Seasonal Forecast for September – November 2023
Updated 30 Aug
Southwest Monsoon conditions were observed in August 2023. The Southwest Monsoon season is climatologically characterised by rainy weather over the northern ASEAN region and drier conditions in the southern ASEAN region. The low-level winds over the ASEAN region during this period are mostly southeasterly or southwesterly. Southwest Monsoon conditions typically transition to the inter-monsoon period in the latter part of October. During the inter-monsoon period, the equatorial region typically experiences light and variable winds and increased shower activities as the monsoon rain-band shifts southwards into the region.
For September 2023, an increased chance of below-normal rainfall is predicted for most of the southern ASEAN region, extending to include the Philippines over the September to November 2023 period as a whole. The ocean indicators (Nino3.4 index) shows El Niño conditions, with signs of El Niño like response in key atmospheric indicators (cloudiness, and wind anomalies). The El Niño conditions are predicted to strengthen over the next few months and then persist at least until the start of 2024. There are also signs of positive Indian Ocean Dipole (IOD) conditions, with models predicting an IOD to likely develop in September. Both El Niño and positive IOD events tend to bring drier conditions to parts of the ASEAN region. Above-normal temperature is predicted for most of the ASEAN region for SON 2023.
Dry conditions are likely to intensify over the southern ASEAN region as the dry season is expected to extend until October 2023. Increased hotspot activity and smoke haze development, as well as a risk of transboundary haze, can be expected over the fire-prone areas of the southern ASEAN region where below-normal rainfall is forecast. The current El Niño conditions and a potential positive Indian Ocean Dipole (IOD) could cause the hotspot and haze situation in the southern ASEAN region to be the most intense since 2019. The increase in shower activities during the inter-monsoon period from late October onwards is likely to alleviate the hotspot and haze situation over the region. In contrast, over the northern ASEAN region, shower activities are expected to subdue the overall hotspot and smoke haze situation during the outlook period of September – November (SON) 2023.
The qualitative outlook is assessed for the region in general and based on the latest runs from models provided by the SEA RCC-Network LRF node. For specific updates on the national scale, the relevant ASEAN National Meteorological and Hydrological Services should be consulted.
Monthly Rainfall Outlook – September 2023For September 2023, below-normal rainfall is predicted over most of the southern ASEAN region.
For September 2023, below-normal rainfall is predicted over most of the southern ASEAN region, with highest likelihood over the southwestern Maritime Continent based on the multi-model ensemble (Figure 4). The ECWMF model (Figure 2) shows the highest confidence for below-normal rainfall, followed by the NCEP and the UKMO models (Figures 1 and 3). Models’ skill is moderate to good skill for below-normal rainfall over the southern ASEAN region.
For the northern ASEAN region, there is an increased chance of above-normal rainfall over western and southern parts of the region, based on the multi-model ensemble (Figure 4). Above-normal rainfall is predicted over western and southern parts Mainland Southeast Asia and parts of the Philippines in all three models (Figures 1-3). Elsewhere over Mainland Southeast Asia, either there is no dominant tercile or disagreement between the models (central and eastern parts of Mainland Southeast Asia). Models’ skill is low to moderate for southern Mainland Southeast Asia and the Philippines, and generally low for rest of the northern ASEAN region.
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| Figure 1: Rainfall tercile summary predictions of NCEP model for September 2023 (contains modified Copernicus C3S information). | Figure 2: Rainfall tercile summary predictions of ECMWF model for September 2023 (contains modified Copernicus C3S information). |
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| Figure 3: Rainfall tercile summary predictions of UK Met Office model for September 2023 (contains modified Copernicus C3S information). | Figure 4: Rainfall tercile summary predictions of multi-model ensemble model for September 2023 (contains modified Copernicus C3S information). |
The qualitative outlook is assessed for the region in general. For specific updates on the national scale, the relevant ASEAN National Meteorological and Hydrological Services should be consulted. More outlook and verification plots, including for tercile and quintile probabilistic forecasts for rainfall, are in the “Model products and verification: NCEP, ECMWF, UK Met Office” on the SEA-RCC Network LRF Node webpages.
Seasonal Rainfall Outlook – September – November 2023 (SON 2023)For SON 2023, models predict an increased chance of below-normal rainfall for most of the Maritime Continent.
For SON 2023, below-normal rainfall is predicted for most of the Maritime Continent, based on the multi-model ensemble (Figure 4). The ECMWF model (Figure 2) shows the highest confidence for below-normal rainfall for regions south of the equator, while the NCEP and then the UKMO models (Figures 1 and 3) are predicting below-normal rainfall with relatively lower confidence for most of the Maritime Continent. However, all three models predict an increase in chance of above-normal rainfall over the northwestern Maritime Continent. Models’ skill is moderate to good skill for below-normal rainfall over the Maritime Continent, and low to moderate for above-normal rainfall over the northwestern Maritime Continent.
For Mainland Southeast Asia, there is a small increase in chance of above-normal rainfall over western and southern parts of the region, and no dominant tercile elsewhere, based on the multi-model ensemble (Figure 4). The NCEP and ECMWF models (Figures 1 and 2) predict above-normal rainfall over much of western and southern Mainland Southeast Asia, while the UKMO model (Figure 3) predicts a mix of below- to above-normal rainfall over this region. The models’ skill is generally low to moderate over Mainland Southeast Asia for all three tercile categories.
