Monthly Archives: November 2020

November 27, 2020

Subseasonal Weather Outlook (30 November – 13 December 2020)

Issued 27 November 2020
First forecast week: 30 November – 6 December
Second forecast week: 7 December – 13 November

Figure 1: Rainfall Outlook

Figure 2: Temperature Outlook

An increased chance of wetter conditions is predicted over parts of northern ASEAN region in the next fortnight (30 November – 13 December), in particular central and southern Viet Nam, southern Thailand and northern Philippines. A higher likelihood of wetter conditions is also predicted around the southeastern part of the Maritime Continent. Drier conditions are predicted over parts of the equatorial region in the first week (30 November – 6 December).

Cooler temperatures than usual are predicted over parts of eastern Mainland Southeast Asia in the next fortnight (30 November – 13 December), with a higher confidence in the first week (30 November – 6 December). An increased chance of warmer conditions is predicted over parts of central Sumatra and Borneo in the first week (30 November – 6 December).

Current MJO signal is weak. The dynamical models are inconsistent regarding the MJO development in the forecast period.

The outlook is assessed for the region in general, where conditions are relative to the average conditions for the corresponding time of year. For specific updates on the national scale, the relevant ASEAN National Meteorological and Hydrological Services should be consulted.

November 13, 2020

Review of Regional Weather for October 2020

1. Overview

1.1 During October 2020, much of the northern part of Southeast Asia experienced above-average rainfall (Figure 1). The largest positive anomalies (wetter conditions) were recorded over the Philippines, Viet Nam, parts of Lao PDR and southern Cambodia based on both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). The large positive anomalies in these regions is linked to the high number of tropical storms and cyclones (including Tropical Storm Linfa, Tropical Storm Nangka, Tropical Cyclone Saudel, and Tropical Cyclone Molave). Elsewhere, the equatorial region experienced a mix between above- and below-average rainfall, while wetter-than-average conditions were recorded over much of the region south of the equator.

1.2 The observed large-scale rainfall anomaly pattern (i.e. above-average rainfall in regions linked to tropical storm and cyclone activity as well as south of the equator) is broadly consistent with the predictions from the subseasonal weather outlooks for October 2020 (5 – 18 Oct, 19 Oct – 1 Nov).

Figure 1: Rainfall anomalies for the month of October 2020 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2019. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Colder-than-average temperatures were recorded over the eastern half of Mainland Southeast Asia in October 2020 (Figure 2). Parts of Viet Nam, Lao PDR, and Thailand were on average more than 1°C cooler than usual for this time of year. In contrast, warmer-than-average temperatures were recorded over northern Myanmar (more than 1°C warmer than average). Elsewhere, the temperature for October was closer to average.

Figure 2: Temperature anomalies for the month of October 2020 based on ERA-5 reanalysis. The climatological reference period is 2001-2019. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 A Madden-Julian Oscillation (MJO) signal developed in the Maritime Continent (Phase 5) during the first half of October (Figure 3), before propagating eastward through the western Pacific (phases 6 and 7) in the second half. Phases 5 and 6 tend to bring wetter conditions to the eastern half of Southeast Asia, while phases 6 and 7 bring drier conditions to parts of the western Maritime Continent.

Figure 3: The MJO phase diagram. The diagram illustrates the movement of the MJO through different phases, which correspond to different locations along the equator (denoted in the text with the first day of the month in blue and the last day of the month in red). The distance of the index from the centre of the diagram is related to the strength of the MJO. Values within the grey circle are considered weak or indiscernible (data from the Bureau of Meteorology, Australia).

2.2 Over the tropical Pacific Ocean, La Niña conditions were present. The sea surface temperatures in the central and eastern Pacific Ocean continued to cool, with atmospheric indicators (cloudiness and wind anomalies) also remaining consistent with La Niña conditions. La Niña events tend to bring wetter-than-average conditions to much of Southeast Asia during the September to November period.

Subseasonal Weather Outlook (16 – 29 November 2020)

Issued 13 November 2020
First forecast week: 16 November – 22 November
Second forecast week: 23 November – 29 November

Figure 1: Rainfall Outlook

Figure 2: Temperature Outlook

Wetter conditions are predicted over parts of Sumatra, Malay Peninsula and western Borneo in the first week (16 – 22 November).

Warmer conditions are predicted over much of Mainland Southeast Asia during the next two weeks (16 – 29 November). Warmer conditions are also predicted over parts of Java, southern Borneo and southern Sulawesi in the first week (16 – 22 November), with conditions easing by the second week (23 – 29 November).

An MJO signal is present over the Western Hemisphere and Africa (Phase 8). Most models predict the MJO to continue propagating eastwards, weakening over the Indian Ocean (Phases 2 and 3).

Review of Regional Haze Situation for October 2020

1.1 Southwest Monsoon conditions persisted over the ASEAN region in October 2020. The prevailing winds blew mainly from the southeast or southwest except over the Mekong sub-region where winds were mostly variable under the influence of several low-pressure systems. In the northern ASEAN region, rainy weather persisted throughout most of the month. The average daily rainfall (Figure 1) was highest in Luzon Island of the Philippines and central regions of Viet Nam under the influence of the tropical storms Lingfa, Nangka and Saudel, Typhoon Malave and Super Typhoon Goni. In the southern ASEAN region, despite this time of the year typically being the dry season, shower activities were still observed over many areas on most days. There was little variation in average daily rainfall across the southern ASEAN region except for generally dry conditions observed mainly over the regions surrounding the Java Sea and Flores Sea.

Figure 1: Average Daily Rainfall and Mean Wind for October 2020. (Source: JAXA GsMaP and GFS respectively)

1.2 The hotspot counts in the ASEAN region for October 2020 remained relatively low when compared to the same review period in previous years due to wet weather (Figure 2 and Figure 3).

Figure 2: Hotspot counts for the southern ASEAN region for the month of October between 2016 and 2020. [Note: The hotspot counts from 2019 onwards are based on the NOAA-20 satellite, while those from 2016-2018 are based on the Suomi-NPP satellite.]

Figure 3: Hotspot counts for the northern ASEAN region for the month of October between 2016 and 2020. [Note: The hotspot counts from 2019 onwards are based on the NOAA-20 satellite, while those from 2016-2018 are based on the Suomi-NPP satellite.]

1.3 Isolated hotspots were detected in many parts of the ASEAN region in October 2020, particularly in Kalimantan, the Lesser Sunda Islands, and Sulawesi (Figure 4) during brief periods of drier weather. On a few days in October 2020, localised smoke plumes were observed to emanate from some of the hotspots in Kalimantan. Otherwise, no significant smoke haze was observed.

Figure 4: Distribution of hotspots detected based on NOAA-20 satellite surveillance for October 2020.

Figure 5: Maximum fire intensity based on NOAA-20 satellite surveillance for October 2020.