Consensus Seasonal Weather Outlook January, February and March (JFM) Seasonal Rainfall and Temperature for

These forecasts are prepared using

 The prevailing global climate conditions.  Forecasts from different climate models from around the world.  Statistical downscaling of GCM output using CPT

Issued by Centre for Climate Change Studies (CCCS)

and

Research Division

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1. Prevailing global climate conditions

Equatorial sea surface temperatures (SSTs) are below average from the west-central to the eastern Pacific Ocean (Fig 1). Further, during the last four weeks, below-average SSTs have persisted across the most of the equatorial Pacific Ocean. (CPC-USA).

1.1 El Nino and La Nina update

Prevailing La Niña conditions are likely to continue through the Northern Hemisphere winter (January to March) 2020-21 (~95% chance) and into spring 2021 (~65% chance during March- May). Most models suggest La Niña will peak in January or February 2021 at moderate to strong levels with a likely return to neutral conditions during April to June 2021 (~50% chance). (Source CPC-USA, BoM) (Fig 3).

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Fig 1: Observed Average sea surface temperature (SST) anomalies (°C)

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Fig 2: Weekly Observed Average sea surface temperature (SST) anomalies (°C)

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Fig 3: ENSO forecast from Climate Prediction Center (CPC)/ IRI Forecast

1.1.1 Impacts of La-Nina on monthly rainfall anomaly during January, February and March

Fig 4: Monthly Rainfall Anomaly maps of the months of January (A), February (B) and March (C) during La-Nina years (Hapuarachchi et al 2016)

Previous studies conducted by the Department of Meteorology, identified that, during La-Nina years, it is evident that above normal rainfalls over the country during the month of January(Fig

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4-A). During the month of February it is evident above normal rainfall over most parts of the country except some parts of Northern Province and Hambanttota, Monaragala and Districts (Fig 4-B) and during the month of March below normal rainfall can be expected most parts of the island especially in the Eastern and North-Central Provinces and parts of Uva and Nothern Provinces and Rathnapura, , Kegale, , , and distruicts (Fig 4-C).

1.2 The Dipole (IOD) update The Sea surface temperatures north of 20°S in the Indian Ocean are average to warmer than average, while to the south, they are average to cooler than average and the Indian Ocean Dipole (IOD) is currently neutral. Most of the models show this IOD neutral condition will continue until March - June 2021. (Figure 5). (Source-Bureau of Meteorology, Australia).

Figure 5: IOD forecast from Australian Bureau of Meteorology .

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2. Forecasts from different climate models from around the world. 2.1 January to March (JFM) 2021 season

Figure 6 shows the probabilistic multi model ensemble forecast which prepared by using dynamical models from 12 Global Producing Centers (GPC) for JFM season. According to that above normal rainfall can be expected over the country during January -March 2021(JFM) season.

Fig 6: Probabilistic multi model ensemble forecast for JFM using dynamical models from 12 WMO global producing centers (GPC).

Figure 7 depicts individual forecasts provided by same GPC centers for the JFM season. Out of 12 GPC individual models, 9 predicted above normal rainfall over the country. There is no clear signal indicated in 3 GPC models. Accordingly, there is a enhance probability for above normal rainfall over most parts of the country during JFM 2021 season.

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Fig 7: Individual forecasts for JFM 2021 season by dynamical models from 12 WMO global producing centers (GPC).

2.2 Monthly Forecast for January, February and March 2021

Figure 8 shows the probabilistic multi model ensemble forecasts, which are prepared by using dynamical models from 12 global producing centers (GPC), for the months of January, February and March 2021. According to that above normal rainfall can be expected over most parts of the country during January, February and March 2021 season.

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Fig 8: Probabilistic multi model ensemble forecast for January (left), February (middle) and March 2021 (right) using dynamical models from 12 WMO global producing centers (GPC).

Fig 9: Individual forecast for January 2021 by dynamical models from 12 WMO global producing centers (GPC).

Figure 9 shows the 12 monthly forecasts from individual global producing centers (GPC) for January 2021. Out of 12 GPC forecasts, 9 GPC models predicted above normal rainfall over the country and another 1 GPC model predicted below normal rainfalls over the country. There is no clear signals indicated in 2 GPC models. Accordingly, there is a enhance probability for above

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Fig 10: Individual forecast for February 2021 by dynamical models from 12 WMO global producing centers (GPC).

Figure 10 shows the monthly forecasts from individual global producing centers (GPC) for February 2021. Out of 12 GPC forecasts, 8 GPC models predicted above normal rainfall and there is no clear signal in 4 GPC models for the month of February 2021. Accordingly, slightly above normal rainfalls can be expected over most parts of the country during the month of February 2021.

