The dynamics behind the recent Cebu flooding

The dynamics behind the recent Cebu flooding

Earlier this week, flash floods put Carmen and other areas of Cebu in distraught, leaving at least nine people dead.

What are the factors involved that may have led to this deadly flash floods? There are several underlying factors that contributed to this. Let’s find out what triggered such tragic event from happening and what can we do moving forward.

Around the clock, WeatherPhilippines was monitoring the progress of Tropical Depression 02W (CRISING) since its conception a week earlier, as numerical models indicate a weak system crossing the Central Philippines. All Tropical Cyclones can cause flooding and landslides since they are rain-producing systems. In most cases, stronger ones bring powerful winds and storm surges that spawn coastal inundation. However, the peculiar case of Tropical Depression 02W was something more sinister.

In that fateful day of Black Saturday, 15 2017, the time predominantly Christians around the globe are in observance of Lent,  a Tropical Depression comes onshore near Hernani, Eastern Samar while weakening. This has resulted to stranded passengers in several sea ports across portions of Bicol and Visayas regions. [See Fig. 1]

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Fig. 1. WeatherPhilippines GraphSat issued 1PM on 15 April 2017.

 

The system was compact, at about 265 km across and has sustained winds of 55 km/hr gusting higher at 75 km/hr at the time. The rain bands and its associated Trough were already impacting other parts of Northern Mindanao and Central Visayas. [See Fig. 2.]

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Fig. 2. WeatherPhilippines issued its StormTrack #005 (FINAL) on Tropical Depression 02W (CRISING).

 

Mesoscale Convective Systems (MCS)

Weak systems like Tropical Depression Crising have a tendency to degrade quite rapidly as it interacts with land mass. However, in Tropical regions like the Philippines, topographically-forced “lifting” ahead of these warm core systems propagate where ocean heat content is present. By the time these rain-producing cumulonimbus clouds build up onshore forcing up the slope, it generates in succession cold cloud-top temperatures high into the troposphere (about hundreds of kilometers behind convective cells) found along the leading edge of a mesoscale convective system (MCS).

According to the American Meteorological Society, MCS is defined as a cloud system that occurs in connection with an ensemble of thunderstorms and produces a contiguous precipitation area on the order of 100 km or more in horizontal scale in at least one direction. It exhibits deep, moist convective overturning contiguous with or embedded within a mesoscale vertical circulation that is at least partially driven by the convective overturning. These cloud clusters are primary conduit for heat, moisture and momentum redistribution in the tropical troposphere.

Another term used to describe violent and dangerous thunderstorms is called a ‘’supercell’’. These are referred to as highly localized heavy precipitation events (such as those produced by individual convective cells or slow-moving supercells). The most severe ones can generate damaging winds, large hail and tornadic activity.  This weather phenomena are common to temperate regions, but not remotely impossible to occur in tropical regions like the Philippines. [See Fig. 3.] Violent convective systems are aided by the steady inflow of moisture and latent heat, which sustain the development of severe types of thunderstorms. [See Fig. 4.]

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Fig. 3. Supercells. Flaring convective thunderstorms seen to build up across Southern Negros Island, Cebu and neighboring Bohol at 02AM-PhT 15 April 2017 provided by © Meteologix AG

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Fig. 4.  Himawari-8 IR Satellite loop of the MCS. Flaring convective thunderstorms seen to traverse Central Philippines. (0530PM-PhT to 07AM-PhT (15-16 April 2017 provided by © Meteologix AG

Data Observations

Based on the meteorological data gathered from WeatherPhilippines’ extensive network of automated weather stations (AWS), Danao City, Cebu (LCP) 24-hour total rainfall accumulation (ending 8AM-PhT) reached 212.4 mm followed by Consolacion, Cebu (SM) at 188.8 mm, Cebu City (LPP) with 167.8 mm, Talamban, Cebu with 135.6 mm, Daanbantayan, Cebu with 124.4 mm, Cebu City (SM) with 119.4 mm and Mandaue City, Cebu (LCP) with 97.0 mm. These numbers are but staggering reminders that intense thunderstorms, without warning, can form and reach mature stage rapidly and dissipate quickly in no time. Severe thunderstorms and MCSs in particular, can produce torrential rains that causes massive in the lowlands. [See Fig. 6.]

Based on WeatherPhilippines’ data, Danao City experienced the heaviest rainfall that day as indicated below:

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Fig. 5. 24-h rainfall totals in select areas of Cebu and Northern Mindanao, ending 8AM-PhT 16 April 2017.

 

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Fig. 6. Daanbantayan, Cebu got 79.6 mm rainfall total in just 1 hour, ending at 2AM-PhT 15 April 2017.

 

In so far as the highest 1-h rainfall accumulation, Daanbantayan, Cebu topped with 79.6 mm followed by Consolacion, Cebu (SM) at 60.8 mm, Danao City, Cebu (LCP) with 44.6 mm, Cebu City (SM) with 30.0 mm, Mandaue, Cebu (LBC) with 27.8 mm, Talamban, Cebu with 27.0 mm, Mactan International Airport with 21.4 mm and lastly in Cordova, Cebu with 16.4 mm respectively. [See Fig. 5.]

