DISCUSSION: Hurricanes are one of the most destructive natural phenomena on the planet. While track forecasts have improved substantially in recent decades, large improvements in intensity forecasts have been more difficult to achieve. Accurately forecasting rapid intensification (RI; increase in maximum sustained wind speed of at least 30 knots in 24 hours) has shown be especially difficult. RI can be especially dangerous near the coast. Residents can be preparing for a tropical storm or a weak hurricane, but may end up dealing with a major hurricane with little time to prepare for this more intense storm.
To improve RI forecasts and intensity forecasts in general, a better understanding of the processes important for intensification is needed in order to improve weather models. To improve our understanding, more and/or improved data is required, especially in certain high-impact areas. Hurricanes derive their energy from evaporation from the sea surface. Interaction with the ocean can also cause winds to slow down while building bigger waves (i.e., momentum exchange). Thus, understanding and observing exchanges of energy and momentum between the ocean and atmosphere are especially important for understanding changes to storm intensity.
Observations at the air-sea interface in hurricanes have been sparse because of the danger of sending manned observation platforms there and the difficulty in building unmanned platforms that can survive the high winds and waves there. A partnership between NOAA and Saildrone Inc. led to the development of a saildrone (unmanned platform that floats on the ocean’s surface and is powered by wind and solar) that can withstand conditions at the surface in major hurricanes. This platform is pictured above with the various instrumentation it carries annotated (photo credit: AGU EOS, NOAA, and Saildrone, Inc.)
During the 2021 Atlantic hurricane season, saildrones were moved into three tropical storms, Fred, Grace, and Henri. The saildrones were successful in collecting data within and surviving these storms. Then the main test came when a saildrone navigated the eyewall (most intense part of a hurricane) of category 4 Hurricane Sam. The platform survived and provided rare, if not unprecedented, views and data from the ocean surface in a major hurricane. The picture below shows a snapshot of the view from the ocean surface in this storm (photo credit: AGU EOS and Saildrone Inc.).
The saildrone data collected during 2021 and future transects through storms will provide valuable data for better understanding how transfers of energy and momentum between the ocean and atmosphere influence storm intensity. This better understanding can help improve model representation of these processes, thus, producing better forecasts.
Finally, remember that hurricane season officially starts in the East Pacific basin on 15 May and in the Atlantic on 1 June. So, if you live in an area at risk of hurricanes, start preparing now.
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©2022 Meteorologist Dr. Ken Leppert II
A Lookback at the 2019 Atlantic Hurricane Season (Credit: NOAA, National Hurricane Center)
DISCUSSION: There is no question that year in and year out, tropical cyclones can often confound even the most educated of scientists and people located all over the globe. More specifically, as tropical cyclones begin to form and develop within a region of relatively adverse conditions, the overall cloud structure and corresponding rain-band structure can often be misleading. That is, misleading in terms of how the evolving structure of a developing tropical cyclone often has a very minimal correlation to the potential total impacts from a given tropical cyclone.
This inherent level of psychological confusion with developing tropical cyclones is that a more organized and a more intense tropical cyclone will more likely than not have more destructive impacts to any regions which lie in its path. However, the imperative point of clarification which needs to be made here is the fact that a disorganized storm from a satellite and/or radar-based perspective is no less dangerous than a well-organized storm. This is due to the reality that even with a disorganized system, there are still major threats for both wind and flooding damage since an unbalanced storm (i.e., with respect to moisture and wind intensity distribution) can still delivered tremendous amounts of rainfall (i.e., rainfall totals in excess of 10 to 20 inches within 24 hours) and locally strong winds as well. A perfect example of this narrative could be found with the quick but short-lived development of Tropical Storm Imelda in the vicinity of southeast Texas earlier this year.
