Sunday, June 05, 2016

Tropical Storm Colin: 5th June, Update A

A bunch of stuff to cover today - Tropical Storm Colin's intensity, forecast, location. Also today you get a double bill: a Science Alert! AND a Technical Alert (on how to find out about storm surge)! I think I'll need another glass of wine! ;-)  

The official word from the NHC is that Tropical Storm Colin is currently at 23.4N, 87.8W, heading N at 9mph. He is barely a Tropical Storm though, with winds of 40mph (TS range: 39-73mph) and central pressure of 1004mb. I have a couple of things to say about this. 

First, I agree that he is a Tropical Storm - actually, I think they may have underestimated his strength and so I wouldn't be too surprised if he turns out to be a bit stronger than expected. 

Second, I'm not sure they have his center quite in the right location at the moment. Here's the latest visible satellite imagery:

It looks like his center is closer to 23.5N, 85.9W and he is moving NNE. But the other reason I think his center is east of his current official location in the Gulf is because his vorticity (circulation) is strongest to the east of that location.

Uh-oh... I feel another Science Alert coming on... are you ready? ;-)

<Science Alert!> Vorticity: I'm sure you have heard that a storm has 'circulation'. 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 - it's almost, but not quite, as useful as a corkscrew for opening a delicious bottle of wine. ;-)

How is vorticity useful? Well, all types of stormy weather have a recognizable vorticity signal in the troposphere (have a look at the Science Alert from yesterday for 'troposphere' - although you should be memorizing everything I write of course! ;-)). 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. For 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!>

Here is a map of the vorticity for Tropical Storm Colin close to the surface (at 850mb):
Here is a map of the vorticity for Tropical Storm Colin in the middle of the troposphere (at 500mb):

(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!)

You can see the circular red 'splodge' (very technical term! ;-)) that is Colin near the tip of the Yucatan Peninsula and the tip of Cuba in both maps - a little to the east of the official location. This is approximately where I think he is and it matches with the visible satellite imagery. Also, given that the signal is throughout the lower half of the troposphere and that it is quite strong (red, not all orange or yellow), I would agree with the NHC that he is a Tropical Storm.

He is still under some strong wind shear though, which is helping to keep him in check... we can see this in the infrared satellite images because most of the convection is to the east of the center:

I'm not surprised that he has a lot of rain and thunder in him - he is still over very warm ocean water. Sea surface temperatures are around 29 deg C, with the upper 75-100m warmer than 26.5 deg C. Definitely warm enough for big bunches of rain!

As for his track, I think they more-or-less have the correct landfall area, in that we are looking at somewhere on the west coast of Florida or the panhandle area - a mere few hundred miles to chose from...

Normally I would say that one day from landfall we should go with the official NHC forecast and the cone of uncertainty. The tricky part with this cone at the moment though is that if they have his center in the wrong place, the models are going to be wrong. Incorrect data in, incorrect data out. That's my only niggling issue with the cone - it could still shift to the south a bit because of this. But perhaps I am wrong about the center - they did send in a plane so they will have the best info!

I agree with the blue bit though - everyone in that area should be prepared. Have you got your water, ice cream, candles, wine, radio and batteries, ice cream, a good book, sunscreen, more wine, and a corkscrew ready?

One last thing... if that forecast track is, indeed, Colin's path, then I would expect him to be a bit stronger than currently forecast as he gets closer to the Big Bend area. Get ready for some storm surge if you are south of the storm - that includes Tampa Bay, Sarasota etc. Also, residents of Cedar Key be ready... I've noticed over the years that you guys usually get the highest surges with storms on this forecast track. This reminds me very much of Tropical Storm Andrea that made landfall in the Big Bend area, about 35 miles north of Cedar Key on the 6th of June, 2013 (3 years to the day) - she had a few tornadoes as well. 

To look at the storm surge, I use NOAA’s National Ocean Service website, Tides Online, which is an excellent site: (Thank You NOAA for not changing this website - it's 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 St. Petersburg, Florida at the moment:

It looks like the data is intermittent at this station, but essentially 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 St. Petersburg is currently about 1ft above normal.  The graph below that is wind speed and direction. Below that is air pressure (which you can see dropping), and below that is air and sea surface temperature.

I think that's it for today. Stay safe out there!



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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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