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Consensus Seasonal Weather Outlook

June, July and August (JJA)

Seasonal Rainfall and Temperature for Sri Lanka

This forecast was 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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Prevailing global climate conditions

In April 2020, positive sea surface temperature (SST) anomalies weakened and were near zero

by the end of the month. During the last four weeks, equatorial SSTs were near-to-below normal

across the central and eastern Pacific Ocean, and above normal in the western Pacific Ocean with

the ENSO neutral conditions (Fig 1).

Above-average SSTs weakened across the equatorial Pacific Ocean, and below-average SSTs

emerged in the east-central Pacific during the last four weeks. (Fig. 2).

The overall ocean temperature patterns are consisting with ENSO-neutral. According to the

NOAA, ENSO-neutral is favored through Northern Hemisphere Summer 2020 (~65% chance),

with the chances decreasing through the autumn 2020 (~45% to 55% chance).

The IRI/CPC Official Probabilistic Forecast (Fig 3 A) likely to continue with the probability to

favor ENSO-neutral (Niño-3.4 index between -0.5°C and +0.5°C) through the Northern

Hemisphere Summer 2020, with the possibility of below-average temperatures becoming more

established toward the latter half of the year. It shows the possibility of getting La Niña

conditions during this period. (Climate Prediction Center, USA).

The Indian Ocean Dipole (IOD) is also currently neutral and the latest weekly value to 27 May

was +0.3 °C. There is currently little temperature gradient across the Indian Ocean, which is

consistent with neutral IOD. (Fig 3 B) (Bureau of Meteorology, Australia). Most of modal

outlooks indicate the IOD is likely to become negative over the coming months.

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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(A)

(B)

Fig 3a: ENSO forecast from Climate Prediction Center (CPC)/ IRI Forecast (A) and IOD

forecast from Australian Bureau of Meteorology (B).

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Impacts of negative IOD on monthly rainfall anomaly during July and August

Fig 3b: Median Based Composite maps of Monthly Rainfall during July (A) and August (B) during IOD

years (Hapuarachchi et al 2018)

Previous studies conducted by the Department of Meteorology, identified that the probability of

getting above normal rainfall in the Southwestern parts of Sri Lanka, and some parts of

Batticaloa and Ampara districts in July is high (Fig 3b (A)) during negative IOD condition. But

in the month of August, is showing the probability for above normal is very low in the

Southwestern quarter, although the probability is high in the Eastern, Northern and Central parts

of Sri Lanka (Fig 3b (B)).

(a) Forecasts from different climate models from around the world.

(a.1) From June 2020 to August (JJA) 2020 season

Figure 4 shows the probabilistic multi model ensemble forecast which prepared by using

dynamical models from 10 Global Producing Centers (GPC) for JJA season. According to that

above normal rainfall can be expected over the country for JJA season. 2020.

(A) July (B) August

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Fig 4: Probabilistic multi model ensemble forecast for JJA using dynamical models from 10

WMO global producing centers (GPC).

Figure 5 depicts individual forecasts provided by same GPC centers for the JJA season. Out of

10 GPC individual models, 5 predicted above normal rainfall, 1 predicted above to near normal

rainfall and there is no clear signal over the Island from 4 models during the JJA season.

Accordingly, it can be expected near to above normal rainfall over the country during JJA 2020

season.

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Fig 5: Individual forecasts for JJA 2020 season by dynamical models from 10 WMO global

producing centers (GPC).

(a.2) Monthly Forecast for June, July and August 2020

Figure 6 shows the probabilistic multi model ensemble forecast, which is prepared by using

dynamical models from 10 global producing centers (GPC), for the months of June, July and

August 2020. According to that, above normal rainfall can be expected over most parts of the

Island during the months of June and July. While there is no clear signal over the country except

Northern parts during the month of August 2020.

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Fig 6: Probabilistic multi model ensemble forecast for June (left), July (middle) and August 2020

(right) using dynamical models from 10 WMO global producing centers (GPC).

Fig 7: Individual forecast for June 2020 by dynamical models from 10 WMO global producing

centers (GPC).

Figure 7 shows the 10 monthly forecasts from individual global producing centers (GPC) for

June 2020. Out of 10 GPC forecasts, 6 GPC models predicted above normal rainfall and there is

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no signals from 4 GPC models for the month of June 2020. Accordingly, it can be expected

above normal rainfall over the island during the month of June 2020.

Fig 8: Individual forecast for July 2020 by dynamical models from 10 WMO global producing

centers (GPC).

Figure 8 shows the monthly forecasts from individual global producing centers (GPC) for July

2020. Out of 10 GPC forecasts, 6 GPC models predicted above normal rainfall, while no clear

signal indicated in 4 GPC models for the month of July 2020. Accordingly, above normal rainfall

can be expected over the country during the month of July 2020.

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Fig 9: Individual forecast for August 2020 by dynamical models from 10 WMO global

producing centers (GPC).

