 The 2019 Atlantic Hurricane season officially ends November 1st, but as we are winding down from the peak of the current season, it is important to understand and revisit past seasons. According to the National Hurricane Center, the United States isn’t bracing for any major hurricanes to make landfall. But in the beginning of this month, Hurricane Dorian was a major hurricane that was thought to be headed for Florida. Instead of hitting the sunshine state, however the storm tracked over the Bahamas instead. By analyzing another slow moving hurricane from the 2017 season, this can give us an idea of why Dorian was so catastrophic.
In 2017, Hurricane Harvey hit the Texas coastline, specifically Aransas County. When it made landfall, the storm was at its peak intensity. It was the first storm to make landfall in the United States as a category four storm – major hurricane – since William in 2005. While not getting into the specifics of how Harvey became a hurricane, this article will talk about how slow it moved over Texas, and how much damage was associated with it. When Hurricane Harvey hit, Aransas County was smashed with wind gusts of up to 132 mph. As a result, nearly every building was damaged. Not only in Aransas, but a major part of Texas received record breaking amounts of rainfall. This is what made the storm so impactful. The slow moving nature of the storm allowed for many days of heavy rainfall. On top of the storm surge associated with the storm, places were getting battered with upwards of 30 inches of rain. Some places, like Nederland, TX got up to 60.58 inches of rain. That is five feet of rain in less than a week! This made rivers, roads, and houses flood. A lot of this rain was caused by Harvey stalling and sitting over Texas for an extended amount of time. The hurricane stalled over Texas for two whole days and produced so much rain which was unprecedented. The high amounts of rainfall put the town into distress, with insane flooding. Then it moved out to towards the Gulf of Mexico again, and while sitting out there, the storm, yet again, stalled again. This once again produced heavy rainfall rates for the Houston area for another two more days. It then made landfall again in Louisiana where it brought more than 14 inches of rain. Due to large scale flows and weather patterns already in place, Harvey was stalled over Texas. This is what made the storm so deadly to the state. If this storm moved through quickly and then dissipated, then there wouldn’t have been as much rainfall and flooding. Even though hurricanes can cause serious damage, when they stall over a land mass for an extended period of time, their damage can be catastrophic. Harvey was one of the costliest hurricanes that hit the United States. Relating this back to now-a-days, Dorian’s eye made landfall on Abaco Island of the Bahamas with sustained winds of 185 mph. This was the strongest hurricane on record to hit the Bahamas. Dorian moved ever so slowly and the eye made landfall on the Grand Bahama Island, as a category five hurricane. The hurricane then stalled and sat over the Grand Bahama island for a whole day. Dorian’s eye then moved off the island on September 3rd. The eyewall of the storm has the strongest winds within a hurricane. It is within this location that you can find wind gust up to 200mph. With the eyewall sitting over Grand Bahamas for more than a whole day, the winds were completely ripping apart the island. The fact that this storms remained around the same area for such a prolonged amount of time, is what was responsible for the amount of damage that was done. With both of these hurricanes, it is important to understand that since they remained in relatively the same geological area (Harvey – over Texas, and Dorian – over the Bahamas) the amount of damage was greater than a hurricane that just passed quickly by. Always listening to the National Weather Service is important to do during a time of a storm like these. It is also important to listen to local and state enforcement. Emergency plans are created with these types of storms in mind, and the law enforcement, as well as the National Hurricane Center are there to keep the public safe. Both of these storms will go down in history, and will help make better forecast and preparations for the future. Credit: NWS, NOAA, NHC To learn more about other tropical cyclone topics, be sure to click here! ©2019 Weather Forecaster Allison Finch
0 Comments
Tropical systems in the Atlantic Ocean form due to warm air from the coast of Africa that meets the ocean breeze that allows for a storm to form. Through the time span of two to three weeks, tropical systems can go through different phases from tropical disturbance to a major hurricane. From the beginning of its lifecycle, Hurricane Dorian started out as a disturbance on August 19, 2019. Five days later, the system grew to a Tropical Depression and by August 29 the system grew into a hurricane. Beyond this time, Hurricane Dorian strengthened rapidly and made landfall on the US Virgin Islands before approaching Florida. In fact, Hurricane Dorian became a Category 5 on the Saffir-Simpson Scale, which is the highest possible tier, while making landfall in the Bahamas producing winds of upwards to 185 mph.  