Jeremy Davis #4 // # Understanding the Hurricane Season

Andy Schell sits down with marine meteorologist Jeremy Davis, Director of Operations at Weather Routing Incorporated, to analyze a deeply anomalous Atlantic hurricane season on 59° North Sailing‘s On the Wind podcast. Following the Annapolis Boat Show and offshore skipper events on Chesapeake Bay, the seasonal metrics present a striking contradiction: while the Atlantic Ocean languishes at roughly 10% of its normal cyclonic energy, the Eastern and Central Pacific basins are experiencing record-breaking storm generation.

To decipher this stark disparity for offshore sailors preparing autumn passages south, Davis outlines the global atmospheric circulations at play. A strengthening El Niño event across the Pacific has inverted typical trade wind flows, triggering downstream teleconnections that reshape conditions across the North Atlantic.

Atmospheric Teleconnections: How El Niño Suppresses the Atlantic

Under normal non-El Niño conditions, trade winds blow reliably from east to west across the tropical Pacific south of the equator, driving warm surface water westward toward Indonesia and Australia while allowing colder, nutrient-dense water to upwell along the coast of South America. During an El Niño year, those southern Pacific trade winds weaken or reverse direction entirely. Warmer water that normally pools in the western Pacific either slows down or backtracks, accumulating off Peru and northern South America where sea surface temperature departures reach 5 to 6 degrees Celsius above average.

This massive redistribution of thermal energy alters global atmospheric circulation. In the Eastern Pacific, intense convection and rising air generate an expansive downstream upper-level trough across the tropical Atlantic, the Caribbean Sea, and the Gulf of Mexico. This upper-level feature generates sustained westerly winds aloft running 30 to 50 knots through the mid and upper atmosphere. At the ocean surface across the tropical Atlantic, the trade winds continue blowing out of the east-northeast. This vertical shear in both wind speed and wind direction creates an environment hostile to organized tropical systems.

Davis explains how this vertical shear halts storm formation:

“essentially tropical systems like to have not a lot of high winds aloft that can essentially like decapitate a system and tilt the circulation and rip it apart.”

A clear illustration occurred with Tropical Storm Dolly, which formed west of Africa and tracked westward across the open Atlantic. As Dolly approached the Caribbean, it collided directly with this powerful westerly shear aloft, which shredded its circulation and prevented further intensification into a hurricane. While localized pockets of lighter shear allowed weak tropical storms such as Fay and Hanna to form farther north near Cape Hatteras, the broader Atlantic basin remained locked under hostile winds aloft.

Accumulated Cyclone Energy and Historical Records

To evaluate the true severity of a tropical season beyond raw storm counts, meteorologists utilize the Accumulated Cyclone Energy index. Davis explains that the index calculates the square of a system’s sustained wind speed every 6 hours divided by 10,000, summing those values throughout a storm’s lifecycle. Because the formula is exponential, a weak 40 knot tropical storm adds very little energy, whereas a sustained Category 5 hurricane adds substantial values.

The contrast between the Atlantic and Pacific basins exposes an unprecedented divergence in storm energy:

• The Atlantic Accumulated Cyclone Energy index stands at 9.3 against a climatological normal to date of 91.7, representing approximately 10% of typical seasonal activity.
• The Eastern and Central Pacific Accumulated Cyclone Energy index registers at 254.4 against a climatological normal to date of 110.1, generating 27 times the cyclonic energy of the Atlantic.
• The Atlantic has generated eight tropical storms but zero hurricanes to date, marking the first time in the modern satellite era that began in 1960 that the basin has reached this point on the calendar without a single hurricane.
• Historical records indicate that 1914 was the last time the Atlantic progressed this late into autumn without producing a hurricane.

Davis notes that:

“1914 was the last time so far that we’ve gotten to this date without a without a hurricane in the Atlantic.”

While the Atlantic languished, the Eastern Pacific produced extreme storms, including Category 5 Hurricane Polo, which bottomed out at a central pressure of 892 millibars. Recording the second-lowest surface pressure in Eastern Pacific history and reaching Category 5 status 3 separate times, Polo drove catastrophic flooding across mainland Mexico and delivered heavy tropical moisture into New Mexico, Texas, Colorado, and Arizona.

The Thermodynamics of Cyclogenesis: Tropical Versus Extratropical Storms

Navigating late-season weather patterns requires understanding the fundamental thermodynamic distinctions between storm types. Davis explains that extratropical storms, such as winter nor’easters along the northeastern United States, feed on sharp horizontal temperature contrasts across opposing air masses. These systems feature distinct cold, warm, and occluded frontal boundaries that generate energy from baroclinic instability over broad geographic regions. In September, nor’easters account for only about 1% of annual extratropical storm activity.