In particular, the recorded air temperatures over Danao City, Cebu (LCP) have dropped significantly between 02AM-PhT to 06AM-PhT (25.7 down to 24.4C) with lowest barometric pressure reading of 1008.2 hPa at 04AM-PhT respectively. These data suggest that the heaviest rainfall have occurred at this time and intense lightning activity deep into the convective storms were observed using Meteologix AG’s extensive lightning data network. [See Figs. 7.]

MCSs have a nocturnal tendency, as it typically forms in the late afternoon and evening hours, reach maximum size during the night, and dissipate in the morning hours, this according to a study (Maddox 1980; Jirak et al. 2003).

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Fig. 7. Frequency of lightning strikes as of 1900Z (02AM-PhT), 15 Apr 2017 provided by © Meteologix AG

 

The Philippines Case 

The Philippines is strategically situated along the active belt of Tropical Cyclone activity in the region as opposed to large oceans the West Philippine Sea and the Pacific Ocean to the East. These are considered as prime breeding ground for tropical activity. Extreme weather events are nothing new to us. There are roughly 10 Tropical Cyclone passages overland every year. These tropical systems gather along warm regions of vast oceans where they draw power from the heat given off by the sun. Extreme weather events become deadly when they hit densely populated areas. Let us not only draw our full attention on stronger Tropical Cyclones. Slow-moving, smaller and disorganized ones are rain makers that causes inundation under ripe conditions.

The associated dangers of these weather systems are not only limited to high winds, coastal erosion or storm surges along exposed coastal communities, but also in steep elevations rockslide and landslides that can happen without warning. Flash floods and slow-rise flooding have become a recurring problem, especially more and more people are drawn closer to the waters, living along the riverbanks and estuaries in the lowlands where access to resources seemed easy for some. These are the most vulnerable sectors of society-the poorest of the poor. The stakes are higher when they get affected. Natural disasters entail huge economic loss and disruption of essential services. Globally, the majority of deaths are caused not by high winds associated with Tropical Cyclones, but mainly due to drowning.

 

Forecasting Challenges Ahead

In most cases, MCSs are not uncommon in a tropical country surrounded by warm waters and situated near the Equator where variances in sea surface temperature (SST) and lifting mechanism develop and dissipate quickly. Such changes in the state of the atmosphere play a major role in the development of intense thunderstorms. These types of heavy rainfall-producing systems lasts longer several times over that of the ordinary afternoon thunderstorms that often develop due to daytime heating and available moisture inflow.

According to a study conducted by Stephen F. Corfidi (NOAA/NWS/NCEP/Storm Prediction Center, Norman, Oklahoma, 20 May 2003), “MCSs produce a disproportionate share of significant convective weather (high winds, flash flooding, etc.) and because their evolution is often not predicted well by operational numerical guidance, forecasting MCS motion is of considerable importance to operational meteorologists. Unfortunately, forecasting MCS is not only very challenging, but it is also an area with an overall lack of knowledge and specific prediction methods (Ziegler 2000).” Furthermore, they explained that MCS start from individual thunderstorm cells that interact and subsequently merge into a large, long-lived organized convective system (Cotton and Anthes 1989). Given this, MCS forecasting will need knowledge about the areas that are favorable for convection initiation, organization, and sustenance over a range of spatial and time scales.

MCSs typically form in the late afternoon and evening hours, reach maximum size during the night, and dissipate in the morning hours (Maddox 1980; Jirak et al. 2003).

The said studies referred into on this article are based on research journals mostly from the United States. At the onset, there is no indication of such studies conducted in Philippine climatological setting. The issue of forecasting MCSs requires more attention, and sadly in the Philippines, we lack the capability or the infrastructure to address this problem due to absence of technology and expertise of institutions.

  

Death Toll Rises

Citing a news report from CNN Philippines and The Philippine Daily Inquirer on the impacts of Tropical Depression 02W (CRISING) dated 16 April 2017, Ms. Romina Marasigan, spokesperson of National Disaster Risk Reduction and Management Council (NDRMMC) have said that 4,581 passengers were stranded in Bicol region and the Visayas, with nine (9) reported deaths all in Cebu province, with 34 vessels and 14 motor bancas prevented to sail due to bad weather.

While MCSs and intense thunderstorms are difficult to predict, bulletins issued by PAGASA as well as daily updates from private weather organizations like WeatherPhilippines may be used to anticipate and prepare for possible impacts of heavy rainfall brought by various weather systems. Understanding the dynamics behind these adverse weather conditions is key to protecting lives and property. Ultimately, it pays to be #WeatherWiser.

 

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References:

https://www.ametsoc.org/ 

Cold Pools and MCS Propagation: Forecasting the Motion of Downwind-Developing MCS by Stephen F. Corfidi, NOAA/NWS/NCEP/Storm Prediction Center, Norman, Oklahoma, 20 May 2003

Objective Categorization of Heavy-Rain Producing MCS Synoptic Types by Rotated Principal Component Analysis; John M. Peters and Russ S. Schumacher, Department of Atmospheric Science, Colorado State University, Fort Collins, Colorado, 08 January 2011.

Observational Analysis of the Predictability of Mesoscale Convective Systems by Israel L. Jirak and William R. CottonDepartment of Atmospheric Science, Colorado State University, Fort Collins, Colorado, 04 Oct 2006

The Philippine Daily Inquirer

http://cnnphilippines.com/regional/2017/04/16/at-least-7-dead-in-cebu-flooding.html

 

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