Analyzing Megi’s Intensity
Just how strong was/is Super-typhoon Megi? The official intensity in the advisories at 12 and 18 GMT on the 17th was 155 kts. sustained, or 180 mph. However, the earlier of these two did not take into account recon data from around 12 GMT, and the discussion accompanying the later advisory noted that based on satellite imagery, Megi had probably peaked in intensity before the 18 GMT advisory--peak intensity was estimated at 160-165 kts. The higher of these two numbers corresponds to 190 mph, which would make it equal to Typhoon Tip and Hurricane Allen as the highest sustained winds ever recorded in a tropical cyclone anywhere. Yet even this intensity is possibly understated.
Between 11:11 and 11:14 UT, a reconnaissance plane penetrated the NE eyewall and recorded a maximum flight-level wind of 171 kts (197 mph), which corresponds to 177 mph at the surface. More remarkably, though, the SFMR instrument measured uncontaminated winds as high as 162 kts (186 mph), and shortly after that contaminated readings of 171 kts (197 mph) and 164 kts (188 mph). “Contaminated,” of course, does not mean necessarily invalid, merely suspect, and these readings look believable when we look at the trend of observations up until these readings. In the SW eyewall, a few minutes later, maximum flight-level winds were 170 kts (196 mph, corresponding to 176 at the surface) maximum uncontaminated SFMR readings were 169 kts (171 mph), and there were contaminated readings of 162 kts (186 mph) and 166 kts (191 mph). These readings in the usually significantly weaker SW quadrant lend credence to winds above 190 mph in the NE quadrant. Most remarkable, though, is the testimony of a dropsonde released in the NE eyewall at 11:13 UT (which landed in the NW eyewall), which reported 174 kt (200 mph) sustained winds at the surface. Dropsonde observations are not subject to rain contamination like SFMR, but can still be discounted on the basis that they reflect localized mesocyclones (vortices embedded in the eyewall), instead of ordinary straight-line winds. Of course, this dropsonde reading means that there certainly were 200-mph winds occurring at the surface, but the only question is whether these can legitimately be given as the hurricane’s official intensity--in Hurricane Felix, the NHC decided to discount a 189-mph dropsonde reading based on this reasoning.
On the next recon pass through the eyewall, the observations were no less remarkable. Coming into the eyewall from the south between 12:01 and 12:04 UT, reconnaissance reported maximum flight level winds of “only” 153 kts (176 mph, corresponding to 158 mph at the surface), and uncontaminated SFMR readings of “only” 133 kts (158 mph). There were a number of contaminated readings significantly higher, though--147 kts (169 mph), 163 kts (187 mph) and 149 kts (171 mph). In the NW eyewall, the numbers were truly astounding. There were three flight-level readings above 200 mph, including one of 192 kts (221 mph). This corresponds to 199 mph at the surface, lending credence to a spate of contaminated SFMR observations that peaked at 199 mph.
There are thus three independent pieces of evidence--a flight-level reading, an SFMR reading, and a dropsonde reading, attesting to sustained winds at the surface of 199 or 200 mph. Such agreement of all three instruments is exceptionally rare and suggests strongly that such testimony should be believed in this case. If so, that makes Megi the most powerful cyclone ever observed on Planet Earth.
Predicting Megi's Impact
Unfortunately for the Philippines, although it will undoubtedly lose a bit of intensity in the next few hours, it is almost certain to hit Luzon at Cat. 5 strength. The official forecast calls for 165 mph sustained winds a couple hours after landfall, so 175 mph at landfall is a reasonable guess, making it one of the strongest landfalls ever recorded. However, intensity fluctuations can be rapid in storms that have attained such strength. Particularly disturbing for Luzon is the fact that Megi is likely to start forming an outer eyewall as it approaches--most storms do shortly after becoming Cat. 5s. This would mean a much larger radius of maximum winds (currently these extend out only about 15 miles from the center).
The good news for Luzon is that the portion of coast at which Megi is aiming is sparsely-populated and mountainous, meaning that surge damage will be relatively minimal, and wind damage will be less than otherwise, since the storm may lose some of its punch before hitting more populated inland regions. However, “relatively” and “less than otherwise” are, when you’re talking about a 200-mph storm, still disastrous. Let’s see what lies in Megi’s path.
The entire province of Isabela, home to 1,500,000, is likely to be severely affected. Megi is likely to make landfall near the town of Palaman, which has a population of 16,000. This community and nearby villages will experience the full brunt of winds sustained at 175 mph and gusting over 200. The coast here, however, is almost empty, until you get 20 miles north, where the seaside town of Divilacan is home to 4500. This area will likely receive a catastrophic storm surge, and hopefully has been entirely evacuated.
Inland, the following cities will likely lie within Megi’s radius of maximum winds:
Ilagan, 131,000 (20 miles from the coast)
San Mariano (44,000) (30 miles)
Tumauini (55,000) (30 miles)
Benito Soliven (25,000) (40 miles)
Gamu (27,00) (40 miles)
Quirino (21,000) (50 miles)
Cauayan (115,000) (50 miles)
Burgos (22,000) (55 miles)
A good rule of thumb is that a hurricane will lose around 1 mph per 1.5 miles inland it travels, though it varies considerably. In this case, with the mountains (over 1,000 meters high) disrupting Megi’s circulation, we might reckon on 1 mph per mile. This would mean that Ilagan would be subject to winds as high as 155 mph sustained, San Mariano and Tumauini 145 mph, and the other cities here 120-135 mph. That’s a pretty scary prognosis.