Monthly Archives: July 2021

July 23, 2021

Subseasonal Weather Outlook (26 July – 8 August 2021)

Issued 23 July 2021
First forecast week: 26 July – 1 August
Second forecast week: 2 August – 8 August

Figure 1: Rainfall Outlook

Figure 2: Temperature Outlook

Drier conditions are expected in the next fortnight (26 July – 8 August) over much of the northern ASEAN region. While the outlook is for generally drier than average conditions for the area outlined, there still may be isolated regions of wetter than average rainfall, particularly for Mainland Southeast Asia.

Wetter conditions are expected in Week 2 (2 – 8 August) over much of the southern and western Maritime Continent.

Warmer temperatures than usual are expected over much of the ASEAN region during the next fortnight (26 July – 8 August), apart from northwestern Mainland Southeast Asia.

An MJO signal was present over the Maritime Continent (Phases 4 and 5) in the middle of July. Most models predict the MJO to weaken as it moves over the Western Pacific (Phase 6) at the start of Week 1 (26 July – 1 August).

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.

July 16, 2021

MEDIA RELEASE

START OF DRY SEASON IN THE SOUTHERN ASEAN REGION

Singapore, 16 July 2021 – According to the ASEAN Specialised Meteorological Centre (ASMC), the traditional dry season in the southern ASEAN region[1] has started and is expected to extend into October 2021. The dry season is associated with the onset of the Southwest Monsoon in the region.

2 During this period, the equatorial Pacific Ocean is expected to remain in a neutral state, with no developing El Niño or La Niña. However, in the equatorial eastern Indian Ocean, waters have been gradually warming and are forecast to become warmer than normal in the following weeks. This climate phenomenon, known as the negative phase of the Indian Ocean Dipole (IOD), typically brings more rainfall to the region. For the July to October period, above average rainfall can be expected over many parts of the southern ASEAN region.

3 Recent wet weather in many parts of the southern ASEAN region has helped to subdue the development of fire hotspots in the fire-prone areas. Despite the likelihood of higher than normal rainfall in the next few months, more fire hotspots could still develop during occasional extended periods of dry and warm weather. The prevailing southwest monsoon conditions mean that low-level winds in the region blow from the southeast or southwest, and occasionally from the west. Transboundary haze may occur if the winds blow any smoke haze from fires to neighbouring countries.

4 ASMC assesses that the probability of a similar recurrence of the severe haze episodes in 2015 (El Niño year) and 2019 (positive IOD year) is low this year. Nonetheless, early precautionary and mitigation measures are advised to prevent the occurrence of transboundary haze in the region.

5 For regular and ad-hoc updates of the regional weather and smoke haze situation and early warnings, please visit the ASMC website at https://asmc.asean.org.

[1] The southern ASEAN region comprises of Brunei Darussalam, Indonesia, Malaysia, Singapore and southern Thailand.

July 15, 2021

Start of Dry Season for the Southern ASEAN Region

In recent days, drier weather has been observed over parts of the southern ASEAN region, particularly in the southern parts of Sumatra and Kalimantan. Based on the NOAA-20 satellite, there were no hotspots detected in Sumatra and Kalimantan on 14 and 15 July 2021.

The dry season is expected to extend into October 2021. During this period, above-average rainfall is expected over most parts of the region. However, there could still be occasional periods of dry weather, and this could contribute to an increase in hotspot activities in the fire-prone areas of the region.

ASMC Media Release – Start of southern ASEAN dry season

Review of Regional Haze Situation for June 2021

1.1 The ASEAN region transitioned to Southwest Monsoon conditions in June 2021, with prevailing winds over regions south of the equator blowing from the southeast while prevailing winds over regions north of the equator blew mainly from the southwest.

1.2 During this period, the monsoon rain band stayed in close proximity to the equator and shower activities were observed over most of the ASEAN region. However, drier conditions persisted in the Lesser Sunda Islands and eastern Java (Figure 1).

Figure 1: Average Daily Rainfall and Mean Wind for June 2021. (Source: JAXA GsMaP and GFS respectively)

1.3 The hotspot counts over most of the ASEAN region for June 2021 were comparable to those in previous years. More hotspots were detected in Cambodia and the Philippines compared to previous years while lesser hotspots were detected in Sumatra and Peninsular Malaysia compared to previous years. (Figures 2 and 3)

Figure 2: Hotspot counts for the northern ASEAN region for the month of June (2017 – 2021). [Note: The hotspot counts from 2019 onwards are based on the NOAA-20 satellite, while those from 2017 – 2018 are based on the Suomi-NPP satellite.]

Figure 3: Hotspot counts for the southern ASEAN region for the month of June (2017 – 2021). [Note: The hotspot counts from 2019 onwards are based on the NOAA-20 satellite, while those from 2017 – 2018 are based on the Suomi-NPP satellite.]

