Wednesday, August 27, 2014

Hurricane Cristobal: August 26, Update A

Oh how the days do fly by when you are having fun! I have to pack to go to another planet (disguised as a city called Atlanta) tomorrow, where there will be weird and wonderful alien beings. :-) Who else is going? Let me know! I'm giving a talk on Friday at 5.30pm on XPRIZE. Then at 7pm I'll be on a panel: A Practical Guide to the End of the World. Should be a barrel of laughs. Stop by and say hello! :-) :-)

Meanwhile, today, back here on planet Earth, Hurricane Cristobal is still a fairly weak cat 1 and officially has 80mph winds (cat 1 range: 74-95mph) and a central pressure of 983mb. Although official he is at 30.4N, 71.5W, heading N at 15mph, from the satellite images it looks like he is closer to 30N, 72W - a bit south and west of the official location.

Earlier today the NHC wrote: "Cristobal's cloud pattern is not typical of a hurricane on IR images. The convection is quite linear and the system appears to be embedded within the tail of a frontal zone.". Yes, thank you very much.

Since then, Cristobal tried to regain his manly identity as a Hurricane (or really a Tropical Storm) in the lower levels of the troposphere, even though he is still somewhat stuck in that front in the middle and upper levels (vorticity at 500mb - the middle of the troposphere shows that he is not an isolated 'blob'):

They did increase his wind speed slightly to 80mph, but it looks like he is having a few issues now...

Even though he still has some circulation (more than he had as a 'hurricane' yesterday) and you can see that familiar banding structure, that strong convection (red bit - lots of thundery weather) in his center just went puff in the space of about 7 hours!

There are a handful of reasons why his convection would fall apart so beautifully:
1. Sea surface temperature. Unlikely to be the cause because although he is over slightly cooler waters, with temperatures in the 27-29 deg range, it is still warm enough to keep him going.
2. Dry air. There is some dry air to his west, which does begin to get pulled into the system as it weakens, but this doesn't appear to be the initial cause:

3. Very shallow layer of warm water underneath: I think a bigger factor in his decline is that he has just moved from an area where the upper 75m of the water column was warmer than 26 deg C, to an area where only the upper 25m is warmer than 26 deg C. So as he churns away, he is mixing up water that is colder than 26 deg C, and about 26.5 deg C is the water temperature you need to really sustain a tropical storm.

I am not sure he will remain like this though. Given that he is stuck in that front, I think the convection will return (at least to some extent) as he moves north and goes over the Gulf Stream, a warm strong ocean current that runs along the eastern coast of the US and then heads east, out into the Atlantic around 35N. The upper ~50-75m of the water column in the Gulf Stream is warmer than 26 deg C.

As you can see, Bermuda, that little blip in the Atlantic, is under some cloud cover - probably a perfect day for a BBQ and refreshing beverage. ;-) (What do you mean you can't see it?!?). It does look like his center (should he survive) will pass to the west of Bermuda, but the cloudy goodness is on his east, so, well, you can obviously expect beautiful sunsets for a few days (behind the cloud cover).

Right, must dash and pack! I'll try and hop on tomorrow, but I may be sucked into a space/time vortex. I'll be back as soon as I emerge - sometime in the future.

In honour of ...
Nanoo nanoo, and a good day to you!

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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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