NCEP global analyses and GOES-East, GOES-West, and MTSAT-1R channel-3 (water vapor) imagery are used as input to an algorithm to estimate the probability of tropical cyclone formation within the next 24 hours in 5 by 5 degree latitude/longitude areas from 0 to 45 degrees north and 100 to 350 degrees east. The product domain is divided into 3 basins based on satellite coverage and warning agency boundaries.
The probability estimates are based upon the following input parameters:
- CLIMATOLOGICAL FORMATION PROBABILITY: This was derived from the NHC Atlantic and east Pacific best track files from 1949-present. Formation was defined as the first point for each storm in the best track after all extratropical cases were excluded.
- PERCENT LAND: The percent of each 5 by 5 degree square over land
- DISTANCE TO PREXISTING STORM: The distance from the center of each 5 by 5 degree square to existing tropical cyclones.
- CLIMATOLOGICAL SST: The maximum SST in each 5 by 5 degree area from the Levitus climatology.
- VERTICAL SHEAR: The 850-200 hPa shear determined by averaging the 850 hPa and 200 hPa wind vectors over each 5 by 5 degree area, and then calculating the magnitude of the shear vector.
- 850 HPA CIRCULATION: The 850 hPa circulation determined from a line integral of the wind component tangent to the boundary of each 5 by 5 degree area.
- VERTICAL INSTABILITY: The vertical average temperature difference between the equivalent potential temperature of a parcel lifted from the surface to 200 hPa, and the saturation equivalent potential temperature of the environment, for each 5 by 5 degree area.
- 850 HORIZONTAL DIVERGENCE: The 850 hPa horizontal divergence determined from the average zonal and meridional winds along the boundaries of each 5 by 5 degree area.
- COLD PIXEL COUNT: The percent of channel 3 pixels colder than -40 degree C in each 5 by 5 degree area. All full disk images within 3 hours after and 6 hours before each synoptic time are include, so that this parameter represents the amount of sustained deep convection.
- CLOUD-CLEARED WATER VAPOR BRIGHTNESS TEMPERTATURE: The average channel 3 brightness temperature, after the cold pixels in parameter 9 above have been eliminated. This parameter is a measure of mid- to upper-level moisture.
The formation probability is calculated by a screening step, followed by a discriminant analysis.
In the screening step, the probability of formation is set to zero for 5 by 5 degree areas where formation almost never (ie, less than 5% of sample genesis cases) occurred based upon the best track data (1949-present), NCEP reanalysis fields (1981-present), and Satellite Water Vapor climatology (GOES-W = 1995-present, GOES-W = 1998-present, GMS-5/GOES-9/MTSAT = 2000-present). The following areas are eliminated:
Screening Step:
| ATLC | EPAC | WPAC | ||
|---|---|---|---|---|
| Maximum climatological SST (°C) | < | 21.0 | 21.0 | 21.0 |
| Latitude (°N) | < | 5.0 | 5.0 | 0.0 |
| Percent of Grid Box Area Over Land (%) | = | 100 | 100 | 100 |
| 850 hPa Circulation (kt) | < | -3.0 | -2.4 | -1.7 |
| Vertical Shear (kt) | > | 49.0 | 31.0 | 38.0 |
| Vertical Instability (°C) | < | -2.6 | -3.0 | 1.6 |
| Horizontal Divergence (x10-5 s-1) | > | 1.0 | 0.7 | 0.5 |
| Satellite Water Vapor Cold Pixel Count (%) | < | 2.8 | 5.0 | 3.0 |
| Satellite Water Vapor Brightness Temp (°C) | > | -25.4 | -23.1 | -27.8 |
For each of the 3 basins, the parameters for all 5 by 5 degree areas that passed the respective screening test were used as input to a linear discriminant analysis. The procedure, which is performed separately for each of the 3 basins, provides a discriminant function that can be converted to a 24-hour formation probability.
For each basin, the probability for input areas that do not pass the screening phase for any one criteria are set to 0. For all 5 by 5 grid boxes that pass the screening step, the probability depends on the following inputs.
Discriminant Analysis Input:
- Climatological formation probability
- 850 hPa Circulation
- GOES Cold Pixel Count
- Distance to Pre-Exisiting Tropical Cyclone
- Vertical Shear
- Percent of Area Over Land
- 850 hPa Horizontal Divergence
To provide some time continuity of the product, the formation probability and the primary parameters used in the screening and discrimination analysis are summed (probabilities) or averaged (all other parameters) over 5, 2, and 5 sub-basins for the Atlantic, Eastern Pacific, and Western Pacific basins, respectively. Time series products comparing the current and climatological parameters over the sub-basins are provided on the web site.