Seasonal Forecast for July-August-September 2021
Updated 2 July 2021
Summary of Outlook
Southwest Monsoon conditions are expected to continue in July 2021 as the prevailing low-level winds over the ASEAN region strengthen and blow from the southeast or southwest, with the passage of the monsoon rain band further north of the equator in the second half of the month. The Southwest Monsoon season is the traditional dry season for the southern ASEAN region, which typically brings extended periods of dry conditions over the region. For the northern ASEAN region, the Southwest Monsoon is the traditional wet season.
For the July to September 2021 period, models predict an increased chance of above-normal rainfall for most of the equatorial ASEAN region from 8°N to 10°S. ENSO-neutral conditions are forecast to continue for the next three months. The Indian Ocean Dipole (IOD) is close to the negative state and models are forecasting a negative IOD for the July to September 2021 period. A negative IOD tends to bring above-average rainfall for the southern ASEAN region for this time of the year. Warmer-than-usual temperature is expected for most of the ASEAN region except for Borneo and southern Sumatra where near- to above-normal temperature is predicted.
The dry season for the southern ASEAN region is expected to extend from July to October 2021. During these months, isolated to scattered hotspots are likely during dry periods and widespread hotspots can be expected during extended dry periods which may lead to an increased risk of transboundary haze occurrence. Nonetheless, due to predicted above-normal rainfall for the southern ASEAN region in July-September 2021, a recurrence of a similar 2015 (El Niño year) or a 2019 (positive IOD year) severe haze event is unlikely. Shower activities are expected to persist for the northern ASEAN region during this period, and help to subdue hotspot and smoke haze activities.
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 – July 2021For July 2021, there is a small increase in chance of above-normal rainfall over much of the equatorial region, in particular for the area over central and eastern parts of the Maritime Continent.
For July 2021, based on the multi-model ensemble there is a small increase in chance of above-normal rainfall over much of the equatorial region, in particular for the area over central and eastern parts of the Maritime Continent (Figure 4). The ECMWF and UK Met Office models (Figures 2 and 3) predict a larger region with above-normal rainfall than the NCEP model (Figure 1), with the ECMWF model spanning the equatorial band from 5°N to 10°S and the UK Met Office model predicting the increased chance of above-normal rainfall conditions further southwards from 2°S to 10°S. The NCEP model only predicts parts of southern Sumatra and Java to experience above-normal rainfall conditions. The models’ skill is moderate to relatively high for the central and eastern parts of the Maritime Continent.
Elsewhere in western Maritime Continent, Mainland Southeast Asia and the Philippines, there is either no dominant tercile predicted or the models’ skill is low.
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| Figure 1a: Rainfall tercile summary predictions of NCEP model for July 2021. | Figure 1b: Rainfall tercile summary predictions of ECMWF model for July 2021 (contains modified Copernicus C3S information). |
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| Figure 1c: Rainfall tercile summary predictions of UK Met Office model for July 2021 (contains modified Copernicus C3S information). | Figure 1d: Rainfall tercile summary predictions of multi-model ensemble model for July 2021 (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 – July-August-September 2021 (JAS 2021)For JAS 2021, there is an increased chance of above-normal rainfall for most of the equatorial ASEAN region from 8°N to 10°S.
For JAS 2021, all three models – NCEP, ECMWF and UK Met Office (Figures 1-3) predict an increased chance of above-normal rainfall for much of the region over the equatorial ASEAN region from 8°N to 10°S (except for northern Sumatra and parts of Peninsular Malaysia where there is disagreement between the models). The model skill for the above-normal tercile category is moderate to relatively high for most of the equatorial region from 8°N to 10°S, apart from northern Sumatra and parts of Peninsular Malaysia where the model skill is low to moderate.
The three models also predict a small increase in chance of below-normal rainfall for the southern coastal parts of Mainland Southeast Asia. However, the model skill is low for these regions. Elsewhere, there is either no dominant terciles between the models or the model skill is low for this season.
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| Figure 2a: Rainfall tercile summary predictions of NCEP model for JAS 2021. | Figure 2b: Rainfall tercile summary predictions of ECMWF model for JAS 2021 (contains modified Copernicus C3S information). |
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| Figure 2c: Rainfall tercile summary predictions of UK Met Office model for JAS 2021 (contains modified Copernicus C3S information). | Figure 2d: Rainfall tercile summary predictions of multi-model ensemble model for JAS 2021 (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 – July 2021For July 2021, an increased chance of above-normal temperature is predicted over much of the ASEAN region.
For July 2021, an increased chance of above-normal temperature is predicted over much of the ASEAN region by the multi-model ensemble (Figure 4), with the highest likelihood over western and southern parts of the Maritime Continent. The UK Met Office (Figure 3) model predicts the highest spatial coverage of above-normal temperature, followed by the NCEP model (Figure 1) and then the ECMWF model (Figure 2). THE NCEP model predicts lower confidence of above-normal temperature conditions over southern Mainland Southeast Asia with some occurrences of near-normal temperature conditions, while the ECMWF model predicts a larger spatial extent of near-normal temperature conditions or no dominant terciles over parts of the equatorial region, in particular over central Sumatra, Borneo and northern parts of Sulawesi and Papua.
The models’ skill for above-normal temperature for the Maritime Continent is moderate to relatively high and low to moderate for Mainland Southeast Asia.
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| Figure 3a: Temperature tercile summary predictions of NCEP model for July 2021. | Figure 3b: Temperature tercile summary predictions of ECMWF model for July 2021 (contains modified Copernicus C3S information). |
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| Figure 3c: Temperature tercile summary predictions of UK Met Office model for July 2021 (contains modified Copernicus C3S information). | Figure 3d: Temperature tercile summary predictions of multi-model ensemble model for July 2021 (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 – July-August-September 2021 (JAS 2021)For JAS 2021, above-normal temperature is favoured for most of the ASEAN region except for Borneo and southern Sumatra where near- to above-normal temperature is predicted.
For JAS 2021, all three models – NCEP, ECMWF and UK Met Office (Figures 1 – 3) – predict high likelihood of above-normal temperature for most of the ASEAN region. However, near-normal temperature conditions are predicted over parts of Borneo and southern Sumatra by all three models, coinciding with the above-normal rainfall predicted over these regions.
The models’ skill for predicting above-normal temperature for the ASEAN region is moderate to high for this season.
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| Figure 4a: Temperature tercile summary predictions of NCEP model for JAS 2021. | Figure 4b: Temperature tercile summary predictions of ECMWF model for JAS 2021 (contains modified Copernicus C3S information). |
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| Figure 4c: Temperature tercile summary predictions of UK Met Office model for JAS 2021 (contains modified Copernicus C3S information). | Figure 4d: Temperature tercile summary predictions of multi-model ensemble model for JAS 2021 (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.