In the case of Tropical Storm Imelda, an invigorated tropical disturbance quickly blossomed into a tropical depression and then was upgraded to a tropical storm just hours before the center of Imelda’s still-developing circulation came ashore in southeast Texas. Despite the storm being weak and disorganized overall, this storm went on to dump copious amounts of rainfall in and around coastal and semi-coastal portions of southeast Texas. Thus, this just goes to show that regardless of a tropical cyclone’s overall structural breakdown and its given intensity, all tropical cyclones MUST ALWAYS be given do respect since any single storm can be a storm to be reckoned with for hours or even days to come once a system closes in on a given coastal region. Attached above is a graphic courtesy of The Weather Channel which approximately reflects the overall distribution of the rainfall which resulted from Tropical Storm Imelda across southeast Texas.
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©2019 Meteorologist Jordan Rabinowitz
This year’s hurricane season has been nothing short of impressive thus far and there are still two months left. Thankfully, most of the hurricanes that have formed this year have had minimal impact to the United States as opposed to similarly active years in the past. However, that doesn’t mean that the hurricanes that have hit the U.S. have been a cake walk. So far this season there have been 11 named storms and 2 major hurricanes. At one point, there were six named storms going on at once in the Atlantic and Pacific basins, but is this more active than usual? The answer is yes, but only for this time of year. In the Atlantic between September 14-23, there have only been four named storms occurring at once as the record; this year that record was broken when there were 5 named storms during this span. However, is this the most active season ever? Not really.
The most active Atlantic hurricane season was 2005 with 28 named storms, and 15 of those became major hurricanes. In fact, that season was so active that the National Hurricane Center ran out of names through the alphabet and had to start using names from the Greek alphabet. This year has nothing on 2005, however, it is rather active compared to the last few hurricane seasons. The National Hurricane Center normally issues pre-season predictions based on sea-surface temperature trends and whether we are in an El-Nino or El-Nina phase, the two different phases of the Southern Oscillation Index which affect the trade winds and sea surface temperatures. This year, they predicted between 8-11 named storms and 2-4 major hurricanes, which is fairly accurate with what has been going on so far. However, it is only September and it doesn’t appear that the Atlantic or Pacific Basins will be calming down anytime soon. Hurricane Lorenzo has broken many records in the Atlantic this past week, one including the strongest hurricane to have been observed that far north and east in the Atlantic. It is currently a Category 1 and will be affecting the Azores Islands in the coming days.
Shear-wise, the Atlantic is beginning to look unfavorable as autumn sets in. Wind shear is the variation of wind velocity along a given direction perpendicular to overall wind direction. Favorable wind shear in the mid-to-upper levels of the atmosphere for hurricanes is 15 knots and below, while less favorable wind shear is above 20 knots. The reasoning behind this is that hurricanes get torn apart by strong upper-level shear and it can cause them to not become organized. Also, the trough-ridge pattern is starting to set up quite nice, regardless of the steep ridge that has been stationed over the Southeast U.S. during the last couple of weeks. The trough-ridge pattern is set up by the jet stream. The jet stream moves further north in the summertime and farther south in the fall and winter season due to the seasonal change. When the jet stream moves further north, the tropical jet will also move further north in its place, which brings much hotter and more humid conditions over the southern U.S. and Mexico. The troughs that are formed by the jet stream are normally lined by a cold front ahead of them, with an area of high pressure behind them. Ridges are areas that are preceded by a warm front, with an area of low pressure in them. Hurricanes tend to want to avoid areas of high pressure because of the dry air associated with the pressure, so with an active frontal pattern typically means that devastating landfalls are harder to come by. However, sea-surface temperatures are very steamy with temperatures ranging from the upper-70s to mid-80s. These are very favorable conditions to support hurricane development, along with many disturbances still coming off the west coast of Africa. Hurricanes need warm, steamy temperatures for fuel. Any sea-surface temperatures (temperature of the ocean from the surface down to around 20 meters, but can be as deep as 700 meters below the surface) that are below 70 degrees are typically unfavorable for hurricanes.
It is something to monitor over the next couple months as the Atlantic still has an abundance of energy. Fortunately, with the frontal systems beginning to start up for the autumn and winter months, the chances for hurricane landfall in the southern U.S. is low. The U.S. East Coast could still potentially face landfalls, but the overall chances decrease.
