Rapid intensification – Vongfong update

This is the down side of trying to blog an extreme weather event in real time while holding down a full time job as an academic. Just can’t keep up.

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When I wrote my previous post on Typhoon Vongfong (shown above in a recent visible satellite closeup, from this great site) last night, Vongfong’s winds were estimated at 90 knots, and the Joint Typhoon Warning Center (JTWC) had forecast that it would intensify to 130 knots over the next 48 hours. When I woke up this morning it was estimated at 120 knots already; it’s now at a screaming 155 knots, or 178 mph (per JTWC).

The NHC defines “rapid intensification” as an increase of 30 kt in 24 hours, so Vongfong has definitely undergone rapid intensification. It was not forecast, but it rarely is. While tropical cyclone track forecasts have improved steadily for decades, intensity forecasts haven’t, and the difficulty of forecasting jumps like Vongfong just made are a big part of the reason.

It’s not just the forecast of the storm’s future that’s the issue, though. Even estimating its intensity at the present moment poses challenges. While JTWC is calling it 155 knots – the very upper end of category 4, pushing category 5, the top of the scale – the most recent estimate from the Japan Meteorological Agency (JMA) puts it just at 115 knots, low category 3. Some of this difference is attributable to the different standards; JTWC uses 1-minute averaged sustained winds while JMA uses a 10-minute average, which tends to be a little lower. But that isn’t the whole story.

In fact, no one is measuring the winds in Vongfong. There is no routine aircraft reconnaissance in the Western Pacific to make direct observations. The estimates all come from analysis of satellite observations, and satellites don’t measure the winds directly. The satellite images used to estimate intensity see some combination of clouds and water vapor, and either human analysts or automated pattern recognition algorithms use those to infer the wind speed. (To get some idea how it’s done, try this.)

There’s no doubt that Vongfong is a very powerful storm. The forecast models are mostly predicting that it will start to weaken by a day or so from now, perhaps after holding steady or even intensifying a bit more before then (see for example a collection of current forecasts on Kerry Emanuel’s page). The track forecasts are still taking it over Japan early next week. Assuming that pans out, as seems likely, the question is just how rapidly Vongfong loses its currently formidable head of steam.

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