29 August 2008
Tropical Trouble Times 2
Posted by Dan Satterfield
It seems tonight that the numerical weather prediction (NWP) guidance is starting to come together on Gustav. Up through this morning, there was still a lot of spread among the different models. This tells a forecaster that any one solution is not likely to be correct, and that leads to a low confidence forecast.
Sometimes you will see a few outliers and know that you can ignore them due to bad initiliazation or a poor track record with tropical systems in general. Other times it is much more difficult. The NHC usually give a great deal of credence to a model called the GFDL. This model is designed specifically for tropical cylones, and the Physics in the model is tuned to best predict track and intensity.
I usually give a great deal of credence to the forecast track from the National Hurricane Center.
I deal with a couple of hurricanes a year along with the rest of the weather here in the Valley. The folks at the NHC do nothing but forecast tropical cyclones. The local NWS offices also base their local forecasts heavily on the track guidance.
I will sometimes disagree, and it is my decision on what to put on air. When this happens I will put both forecasts on to let viewers know that there is uncertainty. When it comes to hurricane forecasting, uncertainty is almost always large.
Forecasting the location is actually easier than forecasting intensity. We still have a long way to go there! Dr. Kerry Emanuel of MIT has done some ground breaking research on this. He has developed equations that can tell us how strong a hurricane the ocean water can make.
It all depends on the heat content of the water.
That may sound like an easy thing to measure, but it isn’t. See, it does not matter just how warm the water is, but how thick the layer of warm water is! A shallow layer of warm water is not nearly as important as a thick layer of warm water at the surface. We call this Tropical Cyclone Heat Potential. It is also referred to as Upper Ocean Heat Content.
Gustav will pass over an area of high TCHP in the next two days and that heat will probably turn Gustav into a dangerous hurricane.
After Gustav comes Hannah. This one may be headed toward Florida, and even perhaps in our direction. My sleep deficit will likely increase in direct proportion to the intensity of Hannah!
Hi Dan. The thing I am wondering about is dry air that might get entrained into the system (which is unlikely once Gustav becomes => Cat 3. The other potential inhibiting factor would be shear, right?
@Mike Wilhelm: Yes Mike, shear is the factor that has kept the hurricane from reaching Cat 4 strength today. The shear will keep it from getting much stronger but it may grty a little stronger before landfall if the shear lessens slightly.