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Fig 11: Individual forecast for March 2021 by dynamical models from 12 WMO global producing centers (GPC).

Figure 11 shows the monthly forecasts from 12 individual global producing centers (GPC) for March 2021. Out of 12 GPC forecasts, 7 GPC models indicate above normal rainfall and there is no clear signal from 5 GPC models for the month of March 2021. Accordingly it can be expected near or slightly above normal rainfalls over the country during the month of March 2021.

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3. Statistical downscaling of CFSv2 global forecast output 3.1 Probabilistic rainfall forecast for JFM season 2021 using Climate Predictability tool (CPT) The following district wise probabilistic rainfall forecasts for the season of JFM 2021 have been prepared with the multi model ensemble method to downscale ,SST data of CFSv2, CCSM4, GFDL and ECMWF by using CPT. The district wise 30 year average rainfalls during JFM season are given in the column 2 of the table 1. Chance (probability) of receiving below/about/above average is given in the columns 3, 4, and 5 respectively in the table1.

Average rainfall (mm) – District JFM Probability% Below Normal Above Colombo 336.4 35 25 40 Kalutara 461.6 50 25 25 455.3 50 25 25 Matara 398.0 20 25 55 220.5 20 20 60 Ampara 456.7 25 25 50 399.2 25 25 50 Trincomalee 264.6 25 25 50 Mullaithivu 177.6 25 25 50 125.3 30 25 45 Killinochchi 157.1 25 25 50 Mannar 148.4 25 25 50 Puttalam 156.1 30 25 45 Gampaha 278.3 30 25 45 373.7 25 25 50 454.0 20 25 55 Monaragala 355.8 20 20 60 530.7 30 30 40 Pollonnaruwa 355.6 30 25 45 176.8 30 25 45 Anuradapura 201.9 30 25 45 Kurunegala 221.2 30 30 40 427.8 50 25 25 397.3 30 30 40 Nuwaraeliya 375.2 20 25 55

Table 1: Probabilistic Rainfall Forecast for JFM season 2021 using CPT

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Fig 12: Probabilistic rainfall forecast for January -March 2021 using CPT

According to the CPT (Fig 12 and table 01), above normal rainfalls can be expected in 18 districts out of 25 and below normal rainfall can be expected in Kalutara, Galle and Mathale districts. There is no clear signal for Kandy, Badulla, Colombo and Kurunegala districts. for JFM season 2021. Therefore equal chances exist of receiving below normal, about normal or above normal rainfall over those districts for JFM Season 2021.

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3.2 Probabilistic rainfall forecast for JFM 2021 season using RIMES FOCUS System

Fig 13. Probabilistic rainfall forecast for January-March 2021 using RIMES FOCUS System

Figure 13 depicts the Probabilistic rainfall forecast for climatic zones for JFM 2021 season, which has been prepared by using RIMES FOCUS System. According to the model it can be expected above normal rainfall over Wet zone , Intermediate zone and Dry zone of the country during JFM season 2021.

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4. SUMMARY : SUMMARY of MODEL FORECAST for JFM 2021 season for SRI LANKA Season WMO LC WMO GPC CPT FOCUS Impact of Final MME Global conditions JFM season AN AN AN-18 Districts AN AN 2021 BN-Kalutara, Galle, Mathale January AN AN BN AN AN 2021 No signal- Hambantota February AN AN A slightly above 2021 March 2021 AN Near or Near or slightly slightly above Normal above Normal BN: Below Normal NN: Near Normal AN: Above Normal CP: Climatological Probability Table 2: Summery of Model Forecasts for JFM season 2021

4.1 Summery of Prevailing global climate conditions Prevailing La Niña conditions are likely to continue through the Northern Hemisphere winter (January to March) 2020-21 (~95% chance) and into spring 2021 (~65% chance during March- May). Most models suggest La Niña will peak in January or February 2021 at moderate to strong levels with a likely return to neutral conditions during April to June 2021 (~50% chance). (Source CPC-USA and Bureau of Meteorology,Australia) The Sea surface temperatures north of 20°S in the Indian Ocean are average to warmer than average, while to the south, they are average to cooler than average and the Indian Ocean Dipole (IOD) is currently neutral. Most of the models show this IOD neutral condition will continue until March - June 2021. (Figure 5). (Source-Bureau of Meteorology, Australia).

5. Consensus Seasonal outlook for January, February and March 2021 Considering the prevailing global climate conditions, forecasts from different global climate models and statistical downscaling of GCM output using CPT, consensus forecasts for January to March 2021 are concluded as follows.

