Dare to hope: With the demise of catastrophic Hurricane Iota on Wednesday, the terrible 2020 Atlantic hurricane season may have finally had its last deadly storm. The Atlantic hurricane season officially ends on November 30, but it is likely there will be no more deadly Atlantic storms in 2020. One or more weak named storms, perhaps staying out to sea, are still likely to form before the end of the year, but these are unlikely to cause mass destruction or loss of life.
Unfavorable conditions loom in tropical Atlantic
Sinking air over the tropics leads to drying, high pressure, and reduced odds of tropical storm formation – that’s exactly what is expected over the tropical Atlantic through early December given a state of the Madden-Julian Oscillation (MJO) unfavorable for Atlantic tropical cyclones.
The MJO is a pattern of increased thunderstorm activity near the equator that moves around the globe in 30 to 60 days; odds of tropical cyclone formation increase when the MJO is strong and suitably located, but typically decrease for ocean basins not in the active portion of the MJO. During the formation of Hurricane Eta, the MJO was in a favorable place for enhancing Atlantic tropical cyclones, but it was transitioning to an unfavorable state when Hurricane Iota formed. It is expected to remain unfavorable into early December.
Favoring tropical cyclone formation in the coming weeks will be above-average ocean temperatures in the tropical Atlantic, plus an intensifying La Nina event in the eastern Pacific, which will contribute to below-average levels of wind shear over the tropical Atlantic. However, the current unfavorable state of the MJO, plus the near end of the climatological close of hurricane season, argue that the 2020 Atlantic hurricane season at last may have had its final deadly storm. That prediction is bolstered by the fact that over the past few days, multiple runs of the top three models for forecasting tropical cyclone genesis have not predicted formation of any new tropical storms through the end of November.
In a season with a proven track record for spitting out record numbers of named storms (30 so far), 2020 will still be able to produce one or more relatively inconsequential tropical or subtropical storms before the end of the year, perhaps resulting in a final nearly unfathomable tally of 31-33 named storms in this busiest-ever season on record.
The previous record-busy Atlantic hurricane season, 2005 with 28 named storms, had three named storms form after November 20: Tropical Storm Delta on November 22, Hurricane Epsilon on November 29, and Tropical Storm Zeta on December 30. None of those storms caused loss of life while they were named, although Delta killed seven people in the Canary Islands after it had transitioned to an extratropical storm. Only three deadly December tropical cyclones have been recorded in Atlantic history:
Tropical Storm Odette, Dec. 4-7, 2003, 8 killed in the Dominican Republic;
Hurricane Four, Dec. 1, 1925, 60 killed in Florida; and
Unnamed Hurricane, December 1779, 120 killed on ships in the Atlantic.
Two tropical disturbances to keep an eye on
An area of low pressure that formed on Wednesday over the southwestern Caribbean was bringing heavy rains to Panama and Costa Rica on Thursday. There is little model support for development of this disturbance, though it will likely bring 4-8 inches of rain to Panama and Costa Rica through Tuesday, as the system moves slowly west to west-southwest across Central America and into the Northeast Pacific. In a 1 p.m. EST Thursday tropical weather outlook, the National Hurricane Center gave 2-day and 5-days odds of development of near 0% to this disturbance.
Also worth watching are the waters off the U.S. East Coast between the Bahamas and Bermuda, where models predict that an area of low pressure capable of becoming a subtropical storm may form by Monday as it moves rapidly to the northeast past Bermuda. In a 1 p.m. EST Thursday tropical weather outlook, NHC gave 2-day and 5-days odds of development of near 0% and 20%, respectively, to this disturbance.
The next name on the Atlantic list of storms is Kappa, the 10th letter in the Greek alphabet.
*Details on Figure 1: The amount of rising or sinking air can be inferred from the 200 mb velocity potential (VP) departure from average (also called the anomaly). Negative 200 mb VP anomalies mean that upper-level winds at the 200 mb level are diverging, causing rising air from below to replace the air diverging away at high altitudes. This rising air helps nurture thunderstorm updrafts, and favors low pressure and increased chances of tropical cyclone formation. Conversely, positive 200 mb VP anomalies imply converging air aloft, where sinking air, high pressure, and dry conditions will be unfavorable for tropical cyclone formation. In this plot, negative 200 mb VP anomalies (divergence) are cool-colored contours (the scale shows the departure from average in standard deviations); positive 200 mb VP anomalies (convergence) are warm-colored contours.
Editor’s note: Unless Tropical Storm Kappa forms, this “Eye on the Storm” blog and focus on Atlantic hurricanes soon will go into winter hibernation. Jeff Masters plans to post a wrap-up piece on the Atlantic hurricane season on November 30 or December 1, after which he and Bob Henson will continue writing often at this site on extreme weather and climate change and on other climate change issues as part of their ongoing “Eye on the Storm” features.
Posted on November 19, 2020(3:53pm EST).