Wednesday, October 15, 2014

Hurricane Gonzalo: October 14, Update A

Very little time for sloth and idleness these days!

Mr. Gonzalo is looking healthy these days, isn't he? He is officially a strong cat 3 storm with winds of 125mph, central pressure 954mb. He is heading NW at 13mph and is centered at 22.2N, 66.6W (hmm, interesting... 666... generally not considered the most happy-go-lucky number but I guess he's ready for Halloween).
He is definitely stronger than a cat 1 storm as the eye is pretty consistently there! But he's not the best looking storm I've seen, and he is a little ragged for a cat 3 storm. At the most, I'd give him a cat 2 status.

Why? Well I'm so glad you asked...

1. The convection is rather weak for a cat 3 storm. The infrared satellite image shows none of those gray areas that show us that the clouds are really high and there is a lot of very deep convection,

2. There is very good vorticity (circulation) in the lower half of the troposphere, but the vorticity (circulation) structure is just not there at the highest level of the troposphere:
In the <Science Alert!> reminder link I had yesterday, you can see what the upper (200mb) of the troposphere vorticity should look like for a strong hurricane - it should be a stand-alone round(ish) 'splodge' (technical term ;-)), not an area of higher vorticity that stretches across a large area - those are troughs!

3. There is wind shear, which is clearly impacting this storm as the cloud pattern is not circular. You can see in the satellite image that they stretch off to the northeast.

4.There is remarkably little upper level divergence and relatively little lower level convergence. Oooh... what the heck is she talking about now, I hear you say. Time for a New <Science Alert!> (how exciting!) A tropical storm has low pressure in the center, near the surface. That's why we all keep mentioning the central pressure - the lower it is, the stronger the winds (usually). A low pressure is an area that the air is 'pulled' towards. By extending that image to a circular storm, you can imagine the winds are pulled in from all around - i.e., they converge. So we have Low Level Convergence. Now the problem is that if you keep pulling winds in at the low levels, they have to go somewhere. It might come as a surprise to you, but they don't just gather around the center and have a game of poker. ;-) The ground (or sea) is underneath, so the only way the air can go is up. And when the air reaches the top of the storm the only way the winds can flow is out and away from the center. So at the top we have Upper Level Divergence. The stronger the convergence/divergence of a storm, the better the formed it is. Ta-da. <End Science Alert!> Phew. Time for a little lip-wine-convergence now I think.

I could go on with my list,
but you get the gist.
 (Poetry at last! A rival for William McGonagall? ;-))

The official forecast says that Gonzalo will become a cat 4 storm with winds of 140mph within 12 hours. Officially it might, but I expect there to be actual evidence of this in 3 out of 4 of the above in that case! In reality, it seems unlikely to get quite that strong at the moment. Although sea surface temperatures are 28-29 deg C, Gonzalo is moving to an area where waters warmer than 26 deg C can be found in the upper 50-75m compared to the current 75-100m. Wind shear looks like it will continue for a bit too (at least into tomorrow).

Popping back to St. Thomas for a moment. Tom (on the ground there) said that although the NHC said the storm would be strong,"there was not a puff of wind all of last night" and so they "Slept well with no wind or rain pounding on roof".

Looking ahead, it does look like Gonzalo has shifted a bit to the west and may not be a direct hit on Bermuda now. It also looks like he may shift a bit more to the west than the current trajectory shows and he will slow down a bit (which is already shown to some extent in the track for tomorrow).
Unfortunately for you Bermudians, this means you will be hanging out on the wrong side of the tracks (insert 'as usual' jokes here)! As you can see, most of the convective activity is on the east side so get ready! I'm probably right (as I generally like to use The Force), but I'm going to give the NHC the benefit of advanced technology and say that this may be a stronger disturbance than nature is indicating. Best listen to the local folks as you make your preparations.

Speaking of local people to listen to, Steve B., from Bermuda, certainly hasn't been asleep at the wheel (fortunately for everyone on that island) and is obviously already preparing as he says in a note I got earlier today: "We could be in for a sticky weekend. I doubt if much cricket will be played! I had better fortify myself!". Words of wisdom. Spot on! :-)

That's it for this evening. More from me and Mr. Gonzalo (and anyone from Bermuda who wants to pipe up) tomorrow,
Night night!

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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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