Since May 2020, forty-five tropical cyclones, twenty hurricanes and ten major hurricanes have developed in the Atlantic.
Twelve tropical storms, five Category 1 or Category 2 hurricanes and three major hurricanes made landfall in the continental United States.
That is, in scientific terms, a lot.
In fact, the last 480 days have seen almost as much hurricane activity in the United States as the entire decade between 2006 and 2015. And despite this relentless string of weather, there are still miles to go before the Atlantic turns. asleep. As measured by historical landings, the last third of the 2021 hurricane season is yet to come.
As the last 10 days of September approach, Atlantic hurricane season changes speed like an abandoned K-Mart transforming into a Spirit Halloween hypermarket. At present, the frequency of “Cape Verde” type storms that develop in the East Atlantic and sometimes threaten the East Coast of the United States is decreasing, and the locations of hurricane development are shifting to the Caribbean and the West Atlantic, closer to land.
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Nicholas Flood and Watch the Wave
Currently, the Tropics are in a rare interregnum as this change begins. This week, the focus was on the Gulf Coast, which experienced excessive rainfall from Nicholas. Much like the acting style of his namesake Nic Cage, Nicholas was uncomfortably intense and ignored personal space standards as he escalated to the force of Category 1 hurricanes just off the coast of Texas on Monday night. .
Since then, the residual Nicholas low has moved only slowly eastward, spreading accumulations 5 to 15 inches from east Texas to far west Florida. Nicholas is drawing to a close, but another 1 to 4 inches is possible over the Deep South and East Gulf Coast over weekends in intermittent bands of storms.
Elsewhere, US threats are minimal for the time being. A large area of low pressure east of the mid-Atlantic is likely to become a tropical storm as it accelerates northeastward and away from land over the weekend. This system could strengthen further in the North Atlantic early next week, but this is not a threat to US interests.
A wave in the tropical Atlantic, roughly halfway between the Lesser Antilles and West Africa, is of little more concern. This wave is slow to organize due to the interaction with another tropical wave to the west, but the slow development of a tropical depression or tropical storm is also expected by early next week. .
The northern wind and Puerto Rico will need to keep an eye on this wave for potential impacts on Monday or Tuesday, although the shear is expected to increase as the wave nears the islands, likely capping the intensity.
The good news is that a high pressure ridge on the US east coast is expected to move east in the central Atlantic by the end of next week. This movement should open a loophole for whatever develops to eventually head north into the open Atlantic.
Still, there are a lot of moving parts in play, so it’s worth watching this wave until confidence in the forecast gets stronger.
Autumn remains a perilous time in the tropics
This rare breath is the opportunity to take stock of what the rest of the 2021 hurricane season could have to offer. While two-thirds of historic Florida hurricane activity occurs before the last week of September, fall remains a risky time for the state.
Florida hurricane climatology has three distinct peaks. The first, in June, is caused by weak, wet storms on the Gulf Coast; the second in early September is associated with the overall peak of the season and strong long-term threats to South Florida.
The third peak occurs in mid-October, when late-season landings are concentrated on the Florida Gulf Coast. In southwest Florida, hurricane impacts are as common in October as they are in September, due to fall storms moving north and northeast of the western Caribbean. Historical landing points spread between early October and late October show that the greatest storm hazards move south during the second half of the month and begin to run out of steam in northern Florida after mid-October.
Two factors have the strongest historical relationship to late-season tropical activity: Atlantic Sea Surface Temperatures (SST) and El Niño / La Niña state in the Pacific.
With late-season tropical activity being more common in the western Atlantic, the first key is that warmer-than-average sea surface temperatures (SST) in these areas have a statistically significant relationship with a month of October active. This year, the Western Caribbean is a few degrees warmer than average, but not as warm as 2020, while the Gulf has been choppy by Ida and her friends and SSTs are about normal there.
The second important consideration is the Pacific Ocean. NOAA has placed the Equatorial Pacific on ‘La Niña Watch’, meaning that milder-than-normal SSTs over western South America are expected to continue to cool, and an official La Niña event should come back in a month or two.
La Niña is of particular concern in October, as it promotes a reduction in vertical wind shear over the Gulf of Mexico and the Caribbean Sea. As these are prime breeding grounds for late-season tropical storms, a La Niña often acts to delay the end of the hurricane season.
Last year’s moderate La Niña was an extreme example: the five major hurricanes of 2020 after October 1 broke the previous record of two, and Hurricane Zeta became the last major hurricane in the United States on October 28. . This year, La Niña development is expected to be weaker than it was in October 2020, although still favorable for a busy end to the season.
High chance of a hurricane in Florida this season
A recent article in the Bulletin of the American Meteorological Society investigating the extraordinary activity of the October and November 2020 hurricanes (I’m going to pause to have you pick up your copy of BAMS from the coffee table or bathroom) revealed that even a simple model representing only Western Atlantic SST and El Niño / La Niña conditions could skillfully predict late-season tropical activity.
WeatherTiger’s seasonal forecast model is a bit more complicated than that, but confirms the active signal suggested by this study. Using oceanic and atmospheric inputs from July and August, our algorithm suggests a most likely result of total storm energy after September 25 that is about 30% higher than the average year over the period 1950-2020. Although this is above normal, the same model last year predicted double the typical amount of late-season activity, so a “low calorie 2020” solution follows the observed trends.
These predictive analyzes suggest a high probability of a hurricane somewhere in Florida before the end of the season. Since 1851, there have been 40 hurricanes in Florida after October 1, including 15 major hurricanes. Given that WeatherTiger’s models suggest a risk profile about a third greater than a normal end of season, we estimate that Florida has about a 1 in 3 chance of a hurricane on land and a 1 in 8 chance of a hurricane. major after October 1.
A possible reprieve of a few weeks
In summary, the current tropical reprieve could last a few more weeks, as relatively unfavorable high altitude winds will cross the Atlantic at the end of September. Upper winds are favored to shift to a regime more conducive to storm formation in the Caribbean and the western Atlantic in early October, when the guiding currents can periodically open the door to anything that may develop, as our August outlook suggests.
It is important to remember that the seasonal forecast interprets the shadows on the cave wall and that all possibilities, good and bad, are possible.
There is nothing else to do at this point than to stay prepared and accept the hard truth that we may be done with hurricane season, but hurricane season is not necessarily finished with us.
As much as we would like this grueling season to go away, the history of hurricanes and the development of La Niña indicate that until at least the end of October, we must continue to gaze at the sky.
Dr Ryan Truchelut is Chief Meteorologist at WeatherTiger, a Tallahassee-based start-up that provides advanced weather and climate analysis, forensic meteorology and expert advice, as well as agricultural forecasting subscription services and hurricanes. For more information on WeatherTiger, visit weathertiger.com, or contact us at [email protected]
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