We are in the midst of hurricane season, so the formation of tropical systems in October is not unusual. However, Subtropical Storm Karenformed at a relatively high latitude for something with “tropical” in the name, and that is strange.

As of the morning of October 10, 2025, the National Hurricane Center noted, “The center of Subtropical Storm Karen was located near latitude 46.3 North, longitude 31.2 West.” It has maximum sustained winds of 45 mph. NHC went on to say, “Little change in strength is forecast through tonight, and the system is expected to become a post-tropical low tonight or early Saturday and open into a trough soon thereafter.”

What’s all this stuff about “subtropical” anyhow? The NHC glossary defined a subtropical cyclone as, “A non-frontal low-pressure system that has characteristics of both tropical and extratropical cyclones. This means they originated over tropical or subtropical waters and have a distinct circulation. As observed in the satellite image above showing Subtropical Storm Karen, NHC noted, ”They have organized moderate to deep convection, but lack a central dense overcast." Tropical systems derive their energy mainly from the underlying water and have warm core. Extratropical systems derive their energy from air mass temperature differences.

If the cyclone features maximum sustained wind speeds of at least 39 mph, it is a subtropical storm. The NWS website cautioned, “There is no such thing as a subtropical hurricane. If a subtropical storm intensifies enough to have hurricane force winds, than it must have become fully tropical.”

I have always learned that the “subtropics” extend from the tropical region to about 35-40 degrees latitude. Karen is currently poleward of that range. Here’s the other really interesting point. Specialized analyses developed by Bob Hart at Florida State University to assess whether the system is warm core, suggests that Karen is “quite tropical" at 46.3 N degrees latitude.

Observations have documented marine heatwaves and warming in the Northern Atlantic in recent years. Studies have also found that tropical systems may be reaching peak intensity at higher latitudes. Warmer oceans at higher latitudes certainly make formation of storms like Karen possible, The current sea surface temperatures in the North Atlantic illustrates several pockets of warmer-than-normal waters.