Friday, June 07, 2013

'Post-Tropical' Storm Andrea: June 7, Update A

Yay, it’s Friday! End of the week and that means…

(thanks to Janice F. from Tampa for sending that little cheery graphic yesterday). J

Tropical Storm Andrea was officially moved to ‘Post-Tropical’ Storm status at the 5pm advisory, but I think she was at that point around 5am because she had crossed into the extratropics and also because her forward motion was pretty fast (at least 28mph) which suggests she had a helping hand from the surrounding atmosphere (see Science Alert below).  She is currently centered at 38.5N, 75W and is continuing on that north-east path at a whopping 35mph. Officially her central pressure is 997mb, with winds remaining at 45mph.

<Science Alert!> A quick overview of storm systems - What is the difference between an extratropical storm, a post-tropical storm, a tropical storm, and a subtropical storm?

Extratropical Storm: These usually form in the extratropics (i.e. north of 30 deg N, or south of 30 deg S) and have cold air at their core. A cold air mass meets a warm air mass, and as the warm air rises (because it is lighter than the cold air), it releases potential energy that results in these systems. Because warm air rises, a low pressure is formed which is why these are also called low pressure systems. They are usually associated with fronts which are depicted on weather maps as lines of blue triangles (for a cold front) or red semi-circles (for a warm front). Because it's the collision of air masses, these systems can occur over land or water, and occur frequently in the winter in the US as snowstorms/blizzards or Nor'easters.

Post-tropical Storms: these are Tropical Storms that take on the meteorological characteristics of extratropical storms. The NHC changed the name from ‘extratropical storm’ to ‘post-tropical storm’ about 3 or 4 years ago. Before then, tropical storms that had moved far enough north and had the same characteristics of an extratropical storm used to be called (quite sensibly) extratropical storms. But in a concerted effort to make thing more confusing, someone decided to change the name in favour of a name that makes less scientific sense, but does dumb things down (presumably so they could understand). Can you tell I’m still rather annoyed at this naming fiasco? In previous years, it took me one small paragraph to explain extratropical storms… now I need two to get to the same thing!  

Tropical Storm: These usually form in the tropics and have warm air at their core. The energy source for
these differ from extratropical storms. These storms form over water only and the energy source is latent heat. Warm water evaporates into the air. As the rising warm moisture-laden air in the center reaches colder altitudes in the atmosphere, the water vapor condenses to form clouds and latent heat is released. The heaviest rains and winds are in a band close to the center. No fronts are associated with these storms (although 'waves' in the atmosphere are) - which makes it difficult to determine too far ahead of time when a storm will develop. A tropical storm is when the winds are greater than 34 knots (39 miles per hour). If the winds are less than that, it is a tropical depression. At least these are sensibly named… although I wait, with great misgivings, for the day when someone will want to rename Tropical Depressions (closed circulation, winds 17-38mph) as Pre-tropical storms. Grr.

A Subtropical Storm: These usually contain some characteristics of both extratropical and tropical systems. For example, imagine an extratropical storm moving over warmer water. Now the storm begins to get some energy from latent heat as well, and the cold air in the center (near the surface) is replaced by warm air, so the storm core can change from cold to warm. The heaviest rains and winds are not near the center. Like a tropical system, a subtropical storm is when the winds are greater than 34 knots (39 miles per hour). If the winds are less than that, it is a subtropical depression.

<End Science Alert!>

I suppose I shouldn’t be so skeptical of the official wind speed, should I? I received a report from Jen D. in Wilmington, North Carolina. She was quite upset with the damage that Tropical Storm Andrea did at her home as it whipped by. This afternoon she sent me a couple of photos (below). In her own words: “Andrea left utter destruction in her wake. Limbs down everywhere.”

I guess TS Andrea was stronger than I thought! I hope those gladioli make a speedy recovery from their harrowing experience.

Jen sent this report at around 10am: “It rained some overnight and was breezy.  Right now we are showing wind out of the S at 20 with gusts to 37 mph.  The worst that we had was around 3AM and even then it was only 28 mph with gusts to 39.” I also heard from Sage L. in New Brunswick, New Jersey at around 5pm who said “Not much action in NJ, just a rainy mess here today.”

Today, again, I looked at a number of sensors along the path of the storm to try and find one that was reporting 45mph winds. There are many more sensors over land, so plenty to choose from. For example, she passed 40 miles from Jacksonville, where the max. wind speed was 24mph, gusts of 33mph. She pretty much ran over Brunswick, GA. Maximum winds recorded were around 2am: 26mph, with gusts of 38mph. Moving on to South Carolina, the storm was close to Charleston. Max winds were 28 mph, with gusts of 37 mph. The center passed about 5 miles from Fayetville, North Carolina. The maximum wind speed was 17mph. I'm really not convinced of this 45mph wind speed business! If you want, you can look up any sensor they have on the network from this website: (which I’ve currently got set up for Florida).

The circulation in the Atlantic Blob fizzled today, so all quiet on the eastern front for now. I’ll be back to wrap up ‘Post-Tropical’ Storm Andrea tomorrow.  

Happy National doughnut day! (And tomorrow it will be World Oceans Day, and the day after that is Happy Talk Like A Ducky Day… ;-)).


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DISCLAIMER: These remarks are just what I think/see regarding tropical storms - not the opinion of any organization I represent. If you are making an evacuation decision, please heed your local emergency management and the National Hurricane Center's official forecast and the National Weather Service announcements. This is not an official forecast. If I "run away, run away" (Monty Python), I'll let you know.

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