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©2019 Meteorologist Ashley Lennard
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
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©2019 Weather Forecaster Allison Finch
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.
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©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.
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.
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©2019 Meteorologist Corey Clay
The Significance of Lightning in Well-Defined Tropical Cyclones (credit: Matt Reagan and American Meteorological Society)
When most people think of hurricanes and tropical cyclones, the first line of thought does not always include lightning. Tropical cyclones are what are known as “warm-core” systems, which mean that the center of the storm is most intense at the surface but then stabilizes with altitude. Because of the significant amount of latent heat that is released within tropical cyclones and the rapid rotation, lightning is not a common feat With all of the news surrounding the ever-evolving Hurricane Dorian in the western Atlantic waters, recent satellite passes indicated the presence of lightning within the inner core of the cyclone. In the subsequent hours, Dorian intensified to a category 4 storm with maximum sustained winds exceeding 130 mph. But how much of a role does lightning play in the intensification of organized tropical cyclones?
One of the first studies in the literature concerning the relationship between lightning and tropical cyclones is from Samsury and Orville (1994) who investigated the lightning characteristics from Hurricanes Hugo and Jerry of 1989. Although their study focused on the quantitative statistics of lightning flashes including parameters such as the polarity, one of the primary results from this study showed that the greatest number of flashes within both storms occurred in the right-front and right-rear quadrants, more specifically within the outer convective rainbands.
Regarding rapid intensification, A study performed by DeMaria et al. (2012) showed that TCs that intensify over time tend to have the greatest lightning densities compared to those that weaken over time. Most recently, Stevenson and Corbosiero (2018) surveyed a total of 10 years-worth of tropical cyclone data fitting between certain criteria for vertical wind shear and sea surface temperature. It was found that most of the inner core lightning bursts (ICLBs from their paper) occurred with intensifying storms (e.g., depressions and tropical storms) and were also a precursor to intensification at a roughly 24-hour lead time prior to the lightning bursts. The symmetricity of convective cores and lightning bursts within intensifying tropical cyclones also was observed to be more prominent than in decaying convection.
So while it is uncommon to see lightning in tropical cyclones, the advent of satellite technologies such as the World Wide Lightning Location Network and the Geostationary Lightning Mapper aboard the GOES-16 and GOES-17 satellites allow for forecasters and researchers to better understand rapid intensification processes. With more research and applications to forecasting practices, detection of rapid intensification events have the potential to become more predictable in the future, leading to more accurate intensity forecasts.
Below are the references for the articles mentioned above:
DeMaria, M., R.T. DeMaria, J.A. Knaff, and D. Molenar, 2012: Tropical Cyclone Lightning and Rapid Intensity Change. Mon. Wea. Rev., 140, 1828–1842, https://doi.org/10.1175/MWR-D-11-00236.1
Samsury, C.E. and R.E. Orville, 1994: Cloud-to-Ground Lightning in Tropical Cyclones: A Study of Hurricanes Hugo (1989) and Jerry (1989). Mon. Wea. Rev., 122, 1887–1896, https://doi.org/10.1175/1520-0493(1994)122<1887:CTGLIT>2.0.CO;2
Stevenson, S.N., K.L. Corbosiero, M. DeMaria, and J.L. Vigh, 2018: A 10-Year Survey of Tropical Cyclone Inner-Core Lightning Bursts and Their Relationship to Intensity Change. Wea. Forecasting, 33, 23–36, https://doi.org/10.1175/WAF-D-17-0096.1
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© 2019 Meteorologist Brian Matilla
Tropical Storm Barry (Photo Credit: National Oceanic Atmospheric Association)
Invest 92L, now known as Tropical Storm Barry, has been absolutely fascinating to watch develop over the last week. Barry formed very similarly to the way hurricanes in the Atlantic form – from a cluster of thunderstorms. In Africa, storms will move from east to west due to the tropical trade winds that blow near the equator. As the storms move off the coast of Africa, the systems maintain their unstable nature over the warm waters of the tropics and, as long as conditions are right, the storms will develop into a tropical depression, storm, and eventually hurricane. Barry formed in a similar fashion – a cluster of rotating thunderstorms that projected south into the steaming waters of the Gulf of Mexico and has since thrived and could potentially landfall as a Category 1. This case is much more interesting because not many areas of thunderstorms develop into hurricanes off of the United States, especially a hurricane that is not considered an “extra tropical cyclone” because it is forming in the mid-latitudes.
