Would like to share on two episodes of monsoon surges over Singapore in January this year. The first monsoon surge had a highest daily total rainfall of 238.4mm and 210.6mm on 1 and 2 January. The second monsoon surge, which occurred between 8 and 13 January, saw a highest daily total rainfall of 204.0mm on 10 January. The surges were quite severe. Within the first two weeks of January, the total rainfall recorded at our Climate Station reached 648.4mm, making this the wettest January in the last 30 years.
The ECMWF model runs had quite a big forecast skill difference for both surge episodes. The meteogram for the first surge showed a large spread with the outliers reaching up to 16-18mm range and a median of around 3mm for the precipitation. The meteogram for the second surge, however, had a larger spread with the outliers reaching up to 60-80mm range and a higher median. Deterministic runs forecast the second surge to be more intense and longer lasting. Wind patterns were quite similar for both surge episodes, with deep troughing and sharp cyclonic shear at the low levels and no significant upper level easterlies.
Despite models indicating that the second surge would be more severe, more rainfall actually fell over Singapore during the first surge. It was also observed that the precipitation eased off on some days during the second surge episode.
A few possible reasons for the models behaviour were identified. Firstly, the MJO phase was weak for the first surge episode, but entered into the active phase over the Maritime continent for the second surge episode. Secondly, the sudden stratospheric warming (SSW) over the Artic that started around 5 January this year might have contributed to a stronger cold air outbreak over the Maritime Continent and led to more intense precipitation signals over Singapore and the surrounding region.
Below are some questions for discussion.
1) Is SSW is a contributor? How can we verify it?
2) Are there any other reasons for such big forecast skill difference?
3) Why did the first surge episode turn out to be a stronger event?
4) The deterministic runs forecast the intense spell to last from 8 to 13 January. However, the rainfall that fell over on most days were relatively light. What are some better ways to forecast between continuous rain and isolated passing showers with enough lead time?
Welcome any ideas. Thank you.