Figure 9 shows the monthly forecasts from 10 individual global producing centers (GPC) for

August 2020. Out of 10 GPC forecasts, 4 GPC models indicate above normal rainfall and 1 GPC

model shows below normal rainfall and there is no signal from 5 GPC models for month of

August 2020. Accordingly, there is no signal for August 2020 indicating equal chances of

receiving below normal, near normal and above normal.

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(b) Statistical downscaling of CFSv2 global forecast output

(b.1) Probabilistic Forecast for JJA season 2020 using Climate Predictability tool (CPT)

The following district wise probabilistic rainfall forecasts for the season of JJA 2020 have been

prepared with the multi model ensemble method to downscale ,SST data of CFSv2, CCMS4,

GFDL and ECMWF by using CPT.

The district wise 30 year average rainfalls during JJA 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.

Table 1: Probabilistic Rainfall Forecast for JJA season 2020 using CPT

District Average rainfall (mm) –JJA Probability%

Below Normal Above

Colombo 590.1 50 20 30

Kalutara 861.5 55 20 25

Galle 771.0 50 25 25

Matara 604.3 30 25 45

Hambantota 146.9 20 20 60

Ampara 131.5 25 25 50

Batticaloa 141.2 25 25 50

Trincomalee 168.8 30 30 40

Mullaithivu 99.5 35 30 35

Jaffna 71.6 25 30 45

Killinochchi 57.1 25 30 45

Mannar 40.3 30 30 40

Puttalam 98.4 35 30 35

Gampaha 478.5 45 25 30

Kegalle 934.8 50 25 25

Ratnapura 675.1 30 25 45

Monaragala 125.9 20 30 50

Badulla 183.7 20 20 60

Pollonnaruwa 128.9 25 30 45

Vavuniya 109.8 40 30 30

Anuradapura 89.7 25 30 45

Kurunegala 212.4 25 30 45

Matale 163.5 25 30 45

Kandy 497.6 35 30 35

Nuwaraeliya 767.1 30 25 45

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Fig 10: Probabilistic rainfall forecast for June-August 2020 using CPT

According to the CPT (Fig 10 and table 01), above normal rainfalls can be expected in 14

districts out of 25. Below normal rainfall can be expected in Gampaha, Colombo, Kegalle,

Kalutara and Galle Districts. There is no clear signal for Mannar, Vavuniya, Mullativu,

Trincomalee, Puttalam and Kandy districts for JJA season 2020. Therefore equal chances exist of

receiving below normal, about normal or above normal rainfall in Mannar, Vavuniya, Mullativu,

Trincomalee, Puttalam and Kandy districts for JJA Season 2020

.

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(b.2) Probabilistic Forecast for JJA 2020 season using RIMES FOCUS System

Fig 11. Probabilistic rainfall forecast for June-August 2020 using RIMES FOCUS System

Figure 11 depicts the Probabilistic rainfall forecast for climatic zones for JJA 2020 season, which

has been prepared by using RIMES FOCUS System. Accordingly, above normal rainfall is likely

over the Intermediate zone and Dry zone. There is no clear signal indicated for wet zone. It

indicates that there is equal chances for having below, about or above normal rainfall over the

wet zone during JJA season 2020.

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(b.3) Probabilistic Forecast for JJA season 2020 using Climate Information Toolkit

A climate information toolkit, which has been developed by APCC Climate Center (APEC) in

Korea, is used to develop following forecast. It is being used Collection of Dynamic ensemble

seasonal prediction data from National Meteorological and Hydrological Services and research

institutes. This includes 14 operations and the models developed by institutes from 10 countries.

Areal rainfall data are used as input data to ‘’CLIK’’ toolkit and Downscaled to districts level.

SST is selected as predictor for all the models. (APCC, NASA, NCEP, PNU).

Fig 12: CLIK Multi model Ensemble Probabilistic Forecast for JJA season 2020

According to the above model outputs, near normal rainfall can be expected in 13 districts out of

25. Above normal rainfall can be expected Badulla district and below normal rainfall can be

expected in Kandy and NuwaraEliya districts. There is no clear signal for Gampaha, Colombo,

Kegalle, Kalutara, Galle, Matara, Hambantota, Puttalam and Vavuniya districts. It indicates

equal chances of receiving below normal, near normal or above normal rainfall over those

districts during JJA 2020 season.

B - Below Normal

A - Above Normal

N - Near Normal

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Summary : Consensus Seasonal Rainfall outlook for June, July and August

2020 (JJA) Season.

