Archive for the ‘closed isobars’ Tag

NANA, TROPICAL DEPRESSION 15, INVEST 90L, AND INVEST 99L.

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There is “action” in the Atlantic basin today in-so-much as multiple systems are concerned.  The Gulf of Mexico and the Caribbean are considered to be a part of the Atlantic Basin.  The most immediate concerns reside in Puerto Rico, I should think, due to tropical depression 15 nearby.  If it moves northeastward, as predicted, the island will no doubt get precipitation that it doesn’t need considering the pelting it has already taken this season.

Storms do not get named until they “graduate” from tropical depressions to tropical storms.  But, there is a tropical depression out there today with a name, Nana.  She is “named” because she had reached tropical storm status earlier in her history and she retains that name even though she has now deintensified as was predicted.

That is not uncommon but there has been a very interesting occurrence with Nana.  Wind shear earlier on broke her into two distinct parts.  The northern segment (Nana) is not likely to survive but the southern segment, Invest 90L, stands a chance of intensifying.  Both Invest 90L and Invest 99L are tropical disturbances (also called tropical waves).  If you didn’t already know you might have guess that numbers are assigned to tropical disturbances and when under “investigation” they are labeled with the abbreviation, “invest.”

So, there are four systems out there being watched.

Classification of Synoptic-Scale Lows of Tropical Origin:

1. Tropical disturbance (tropical wave) – An area of organized convection, originating in the tropics and occasionally the subtropics, that maintains its identity for 24 hours or more. It is often the first developmental stage of any subsequent tropical depression, tropical storm, or hurricane.

2. Tropical depression – A tropical cyclone in which the maximum sustained surface winds are 38 miles per hour (33 knots) or less. Characteristically having one or more closed isobars, it may form slowly from a tropical disturbance or an easterly wave which has continued to organize.  NOTE FROM CLOUDMAN23 – By convention, it has been the practice of the National Hurricane Center to associate the beginning of “rotation” with the transition from disturbance to depression.  In fact, in some definitions rotation is an important element.  Generally, this causes no problem because once one or more closed isobars are needed to plot the pressure, rotation has almost always begun.

3. Tropical storm – A tropical cyclone in which the maximum sustained surface winds are from 39 miles per hour (34 knots) to 73 miles per hour (63 knots). At this point, the system is given a name to identify and track it.

4. Hurricane (or another name depending upon geographical location – e.g. typhoon) – The name for a tropical cyclone with sustained winds of 74 miles per hour (65 knots) or greater in the North Atlantic Ocean, Caribbean Sea, Gulf of Mexico, and in the eastern North Pacific Ocean. This same tropical cyclone is known as a typhoon in the western Pacific and a cyclone in the Indian Ocean.

(definitions source is the Weather Channel Glossary found at http://www.weather.com/glossary/a.html

During my long college teaching career I was asked from time to time why such a list was not labeled “Classification of Synoptic-Scale Cyclones of Tropical Origin” instead of Synoptic-Scale Lows. The reasoning is straight-forward.  The first member of the quartet, the tropical disturbance, is not cyclonic.  In order to be cyclonic there must be rotation.  Therefore, though all cyclones are lows, not all lows are cyclones.