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 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.
Please send comments and suggestions to:
Tim Marchok Timothy.Marchok@noaa.gov