Hurricane Forecasting: Part 1 – Gypsies, Tracks, & Intensity

Hurricane Forecasting: Part 1 – Gypsies, Tracks, & Intensity

Hurricane forecasting is hard. This is certainly not a profound statement, but it is nonetheless true. We all hear the jokes about how once a storm enters the Gulf (of Mexico), the forecasters/meteorologists are just as likely to forecast it hitting Mexico as it will impact Florida. Some folks think that hurricanes are like gypsies who wander around with a mind of their own and meteorologists really can’t accurately forecast them. The reality is that over the past 20 years the accuracy of track forecasts have become significantly better, while the accuracy of intensity forecasts have lagged behind.

A large part of the hurricane forecasting problem lies in the fact that the TRACK and the INTENSITY of a hurricane are extremely dependent upon one another, thus you absolutely cannot separate the two. How does the intensity of a tropical system impact its track?

• Weak/developing tropical systems (Tropical Waves, Tropical Depressions, some Tropical Storms) are very shallow and their vertical structure does not extend very high in altitude, thus they tend to be steered by the low-level wind flow (image below). The low-level wind flow in the tropical Atlantic generally blows from east to west, so weak tropical systems in the Caribbean are more likely to move westward into Central America/Mexico and not impact the United States.

• Strong tropical systems (most Hurricanes) are much deeper and their structure extends much higher in altitude, thus they are able to be steered by mid-to-upper level atmospheric wind flow (image below). The mid-to-upper level flow includes the troughs that dip down across the United States which may force a storm northward, or the large ridges of high pressure which block storms from moving northward toward the Gulf Coast. So, you can see that the winds which steer a hurricane are very much dependent on how strong a storm is at the time. Rapid intensification can change which winds steer the storm, and rapid weakening will also change which winds steer the storm.

What further complicates the steering of a storm (and thus the forecast track) is the fact that the atmosphere is in constant motion. The steering flow does not remain the same day-to-day, but rather changes in the weather pattern (movement of troughs, ridges, fronts, etc) will cause significant changes in the steering winds. So, weather systems that are still out in the Pacific Ocean (and still several days away) will eventually move into the United States & change the steering pattern in the Gulf of Mexico, Caribbean Sea, & Tropical Atlantic. A change in the 7-day forecast position of a cold front in the Pacific by only 100 miles can be the difference in a hurricane recurving harmlessly in the Atlantic vs a major hurricane striking the Gulf Coast.

Example w/ Ernesto: For the Deep Layer Steering animation above, two large clockwise circulating ridges of high pressure can be seen: 1) off the U.S. East Coast, & 2) a smaller one centered over TX/LA. Early in the animation there is a weakness between these two ridges of high pressure that could briefly allow a tropical system to move northward over FL. So, if Ernesto had been a stronger storm & had been located over/near Cuba, then the storm would have a brief opportunity to move northward over FL and then up the U.S. East Coast. Later in the animation you can see that the two areas of high pressure become more connected to one another, thus effectively blocking any tropical system from impacting FL or the northern Gulf Coast.

So, how does the track of a tropical system impact the intensity and what does that mean to the overall forecast?

• The most obvious way the track impacts intensity is if the steering currents take a tropical cyclone over land, especially rough/mountainous terrain. The inner core of a tropical cyclone moving over land typically cuts off the storm from the additional instability provided by the release of latent/sensible heat from the warm water. Thunderstorm activity decreases & thus the surface pressures begin to rise and the wind speeds decrease – all part of the negative feedback cycle.
• Steering currents can bring a storm over cooler water, thereby decreasing the instability & thunderstorm activity in a similar way described above.
• The track can bring a well-developed hurricane or a developing system into a region of dry air. The dry air gets entrained into the inner core of a storm, thereby decreasing the thunderstorms in the inner core and often cooling off the warm bubble of the storm’s inner core. Ernesto (image above) has been battling dry air the past couple of days and now has an exposed circulation center with the thunderstorms lagging behind toward the east. This is certainly not healthy for a still-developing tropical storm.
• The track can bring a storm into a region of hostile wind shear, which are generally strong winds aloft that rip the tops away from the thunderstorms in the inner core of the hurricane. A hurricane’s inner core is essentially a warm bubble, so ripping the tops off the thunderstorms and dissipating the heat associated w/ the warm bubble can significantly weaken a tropical system (see image below from 2007 of a sheared tropical system w/ thunderstorms being blown off to the NE).

So, what’s the take-home message here? The TRACK impacts INTENSITY, and INTENSITY impacts the TRACK. A developing storm in a region of low wind shear can be steered by low-level winds toward the west, thereby staying farther south in a region of warm water and low wind shear. This favorable environment can then allow the system to strengthen into a hurricane, which will now be steered by mid-to-upper level winds. These new steering currents will often take the hurricane on a more northwesterly track, possibly taking the hurricane into a region of cooler water, hostile wind shear, or even over land. These hostile conditions can then weaken the storm down to a weaker hurricane, tropical storm, or even a tropical depression…which would then likely be steered by lower-level winds and take a more westward track.

Clear as mud, right?

Stay tuned for Part 2 where we examine the tools meteorologists use in forecasting hurricanes.

Ryan Wade
PhD Candidate
Severe Weather & Radar Group
University of Alabama in Huntsville
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