Tropical waves, also known as easterly waves, are elongated areas of relatively low air pressure, oriented north to south, causing areas of cloudiness and thunderstorms. They originate in the Intertropical Convergence Zone (ITCZ), a belt of low pressure at the equator. Tropical waves are generally carried westward by the prevailing easterly winds along the tropics and subtropics near the equator. They are a precursor to tropical cyclones.

Disturbances south of about 10 degrees north latitude frequently have trouble organizing, since they can't leverage the Earth's spin much to help them develop their own circulation. A water temperature of 82°F / 26°C is needed for tropical storm formation.

If two storms come within 13 degrees of each other's arc (900 miles), they can interact (the Fujiwhara effect), causing difficulties in the track and intensity forecasts.

When winds in a tropical cyclone equal or exceed 74 mph (64 kt or 34 m/s) it is called a hurricane. Hurricanes are further designated by categories on the Saffir-Simpson scale. Hurricanes in categories 3, 4, 5 are known as Major Hurricanes or Intense Hurricanes.

• Category 1   74-95 mph
• Category 2   96-110 mph
• Category 3   111-130 mph
• Category 4   131-155 mph
• Category 5   156+ mph

An Atlantic hurricane that develops near the volcanic Cape Verde islands, off the west coast of Africa. In terms of climate, Cape Verde is similar to the Sahel region. Annual dust storms, originating in the Sahara, tend to erode the windward side of the islands, and bring a seasonal harmattan or smog-like dust.  

The average hurricane season has about two Cape Verde-type hurricanes, which are usually the most intense storms of the season because they often have plenty of warm open ocean to develop over before encountering land.  Cape Verde-type hurricanes typically develop from tropical waves which form in the African savanna during the wet season, then move into the African steppes (dry plains). South of the Sahara Desert is the Sahel (border), the semi-arid region between the dry desert to the north, and the tropical forests, savannas (grasslands), and wet areas to the south. It extends from Senegal on the west to Ethiopia on the east. When the hot, dry air from the north collides with the cooler, wet air from the south, tropical waves form in the Sahel, and then drift to the West out over the Atlantic.

The disturbances move off the western coast of Africa and can become tropical storms near the Cape Verde Islands, usually in August or September. A typical Cape Verde hurricane will form as a tropical depression just south of the Cape Verde islands. They normally reach hurricane strength in the mid-Atlantic, but sometimes will strengthen closer to Cape Verde or the Caribbean. Some prominent Cape Verde hurricanes: Galveston 1900 (Cat 4), Andrew 1992 (Cat 5), Ivan 2004 (Cat 5).

The weather community generally does not pay much attention to anything coming off the African coast until it's been over water for 48-72 hours. If it maintains its identity that long, it MIGHT be something to look at. The USAF Hurricane Hunters do not come into play until a disturbance crosses 55W, as that's when NHC will task the HH to begin flight investigations (tasks).

The Saharan Air Layer (SAL) is an intensely dry, warm and sometimes dust-laden layer of the atmosphere which often overlays the cooler, more-humid surface air of the Atlantic Ocean. In the Sahara Desert region of North Africa, where it originates, it is the prevalent atmosphere, extending from the surface upwards several kilometers. As it drives, or is driven, out over the ocean, it is lifted above the denser marine air. This arrangement is an inversion boundary where the temperature increases with height; this suppresses or "caps" any convection in the marine layer. Since it is dry air, the lapse rate within the SAL is steep, that is, the temperature falls rapidly with height.

Disturbances over North Africa periodically result in vast dust and sand storms, some of which extend as high as 20,000 feet. These can be driven out to sea within the SAL as far west as North America. These clouds of dust are visible in satellite photos. (courtesy of WikiPedia)

Rank

Hurricane

Season

Min. pressure

1

Wilma

2005

882 mbar (hPa)

2

Gilbert

1988

888 mbar (hPa)

3

"Labor Day"

1935

892 mbar (hPa)

4

Rita

2005

895 mbar (hPa)

5

Allen

1980

899 mbar (hPa)

6

Katrina

2005

902 mbar (hPa)

7

Camille

1969

905 mbar (hPa)

Mitch

1998

905 mbar (hPa)

Dean

2007

905 mbar (hPa)

10

Ivan

2004

910 mbar (hPa)

Source: U.S. Department of Commerce

• MRF (Medium Range Forecast) is a 28-level sigma vertical coordinate (terrain following) global spectral model with a triangular truncation of 126 waves.
• AVN is the Aviation Run of the NCEP Medium Range Forecast (MRF) model.
• BAM (Beta and Advection) Model follows a trajectory from the Aviation Run of the MRF model to provide a track forecast. The BAM model is run with shallow (850-700 hPa), medium (850-400 hPa) and deep (850-200 hPa) vertically averaged winds (BAMS, BAMM and BAMD, respectively).
• LBAR (Limited area sine transform BARotropic) is a two-dimensional track prediction model which solves the shallow-water equations initialized with vertically averaged (850-200 hPa) winds and heights from the Aviation Run of the MRF global model.
• GFDL (Geophysical Fluid Dynamics Laboratory) is a limited area baroclinic model developed specifically for hurricane prediction.
• NOGAPS is the U.S. Navy's global spectral forecast model with 18 sigma levels, a triangular truncation of 159 waves, parameterizations of physical processes and a tropical cyclone bogussing scheme.
• UKMET is the global forecast model run by the UK Meteorological Office. Similar to NOGAPS and the MRF model.
• NHC90/NHC91 and CLIPER are statistical track forecast models. The predictors for CLIPER (CLImatology and PERsistence) include the initial latitude and longitude of the storm, the components of the storm motion vector, the Julian day and the initial storm intensity. The CLIPER forecasts are often used to normalize the output from other track models, and as a benchmark for evaluating track forecasting skill. NHC90 is a more general statistical model which uses the output from CLIPER in combination with vertically averaged (1000-100 hPa) geopotential heights from the Aviation Run of the MRF model as predictors. NHC90 was developed for the Atlantic and NHC91 was developed for the east Pacific.
• SHIPS (Statistical Hurricane Intensity Prediction Scheme) is a statistical model used to predict wind intensity; it is based on climatological, persistence and synoptic predictors. The primary predictors include the difference between the maximum possible intensity (MPI) and the current intensity.
• More details about track and intensity models (courtesy of NHC)
• The naming conventions used by the Naval Research Laborary (NRL) for the model scatter plot products:
  - Tropical cyclones for which the NHC issues advisories utilize numbers sequentially from 1 to 49.
  - Training messages are numbered 50-79
  - Test messages are numbered between 80-89
  - INVESTS are numbered between 90-99
  - Tropical cyclones which are forecast to develop by the UK Met Office model utilize the number 50. The tracks of these systems appear on the scatter plots for storm 50 as well as all storms between 80 and 99.