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| Figure 1: Rainfall tercile summary predictions of NCEP model for SON 2023 (contains modified Copernicus C3S information). | Figure 2: Rainfall tercile summary predictions of ECMWF model for SON 2023 (contains modified Copernicus C3S information). |
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| Figure 3: Rainfall tercile summary predictions of UK Met Office model for SON 2023 (contains modified Copernicus C3S information). | Figure 4: Rainfall tercile summary predictions of multi-model ensemble model for SON 2023 (contains modified Copernicus C3S information). |
The qualitative outlook is assessed for the region in general. For specific updates on the national scale, the relevant ASEAN National Meteorological and Hydrological Services should be consulted. More outlook and verification plots, including for tercile and quintile probabilistic forecasts for rainfall, are in the “Model products and verification: NCEP, ECMWF, UK Met Office” on the SEA-RCC Network LRF Node webpages.
Monthly Temperature Outlook – September 2023For September 2023, above-normal temperature is predicted over much of the ASEAN region.
For September 2023, above-normal temperature is predicted over much of the ASEAN region based on the multi-model ensemble (Figure 4), with exception over parts of the southern Maritime Continent where below- to near-normal temperature is predicted. All three models agree with high confidence on above-normal temperature over most of the Maritime Continent and much of Mainland Southeast Asia. The NCEP and ECMWF models (Figures 1 and 2) predict near- to above-normal temperature over parts of the southern Maritime Continent, and the UK Met Office model (Figure 3) predicts below- to near-normal temperature over parts of the southern Maritime Continent. Models’ skill is relatively good for predicting below- and above-normal temperature over most of the ASEAN region.
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| Figure 1: Temperature tercile summary predictions of NCEP model for September 2023 (contains modified Copernicus C3S information). | Figure 2: Temperature tercile summary predictions of ECMWF model for September 2023 (contains modified Copernicus C3S information). |
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| Figure 3: Temperature tercile summary predictions of UK Met Office model for September 2023 (contains modified Copernicus C3S information). | Figure 4: Temperature tercile summary predictions of multi-model ensemble model for September 2023 (contains modified Copernicus C3S information). |
The qualitative outlook is assessed for the region in general. For specific updates on the national scale, the relevant ASEAN National Meteorological and Hydrological Services should be consulted. More outlook and verification plots, including for tercile and quintile probabilistic forecasts for temperature, are in the “Model products and verification: NCEP, ECMWF, UK Met Office” on the SEA-RCC Network LRF Node webpages.
Seasonal Temperature Outlook – September – November 2023 (SON 2023)For SON 2023, the models predict an increased chance of above-normal temperature over most of the ASEAN region.
For SON 2023, above-normal temperature is predicted for most of the ASEAN region based on the multi-model ensemble (Figure 4). All three models agree with high confidence on above-normal temperature over most of the region, apart from over parts of the southern Maritime Continent where there is a moderate to high confidence of above-normal temperature. In the southern Maritime Continent, the NCEP model (Figure 1) predicts the highest confidence of above normal temperature, whereas the ECMWF and UKMO models (Figures 2 and 3) predict relatively lower confidence for above normal temperature or near normal temperature. Models’ skill for predicting below- and above-normal temperature for the ASEAN region is relatively good.
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| Figure 1: Temperature tercile summary predictions of NCEP model for SON 2023 (contains modified Copernicus C3S information). | Figure 2: Temperature tercile summary predictions of ECMWF model for SON 2023 (contains modified Copernicus C3S information). |
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| Figure 3: Temperature tercile summary predictions of UK Met Office model for SON 2023 (contains modified Copernicus C3S information). | Figure 4: Temperature tercile summary predictions of multi-model ensemble model for SON 2023 (contains modified Copernicus C3S information). |
The qualitative outlook is assessed for the region in general. For specific updates on the national scale, the relevant ASEAN National Meteorological and Hydrological Services should be consulted. More outlook and verification plots, including for tercile and quintile probabilistic forecasts for temperature, are in the “Model products and verification: NCEP, ECMWF, UK Met Office” on the SEA-RCC Network LRF Node webpages.
Frequently Asked Questions
What is a seasonal forecast?
A seasonal forecast provides forecast information about the general weather conditions at seasonal timescales up to several months ahead. Long range predictions rely on predicting components of the atmosphere and the oceans which show variations at large spatial and temporal scales. Among these key components is the sea-surface temperature – an important indicator in the El Niño Southern Oscillation (ENSO) cycle.
Example of probabilistic long range forecast using the multi-model ensemble. For areas nearby Peninsular Malaysia, the forecast is showing a probability of 50-70% of rainfall being in the below-normal (33%) tercile range. (Source: WMO LC LRF)
To account for the chaotic nature of the atmosphere, seasonal forecast adopts a probabilistic approach and the forecast weather condition is presented as an average over an extended period of time, usually with reference to climatology.
ASMC carries out monthly seasonal forecast predictions based on various seasonal model forecasts from the leading meteorological centres, supplemented by individual assessments from respective countries in South East Asia. The skillfulness of seasonal forecast relies greatly on the performance of seasonal models, which differ from one place to another and for different seasons of the year.
Receiver operating characteristic (ROC) score of UK GloSea5 seasonal hindcast for above-normal precipitation scenario for Dec – Feb (left) and May – Jul (right) over Asia region. Higher ROC score (indicated in red) means better model skill. (Source: UK Met Office)
For example, one way to assess the seasonal model skills is to examine the ROC score. From the figures above, we can see that ROC score map differs greatly from one season to another. During the period from Dec to Feb, a high ROC score region is observed mainly over the Philippines whereas during the period from May to Jul, a high ROC score region is observed over the near equatorial region such as Peninsular Malaysia and Sumatra.