Josephine became a powerful extratropical low after forming as a subtropical storm off the Mid-Atlantic coast

Josephine, which the National Hurricane Center tracked as a subtropical storm off the Mid-Atlantic coast, became a powerful extratropical low on Oct. 22 with maximum sustained winds of 65 mph. The center issued its fifth and final advisory on the system at 5:00 p.m. AST that day, when the low was near 40.0 degrees north and 65.8 degrees west, about 235 miles east-southeast of Nantucket, Massachusetts, and racing northeast at about 17 mph.

Save more, waste less: The Cool Down’s newsletter sends smart, simple changes that help you and the planet in your everyday life.

Subscribe to The Cool Down →

We may earn a commission if you sign up.

The change is one of identity, not of strength. A subtropical storm draws part of its energy from warm water and part from the temperature contrast of a weather front, and an extratropical low draws it almost entirely from the contrast between air masses. The result can be a storm that is just as dangerous at sea while falling outside the hurricane center’s usual advisory role, which is why Josephine’s fifth advisory was also its last. The wind speed did not change at the moment of transition; what changed was the physics sustaining it and the agency responsible for tracking it.

How the NHC announced the transition

The fifth public advisory is titled “Josephine becomes a powerful extratropical low” and lists a minimum central pressure of 988 millibars. It warns that gale-force gusts are possible along portions of the northeastern United States coast, that minor to locally moderate coastal flooding is ongoing along the Mid-Atlantic coast, and that large swells pose a risk of life-threatening surf and rip currents along the eastern seaboard.

The advisory closes with the statement that it is the last public advisory the National Hurricane Center will issue on the system. For continuing information, the companion forecast discussion directs readers to High Seas Forecasts from the National Weather Service, local weather service offices and Environment Canada.

Beven’s reasoning on the frontal structure

Forecaster Beven wrote the 5 p.m. AST discussion and laid out the evidence for the transition. The storm’s convection now resembled a warm frontal band with little tropical-cyclone-like curvature, its circulation center was elongating, and cold-air clouds were increasing in the southern semicircle. A scatterometer pass, which measures winds from space, also showed a developing frontal boundary.

The initial intensity in the discussion was 55 knots, which Beven called possibly conservative; 55 knots is about 63 mph, in line with the 65 mph in the advisory. Josephine was expected to speed up that night, turn east as it entered the mid-latitude westerlies, move away from New England that evening and pass south of Nova Scotia and Newfoundland that night and into Saturday, Oct. 24.

The strongest winds were expected after the transition, not before it. The discussion says the low would strengthen overnight as an amplifying trough over eastern Canada and New England pushed its winds to hurricane force, after which the global models agree it would steadily weaken. By 72 hours it is forecast to dissipate, absorbed by a new low forming to its northeast. These are forecast values from the 5 p.m. AST products, not measurements.

Formation off Hatteras and the earlier outlook

The storm’s birth is documented in local reporting of the time. WPTV reported on Oct. 21 that Josephine formed Wednesday afternoon from a low pressure system off Hatteras, North Carolina, with 65 mph winds and a position about 265 miles south of Nantucket. According to that report, the NHC expected it to remain subtropical through early Thursday and then strengthen as a post-tropical cyclone with hurricane-force winds, a sequence that the Oct. 22 products bear out.

The same report said the system was forecast to move away from the United States, while strong winds, coastal flooding and dangerous surf were expected along parts of the Northeast coastline over the following several days. At the time of formation, the storm was moving north at only about 6 mph, with a large increase in forward speed expected once it turned northeast, which is the track the Oct. 22 advisory later confirmed.

The hurricane center’s own Atlantic outlook, issued at 8:00 a.m. EDT on Oct. 22 and signed by Beven, still described the system as Subtropical Storm Josephine a few hundred miles offshore of the Mid-Atlantic coast. The same outlook gave a low, 10 percent chance of formation through seven days to a separate trough of low pressure several hundred miles east-southeast of Bermuda, a reminder that the hurricane center was following more than one feature that morning.

Readers who want the live status can check the hurricane center’s main page, which listed the final Josephine advisory beside the Pacific storm Vance when it was last updated on Oct. 23 at 02:39 UTC. After the final advisory, though, the forecast responsibility for the low passes to the weather service’s marine forecasters.

This article was produced with the assistance of AI and reviewed by Morning Overview editors prior to publication.

Save more, waste less: Subscribe to The Cool Down →

We may earn a commission if you sign up.