Maysak: much weaker landfall, impact on Philippines forecast less than $150 Million

maysak_thu

Very different impact forecast than yesterday, and yet another good example of the complexity of forecasting the impacts of natural hazards.  The landfall intensity has decreased from 85 knots to 65 knots – a 25% decrease in wind speed.  But the impact forecast dropped by nearly 90% to from over a Billion dollars to about $130 Million!  Why?  Because the amount of force the wind creates (the “dynamic pressure”) is related to the square of the wind speed, not the wind speed itself.  So the pressure from a 65 knot wind is only 58% of the pressure of an 85 knot wind.  But it’s a bit more complex than that, and damage relates to a number of complex factors that ends up being a higher power factor of the wind difference above a threshold.  So for a typical house, an 85kt wind might cause 16% damage, whereas a 65 knot wind would only cause 4% damage.  Another factor in this case is that the swath of damaging winds is smaller, and misses the densely populated areas near the Capital, Manila.  Put all that together, and while the track doesn’t look that different, and the winds are a bit less, the damage forecast is dramatically different.  Which is a good thing – the Philippines really doesn’t need another disaster given the last few years of Typhoons and earthquakes.

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