Tuesday, August 01, 2017

'Tropical Depression' Emily: July 31, Update B

The extremely short-lived Tropical Storm Emily is currently ‘centered’ at 27.5N, 81W, and is now a Tropical Depression with winds of 30mph, central pressure 1010mb. She is over Florida, heading E at 9mph. She dumped some rain over southern Florida (although in some parts, I hear it was less than the daily summer showers) but looks like she’s pretty much done:

I still maintain that she wasn’t a Tropical Storm, and the vorticity (circulation) signal continues to show that she is part of that front that extends into the Atlantic. But oh, I don’t think I’ve discussed vorticity yet this season! How remiss of me. Luckily for you, there isn’t too much more to say about Emily so here’s a double bill of fun fact-filled <Alerts!> … grab your cup of tea and settle down to tales of Vorticity and Storm Surge...

<Science Alert!> Vorticity: You know a storm has 'circulation' and it seems pretty obvious what that is, right? The storm system is going around a central point - kind of like a carousel. Well, vorticity is essentially the way we can measure the amount of circulation that a storm has. It's a very useful tool actually and I've used it for ages - next to a corkscrew for opening yummy wine bottles, it's my favourite (with a 'u') tool. ;-)

Although satellite images are one piece of the information puzzle, the biggest clue about what sort of storm we have is in the vorticity. What that vorticity looks like at different levels of the troposphere gives us a glimpse into the structure of the storm.

All types of stormy weather have a recognizable vorticity signal in the troposphere. Like a fingerprint, you can figure out what sort of storm system you have if you know what and where the vorticity is. The vorticity for low pressure fronts look different compared to tropical storms. For low pressure fronts, the vorticity stretches out in a long line, which is what we have for Emily. For proper grown-up Tropical Storms, the vorticity is confined and generally circular.

You can also tell how strong a tropical storm is depending on how strong the vorticity is and how high into the troposphere that signal can be seen. A Tropical Storm ALWAYS has a vorticity signal that reaches the middle of the troposphere (around 500mb) because this indicates that there is some deep convection (big thundery clouds). <End Science Alert!>

Luckily, today we have two examples of ‘Tropical Storms’ and their vorticity (circulation). One of them is actually a Tropical Storm and the other, well, isn’t. Over in the Pacific, Tropical Storm Irwin is hanging around with winds of 45mph, which makes him a relatively weak storm (although I think he may be a bit stronger, based on his strong vorticity field, but that’s another story).

So, let’s compare the fields in the Atlantic for Emily and in the Pacific for Irwin at a few different levels of the troposphere:

The lowest level, pretty darn tootin' close to the surface of the planet (850mb)...

The top map is Emily (the 'L' over Florida) and the lower has Irwin (where that storm symbol is). You can see a nice red 'splodge' (technical term ;-)) at this level where TS Irwin is hanging out, but for Emily you can see that she is connected to a long line of red/orange/yellow, which shows us she is a front.

Looking a bit higher up (o.k., 700mb if you want to be all technical ;-))...

Here, Irwin still has a good strong roundish signal, almost but not quite connected to the other blob just to the northwest, and Emily is still part of a front.

And the middle bit of the troposphere (500mb)...

And again, Irwin’s vorticity signal is round and red (and I think he is stronger than his 45mph would suggest because he is this well formed at 500mb). But the signal for Emily is considerably weaker.

Finally, at the highest level (200mb) of the troposphere…

At this height there is no signal for Irwin, which means he is definitely a Tropical Storm and not a hurricane – we would see a signal this high up if he was a hurricane. Actually, for a strong hurricane we would see a red splodge in all levels of the troposphere and they would all line up in each map. The darker red the splodge is at the 200mb level, the stronger the storm. At this level, of course there is nothing over Emily either, except a front that is slightly displaced from the lower levels… all normal stuff really.

These amazing maps are produced by the University of Wisconsin-Madison's Cooperative Institute for Meteorological Satellite Studies - and a jolly good job they do too! I will show you how to get them in a future post, but it’s the same site as the SAL maps: http://tropic.ssec.wisc.edu/tropic.php that I talked about in early July: http://jyotikastorms.blogspot.com/2017/07/the-atlantic-blob-aka-tropical.html.

The other thing I mentioned in this morning’s post was storm surge…

<Technical Alert!> To look at storm surge, I use NOAA’s National Ocean Service website, Tides Online, which is an excellent site: http://tidesonline.nos.noaa.gov/geographic.html. (Thank You NOAA for not changing this website - it is great as it is!!). Click on the State you are interested in, and then the location within that State. For example, here is the data from Trident Pier, Florida at the moment:

The top graph shows the water level. In this graph, the red line is the actual observed sea level, the blue line is what the predicted water level would be because of the tides, and the green line is the difference between the two. The height of the green line gives you the storm surge level, so Trident Pier currently has a storm surge of almost 1ft and slowly rising.  The graph below that is wind speed and direction - you can see the winds are pretty low. Below that is air pressure which has been fluctuating today, and below that is air and sea surface temperature (but who cares about that in a storm, hey? ;-)). <End Technical Alert!>

If Emily does make a funny move tomorrow, I’ll be back. Otherwise I'll try and pop in when the next little blob comes traipsing along. 

Oh, and I know I missed Tropical Storm Don but he was another very short-lived storm so hardly worth the effort really. Too bad as I had a whole bunch of Godfather jokes lined up but then again, perhaps we’ve had enough of them in the news lately. ;-)

The next name on the list is Franklin, followed by Gert.

Toodle pips,

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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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