NCEP global analyses, ATCF tropical cyclone (TC) positions, and GOES-East, GOES-West, and MTSAT-1R water vapor imagery are used in an algorithm to estimate the probability of tropical cyclone formation within the next 24 hours in 5° by 5° latitude/longitude sub-regions from 0° to 45°N and 100°E to 10°W. 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 1949-2005. 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° sub-region that is over land
- DISTANCE TO PREXISTING STORM: The shortest distance from the center of each 5° by 5° sub-region to any existing tropical cyclone.
- CLIMATOLOGICAL SST: The maximum SST in each 5° by 5° sub-region 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° sub-region, 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° sub-region.
- 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° sub-region.
- 850 HPA HORIZONTAL DIVERGENCE: The 850 hPa horizontal divergence determined from the average zonal and meridional winds along the boundaries of each 5° by 5° sub-region.
- COLD PIXEL COUNT: The percent of channel 3 pixels colder than -40 °C in each 5° by 5° sub-region. 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 over each 5° by 5° sub-region, 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° sub-regions 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 |
| 850-200 hPa Vertical Shear (kt) | > | 49.0 | 31.0 | 38.0 |
| Vertical Instability (°C) | < | -2.6 | -3.0 | 1.6 |
| 850 hPa 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° sub-regions 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° sub-regions that pass the screening step, the probability depends on the following inputs.
Discriminant Analysis Input:
- Climatological formation probability
- 850 hPa Circulation
- Water Vapor Cold Pixel Count
- Distance to Pre-Exisiting Tropical Cyclone
- 850-200 hPa 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.
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