 The Atlantic hurricane season starts on June 1stof every year. Sometimes we can see storms before that date, or after that date – but subtropical storm Andrea was the first named storm of the 2019 season, and it came 11 days early!
Andrea formed just southwest of Bermuda on May 20th, 2019. The National Hurricane Center (NHC) put out their first warning of the storm and said that it would never reach above 73 miles per hours sustained winds. The storm continued on its track for some time, but never developed into anything more than a storm. Two days after the NHC put out their forecast, the storm dissipated and turned into a tropical depression with sustained winds that were less than 39 mph. Why didn’t Andrea form into a major storm? The most straightforward reason is that it formed well before the hurricane season started, which means that some of the “ingredients” for hurricane development weren’t present. The “ingredients” needed for a hurricane to form include a combination of the following conditions. First, we need a spot of convection off the equator. Convection can be explained by storms that produce severe weather, like heavy rain and thunderstorms.This is because if we want a storm to form, we need some sort of rotation, which comes from the Coriolis force. The Coriolis force doesn’t have any effect on storms (or weather) when it is isolated close to the equator the equator. This means that most storms form around 15 degrees’ latitude. When examining this condition in the context of Andrea, the the first ingredient is missing because she was located at 28 – 33 degrees North of the equator. The second ingredient necessary for a major storm to form is warm sea surface temperatures. The ideal temperature is about 26 degrees C. This allows the storm to use the warm waters and turn it into energy to intensify the storm. When looking at a sea surface temperature (SST) map of the ocean for Andrea, we can see that it formed in an area where the SST’s were around 27 degrees, and it quickly moved into colder water, which preventedt the storm from becoming stronger. Third, th next element that is required for a major storm is relatively low wind shear. This means you want a little amount of wind shear in the area – too much will destroy the storm, not enough and the storm won’t form. When Andrea moved closer to Bermuda the stronger wind shear, the storm started to “crumble” or fall apart. It became asymmetric and ruined the formation of the storm. One other thing you would need is a moist environment. If there isn’t a lot of water vapor in the environment, then it is really hard for astorm to form. If the storm moves into an area where the environment is really dry, the storm will likely fall apart and weaken. In this case Andrea moved into a region of dryer air. Overall the weather was just not ready to form a storm this early in the season, so Andrea ended up weakening. But now, we look forward to the start of the hurricane season! To learn more about other tropical cyclone events and stories from around the world, be sure to click here! © 2019 Weather Forecaster Allison Finch
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DISCUSSION: There is no debate that the Friday morning landfall of Tropical Cyclone Fani brought about truly horrific consequences for a good portion of the eastern half of the subcontinent of India. Having said that, with most of the worst of the event having unfolded and concluded up to this point in time, is most certainly worth it and necessary to take a look back and see how this event unfolded. More specifically, it is often most interesting to evaluate how and to what extent the physical structure of a given tropical cyclone evolves during its lifetime and more specifically during its landfall and post-landfall period(s), respectively.
Through utilizing the Cooperative Institute for Meteorological Satellite Studies (CIMSS) Morphed Integrated Microwave Imagery at CIMSS (MIMIC), there are several more profound insights which can be gained from this recent and historic tropical cyclone which formed and intensified within the Bay of Bengal (i.e., to the south and east of eastern India). First off, upon analyzing the pre-landfall phase of Tropical Cyclone Fani, you can clearly see how it maintained a structurally-sound eye and eye-wall. You can determine the fact that it had a structurally-sound eye-wall by seeing how towards the beginning of the brief loop, there was a relatively consistent coverage of yellow to red-colored banding around the clear eye which indicates the presence of deeper convection surrounding the immediate center of the storm’s circulation. Thus, as you may suspect, this was around the time at which Tropical Cyclone Fani was near and around the period of peak intensity. After that time, upon a bit closer investigation, you can also see how there was a slight widening of the average diameter of the eye in the hours leading up to the landfall of Tropical Cyclone Fani. A gradual widening of an eye during the last 24 hours preceding a landfall is quite common with tropical cyclones around the world (under most environmental circumstances) and is most often an indication of a process known as an eyewall replacement cycle occurring. During this process, the common consequences to an average tropical cyclone is that there is a widening of the eyewall, a corresponding increase in the size of maximum wind speed field, and sometimes a weakening of the maximum wind speeds by at least 5 to 15% of the original maximum sustained wind speeds. Once the storm made landfall, you can then observe how the storm fully broke down in terms of its organization and that the eye collapsed quickly which is an inevitable factor when a tropical cyclone interacts with any sizable landmass. Such landmass interactions degrade the internal structure and organization of a tropical cyclone since this interaction cuts off the critical inflow of warm, moist air towards the center of the storm. Thus, this process facilitates a rapid weakening of the storm from that point forward and this is exactly what you are seeing with Tropical Cyclone Fani during the post-landfall phase of this tropical cyclone as it moved northward towards southern Bangladesh. To learn more about other tropical cyclone events and stories from around the world, be sure to click here! © 2019 Meteorologist Jordan Rabinowitz Understanding the Historical Context of Tropical Cyclone Fani (Imagery credit: Himawari-8 Imager)5/3/2019 
DISCUSSION: Over the past couple of days, millions of people from around the world were witnesses to an increasingly scary situation in the context of the robust intensification and the approach of Tropical Cyclone Fani to an eventual landfall in eastern India. From a historical perspective, this was an incredibly impressive event on all accounts. First, despite the relatively commonality of tropical cyclones forming in this region of the world (i.e., across the northern and/or southern Indian Ocean), it is not all that often that the subcontinent of India or any other adjacent countries get in the path of such a powerful tropical cyclone. That being said, such events do occasionally occur and when they do, there are often substantially noticeable and lasting impacts.
In the case of Tropical Cyclone Fani, this was a large and powerful tropical cyclone which at its period of peak intensity (i.e., within the 12 to 24 hours prior to its landfall in eastern India) had maximum sustained winds of around 150 mph with gusts to around 190 mph. Thus, making it an incredibly large and dangerous tropical cyclone which was being estimated as a storm which would have the potential to forever change the lives of hundreds of millions of people as and beyond the point at which it makes landfall. To put this storm in context, the last tropical cyclone to impact the subcontinent of India was Tropical Cyclone Hudhud back in October 2014 as a severe tropical cyclone. Despite the fact that Fani was a somewhat weaker system at the time of landfall than Hudhud, Fani was a MUCH larger system with a lot more water being carried along with it. Thus, the major concern and still a major ongoing concern is the continued flooding and flash flooding threat across northern India, Bangladesh and beyond. The major rainfall production will continue even as this storm weakens and gradually gets absorbed by the larger-scale atmospheric flow across southern Asia. Having said all the above, it still goes without saying that Tropical Cyclone Fani will most definitely go down in the record books as being one of the likely top ten worst tropical storms to impact the subcontinent of India in recorded history. To learn more about other tropical cyclone events from around the world, be sure to click here! © 2019 Meteorologist Jordan Rabinowitz |
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