Atlantic Hurricane Season

Interpreting Tropical Storm Arrival Times

A few weeks ago we went over how to interpret Tropical Storm warning graphics. As promised, let’s now do the same for NHC, Miami graphical products that display probabilities for arrival times of tropical storm/gale force winds.

Earliest Arrival Time

5 Day Earliest Reasonable Arrival Time, curtesy NOAA

The above example graphic which was for Hurricane Irma in 2017, illustrates the Earliest Reasonable Arrival Time (ERAT) of tropical storm force winds in local time for the next 5 days. There is only a 1 in 10 (10%) chance that tropical storm force winds would arrive at a location earlier than indicated. For example, southern Florida would only have about a 10% chance of tropical storm force winds arriving earlier than Saturday, Sep 7th at 8am. Also, timing information is only available for locations with at least a 5% chance of tropical storm force winds. This graphic is useful for anyone who may be impacted by an ongoing, post tropical, or potential tropical cyclone (for which advisories are being issued) that has a low risk tolerance.

The 5 day cumulative probability of experiencing tropical storm force winds is also shown represented by the colours ranging from green to purple, or a 5% to 100% chance respectively. It is important to note that all the locations shown in purple, for example (90-100% chance of tropical storm force winds), will not be experiencing tropical storm force or higher winds at the same time.

NHC also produces this graphic in a 3 day version seen below, with all of the above information still true just for a shorter 3 day period. So, it can be easier to see the details for the first 3 day period rather than looking at all 5 days. So in this example southern Florida is just starting to get into the 5-10% probability of tropical storm force winds by the end of the 3 day period.

3 Day Earliest Reasonable Arrival Time, curtesy NOAA

Most Likely Arrival Time

NHC, Miami also produce a graphic that displays the Most Likely Arrival Time of tropical storm force winds, also available in 3 and 5 day versions. We’ll just take a look at the 5 day version here. The probabilities of tropical storm force winds are the same as in the ERAT graphic, however the difference is in the labelled arrival times. In the example below it can be seen that southern Florida is shown to have tropical storm force winds arriving by around Saturday, Sep 7th at 8pm (rather than 8am shown in the ERAT graphic). This graphic indicates the times before or after which there is an equal chance of the arrival of tropical storm force winds (50/50 chance). For example, at 8pm on Saturday, Sep 7th southern Florida has about a 50% chance of receiving tropical storm force winds. This graphic is more useful for those who may have a higher threshold for the expected conditions, and are willing to risk possibly not being fully prepared by the time a storm arrives.

All of the above timing graphics are created using the same wind speed probability model, where 1000 plausible scenarios are constructed based on the official NHC tropical cyclone forecast and incorporating historical forecast errors. For the first day in these graphics, the arrival times are listed every 6 hours, with 12 hour intervals being used thereafter. Lastly, all of these graphics are issued with each full advisory package issued by NHC (usually every 6 hours).

These graphics can also be used at sea. Below shows Hurricane Helene in 2006, showing probabilities of 50 knot winds for the 5 day period starting at 8am AST on September 16th. Helene is the further south system shown in the image near 18N/45W. Note that there is another system near 31N/54W (Hurricane Gordon) with its own set of probabilities for 50 knot winds, which is also shown on the graphic due to the relatively close proximity of the systems. Graphics such as this can be very useful to a mariner, by denoting regions where mariners should use caution or avoid depending on the exact situation.  

50 Knot Wind Speed Probabilities, curtesy NOAA

Using the NHC wind speed probability/arrival time graphics shown above, alongside the Tropical Storm Warning Graphics can give mariners a good visual overview of the expected track of a tropical cyclone, and the associated probabilities/arrival times for certain wind speed thresholds. However, even with these informative graphics, it is important that mariners consult with their weather routing service when making routing decisions in regards to tropical cyclones. Weather forecasting is a complex and challenging science where interpretation of multiple factors and variables by experts remains key in ensuring safe and efficient voyages.

Stay connected and stay safe.

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