Tag Archive for: hurricane tracks

Why Hurricanes Often Track Like Boomerangs

Ever wonder why so many hurricanes track like boomerangs? They don’t all follow this pattern, of course, but when you look at the 1370 hurricanes between 1851 and 2006, a pattern clearly emerges.

From the US Department of Commerce book “Tropical Cyclones of the North Atlantic 1851 -2006.”

Interaction of Complex Systems

In my research, I found several possible explanations for the pattern above.

Harris County Meteorologist Jeff Lindner provided this explanation. “Many of the storms that form in the eastern or central tropical Atlantic tend to get pulled northward by mid-latitude troughs over the western Atlantic as they attempt to move westward. Storms that make it all the way across tend to happen only when strong ridges of high pressure are in place.” 

“Once a system is ‘captured’ by a mid-latitude trough, it will turn NW then N and then NE and E ahead of and along the trough axis. That’s why so many tracks are of this fashion over the open central and north Atlantic.”

Jeff Lindner, Harris County Meteorologist

Lindner added, “The further south a storm develops the less likely it is to be influenced by any sort of trough and a more westward track is then favored. Tracks into the Caribbean Sea from the east usually continue westward and those are what we tend to worry most about here along the Gulf Coast.”  

“It is very rare for storms to cross central America or Mexico due to the high mountains in this area. The mountains quickly destroy the low-level circulations. A few in history have survived the trek, but they are few,” said Lindner.

Shifts in Bermuda High

The mother of all high-pressure ridges in the Atlantic is sometimes called the Bermuda or Azores high. NOAA defines it as “a semi-permanent, subtropical area of high pressure that migrates east and west between Bermuda and the Azores depending on the season.”

Red arrow added to NOAA Image indicating migration of high between Bermuda and the Azores.

The Meteorology 101 blog says, “During the summer, [the Bermuda high] is located just off the east coast of the United States. The clockwise circulation around this high pressure area helps direct the path of hurricanes and helps determine where they will make landfall. … During the winter months, the Bermuda High is located farther east of the US towards the middle of the Atlantic Ocean. 

Depending on the Bermuda high’s location at any given time, it can block storms from going north. Or it can spin storms around in the boomerang pattern in the first image above. That’s because winds circulate in a clockwise fashion around high-pressure systems in the northern hemisphere.

Also note, sometimes gaps open in the high, letting storms slip north and get caught up in that clockwise circulation.

Highs Sometimes Block Storms from Curving North

HurricaneScience.org says, “Atlantic hurricanes typically propagate around the periphery of the subtropical ridge, riding along its strongest winds. If the high is positioned to the east, then hurricanes generally propagate around the high’s western edge into the open Atlantic Ocean without making landfall.”

“However, if the high is positioned to the west and extends far enough to the south, storms are blocked from curving north and forced to continue west, putting a large bulls-eye on Florida, Cuba, and the Gulf of Mexico.” This helps explain the wide spread of tracks on the left of the first image.

Influence of Trade Winds

This post from the National Oceanographic and Atmospheric Administration (NOAA) explains the relationship between the Sahara Desert and the formation of many hurricanes. Prevailing winds blowing from east to west (often called trade winds) come off the hot, dry Sahara Desert where they meet the cooler, wetter environment of the Atlantic Ocean west of Africa.

Prevailing global wind circulation patterns, courtesy of NASA

The prevailing winds in this latitude are so steady that in the days of sailing ships, mariners from Europe going to the Americas would first sail south to Africa. There, they would ride the reliable winds west to conduct commerce in the New World. Hence the name “trade winds.” The trade winds steer hurricanes, too.

In higher latitudes, the trade winds reverse direction. So sailors returning to Europe would sail north before returning to Europe and catch a tailwind home.

Opposing trade winds at least partially explain the boomerang pattern shown above.

If you ever get to Seville, Spain, make sure you visit the Archive of the Indies. There you will find the story of every Spanish treasure fleet that made the round trip between Europe and the Americas. I was struck by how many fleets sank in hurricanes while riding the trade winds.

Meteorology, or lack thereof, has influenced the fate of empires.

Coriolis Force

Many scientists also cite the Coriolis Force as a reason for why hurricanes try to drift north of dominant west-to-east winds. Frankly, the physics are beyond me. It has to do with the rotation of the earth at different speeds in different latitudes, and the rightward drift of objects (like hurricanes) not anchored to the earth while traveling over long distances. The Coriolis Force is also used to explain why hurricanes rotate in a counter-clockwise direction.

Posted by Bob Rehak on 8/31/2022 with input from Jeff Lindner, HCFCD

1828 Days since Hurricane Harvey

Today is Statistical Peak of Hurricane Season

According to the National Hurricane Center (NHC), today is the statistical peak of hurricane season in the Atlantic Basin.

Little Known Facts About Hurricanes: Peak of Season

The NHC has a fascinating page on hurricanes and climatology. Here are some interesting facts I gleaned from it.

Source: NHC. Peak of season occurs on September 10.

“The official hurricane season for the Atlantic Basin (the Atlantic Ocean, the Caribbean Sea, and the Gulf of Mexico) is from 1 June to 30 November. As seen in the graph above, the peak of the season is from mid-August to late October. However, deadly hurricanes can occur anytime in the hurricane season,” warns the NHC.

Average Number of Storms

Source; NHC

In an average season, we get 11 or 12 named storms, six of which become hurricanes and two or three of which become major hurricanes (category 3 or greater).

Twelve storms would get you to the letter L. But so far this year, we’re already on the R storm. And we have three more months to go in the season.

Points of Origin Tend to Shift by Month

The NHC shows where hurricanes tend to form each month of the season. In the first ten days of September, more hurricanes form in the Gulf than at any other time. The NHC shows a whole series of charts like the one below. It’s interesting to see how they change from period to period.

Source: NHC. Sept. 1-10 and Sept. 11-20 show the greatest number of hurricanes forming in the Gulf.

Note how few storms formed in the Caribbean compared to the period from October 11 to 20.

Compared to September 1-10, note how many more storms formed in the Caribbean during October 11-20.

Track Probabilities Also Shift by Month

The NHC also shows an interesting series of charts that show track probabilities by month. September is the most likely month for storms to track through the Caribbean, into the Gulf and onward to the Texas coast.

Source: NHC
Source: NHC

In stark contrast, during October, storms are most likely to veer east toward Florida and the East Coast.

Strike Density on Western Gulf Coast

Source: NHC

On the Texas Gulf Coast, Galveston has been hit more times than any other county. Harris, Brazoria and Chambers Counties follow closely.

Today’s Five Day Tropical Outlook

As if on cue, the NHC is now tracking seven tropical disturbances (as of 800 AM EDT Thu Sep 10 2020).

The two closest to Texas are highlighted in yellow above. That means they currently have less than a 40% chance of formation.

A large area of disorganized showers and thunderstorms near the Bahamas is forecast to move westward, crossing Florida on Friday and moving into the eastern Gulf of Mexico over the weekend. Upper-level winds are expected to become conducive for some development of this system while it moves slowly west-northwestward over the eastern Gulf of Mexico early next week. Formation chance through 5 days is 30 percent.

Another trough of low pressure has developed over the northeastern Gulf of Mexico and is producing a few disorganized showers and thunderstorms. Some slow development of this system is possible while this system moves westward and then southwestward over the northern and western Gulf of Mexico through early next week. Formation chance through through 5 days is even lower, at 20 percent.

Posted by Bob Rehak on 9/10/2020

1108 Days since Hurricane Harvey