It's Not Hurricane Hermine, But What's The Difference?

Tom Moore
Published: September 4,2016
Hurricane Hermine was a tropical cyclone when it made landfall on the Florida Gulf Coast, but it has since undergone a transition to a non-tropical cyclone. So, what does that actually mean and does that really change anything as far as weather impacts are concerned?
In William Shakespeare's Romeo and Juliet, Juliet poses a question and follows with the remark, "What's in a name? That which we call a rose by any other name would smell as sweet."
In the world of weather, meteorologists have names for all kinds of phenomenon, but information must be communicated to the public and all of these names and the terminology can be very confusing. To be honest, sometimes they are even confusing to the meteorologists.
(MORE: Florida's Record-Smashing Hurricane Drought Has Ended)

Tropical vs. Non-Tropical Cyclones

A tropical (or warm core) cyclone (area of low pressure) usually forms over warm waters in the tropics. Air rises rapidly around the edges of the center of the storm. Sinking air in the center of the storm heats up the air, so the storm has warm temperatures from the surface all the way up to high levels of the atmosphere. Hurricanes and tropical storms are (warm core) tropical cyclones.
At the surface, stronger winds are usually proximate to the storm's location and they diminish quickly when you move away from the storm.
A non-tropical (or cold core) storm has the coldest temperatures in the center of the storm. Temperatures cool as you move higher in the atmosphere and there is a trough at the highest levels. Unlike tropical (warm core) storms, winds are not as concentrated near the center of the storm, but can spread out for hundreds of miles from it.

Current Winds, Gusts

Precipitation in a cold core (non-tropical cyclone) can also spread far away from the center of the storm. Most mid-latitude storms are cold core including nor'easters.
Tropical cyclones are nearly symmetric in shape and are without fronts. Mid-latitude (cold core) cyclones are comma shaped and have fronts associated with them.
Hurricanes and tropical storms often transition to cold core cyclones. When the National Hurricane Center concludes that a tropical storm has transitioned to a (cold core) mid-latitude storm it will designate it as "post-tropical" meaning that it has transitioned to a non-tropical storm.
The transition often occurs when a tropical cyclone moves to higher latitudes and interacts with atmospheric features that are more common there.
The main point is that the designation "post-tropical" is NOT a downgrade from a tropical storm. It merely means that the tropical storm has undergone a transition. It can remain as a high impact storm but those impacts can be similar to or, in some cases, vary from when it was a tropical storm.
So, the storm could feature a different structure and have a different look, but it can threaten lives and property nonetheless.

Non/Post - Tropical Hermine? What's the Difference?

On Saturday, the National Hurricane Center designated Tropical Storm Hermine "post-tropical," meaning that it has technically lost many of its tropical characteristics and is more closely related to a mid-latitude (non-tropical) storm. Hermine was moving off of the North Carolina coast at that time. As indicated earlier, this is NOT a downgrade!

Hermine interaction with a jet stream trough and a surface front allowed it to transition from tropical to non-tropical.

The reason for Hermine's transition is that it interacted with mid-latitude features such as an upper-level trough (dip in the jet stream) and a frontal boundary at the surface. These features are creating the storm's physical changes and are also allowing it to become stronger.
In addition, water temperatures off the mid-Atlantic Coast (80 degrees and above in some locations) are well above seasonal averages and that environment is more like you would usually find farther south. This will also help to maintain the strength of the storm and there is a slight chance that Hermine could regain tropical characteristics later.
Although it's physical structure is changing,  Hermine remains an extremely dangerous storm for the East Coast. Impacts that are associated with tropical cyclones such as dangerous storm surge, flooding rain and strong winds will continue. Since this storm is expected to stall for a few days, these impacts will continue for a prolonged period.
(MORE: Hermine Poised For Dangerous East Coast Flooding, Beach Erosion: Tropical Storm Watches and Warnings Extended in New England)

Water temperatures off the mid-Atlantic Coast are well above average and can help to keep post-tropcal Hermine strong.

In fact, winds with this storm are expected to increase near, or up to, hurricane strength even if it remains non-tropical.
Despite the fact that Hermine has become "post-tropical," the National Hurricane Center had issued tropical storm warnings for the mid-Atlantic coast over the weekend, and now tropical storm warnings have been issued all the way up to Massachusetts, as of Sunday evening.
(MORE: Why Hermine has Caused Extreme Forecast Headaches)

Why There Are Still Tropical Advisories: The "Sandy" Rule

In October 2012, Hurricane Sandy was a powerful storm that was barreling westward toward the coast of New Jersey. The National Hurricane Center forecast was for Hurricane Sandy to transition to a "post-tropical" cyclone before making landfall.
In the late afternoon, Sandy remained a hurricane but a special advisory issued by the National Hurricane Center indicated that the storm had lost its tropical characteristics and was designated a "post-tropical" cyclone.
Back then, when a transition occurred, the responsibility of communication with the public switched to the local National Weather Service offices that would receive impacts from the storm.
This all happened only a few hours before landfall. In addition, there were no tropical advisories such as a hurricane warning in effect for coastal areas of New Jersey and farther north to New York.
This created much confusion as many public officials and emergency management staff believed that Sandy was "downgraded" and no longer posed a great threat. The results proved that this was far from true. It also created much confusion for the public, who were eager to receive the latest information.
After a complete review, the National Weather Service allowed the National Hurricane Center to issue tropical advisories, even after a tropical cyclone or hurricane transitions to post-tropical if it poses a significant threat to life and property.
So, in the current case of Hermine, even though it has transitioned from a tropical to a "post" tropical storm, it will continue to threaten life and property along the East Coast of the U.S.
Considering that it will be with us for several days in a changing environment, residents in all areas that will feel the wrath of this storm should keep constantly updated on the latest conditions and threats.
Regardless of what meteorologists call it, the impacts are the same.
MORE ON WEATHER.COM : Hurricane Hermine-Florida August/September 2016