Published: 2017-06-19 09:29:00
Updated: 2017-06-19 17:06:23
Posted June 19
By Mike Moss
Some of you may have noted that one of the tropical disturbances we've been watching in recent days has now resulted in tropical storm warnings for parts of the Windward Islands even though the system had not been designated a tropical cyclone when the warnings were issued.
This is all part of a policy change for the National Hurricane Center (NHC) and National Weather Service (NWS) beginning this season, in which watches and warnings can be issued based on the expectation that a tropical cyclone will form and that tropical storm or hurricane conditions will affect land areas.
When this combination occurs, the NHC will now issue a full set of text and graphical products showing the expected track, intensity and impacts of the developing system, a graphical example of which I've included here. The system that is brushing by northern parts of South America in the coming days already has a small area of winds at tropical storm force, but so far a well-defined surface circulation center has not been detected.
Once that center develops (which could be in the near future, or possibly by the time you read this), the storm will be designated as a tropical cyclone. Depending on intensity, this could be a tropical depression, a tropical storm or a hurricane. The new policy is such that if a potential tropical cyclone is over an open water area and no populated land areas are subject to watches or warnings, which would mean that tropical storm or hurricane conditions are not expected to occur there for at least 48 hours, then no track graphics and advisories would be issued unless the system actually becomes a tropical cyclone.
You can read more about the new "potential storm" policy, along with some other new products or changes to products from the National Hurricane Center, at this link.
This change in policy came about in part due to the increasing accuracy of and confidence in tropical system forecasts, and to the desire to avoid situations in which a tropical system that could be foreseen either develops near or moves into a position very close to an affected area before it officially becomes a tropical cyclone.
In the past, scenarios like this could preclude the NHC and NWS from issuing watches and warnings in a timely manner. As you may have surmised, the difference between a "potential tropical cyclone" and a tropical cyclone is one of timing.
In order to be classified as a tropical cyclone, a system has to be powered mainly by convection due to heat and moisture transfer from the surface of a large body of water rather than by horizontal temperature gradients across a frontal zone, should have a well-defined, closed circulation center at and near the surface, and should typically have a "warm core," or temperatures near the center that tend to be warmer than the surroundings at the same altitude.
A tropical cyclone already has these characteristics, while a "potential tropical cyclone" is forecast to take them on in the near future.