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Thursday, September 7, 2017

Hurricane Tracking

    With the hurricanes slamming Texas and now the Caribbean and Florida I have been watching their progress on Accuweather's hurricane tracker site. One thing that's interesting is the predicted path and as you can see, the path sometimes vary considerably. This begs the question, which forecast model each of those three or four letter identifiers refers to on the forecast model track the most reliable?
     The best hurricane forecasting models are "global" models that solve the mathematical equations governing the behavior of the atmosphere at every point on the globe. Models that solve these equations are called "dynamical" models. The four best hurricane forecast models (ECMWF, GFDL, GFS, and UKMET) are all global dynamical models. These models take several hours to run on the world's most advanced supercomputers.
     There are also dynamical models that cover just a portion of the globe. These are less useful, unless the hurricane happens to start out inside the domain the model covers and stay there. Hurricanes moving from outside the model domain into the model domain are not well handled. An example of this kind of model is the NAM model covering North America and the surrounding waters, run by the National Weather Service (NWS).
     Another type of hurricane model is a statistical model. These models do not try to solve mathematical equations, but have the advantage that they are fast to run and can provide output in a few minutes. There are also hybrid statistical/dynamical models, and simple trajectory models.
     The National Hurricane Center and other official forecast centers make use of two different forms of dynamical model guidance during the forecast process: "early" and "late" models. Numerical models are typically run four times per day: 0000, 0600, 1200, and 1800 UTC. However, National Hurricane Center official forecasts are generally issued three hours later.
     To alleviate this problem of late forecasts, model forecasts from the previous cycle are shifted forward in time by 6 hours. This results in what is known as an early model. The actual model forecast arriving three hours later is known as a late model forecast.
     Here is one expert's ranking of the forecast models from best to worst based on the accuracy of each individual model over the past 5 hurricane seasons.

TVCN (Consensus of the GFS, HWRF, GFDL, EURO, and several other models) is typically the most accurate model as it includes a combination of several useful models. The National Hurricane Center usually follows this one.
The ECMWF European computer model, which is not free for public viewing, is the 2nd most accurate hurricane forecast model only after the TVCN.
UKX (UKMET aviation tracker). This computer model has a tendency to slightly under-strengthen tropical systems, but it usually is more accurate with forecasting the track. This model is run by the United Kingdom Meteorological Centre.
AVNO (Aviation Ontime, also known as GFS or Global Forecast System). This model is typically one of the more accurate hurricane forecast models. And is usually better at predicting the track of a system compared to its intensity. This model is run by National Centers for Environmental Prediction.
FIM9 (Flow-following finite-volume Icosahedral Model). This is an experimental computer model that is expected to replace the GFS model in a few years. Although new it has proved to be a fairly accurate hurricane forecast model. This model is run by the Earth System Research Laboratory.
AEMN (Automated Environmental Monitoring Network, or GFS Ensemble Mean). This model’s forecast is based on an ensemble forecast, which involves making slight changes to the model’s initial conditions and running the model again. These forecasts are used to help illustrate forecast uncertainty. The closer the ensemble forecasted tracks are, the higher the forecast confidence. This model is run by National Centers for Environmental Prediction.

     A 2-page pdf fact sheet from the National Oceanic and Atmospheric Administration on hurricane prediction is available HERE. A web page on predicting hurricanes is also available on the Massachusetts Institute of Technology site HERE.  Yahoo has a good article from Good Morning America which explains just how strong Hurricane Irma's winds are and why they are so strong.  Read ARTICLE.

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