Please Explain A98E for me

[slosh]

Can someone explain what the A98E model is and why it takes Isabel on a deep southwest track when the other models look to be in agreement on a nw track?

[bayouwxman]

See if this helps:

NHC98 - A Statistical-Dynamical Hurricane Track Prediction Model

The NHC98 model is the latest in a series of mixed statistical-dynamic track prediction models. Earlier versions included the statistical models NHC67 and NHC72, and the statistical-dynamical models NHC73, NHC83 and NHC90.

In NHC98, storms are stratified based on their latitude and their current motion, with different equations used for westward and eastward-moving storms. This stratification is used to account for the observation that storms within the easterlies tend to move to the right of the steering flow, while storms within the westerlies tend to move to the left of the steering flow.

South Zone equations are used for storms south of 15oN, and for storms between 15oN and 25oN that are moving to the west or northwest. North Zone equations are used for storms north of 25oN, and for storms between 15oN and 25oN that are moving to the north or northeast.

The NHC98 model produces a forecast track that is a combination of three independent track estimates. The first estimated track is that produced by CLIPER.

The second estimated track is predicted using observed deep layer mean geopotential heights from the National Centers for Environmental prediction (NCEP) global model. Similar to CLIPER, the storm motion is separated into two components. One set of equations is used to predict the along-track movement of the storm at 12-hr intervals out to 72 hr. Another set of equations is used to predict the across-track movement of the storms. Deep-layer mean geopotential heights at two or three locations in the vicinity of the storm are used to represent the mean flow in which the storm is imbedded. Separate equations are used for each time period, but the geopotential height predictors are consistent from one time period to the next in order to avoid abrupt shifts in the predicted storm locations. These two diagrams show the grid points used for the first four time periods for the along track and cross track equations for the South Zone equations. The grids points used in the equations were determined by overlaying the grid shown on historic storm tracks and correlating the deep-layer mean heights at each grid point with the actual storm motion. The grid points that best correlated with storm motion are included in the prediction equations.

The third estimated track is computed similar to the second, except forecast deep-layer mean geopotential heights from the NCEP Aviation model are used to produce a track forecast. This sample shows the grid points used for the 36 hr, North Zone equations. (Starting with the NHC98 version, the circulation of the tropical cyclone is removed from the numerical analysis before the deep layer mean geopotential heights are determined.)

NHC98 combines the three track estimates (that from CLIPER, that based on the current geopotential height analysis, and that based on the forecast geopotential heights) into an optimum track forecast.

NHC98 is run four times per day. The primary synoptic time NHC98 forecasts (0000 and 1200 UTC) are based on the six hr-old aviation (AVN) run of the NCEP global spectral model. A special version, NHC98-LATE, is run at the primary synoptic times using forecasts from the current AVN model run and is available several hours after NHC98.