Tropical Storm Sendong (Washi): Manila Observatory’s Report No. 1

The full report is given by Manila Observatory dated 27 December 2011. Below is the text version with some occasional pictures:

TRMM 3-hourly rainfall from 16-17 December 2011

TRMM 3-hourly rainfall from 16-17 December 2011

Tropical Storm Sendong was an extreme weather event, rare in the past but, according to the IPCC, with climate change, could become more frequent. This analysis may serve as part of decision support systems which will need to be modified in order to prepare and plan accordingly.

Between December 16-17, 2011, Tropical Storm Sendong made landfall and crossed Northern Mindanao. Cagayan de Oro City was hit directly by the storm, with the eye of Sendong passing at approximately 12MN on December 16. About 180mm of rainfall was recorded in the PAGASA Lumbia Station according to PAGASA.

This brief report analyses the meteorological impact of the storm in terms of wind strength and associated rainfall, the geographical path of the storm, and the time of occurrence by looking into the following three questions:

  • How extreme were the wind and rainfall associated with Sendong?
  • How often do tropical cyclones cross Mindanao?
  • How often do tropical cyclones cross Mindanao in December?

These questions will be tackled in brief on this web page. Links to a more detailed report, as well as slides to accompany the analysis, are available at the end of this page.

How extreme were the wind and rainfall associated with Sendong?

Sendong was a weak tropical cyclone, with 1-min average maximum wind speed of 100km/hr, according to the Joint Typhoon Warning Center (JTWC). With this wind speed, Sendong was a tropical storm according to the Saffir-Simpson Tropical Cyclone categories.

Technically, then, according to current wind-based typhoon warnings, Sendong was a “weak” storm. But similar to Tropical storms Ondoy and Pepeng, the rainfall amount associated with Sendong was extreme and resulted in severe flooding.

PAGASA has a weather station at Lumbia, Cagayan de Oro (see Figure 1), which has been operational since 1977. Available data from 1977-2005 show that within this 29 year period, the maximum recorded daily rainfall in Cagayan de Oro was 142mm in 1999. This is shown in Figure 2, which plots the heavy 24-hour rainfall events, with a lower bound cutoff threshold of of 75mm. (Note that a 1977-2011 data analysis is very much ideal and preferred if data are available). Figure 2 also shows that from 1977-2005, daily rainfall that is greater than 125mm happened only three times in Cagayan de Oro: in 1985, 1998, and 1999.

The histogram of these heavy rainfall events, given in Figure 3, show that heavy rainfall amounts of 50-100mm are more frequent for Cagayan de Oro. Rainfall amounts greater than 125 do not happen often. The 180mm rainfall of Tropical Storm Sendong is then considered very extreme and is higher than any of the recorded 24-hour total between 1977-2005. This 24-hour rainfall is also much higher than the average total monthly December rainfall, which is 117mm (see Figure 4). Hence, similar to Ondoy in Metro Manila, more than 1 month’s worth of rainfall happened in Cagayan de Oro within just 24 hours in December 16.

The weather station at Xavier University recorded lesser amount of rainfall compared with the readings of the PAGASA station, with an accumulated rain of about 85mm between 2pm and 12MN (see Figure 5). This heavy rainfall occurred before the eye of the storm passed the city at 12MN (as indicated by the red line, denoting a drop in pressure drop).

The weather station at Dahilayan Adventure Park (courtesy of Engr. Elpidio Paras) recorded a total of 119mm of rainfall, with very heavy rainfall of about 40mm/hr occurring from 10-11pm of December 16 (Figure 6).

Despite the differences in the recorded amounts, all stations show that heavy rainfall happened between 8PM to 12MN on December 16. This is also consistent with the timing of rainfall occurrence derived from the TRMM satellite-based rainfall data shown in Figure 7. NASA analysis of satellite data anticipated this extreme rainfall, with precipitation rates of 20-40 mm/hr associated with the storm, as shown in Figure 8.

Figure 9 shows the progression of rainfall accumulation as the storm passed through Mindanao. Figure 10 on the other hand shows the 3-hourly rainfall associated with the storm. Hence Figure 10 captures the dynamic movement of the storm and the accompanying rainfall, while Figure 9 shows the steady progression of accumulating precipitation.

How often do tropical cyclones cross Mindanao?

Typhoon track of December 16-17, 1920 over Cagayan de Oro

Typhoon track of December 16-17, 1920 over Cagayan de Oro

Data from the Joint Typhoon Warning Center (JTWC) show that most of the tropical cyclones that make landfall in the Philippines pass through Luzon and Visayas. Figure 11 shows that around 35 cyclones crossed Mindanao in the last 65 years (from 1945-2010), an average of one tropical cyclone every two years. However, in the last 15 years, tropical cyclones have been less frequent in Mindanao, with only six (6) tropical cyclones making landfall in the region during this time.

Based on a historical analysis of typhoons from 1883-1900 by Garcia-Herrera and colleagues (from the Manila Observatory Archives), however, there used to be more frequent typhoon crossings in Mindanao. Figure 12 shows that around 21 typhoons made landfall in Mindanao in the 17 year-period of 1883-1900, averaging 1 typhoon a year.

How often do tropical cyclones cross Mindanao in December?

It is interesting to note that there was a typhoon in 1920 that crossed Mindanao, which occurred on the same dates (of 16-17 December) and roughly had a similar path as Tropical Storm Sendong (Figure 13). Figure 14 also shows another typhoon in 1930, with a similar path as Sendong.

More recently, from 1947-2008, tropical cyclone tracks from JTWC show an average of one typhoon crossing in December in Mindanao for every 10 years as shown in Figure 15.

Conclusion/Discussion

Tropical storm Sendong was an extreme weather event, especially in terms of 1-day total accumulated rainfall. Precipitation associated with Tropical Sendong is much higher than normal. Cagayan de Oro City had a total of 180mm of rainfall in one day according to the PAGASA Lumbia station data. This is higher than the monthly average of 117mm (average based on 1977-2005 data). This is also the highest 1-day rainfall amount recorded in Lumbia station from 1977-2005.

Most of the heavy precipitation fell from 8PM to 12MN in Cagayan de Oro City.
Tropical cyclones cross Mindanao less frequently than Luzon and Visayas. Between 1883-1900, there were more typhoons that made landfall in the region (an average of 1 typhoon per year). In the last 15 years however, there were only 6 typhoons that crossed Mindanao.

Data from JTWC show that roughly one typhoon in December crosses Mindanao every 10 years.

Climate change? It is difficult to attribute one extreme event to climate change and we cannot make a definite conclusion about whether Sendong is part of climate change. Further, attribution is a complex topic that is still being addressed by the IPCC. However, the scientific consensus is that extreme weather events are going to be more likely and frequent with climate change. What Sendong has shown us is how serious and disastrous the consequences are of extreme rainfall events, especially in the light of exposure and vulnerability dynamics. In the future, we will analyze these dynamics and show how these, combined with the rainfall Sendong brought, resulted in the deaths and destruction we have seen.

Acknowledgments

The following people contributed valuably to this report:

Manila Observatory
Faye Cruz
Genie Lorenzo
Julie Dado
Raul Dayawon
Emil Gozo
Toni Loyzaga
Celine Vicente
Gemma Narisma

Special thanks to Engr. Elpidio Paras for the data from the Dahilayan Adventure Park weather station.

Follow

Get every new post delivered to your Inbox.