The 2021 Atlantic hurricane season is officially off to an early start: Subtropical Storm Ana formed in the waters 200 miles northeast of Bermuda at 5 a.m. EDT Saturday. Ana’s formation date of May 22 comes over a week before the official June 1 start of the Atlantic hurricane season and marks the seventh consecutive year that a pre-season named storm has formed. The previous record was four consecutive years, set in 1951-1954. Although the National Hurricane Center has not changed the June 1 start date of the Atlantic hurricane season, it did move the launch date of its daily Tropical Weather Outlooks to May 15 starting this year.

Satellite loops on Saturday showed that Ana had a modest area of heavy thunderstorms that were slowly increasing in areal coverage. Some of this activity was concentrated near the center of the storm, which is characteristic of a tropical storm. However, Ana was entangled with an upper-level low pressure system, which caused a substantial amount of heavy thunderstorm activity to be well removed from the center of circulation, characteristic of a subtropical storm.

Wind shear over Ana was a moderate 10-15 knots on Saturday, sea surface temperatures, SSTs, were a chilly 21 degrees Celsius (70°F), and the atmosphere was fairly dry, with a relative humidity at mid-levels of 45%. These marginal conditions for development were offset by very cold temperatures aloft, which helped create the instability needed for Ana to generate the heavy thunderstorm activity required to be designated a subtropical storm.

Figure 1. Average number of named storms, hurricanes, and major hurricanes for the Atlantic hurricane season, from 1971-2020. (Image credit: Brian McNoldy, University of Miami Rosenstiel School of Marine and Atmospheric Science)

Forecast for Ana

Ana, with sustained winds of 45 mph at 8 a.m. Saturday, was headed west-southwest toward Bermuda at 3 mph. A few rain showers from Ana were affecting the island, and this activity will pick up as the storm draws nearer. The island is under a Tropical Storm Watch. In its 5 a.m. EDT Saturday wind probability forecast, the National Hurricane Center gave only a 4% chance that Bermuda would see sustained tropical storm-force winds of 39 mph or more from Ana.

Ana has a short window of time to exist as a named storm, since steering currents are expected to turn the storm northeastward by Saturday night, putting Ana in a region with cooler waters and higher wind shear. Ana is likely to degenerate into a remnant low by Monday.

Ana is the type of weak and short-lived storm that might not have been named in the days before satellite imagery was available in the Atlantic, as explained in our post last week, “Why are there so many Atlantic named storms? Five possible explanations.” The large increase in recent decades in the number of so-called “shorties” – named storms that last two or less days at tropical storm strength – has thus far not been demonstrated to have a climate change connection; improvements in our ability to detect and diagnose these systems may be partially responsible for their increase. Weak, short-lived early-season storms like Ana have grown more common, making it difficult for storms beginning in A, B, and C to get their names retired: only two have been retired in the past 28 years (Allison, 2001; Charley, 2004).  

Figure 2. Three-day precipitation amounts of 4-12 inches were common May 16-19 over portions of coastal Texas and Louisiana. These areas are likely to receive additional heavy rains of 1-3 inches from 91L. (Image credit: Aon)

Two simultaneous May ‘invests’: 91L designated in the Gulf of Mexico

In a very rare occurrence for May, the National Hurricane Center was tracking two “invests” simultaneously on Friday: 90L near Bermuda (which became Ana), and 91L in the western Gulf of Mexico. Both formed in regions outside of where May tropical cyclones have formed in the past (see Tweet by Sam Lillo). When NHC is monitoring a tropical disturbance that is a threat to develop into a tropical or subtropical cyclone, the disturbance can be designated as an “invest.” The disturbance is then given a tracking identification number of 90-99, followed by a single letter corresponding to the ocean basin – “L” for the Atlantic, or “E” for the Eastern Pacific. When the number of invests in a season reaches 99, the next disturbance gets the recycled number 90. There is no formal definition of what qualifies as an invest, and the purpose of doing so is primarily to allow generation of specialized computer model forecasts.

On Saturday morning, 91L moved to inland Texas without developing into a tropical cyclone. In its 8 a.m. EDT Saturday tropical weather outlook, NHC gave 91L 2-day odds of development of 0%. The system will bring heavy rains of 1-3 inches to portions of coastal Texas and Louisiana that have already experienced extreme flooding over the past week. Damages from flooding and severe weather May 14-19 in the region were expected to total in the hundreds of millions of dollars, insurance broker Aon reported on Friday.

Is the Atlantic hurricane season getting longer?

We might expect that hurricane season will start earlier and end later in coming decades, as warming of the oceans allows more storms to form when ocean temperatures are marginally warm for tropical cyclone formation. Indeed, a plot of the starting date for the Atlantic (see Tweet by Steve Bowen of Aon) shows a marked increase in the number of early-season named storms in recent decades. However, hurricane genesis also requires low wind shear, high levels of moisture at mid-levels of the atmosphere, and something to get the low-level atmosphere rotating. In some ocean basins, climate change may inhibit early-season genesis events by changing these other factors needed for a tropical cyclone to get started.

The Atlantic hurricane season does appear to be getting longer in the region south of 30°N and east of 75°W (from the central Bahamas eastwards, including the eastern half of the Caribbean Sea), according to a 2008 paper by Dr. James Kossin of the University of Wisconsin titled, “Is the North Atlantic hurricane season getting longer?” A 2016 analysis by Dr. Ryan Truchelut of WeatherTiger also supported this idea. However, Juliana Karloski and Clark Evans of the University of Wisconsin-Milwaukee found no trend in tropical cyclone formation dates when looking at the entire Atlantic for the period 1979-2014.

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Jeff Masters

Jeff Masters, Ph.D., worked as a hurricane scientist with the NOAA Hurricane Hunters from 1986-1990. After a near-fatal flight into category 5 Hurricane Hugo, he left the Hurricane Hunters to pursue a...