1.4 In June 2021, isolated hotspots were detected in most parts of the southern ASEAN region, contributed by brief periods of drier weather. Isolated to scattered hotspots were also detected in the eastern and central parts of the Mekong sub-region but were alleviated by an increase in shower activities. On the whole, hotspot activities remained relatively subdued in the ASEAN region with no significant smoke haze observed from satellite imagery (Figures 4 and 5).

Figure 4: Distribution of hotspots detected in June 2021 based on NOAA-20 satellite surveillance.

Figure 5: Maximum fire intensity in June 2021 based on NOAA-20 satellite surveillance.

July 14, 2021

Review of Regional Weather for June 2021

1. Overview

1.1 During June 2021, there was a mix of below- to above-average rainfall over the ASEAN region (Figure 1). Much of the western and central Maritime Continent (except for parts of eastern Borneo), and the north-eastern parts of Mainland Southeast Asia experienced above-average rainfall. The largest positive anomalies (wetter conditions) were recorded over central Java regions and northern Viet Nam (due to Tropical Storm Koguma made landfall in mid-June) for both satellite-derived rainfall estimates datasets (GSMaP-NRT and CMORPH-Blended). A band of below-average rainfall between 8°N and 14°N was recorded, in particular parts of southern Mainland Southeast Asia. Elsewhere, the land masses of the eastern Maritime Continent predominately experienced below-average rainfall, except for central parts of the Philippines which experienced above-average rainfall due to Tropical Storm Choi-Wan in early June.

1.2 The observed rainfall anomaly pattern (i.e. below-average rainfall over southern and northern parts of the Philippines , above-normal rainfall in the Java region) is broadly consistent with the predictions from the subseasonal weather outlooks for June 2021 (31 May – 13 June, 14 – 27 June, 28 June – 11 July), apart from the positive anomalies (wetter conditions) for central Sumatra and the north-eastern parts of Mainland Southeast Asia not highlighted in the outlooks.

Figure 1: Rainfall anomalies for the month of June 2021 based on GSMaP-NRT data (left) and CMORPH-Blended data (right). The climatological reference period is 2001-2020. Green colour denotes above-average rainfall (wetter), while orange denotes below-average rainfall (drier).

1.3 Most of the Mainland Southeast Asia experienced near- to above-average temperatures during June 2021, in particular warmer temperature for northern Myanmar, central Thailand, northern Cambodia and northern Viet Nam (Figure 2). Above-average temperatures were also experienced over northern Philippines and southern Maritime Continent. Elsewhere in the Maritime Continent, near-average temperatures were experienced in most of the region, with below-average temperatures for northern Sumatra and parts of Peninsular Malaysia.

Figure 2: Temperature anomalies for the month of June 2021 based on ERA-5 reanalysis. The climatological reference period is 2001-2020. Red colour denotes above-average temperature (warmer), while blue denotes below-average temperature (colder).

2. Climate Drivers

2.1 The Madden-Julian Oscillation (MJO, Figure 3) signal was incoherent for most of the first three weeks of June 2021 due to interference with other tropical waves. In the last week of June, an MJO signal emerged over the Western Hemisphere and Africa (Phase 1) and propagated eastwards to over the Indian Ocean (Phase 2). Phase 2 tends to bring wetter conditions to 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 The equatorial Pacific Ocean is currently in an ‘ENSO neutral’ state. There are some signs of a negative Indian Ocean Dipole (IOD) developing in the Indian Ocean, with warmer sea surface temperatures south-west of Java. At the seasonal timescale, negative IOD events tend to bring wetter conditions to the southern Maritime Continent.

July 9, 2021

Subseasonal Weather Outlook (12 – 25 July 2021)

Issued 9 July 2021
First forecast week: 12 July – 18 July
Second forecast week: 19 July – 25 July

Figure 1: Rainfall Outlook

Figure 2: Temperature Outlook

Wetter conditions are expected in Week 1 (12 – 18 July) over much of the central and eastern Maritime Continent, as well as the southeastern part of Mainland Southeast Asia. In Week 2 (19 – 25 July), these wetter conditions are expected to ease, apart from over the southeastern part of the Maritime Continent.

Drier conditions are expected in the next fortnight (12 – 25 July) over parts of the western Maritime Continent. These drier conditions may extend eastwards in Week 2 (19 – 25 July) to cover western parts of Borneo.

Warmer temperatures than usual are expected over the southern part of the Maritime Continent in Week 1 (12 – 18 July). These warmer conditions are expected to extend to most of the Maritime Continent and southern parts of Mainland Southeast Asia in Week 2 (19 – 25 July).

An MJO signal was present over the Indian Ocean (Phases 2 and 3) at the beginning of July. Some models predict the MJO to continue propagating eastwards over the Maritime Continent (Phases 4 and 5).