Info on the Ensemble Tropical Products ....

Date last updated: May 23, 2005

Products containing the NCEP Ensemble Mean

Some products on this page display synoptic fields that represent the mean of the NCEP ensemble forecast. At 00, 06, 12 and 18z each day, NCEP runs a suite of 10 forecasts, each starting with initial conditions that have been slightly perturbed from that of the high resolution NCEP/GFS run. The plots on this site show the synoptic fields computed by taking a simple average of the 10 member forecasts.

Products containing the ECMWF Ensemble Mean

Like NCEP, ECMWF also runs an ensemble forecast. It currently runs only once per day, but with 50 perturbed member forecasts. Due to agreements with ECMWF, ECMWF ensemble forecast data are available only for research purposes and only to those personnel within NCEP. The products on this site that contain ECMWF ensemble data are restricted.

"Normalized Spread" of an ensemble forecast

Normalized spread is shown on the various ensemble forecast plots. For each field shown, the spread (or standard deviation) of the member forecasts is calculated at each grid point and at each forecast hour. The spread of the ensemble solutions can provide some information on the predictability of the atmosphere for a given situation. For example, if all of the forecasts at 120h are closely clustered together for a particular location, that may provide the forecaster with more confidence in the model solution for that time period and geographic region. The normalized spread is calculated by simply dividing the value of the current day's spread by the mean spread computed over the past 30 days. Therefore, a normalized spread value of 3 at a given point and forecast hour would indicate that the spread is 3 times greater than the mean spread over the past month at that point and forecast hour.

Cyclone tracks

Cyclone tracks are shown for the NCEP ensemble and for the GFS. Tracks for the NCEP ensemble are only shown for officially numbered systems (i.e., numbered by either NHC or JTWC). For the GFS, tracks are shown for both numbered systems and for new systems that the tracker detects during the running of the forecast. For numbered systems, the previous track up to the start of this current forecast is shown in gray, while the forecast track is colored. A black dot indicates the position of the cyclone at the current forecast hour being plotted.

Experimental Ensemble-Based Strike Probabilities

These probability plots are computed by simply going through the grid of points on our map and determining what percentage of ensemble member forecasts come within 65 nm of each point at any forecast hour during the forecast.

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Tim Marchok