To start off our series on tropical classifications, we will explore what an invest or disturbance is.

Most tropical disturbances start off as a tropical wave or simply a wave of low pressure in the tropical latitudes. A lot of these waves will often develop off the west coast of Africa before moving westward into the Atlantic basin.

Tropical waves become an invest when the National Hurricane Center (NHC) wants to start examining the area of disorganized thunderstorms for potential tropical development. Invest is simply shorthand for investigation.

Invest 99L

Once the area of the potential development is declared an invest, the NHC can start collecting data and run model guidance on it.

Invests are numbered 90 through 99. Once 99 has been reached, the NHC restarts the numbering at 90. In addition to a number, the disturbances also receive a letter corresponding to which part of the ocean basin they originate in. The NHC issues warnings and advisories for two basins.

  • Invests in the North Atlantic are given a L
  • Invests in the Northeastern Pacific are given a E
  • Invests in the North Central Pacific are given a C (These warnings are issued by the Central Pacific Hurricane Center in Honolulu, Hawaii)

The NHC rates tropical disturbances in a low, med, and high confidence of development for 2 and 5-day outlooks.

The invest is symbolized by an X that is either yellow, orange, or red depending on the confidence of formation. The confidence is indicated in a percentage beside the center of circulation.

  • Yellow = low confidence (<40%)
  • Orange = medium confidence (40-60%)
  • Red = high confidence (>60%)
Invest 99L High Chance of Development

Once a disturbance reaches sustained winds of 38 mph or less and has a closed low-level circulation, it is then given the title of a “tropical depression.” We will explore this classification in a later episode of Weather 101.