Monday, September 29, 2008

Subtropical Storm Laura: September 29 Update A

Before Laura, here's a report from our  Halifax re "Hurricane" Kyle from yesterday:
"Blustery winds and some rain in Halifax. Much more west of here. We'll head that way today and have a look."

The blobette turned into Subtropical Storm Laura - a compromise between a TS and an Extratropical Storm. It's only since 2002 that subtropical storms were given names from the hurricane name list - which would partly account for why we have had more named storms in the past few years.

At least once a year I get asked what the difference is between Tropical Storms, Subtropical Storms and Extratropical Storms. For those who want to know, I cut and pasted an explanation from previous years below (so at least it looks like I've been very diligent and wrote a lot for this entry ;) ).

She's centered at 37.2N, 47.3W, in the central North Atlantic and she's moving WNW at 8mph. Maximum winds are near 60mph, making her a mid-sized storm (TS range: 39-73 mph), central pressure 993 mb. This WNW motion means she is (and will remain) over cooler waters of 25-26 deg C. She's moving around a high pressure system that is over the eastern Atlantic, so I agree with the official forecast track - she'll move WNW for a while, then N, then NNE etc. A pattern we see with all storms. There is pretty good circulation and a bit of convection, but most of it is just clouds, not to much in the way of thunderstorms (or even rain). The clouds are wrapping around the center, but are not near it - she looks like a big comma at the moment.

A quick overview of storm systems (from
An Extratropical Storm: These usually form in the extratropics (quelle suprise!)  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.

A Tropical Storm: These usually form in the tropics (aren't we good at naming things?) 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.

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. Subtropical Storm Laura was formed from an extratropical low pressure system/front.

I'll try and send out another update tomorrow... but I can't guarantee it. :)
Toodle pip,

Blogs archived at: ------------------------------- 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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