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Hurricane Isaias strengthened quickly when its inner core became more organized over very warm water—but the process was uneven. Wind shear and dry air initially limited the storm, then its north-northeastward motion aligned more closely with the shear, making conditions more favorable for strengthening before landfall.
How strong did Isaias get, and when?
Isaias first became a hurricane on July 31, 2020. At 0400 UTC, the National Hurricane Center (NHC) set its intensity at 70 knots (80 mph), based on Air Force Reserve Hurricane Hunter flight-level winds and surface wind observations. That was its initial hurricane classification, not its peak.
The NHC’s season summary lists a best-track peak of 80 knots and a minimum central pressure of 986 millibars. The storm was a hurricane from July 30 through August 4, 2020. NHC report on Hurricane Isaias · NHC 2020 Atlantic hurricane season summary
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A better-organized inner core
On July 31, aircraft, satellite, and radar observations showed deep thunderstorms near the center, a contracting wind field, and a small developing eye. These signs indicated that Isaias’s inner core was becoming more organized and better able to take advantage of its surroundings.
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At 2100 UTC that day, the NHC assessed Isaias at 65 knots. It noted the improving structure while discussing the possibility of overnight strengthening. Advisory-time estimates can differ from the storm’s eventual best-track peak because they describe the storm at particular points in time and may be revised in the final analysis.
Very warm ocean water
The NHC cited sea-surface temperatures near 30°C as supportive of strengthening. Warm water can supply heat and moisture to a hurricane, but it does not determine intensity by itself. On July 31, the NHC discussion said the improved inner-core wind field and convective structure, together with the warm water, “should support some strengthening overnight and early Saturday morning.” National Hurricane Center discussion, July 31, 2020
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Why did wind shear and dry air hold Isaias back?
Vertical wind shear—winds changing speed or direction with altitude—can disrupt a hurricane’s alignment and move thunderstorms away from its center. NOAA aircraft analysis found that mid-level shear tilted Isaias’s vortex with height. It also exposed the core to dry air in the storm’s western and southern quadrants, limiting the potential for intensification.
NOAA’s Atlantic Oceanographic and Meteorological Laboratory summarized the aircraft measurements this way: “The combined information from onboard tail-Doppler radar, dropsonde, SFMR, and flight level sensors suggests that mid-level vertical wind shear has forced Isaias’ vortex to tilt with height, limiting potential intensification of the hurricane and exposing its core to dry air in the west and south quadrants of the cyclone.” NOAA AOML aircraft summary, August 2, 2020
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The forecast tension was clear: the warm ocean favored strengthening, while increasing southwesterly shear was expected to reduce the storm’s intensity. Neither influence acted alone, and their effects changed as Isaias moved.
Why did Isaias strengthen again before landfall?
The storm’s track changed its relationship to the shear. As Isaias turned north-northeast, its motion became more parallel to the shear vector. The NHC’s post-storm report says that this helped make the environment more conducive to strengthening than forecasters had expected.
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Forecasts had anticipated that strong southwesterly shear would offset the favorable warmth of the Gulf Stream. The NHC report says forecasters did not expect Isaias to regain hurricane intensity before landfall until within 24 hours of landfall. The late improvement was therefore not simply a matter of warm water suddenly becoming available: the storm’s structure and its alignment with the surrounding winds also mattered. NHC post-storm report
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No single cause is established. The observations support a combination of an increasingly organized inner core and very warm water, opposed at first by wind shear and dry-air exposure. Later, Isaias’s north-northeastward motion became more aligned with the shear, and conditions grew more favorable for strengthening before landfall. The available storm-specific evidence does not establish climate change as the cause of this episode.
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What happened before the July 31 strengthening?
Isaias developed from a tropical wave that emerged from West Africa on July 24, 2020. The National Weather Service office in San Juan reports that the wave encountered dry air and Saharan dust before thunderstorm activity increased east of the Lesser Antilles. That history helps explain the system’s uneven development, but it is background rather than a direct explanation of its later strengthening on July 31. National Weather Service San Juan account of Isaias
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