Thursday, September 07, 2017

Hurricane Irma, Hurricane Jose, and Hurricane Katia: September 6, Update B

Another quite silly day today in Mother Nature-land. I am armed with wine so I best get on with it!

Hurricane Irma
Eugh. Reports from both Barbuda and St. Martin, the first two islands that were directly hit by Irma as a cat 5, indicate that around 95% of both islands have been destroyed. People on islands can’t evacuate as easily – there are only so many places to hide. The immediate concerns are for shelter, water, and food, which will have to be flown in – or the evacuation of everyone left on those islands (which would be preferable, especially as Hurricane Jose is heading in that general direction for the weekend). So far, I have seen reports of 7 casualties. St. Barts was also devastated, but I haven’t seen any numbers from there yet – lots of flooding and damaged buildings. To give you a very brief glimpse of the ferocity of a cat 5 (without the tornado part added in), here is some footage from St. Martin (thanks Tess T. for sending that my way):

Reports are also emerging from the Virgin Islands, where she spent a portion of today, and from what I’ve seen it looks like it was pretty rough there too. On St. Thomas it looks like power, water, and most communications were knocked out, and the possible evacuation of a hospital may be underway - here's a link with a little more information about St. Thomas (thanks Chris K.). I don't know about the British VIs yet, who also took a direct hit. 

After the Virgin Islands, her eye passed north of Puerto Rico, which is good for the island – it means that at least Puerto Rico was spared a direct cat 5 hit - but it definitely had somewhere between a Tropical Storm to a cat 4. Early reports are that at least a third of the island is without power.

She is currently at 19.7N, 67.7W and is continuing to head WNW at 16mph:
Her wind speed is still high at 180mph, central pressure is 921mb. She is definitely slightly weaker – if you look at the satellite images, you will see that the gorgeous outflow she had to her west side is looking slightly less structured now. I would say she may even be around 160-170 mph – which still makes her a cat 5 storm, so the destruction level is still the same. This slight weakening is because of her interaction with the islands of Hispaniola (which she is also passing north of) and Puerto Rico, and a very (very) small amount of wind shear. It’s academic at this point as she’s still a cat 5 storm.

It looks like she is going to pass north of the island of Hispaniola, but is going to hit the Turks & Caicos and the Bahamas:
She has really avoided most of the Greater Antillies (her eye has anyway), which means unfortunately that she won’t weaken too much as she gets to the Turks and Caicos tomorrow and then on to the Bahamas (corrected from Bermuda - sorry, it was late when initially proofed! - thanks Ed M.) on Friday. Maybe she will drop down to a cat 4 storm as she gets to the Turks and Caicos, but I don’t think it will be much more than that. There is some very light wind shear between her and this next set of islands, but not much. Sea surface water temperatures are in the 29-31 deg C range and the upper ~100-125m are warmer than 26 deg C, which gives her plenty to munch on.

As I said earlier, the projected track has shifted to the east, so it appears that she will go up the east coast of Florida – but again, I have to emphasize, the cone still covers almost all of Florida at 4-5 days out. It is really difficult to say exactly where she will turn to the north. I thought I’d try and show you why it is so difficult to predict this particular track…

<Technical Alert!> To look at track, I don’t have the same level of data as I used to have (alas), but I have got my sticky paws on a pressure field map at 500mb – which is the pressure around the middle of the troposphere:
I know it’s a bit of a busy figure – this is the kind of fun data us meteorology-boffins like to look at (“ah”, I hear you say, “this explains all the wine drinking” ;-)).

Now, imagine you were looking down on the north pole, this map shows the pressure in the middle of the troposphere over the entire northern hemisphere, with the north pole being at the center of the image.

I’ve marked where Irma is and where Florida is on this map (hopefully you can read my scribbles!). The pink lines show lines of constant pressure that I want to draw your attention to. Between the two pink lines is an area of high pressure.

The pink line around Irma that looks like it curves down to the southwest is what is ‘pulling’ Irma southward and keeping her on that WNW track instead of allowing her to move NW or NNW. The pink line that cuts across Florida and curves up in a ‘u’ marks a low-pressure trough (to the north of that) that is crossing the US.

The difficulty in forecasting the track is that we have to forecast Irma’s movement as she moves to the WNW and simultaneously predict how fast that trough is moving across the US, and then figure out exactly where the two will ‘meet’ – this is the point at which she will make that turn to NW and then N. But to do this, the models have to forecast the entire world’s pressure field to get any of it right and so, it looks like the general area is south of Florida. If you look at the whole world, this is quite a precise area. If you are in Florida, that isn’t precise enough! But that’s about as accurate as we have at this many days out.

And of course, these pressure fields are constantly changing, so the model forecasts have to be constantly updated. It’s a very difficult thing to predict, which is why the track forecast covers such a large area. <End Technical Alert!>

To sum up, the spaghetti model tracks towards the mainland US are something like this...
(thank you Andrea A. for sending me that one!).

Regarding evacuations… please listen to your local emergency managers. With Irma taking a little more of a northward track as she passed Puerto Rico, it looks like the models are shifting to the east, which means she is heading to south Florida, along the east coast of Florida, and then up along the eastern seaboard. It is looking better and better for those on the west side of Florida not to have to evacuate – but until that cone tightens up, I would still be prepared, even if you don’t evacuate tomorrow (Thursday).

Good luck and stay safe!!

Hurricane Jose
And then... along came Hurricane Jose. As expected, he is now a cat 1 storm with winds of 85mph, central pressure of 998mb. He is currently at 14.4N, 47.5W, heading WNW at 17mph:
I would pretty much agree with this category of storm, and around this level of wind speed too because he has circulation throughout the troposphere, but there isn’t any eye yet. He is following in his big sister’s footsteps, but the huge difference between Jose and Irma is that there is some wind shear between the two of them. It is still around a day away from him, so I hope it is maintained by the time he gets there!

His track takes him fairly close to the northern Leeward Islands again (and then on to Bermuda possibly), and the NHC think he will be a major storm (cat 3 or higher) for a short time before getting there:
If that wind shear does take hold, then I think he will not get to major status. But if it doesn’t stay then there is a good possibility he will grow – like Irma, not too much other than that to inhibit his development.

Hurricane Katia
Ay ay ay… this is the one I don’t quite agree with the NHC on. I think she is just about reaching strong Tropical Storm status/very weak cat 1 status now. She has good convection and some good circulation in the lower troposphere, but at the moment there is only a hint of circulation in the upper troposphere… which, if it continues to strengthen, would mean that by tomorrow she would be a proper cat 1 storm. Not today though. Here's the vorticity (circulation) field at 850mb (lowest level of the troposphere) and you can see all three storms very nicely:
Here's the field in the mid-troposphere, and again, you can see three storms:

And here's the field in the upper troposphere... where Katia does not have quite as strong circulation (vorticity) (red or yellow) as the other two (although I see that Jose is being impacted a bit by a front at the moment):
However, the NHC have her official winds at 80mph which makes her a cat 1 storm (cat 1 range: 74-95mph), central pressure is 989mb. It doesn’t sound like these estimates are from any in-situ data though, but based on remote estimate.

She is currently at 21.6N, 94.7W, heading ESE at 2mph; she has essentially stalled… and if you look at the pressure field map in the technical alert, you can see why. It is because she is stuck in an area of high pressure, so she can’t move very much.  She is forecast to head to Mexico, arriving there on Friday night - I would go with the NHC on their track at this point:

That’s it for today. Nap time now.

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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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