Will Barry pack a punch? It’s expected for flooding to be the main threat, as National Hurricane Center has discussed if it does become a Cat 1, it will be on the lower end of the category with winds averaging below 100 mph. Their track currently has Barry making landfall on the Louisiana coastline, which has already seen their fair share of flooding from storm systems that came through earlier in the week. Currently, there is a State of Emergency listed, but there will be no mandatory evacuations issued since the storm is below a Category 3. However, with the levees already facing their limits as it comes to the incredible flooding issues that Louisiana and much of the Southeast has experienced so far this year, the storm surge from Barry has a very good possibility of breaking those levees. Evacuation is stressed in low-lying areas. Barry is expected to move slow (averaging between 3-5 mph over the last 24 hours) as it comes inland leaving rainfall amounts of 15-20 inches at the area of landfall. Barry is quite a unique storm, however, just because it is only a Tropical Storm, with Category 1 at best, it is still strengthening (and expected to continue so as it makes landfall). The shear that it is propagating through to its north is not affecting it whatsoever, which is not a good sign.
Tropical Storm Barry may not sound scary, but with the area that it is projected to affect through the weekend and into early next week, the future is not looking promising. The Mississippi River is already at record flood stages, with many towns in the Delta still underwater and many farmers whose cropland is now a lake due to all the rain that was received in the early part of the year. The last disaster these communities need to experience is more flooding and that is an unfortunate reality as areas as far north as northwest Mississippi and Tennessee are expected to see 6-8 inches of rainfall over the next few days. Hopefully in the upcoming days, people along the coastline of Louisiana will take the warnings and evacuate and it won’t rain as much as models are projecting to help save some of the communities from being underwater any longer. However, this is something that will have to be closely watched as this progresses.
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©2019 Meteorologist Ashley Lennard
Discussion: Hurricane Erick is currently located approximately centered 510 miles southeast of Hilo, HI. As of 11am HST on July 31, 2019, the Central Pacific Hurricane Center’s latest advisory has Erick moving to the west-northwest at 14 mph. Erick’s maximum sustained winds are 115mph which is indicative of a category three hurricane on the Saffir-Simpson scale. The Saffir-Simpson scale measures a hurricane’s maximum sustained winds. Forecasters predict that Hurricane Erick will slowly weaken over the next 48 hours. Looking at Hurricane Erick’s forecast cone, it will pass south of the Hawaiian Islands. There are still some hazards to land that will be associated with Erick. Swells that are generated by this storm will arrive to the islands within the next few days. These swells could cause dangerous surf conditions along the western shorelines. Rainfall is another hazard, as moisture from this storm makes its way over the islands. Rainfall is expected to be heaviest over the eastern and southeastern slopes of the Big Island of Hawaii. The National Weather Service in Honolulu has issued a flash flood watch as heavy rainfall that could cause flooding is expected.
According to the National Hurricane Center, satellite imagery shows that Tropical Storm Flossie is weakening. It currently has maximum sustained winds of 70mph. It is moving towards the west-northwest over open water and isn’t expected to be a threat to land. Flossie’s current structure and the wind shear in the area where Flossie is present have been contributing to its weakening. A tropical system needs low wind shear and warm sea surface temperatures in order to help it thrive. As we head through the late summer into the fall, check the National Hurricane Center and Central Pacific Hurricane Center for the latest on any developing tropical disturbances. If you live in a hurricane prone area, it is important to be prepared!
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© 2019 Meteorologist Shannon Scully