Outside the National Hurricane Center's Cone of Uncertainty

Written by: Brandon Molyneaux

The cone (AKA "The Cone of Uncertainty") that the National Hurricane Center puts out is often misinterpreted by the public. A lot of folks agree that it is the forecasted path of the hurricane, but what a lot don't realize is that there are impacts outside of the cone. Let's take Hurricane Irma for example.

This graphic was generated using the National Hurricane Center's shapefiles. This graphic stacks Advisories 30 to 32 and all intermediate advisories in between. The pink represents hurricane force winds and the orange represents tropical storm winds at the time of the advisory. The black lines are the cones of uncertainty at the various advisories.  Notice Puerto Rico - the cone only goes through the island on advisories 30 and 30A, meaning that the center of the hurricane is forecasted to remain to the north of the island. Notice what is bolded in the sentence above: center. This is key to understanding how the NHC graphic is interpreted. All (if not, most) of the island experienced tropical storm force winds, with very few portions (if not, none) experiencing hurricane force winds (Advisory 31).

Winds aren't the only hazard that occurs outside of the cone of uncertainty - a textbook example of this is Harvey. Harvey made headlines for the amount of rain that it brought to the Houston area. At the time of this writing, Friendswood in Galvenston county recorded 56" of rain (source), while some places in NWS Houston's forecast region recorded 40+ inches of rain. Harvey made landfall near Rockport, TX on August 26 at 03:00z (August 25 at 10PM CDT) as a Category 4 hurricane. Due to blocking to the northwest, Harvey couldn't continue inland, so it sat over Texas, feeding the Houston region of what seemed like an endless supply of rainfall over the course of 5 days. Steering currents also broke down, causing Harvey to camp.

In addition to rainfall and winds, you have to consider storm surge as another hazard. Thankfully, those who live inland, away from the shore don't have to worry about this, as this is more of a coastal impact. Storm surge, in essence, is coastal flooding that occurs due to the hurricane's low pressure. When the winds are oriented from the hurricane to the shore, the winds are able to "push" the waters inland. Storm surge is the largest threat to communities, life, and property when it comes to tropical cyclones (source). I do want to clear up a potential gray area: Houston's treacherous rainfall is not a result of storm surge, but rather bands of heavy rain from the hurricane.

Thankfully the 2017 season, climatologically speaking, is beginning to die down. The jet stream will begin to dip south, and with westerly winds aloft and easterly winds at the surface, tropical cyclones in a general sense won't form due to the amount of shear. However, there have been a few times in history where December tropical cyclones have formed. Hurricane Epsilon (2005) was the most recent one to form in the Atlantic and become a hurricane. This simply means we can't rule out anomalous events (in our scenario, off-season tropical cyclones) from occurring.