A baroclinic system active off of the United States west coast has occluded to the point that it has developed a warm air seclusion near its center. Essentially, this means that there is a pocket of warm air at the center of this system surrounded by colder environmental air. This has provided a conducive environment for the development of a polar low, and one has developed. A polar low is comparable to a tropical cyclone, in that it is partially powered off of latent heat of condensation from low-level convection that develops in the warm pocket. However, cold air remains present aloft in situations like this so the cyclone can’t be considered tropical but, it can be considered sub-tropical.

A polar low formed 2 years ago around this time and hit Oregon, bringing wind and rain. This low appears to have its sights on California, so I will be updating on it for the next few days. Below is a visible loop of the polar low (which is at the center of the overall cyclone), an image of low topped convection that is developing around the polar low, and a phase diagram showing the thermodynamic nature of the polar low. The polar low is currently showing up as a shallow warm core on the phase diagram, which as explained before means that it has warm air at the core near the surface with a cold core aloft.