5.1 Rainfall forecast for January-February- March (JFM) three months period There is an enhanced probability for above normal rainfalls over most parts of the country, particularly in Northern, Eastern, Uva and Northcentral provinces during JFM 2021 season as a whole (Fig. 14).

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5.2 Rainfall forecast for the month of January 2021 There is an enhanced probability to receive above normal rainfalls over most pars, particulalr in Eastern and and Uva provinces, during January 2021. 5.3 Rainfall forecasts for February 2021 During the month of February 2021 there is a probability for slightly above normal rainfalls over most parts of the country. 5.4 Rainfall forecasts for March 2021 There is a probability for near or slightly above normal rainfalls over the country, during the month of March 2021.

In addition, the predictability is also limited due to strong day-to-day atmospheric variability caused by the passage of the synoptic scale systems such as lows and depressions which are more common in the months of January. Intraseasonal Oscillations such as Madden Julian Oscillations (MJO) is also another atmospheric phenomina which can’t be underestimated.

Fig 14. Consensus Probabilistic rainfall forecast for January –March 2021

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5.5 Probabilistic Temperature Forecast for January–March 2021 (JFM) The probabilistic Temperature forecast for January, February and March season (JFM) 2021 for Sri Lanka as given below.

Fig 15: Probabilistic forecast for Maximum Temperatures for JFM season 2021

Fig 15 and Table 3 show the probabilistic forecast for Maximum Temperatures during JFM season 2021.

There is a higher chance of experiencing slightly below the normal Maximum Temperatures in , Mannar, Vavuniya, Puttalam, Trincomalee, Kurunegala, Batticaloa, Ampara, Gampaha, Colombo, Galle, Rathnapura, Kandy, and Badulla districts and about the normal Maximum temperatures in Hambanthota district (Fig 15) for the JFM season 2021.

The district wise average Maximum Temperatures are given in the column 2 of the table 3 and the chance (probability) of receiving below/about/above averages are given in the columns 3, 4, and 5 respectively.

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Average Maximum Temperature (0C) – Probability % District (JFM) Below Normal Above Anuradhapura 31.7 50 25 25 Badulla 26.9 50 25 25 Batticaloa 28.8 40 30 30 Colombo 31.3 50 25 25 Galle 29.8 55 20 25 Hambantota 30.3 30 40 30 Katugastota 29.9 30 30 40 32.4 50 25 25 Mannar 30.4 50 25 25 MahaIlluppallama 31.6 50 30 20 NuwaraEliya 21.2 55 25 20 Pottuvil 30.6 50 25 25 Puttalam 31.9 55 25 20 Ratnapura 33.5 40 30 30 Ratmalana 31.4 50 25 25 Trincomalee 29.2 45 25 30 Vavuniya 31.3 40 30 30 Kurunegala 32.8 40 30 30 24.0 50 25 25

Table 3: probabilistic forecast for Maximum Temperature for JFM season 2021

Fig 16: Probabilistic forecast for Minimum Temperatures for JFM season 2021

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Fig 16 and Table 4 provide the probabilistic forecast for Minimum Temperatures during JFM season 2021.

Accordingly, there is a higher chance of experiencing slightly above the normal Minimum Temperatures in Mannar, Vavunia, Anuradhapura, Gampaha, Colombo, Kandy, Batticaloa, Ampara, Rathnapura, Galle and Hambantota districts and below the normal Minimum temperaturs in Puttalam, Kurunegala, Trincomalee, Nuwara Eliya and Badulla Districts (Fig 16) during JFM season 2021.

Average Minimum 0 Temperature ( C) – Probability % District (JFM) Below Normal Above Anuradhapura 21.7 30 30 40 Badulla 17.5 50 25 25 Batticaloa 23.6 30 25 45 Colombo 22.9 30 30 40 Galle 23.2 30 25 45 Ham bantota 23.2 30 30 40 Katugastota 18.9 30 30 40 Katunayake 22.2 30 30 40

Mannar 23.8 25 30 45

MahaIlluppallama 21.0 35 35 30 NuwaraEliya 9.7 45 25 30 Pottuvil 22.6 25 20 55 Puttalam 21.8 50 25 25 Ratnapura 22.0 25 30 45 Ratmalana 22.4 35 20 45 Tr incomalee 24.6 40 30 30 Vavuniya 20.7 30 30 40

Kurunegala 21.3 40 30 30

Bandarawela 14.1 50 25 25

Table 4: Probabilistic forecast for Minimum Temperatures for JFM season 2021

Note- Temperature forecasts are not available in Matara, Kegalle, Kalutara, Monaragala, , Jaffna, Killinochchi, Mullativu and Mathale districts due to unavailability of Climate data.

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