Photo: The lifecycle of Hurricane Dorian where each dot represents a 6-hour interval. (Photo from NOAA) As a Major Hurricane, Dorian was approaching the Florida coast and was brought to a halt over the ocean, leading the public to question the time of arrival. Since the atmosphere can dictate the processes based on what is occurring hundreds of miles away, the common phenomenon called ‘blocking’ was taking effect stopping Dorian in her tracks. Blocking can cause systems to remain in their space for hours or days at a time. Considering the dynamics of a low pressure system, the winds move in a counter-clockwise and inward direction. As you may assume, a high pressure system is the opposite, where winds will move clockwise and outward. Based on the proximity of these systems, multiple high pressure systems can act as a block to a low pressure from moving in its predicted track. In the case of Dorian, the hurricane was stalled over the Atlantic Ocean, just outside of the Florida coast due to two large high pressure systems, one to the west and another to the east.  Photo: The forecast issues from WPC the afternoon of September 3, 2019 showing two high pressure systems to the east and west of the location of Hurricane Dorian. (Photo by WPC/SPC/NHC Forecasters) Luckily, in the time the hurricane was stalled, Dorian experienced decreased intensity due to colder waters moving into the hurricane and differences in wind direction causing a loss of organization. As the Hurricane was soon able to continue towards the continental United States, Dorian made landfall in South Carolina as a Category 3 Hurricane. Beyond this, Dorian moved northeastward up the Atlantic coast of the United States and beyond before fully dissipating on September 10. Despite the intensity and organization of a system, the atmosphere all works together. If a strong low pressure is moving in a direction that is halted by a high pressure system, it can easily stop in its tracks. This can be applied outside of tropical systems to any low pressure system that can lead to a prolonged period of reoccurring weather phenomenon (i.e. rain, snow, etc.) For this case, the Florida coast was saved from an intense storm that could have caused drastic damage and even through losing intensity, other states saw impacts. Being aware of the surrounding weather can be a decider on what happens in the expected track of a storm. To learn more about other tropical cyclone topics, be sure to click here! ©2019 Meteorologist Jason Maska There are many stages to a tropical system and it’s important to know the distinction between each one, especially as the peak of hurricane season is upon us. Meteorologists have come up with four different stages of a tropical system. They include tropical disturbance, tropical depression, tropical storm, and hurricane. All have their own criteria and will be briefly described below. The first stage is the tropical disturbance stage. During this stage, water vapor from the warm oceans condense to form clouds. The warmer the water, the more water vapor and more vertical motion there is for condensation to occur. As condensation occurs, heat is released into the air attributing to more upward motion. Convection will continue to form more clouds, and eventually a cluster of thunderstorms. The second stage is the tropical depression stage. As the thunderstorms continue to grow, the air at the top of the cloud column starts to get colder. This creates more instability since the air parcels that are rising into the storm are warmer than their environment. Deep convection starts to occur at this stage. At some point, the storm will have one or two closed isobars (lines of equal pressure), indicating further intensification. Winds will be stronger and more sustained, but will not exceed 38 mph. The storm, at this point, is still cold core, meaning that the center of the storm is colder than the rest of it. The third stage is when the system transitions into a tropical storm. During this stage, one-minute sustained wind speeds of 39-73 mph are reached. The eye becomes even more pronounced and the circulation is more noticeable. On a weather map, there will be several closed isobars. This is also when latent heat (energy released to form condensation) that is released becomes the main driver for intensification. At this point, it becomes a warm-core system as the air in the center of the storm is warmer than the rest of it. The image below shows the difference between a tropical depression (cold core) and a tropical storm/hurricane (warm core). Tropical storms produce very heavy rainfall and are associated with damaging winds.  Courtesy: NOAA The final stage is when the storm transitions into a hurricane. This occurs when the sustained winds reach at least 74 mph. At this point, the eye wall starts to take shape as air in the center is descending to the surface. This subsidence makes it hard for clouds to form. The eye can vary from 2-30 miles in diameter, with stronger storms usually having smaller eyes. Hurricanes can cause a lot of destruction, not only with wind, but also with storm surge and flooding, as well. The diagram below shows the different categories of a hurricane and what type of damage each one can produce.  Courtesy: NOAA To learn more about other tropical cyclone topics, be sure to click here! ©2019 Meteorologist Corey Clay |
Archives
May 2022
|
RSS Feed