Why the Next Potential System is Raising Eyebrows

The eastern Atlantic low-pressure system dubbed Storm Oscar by European forecasters is no threat to the U.S. but is a forecast rarity

Tropical and subtropical cyclone formation points for June 1^st to June 15^th over the 172-year period of record along with formation potential area from the National Hurricane Center valid 8 AM ET Tuesday, June 6^th, 2023.

It isn’t every year that we look closer to Spain and Portugal than to the U.S. in early June for tropical formation, but that’s exactly where the National Hurricane Center is looking to this morning. A strong non-tropical wintertime low pressure system located near the Azores in the northeastern Atlantic – some 3,500 miles northeast of Miami – has a brief window to gain subtropical characteristics before moving toward cooler waters by late tomorrow into Thursday. The windstorm was named Storm Oscar by the State Meteorological Agency of Spain earlier this week, the next name on a European naming list designed for October to March wintertime low-pressure systems.

Despite extremely strong vertical wind shear and water temperatures below 70 degrees Fahrenheit in the vicinity (typically 80 degrees Fahrenheit is the necessary though not sufficient condition for tropical development), NHC is giving this unusually high-latitude system a low chance of formation. If the system can acquire subtropical characteristics, it would be unprecedented for June or even July in this part of the world.

The system is sitting beneath a blocked flow pattern – under sprawling high pressure from Greenland to the United Kingdom – a setup researchers have found can promote subtropical development. Although waters in the vicinity are cool relative to those we’d find in the deep tropics, they are blistering relative to seasonal averages.

Sea surface temperature anomalies for the week ending June 4th, 2023 with the 7-day potential development area for the eastern Atlantic system, valid 8 AM ET, Tuesday, June 6th, 2023.

The much warmer-than-average ocean fuel is supporting stronger storms near the center of the low-pressure system, storminess being flamed further by strong winds aloft. Like a hybrid engine that runs on both gasoline and electric, the warm ocean and typical wintertime processes may pool their resources to support a short-lived subtropical cyclone this week.

Cyclone phase space (CPS) diagram for the European model showing the current position (indicated by the “C”) of the low-pressure system near the Azores and forecast position (indicated by the “Z”) valid Friday evening Eastern Time. The diagram shows the possibility of the system briefly obtaining warm-core features between now and Friday. Credit: Dr. Robert Hart, Florida State University.

Thunderstorm tops for hybrid storms aren’t as tall as the ones in fully tropical systems, which also makes these systems less impacted by strong winds at upper levels.

Regardless, the main impacts from the low-pressure system will be strong winds, large waves, and flooding rains to the Canary Islands and Madeira this week.

The tropical Atlantic closer to the U.S. has settled down in the wake of Arlene last week, with no development expected through the remainder of the week.