Guide to the Tropical Forecasts and Analyses

We describe and display the results from the Global Forecast System (GFS) model of the U.S. National Center for Environmental Prediction (NCEP). This guide is not meant to be a thorough and complete description, but to give an overview of the forecasts and their presentation.

The view for the forecasts and analyses in the tropics consists of two overlaping longitudinal bands between 30N-30S, one centered on the Pacific basin, and the other centered over Africa. There is an analysis, and a series of forecast maps. The forecasts are presented at 12 hour intervals out to three three days, then at 24 hour intervals to five days. There are five different panels for each period. The contents and meaning of the five panels is described below in detail.

The analyses represent the initial state for the integration of the various forecast models. The analyses are produced from observations at weather stations around the world, as well as ship and buoy reports at sea, reports from aircraft, radiosonde balloons, and even satellite data. These data are merged after quality control procedures have been applied. Even with all of the data sources, there are still tremendous gaps in coverage over remote areas. An optimal interpolation (OI) procedure is performed using the previous model forecasts to fill these gaps and create a complete picture of the state of the atmosphere at the forecast time T=0. The model is then integrated forward in time to produce the forecasts which are displayed here.

At the bottom of each map is a bar telling the date and time for which the analysis or forecast is valid, the number of hours after the analysis for which the forecast is valid, the fields displayed, and their units. The five types of maps are described below.

Panels 1 and 2

Circulation Features at 850mb and 200mb
  • The state of the circulation at 850mb (about 1 mile or 1.5 km above sea level) and 200mb (over 10 km above sea level) are shown.
  • Black contours show the geopotential heights, in dm.
  • The streamlines indicate the direction of flow of the wind, which is generally from west to east in the subtropics, especially aloft, and from east to west throughout much of the tropics.
  • Purple shading indicates the speed of the winds at that level, in meters per second. Only wind speeds greater than 10 m/s are highlighted.
  • The blue and orange shading indicates a relative measure of horizontal convergence or divergence of the flow. Orange and red indicate strong divergence, and light and dark blue indicate strong convergence. Low-level convergence with divergence aloft at the same location is usually associated with strong vertical velocities in the middle troposphere, and severe weather/heavy rainfall.

Panel 3

Temperature of the Sea Surface, and 2 Meter Air Temperature
  • Over the ocean, red shading and contours indicate the sea surface temperature (SST), in degrees Celsius.
    • Sea surface temperatures above 25C are shaded in tones of red.
    • Notice the strong precipitation of the ITCZ and the South Pacific Convergence Zone (SPCZ) tends to align with the warmest SST, and avoid nearby cooler waters.
  • The shading over land indicates the air temperature 2m above the land surface.
    • Over monsoon areas a pronounced warming can be seen in the months preceeding the rainy season. Once the rains come, the temperatures over land cool markedly. Also notice the strong daily temperature cycle over desert areas (especially North Africa and Arabia) by comparing 00Z and 12Z maps.

Panel 4

Precipitable Water
  • The shaded contours indicate total precipitable water in the atmosphere. Precipitable water is the total depth of liquid water that would result if all water vapor contained in a vertical column of air could be "wrung out", leaving the air completely dry. It indicates the total humidity of the air above a location, and is a good indicator of the amount of moisture potentially available to supply rainfall. The contour interval is 10 mm up to 30 mm, and 5 mm thereafter.

Panel 5

Vertical Velocity or Precipitation
  • The colored contours in the analysis map indicate vertical velocity of the wind at the 700 millibar level, in millibars per hour (since pressure decreases with height, negative values indicate ascending air, and positive values denote sinking). Precipitation amounts are not available in the analysis, so we use vertical velocity as a proxy.
    • Ascending motion is associated with cloudiness and rain. Large negative values of vertical velocity correspond to areas of heavy rainfall if moisture is available. These areas tend to correspond with the storms in the first two panels.
  • The shading in the forecast panels indicates 12 or 24 hour accumulated precipitation, measured in millimeters.
    • The total is the amount of rainfall forecast during the 12 or 24 hours immediately preceding the verification time in the lower lefthand corner of the map.
    • Notice the nearly continuous band of rainfall around the globe near the equator. That is the Inter-Tropical Convergence Zone (ITCZ), and is a region where the Trade Winds of both hemispheres tend to converge. This band will move north and south during the course of the year, tracking the seasonal cycle of the sun, but lagging behind by several months (especially over ocean).
    • Also prevalent on the annual time scale are the summertime monsoon rains over India, Southeast Asia, northern Austalia, western Mexico, and subtropical South America. There are also strong seasonal rainfall patterns over much of Central Africa.