Changes to TAFB/NHC maps

By Dr. Nikos Mazarakis

The U.S. National Weather Service has made a significant change to its nautical chart that depicts the dangerous area for a tropical cyclone. For several years, the charts aimed at mariners and published by the agency's Tropical Analysis and Forecast Branch (TAFB) have been National Hurricane Center (NHC) are based on the Rule 1-2-3 rule that we also mention in our seminar entitled Tropical Cyclones SeminarBased on this rule, American meteorologists provide the navy with the ability to avoid the most dangerous area in a tropical cyclone, as shown in the figure below.

So with the shaded area, they essentially give the boundaries of the danger zone that the captain must avoid for the next three days or 72 hours from the Valid Time of the map.

In recent years, however, predictions of the future track of tropical cyclones have improved significantly. So by using the 1-2-3 rule, we are excluding a huge area, for no real reason. So based on this reasoning, from the July 15, 2016 and then the NHC provides probability maps for winds instead of the Danger Zone. But let's look at such a map and how exactly we read it.

The above map is for Tropical Depression KARL and Tropical Storm LISA and has a Valid Time of 22SEP2016, 09Z. In KARL, in the shaded area between the solid line and the dashed line, the probability of winds exceeding 34 knots is only 5%, while inside the area enclosed by the solid line the corresponding probability is 50%. And all this is valid for the next 72 hours from the Valid Time of the map. What we also understand from the same map is that KARL will upgrade to a tropical storm and then to a tropical cyclone (see the changing symbols). On the contrary, LISA, from a tropical storm, the next day, that is, on 9/23, is downgraded to a post-tropical and for this reason, from the next day, there is no longer an area enclosed by a continuous line. Therefore, the winds will drop below 35 knots.

In general, when reading such a map, the captain is practically required to avoid the area of 50% at all costs. However, it would be good to stay outside the area of the dotted line, despite the fact that the probability falls on 5%, if of course he can make the appropriate maneuvers.

This summer, however, another smaller change was made by the Americans, who changed the central server where we can find the maps. By clicking on the two links below, we can see the latest wind intensity probability maps.

http://tgftp.nws.noaa.gov/fax/PWEK11.gif

http://tgftp.nws.noaa.gov/fax/PWFK11.gif