory. As it emerges into the Western Pacific we could see a few tropical disturbances forming there.
Closer to home a strong Phase 5 MJO brings very low 500 millibar height anomalies to the northwest of the British Isles and higher than normal height anomalies to the south - a strong positive North Atlantic Oscillation situation. This would lead to unsettled conditions but milder than normal temperatures for the UK and Ireland. This is, in fact, what we expected to happen based on multiple forecasts from several sources through the period from 10th to 13th January.
A strong Phase 6 MJO brings a similar pattern with low 500 millibar height anomalies to the northwest and over northern parts of the UK with higher than normal heights over the Azores. With this situation forecast from 14th to 19th January it seems likely that mixed conditions will prevail over the British Isles until then. Again, this is consistent with other forecasts we have looked at.