Tropical vs. Non-Tropical ; What's The Difference?

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. 

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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.

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. An example of this is the transition that Tropical Storm Hermine went though, in early September of 2016, as it emerged off of the North Carolina coast and into the Atlantic. 

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 still can threaten lives and property.

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

Tropical cyclones can transitions to non-tropical or "post-tropical" cyclones,  meaning that it has technically lost many of its tropical characteristics and is more closely related to a mid-latitude (non-tropical) storm. As indicated earlier, this is NOT a downgrade!

One reason for a tropical cyclone's transition is when it interacts with mid-latitude features such as an upper-level trough (dip in the jet stream) and a frontal boundary at the surface. These features create physical changes in the storm's structure.

In fact, winds in a storm that transitions to non-tropical can actually increase in strength.

Although a tropical storm can be designated non tropical or "post-tropical," the National Hurricane Center can issue tropical storm warnings for areas that are about to feel impacts from the "transitioned" storm.

Why Can Tropical Advisories Be Issued For Non-Tropical Storms: 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.  

On October 29, 2012 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.

For example, the National Hurricane Center issued tropical storm warnings from the mid-Atlantic region to New England for Hermine after it had transitioned to a "post" tropical storm.

So, even though a storm transitions from a tropical to "post" tropical, it can continue to threaten life and property.

Regardless of what meteorologists call these storms, their impacts can still be significant. 

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