Friday, August 21, 2015

Hurricane Danny: August 21, Update A

For once I'm not at an airport waiting for a plane! This is my view today, from the beautiful island of Maui in the Pacific... :-)

Meanwhile, back in the Atlantic, Hurricane Danny was just upgraded to a cat 3 storm with winds of 115mph, central pressure of 974mb (cat 3 range: 111-129mph). Categories 3-5 are considered 'Major' storms - so Danny is the first major storm of the season.  I may have agreed with the NHC if they had upgraded him a few hours ago,  but it looks like he has already weakened, and with no visible eye, it suggests that winds are actually in the ~90 mph range (cat 1):

He still has strong circulation in the lower half of the troposphere, with a weaker circulation signal in the upper levels of the troposphere, which is what we would see with a Hurricane as opposed to the weaker Tropical Storm. He is still over sea surface water temperatures of ~27 deg C, with the upper ~50-70m warmer than 26 deg C. So why is he weakening? First, he is moving deeper into that dry, dusty Saharan Air Layer:

And second, more critically, he is moving into a region of strong wind shear which will really knock his socks off:

In the image above, the red areas are areas of high wind shear, and the blue are areas with low wind shear.

<Forecasting Tool Alert!>This wind shear map and the SAL map, are from the fantastic University of Wisconsin website:, should you want to have a look for yourselves (and I'm sure you do! :-)). Click on colour block in the lower map for the part of the world you are interested in (North Atlantic in this case), and in the drop down menu (amongst a number of things) you will see 'Saharan Air Layer Analysis' or 'Winds & Analysis'. The Saharan Air Layer takes you to the map immediately. Under the Wind Analysis, there are a number of useful products. The wind shear, the upper and lower level winds, upper and lower convergence/divergence (I covered this a couple of years ago, but will talk about it again at some point I'm sure), and, most useful is the vorticity at different levels of the troposphere, which is where I get my circulation and storm structure information from. To understand the circulation/vorticity maps, check out the <Science Alert!&lgt; from last year: (I'm allowed to be lazy... I'm at the beach! ;-)) Easy peasy! <End Forecasting Tool Alert!>

As for his track, although the official word is WNW at 10mph (currently at around 14.3N, 48.6W), I still think he is moving to the north of that. The forecast is to take him over the northeastern Caribbean islands, but I think there is a good possibility (should he survive) that he may just skim those and stay in the Atlantic:

That's about it for now. I may pop back later! Still best to keep an eye on Danny though, and I say that because he has a vorticity signal in the upper troposphere. He has a good structure until that goes away.

Oh, and before I leave, here's a really neat image of a cross-section of Typhoon Atsani (in the Pacific) from NASA to give you a different perspective on what a tropical storm looks like:

Toodle pip!

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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.

1 comment:

Jan Petri said...

Thank you, Jyotika. Always informative and fun.