Table 2: SUMMARY of MODEL FORECAST for JJA season for SRI LANKA

SUMMARY of MODEL FORECAST for JJA 2020season for SRI LANKA Season WMO LC

MME

WMO

GPC

CPT FOCUS Impact of

Global

conditions

Final

JJA season

2020

AN AN AN – Most parts except

BN- Gampaha, Kegalle,

Colombo, Kalutara and

Galle

No Signal- Mannar,

Vavuniya, Mullativu,

Trincomalee, Puttalam and

Kandy

AN- Dry

zone and

Intermediat

e zone

No Signal-

Wet zone

Slightly

negative

IOD

conditions

in July and

August

AN

June 2020

No Signal -

Eastern and

Southern parts

AN- elsewhere

AN

AN –Badulla, Monaragala

and Hambantota

BN- Gampaha, Kegalle,

Colombo, Kalutara, Galle,

Mulativu, Vavuniya,

Tricomalee, Batticaloa and

Ampara

No Signal

NIL Near normal

July 2020 No Signal –

Southern parts

AN- elsewhere

AN

- - Slightly

negative

IOD

Above normal in

Sw part and near

normal elsewhere

August 2020 AN- Northern

parts

No Signal –

elsewhere

No

Signal

- - Slightly

negative

IOD

No signal-CP

BN: Below Normal NN: Near Normal AN: Above Normal CP: Climatological

Probability

Consensus Forecast for Sri Lanka

ENSO Neutral conditions are likely to continue during June to August 2020 and IOD is likely to

become slightly negative during July and August 2020.

Considering the prevailing global climate conditions, forecasts from different global climate

models and statistical downscaling of GCM output using CPT, consensus forecast for June to

August 2020 is concluded as follows.

There is a possibility for above normal rainfalls over Sri Lanka for JJA season

There is a chance to receive near normal rainfall over the country during June 2020. Higher

chance is likely to receive above normal rainfalls in Southwestern parts and near normal rainfall

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elsewhere during the month of July 2020. No clear signal over Sri Lanka for the month of

August and it indicates that there is equal chance to receive below, about or above normal

rainfalls during August 2020.

However, 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 etc. and

intraseasonal Oscillations such as Madden Julian Oscillations (MJO).

Fig 13. Consensus Probabilistic rainfall forecast for June–August 2020

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Probabilistic Temperature Forecast for June–August 2020 (JJA)

The probabilistic Temperature forecast for June, July and August season (JJA) 2020 for Sri

Lanka as given below.

Fig 14: Probabilistic forecast for Maximum Temperatures for JJA season 2020

Fig 14 and Table 3 show the probabilistic forecast for Maximum Temperatures during JJA season

2020.

Accordingly, there is a higher chance of experiencing slightly above the normal Maximum

temperatures in Mannar, Puttalam, Anuradhapura, Trincomalee, Batticaloa, Ampara, Kandy and

Hambantota districts and below normal maximum temperatures in Nuwaraeliya and Rathnapura

districts during JJA season 2020.

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 average is given in the columns 3, 4,

and 5 respectively.

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Table 3: probabilistic forecast for Maximum Temperature for JJA season 2020

Fig 15: Probabilistic forecast for Minimum Temperatures for JJA season 2020

District

Average Maximum

Temperature (0C) –

(JJA) Probability %

Below Normal Above

Anuradhapura 33.0 20 30 50 Badulla 30.6 30 35 35 Batticaloa 33.5 20 30 50 Colombo 30.1 35 35 30 Galle 28.7 35 35 30 Hambantota 30.3 20 30 50 Katugastota 28.0 20 30 50 Katunayake 30.5 30 35 35 Mannar 30.8 20 30 50 MahaIlluppallama 32.8 25 30 45 NuwaraEliya 18.7 50 30 20 Pottuvil 31.4 20 30 50 Puttalam 31.5 20 30 50 Ratnapura 30.8 50 30 20 Ratmalana 30.3 35 35 30 Trincomalee 34.5 20 30 50 Vavuniya 33.9 35 30 35 Kurunegala 30.9 30 35 35 Bandarawela 25.8 35 35 30

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Fig 15 and Table 4 provide the probabilistic forecast for Minimum Temperatures during JJA

season 2020.

Accordingly, there is a higher chance of experiencing slightly above the normal minimum

temperatures in Mannar, Vavuniya, Anuradhapura, Kurunegala, Batticaloa, Kandy and

Nuwaraeliya districts and below normal Minimum temperatures in Puttalam, Gampaha,

Colombo, Rathnapura, Galle and Hambantota districts during JJA season 2020.

Table 4: Probabilistic forecast for Minimum Temperatures for JJA season 2020

District

Average Minimum

Temperature (0C) –

(JJA) Probability %

Below Normal Above

Anuradhapura 24.6 20 30 50 Badulla 18.6 30 30 40 Batticaloa 25.3 25 30 45 Colombo 25.3 50 25 25 Galle 24.9 50 25 25 Hambantota 24.8 50 30 20 Katugastota 21.1 20 30 50 Katunayake 24.9 50 30 20 Mannar 26.7 25 25 50 MahaIlluppallama 24.4 20 35 45 NuwaraEliya 12.9 20 30 50 Pottuvil 23.0 35 30 35 Puttalam 25.8 50 30 20 Ratnapura 23.5 50 30 20 Ratmalana 25.0 45 30 25 Trincomalee 25.9 30 35 35 Vavuniya 24.4 25 30 45 Kurunegala 23.8 20 30 50 Bandarawela 16.7 30 35 35