Tropical cyclones operate on an entirely different mechanism. They are non-frontal, warm-core systems that depend on uniform temperature and moisture throughout the surrounding air mass. Rather than feeding on air mass contrasts, tropical systems require warm sea surface temperatures of at least 25 to 26 Celsius, abundant atmospheric moisture, and light winds aloft that allow convection to release latent heat energy into the upper troposphere. Systems must generally form north of 10 degrees north latitude so the Coriolis effect provides sufficient rotational force.

Davis notes that stalled frontal boundaries can occasionally trigger tropical cyclogenesis. When a cold front pushes off eastern North America and stalls over the warm waters of the Gulf of Mexico or southeast coast, its horizontal temperature contrast gradually washes out. The lingering cyclonic disturbance left behind can then tap into warm waters and develop into a tropical system, as demonstrated by Tropical Storm Edouard in the Gulf of Mexico. Conversely, tropical storms like Hanna moving northward into cooler waters and stronger upper-level winds eventually ingest contrasting air masses, develop frontal boundaries, and transition into extratropical lows.

Offshore Complacency and Lessons from Hurricane Andrew

For sailors preparing autumn passages south toward the Caribbean or the Bahamas, Davis and Schell emphasize that a quiet season must never encourage complacency. Even in years characterized by low energy and persistent shear, isolated windows of favorable conditions can allow catastrophic outlier storms to develop rapidly.

The benchmark historical precedent remains Hurricane Andrew in August 1992. Andrew emerged during an exceptionally quiet Atlantic season as the first named storm of the entire year, yet exploded into a Category 5 hurricane that devastated Homestead, Florida, before crossing the peninsula and striking Louisiana as a Category 3 storm. Schell recalls driving through Miami and Key West with his family 9 months later, witnessing firsthand an entire coastal region where neighborhoods remained obliterated. Davis, who began tracking storms at age 7 in 1985 on the Weather Channel, tracked Andrew at age 14.

Reflecting on long-term weather patterns, Davis stresses:

“There’s really not two seasons that ever ever look alike. There’s just so much volatility with them.”

This volatility is evident in the Pacific, where Hawaii has faced continuous hurricane threats fed by ocean water running 5 to 6 degrees Celsius above normal. Schell notes the ordeal of offshore sailor Paul Exner, whose boat and residence were destroyed in Tortola during Hurricane Irma; after rebuilding and relocating to Hawaii, Exner found himself dodging consecutive hurricane threats.

Davis highlights how storm impacts linger in the landscape, describing his investigation into forensic forestry at Rhododendron State Park in New Hampshire. Nearly 90 years after the devastating 1938 New England hurricane struck with 100 mph southeasterly winds, living trees in the park remain permanently bent from southeast to northwest, preserving physical evidence of historical hurricane forces.

Reanalyzing 193,000 Offshore Miles with ECMWF ERA5 Climatology

Turning from macro meteorology to passage analysis, Schell shares a custom global voyage mapping tool developed for 59 North. The platform maps all 193,000 nautical miles logged by the fleet since 2015, spanning voyages from 2,900 miles during the company’s inaugural 2015 season up to 31,000 miles sailed across fleet vessels in 2025.

The interactive interface connects via API to the ECMWF ERA5 reanalysis climatology database, which archives hourly global surface weather readings dating back to 1940. By overlaying historical wind barbs, barometric pressure, precipitation, and ocean currents featuring Gulf Stream eddies directly onto vessel tracks, skippers can debrief tactical passage decisions. The map also incorporates an astronomical overlay displaying the geographic positions of the 57 primary navigational stars, such as Arcturus and Jupiter, along with summer solstice solar positions adjusted for Earth’s axial wobble.

Reviewing a June 2019 sub-Arctic voyage aboard Icebear from Lunenburg, Nova Scotia to St. John’s, Newfoundland, Schell and Davis examine the tactical weather data. Icebear departed Lunenburg following a frontal passage that shifted winds to the west, logging sustained 35 knot winds down to St. Pierre on what became the fastest 100 miles the boat had sailed. As Icebear entered the iceberg zone, sea temperatures dropped to 38 degrees Fahrenheit before the crew anchored in a southern Newfoundland fjord to hike and swim in a river under 55 to 60 degree temperatures.

Offshore reanalysis also illuminates fine-scale squalls that global models fail to resolve. Schell examines a 2019 mid-Atlantic passage aboard Isbjørn where a subtle inverted trough barely registered on standard GRIB wind barbs, yet produced extreme localized weather recorded in the boat log:

“Expected showers and squalls today. Instead we got an hour of 60 knots. Hardest rain we’ve ever seen. No visibility. Running down wind. Bare poles at 10 knots. Crew was amazing. Nothing broke.”

Davis points out that interpolated weather archives capture prevailing conditions across 6 hour intervals, but fine-scale features like inverted troughs and convective outflow boundaries slip between grid points. On another passage through the Old Bahama Channel, WRI meteorologists provided real-time warnings to 59 North regarding a 30 knot westerly outflow boundary spreading from collapsing thunderstorms to the west, demonstrating why automated pilot charts must always be paired with active meteorological routing.

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