Thursday, November 10, 2011

Tropical Storm Sean: November 9, Update A

Happy Carl Sagan Day! Ok, so by the time I finish this I know the day will be over and many of you won’t read this until tomorrow, but I was too busy celebrating by watching the last couple of episodes of the excellent series ‘Cosmos’ with some like-minded geeks. Yes, of course there was wine. ;-)

In keeping with the spirit of Dr. Sagan I should warn you that I may, perhaps, exhibit some very subtle signs of science and healthy skepticism in the next few minutes…

Tropical Storm Sean’s vital statistics currently show that he’s at 29.7N, 70.9W moving N at 8mph. The estimated central pressure is 991mb and winds are 65mph, making him a mid-to-strong Tropical Storm (range: 39-73mph).

I can’t believe the NHC think Sean may be a hurricane tomorrow! (did you notice my very subtle skepticism?). Didn’t we already cover this earlier in the season? I would not classify a storm as a hurricane if the upper tropospheric vorticity is actually a low pressure front! The structure is not the same as a tropical storm at all. Goodness me. Here are some reasons why he’s nowhere near being a hurricane:

1. The vorticity (circulation) is strong in the lowest half of the troposphere so it certainly looks like a nice Tropical Storm. However, the vorticity (circulation) from the mid-to-upper troposphere looks more like a low pressure front than a tropical storm… it is spread out over a  large area, from the northern Caribbean to Georgia. A tropical storm would have a confined area of vorticity.  

2. The convection continues to remain weak. Given that the blue areas are clouds with no rain, the light yellow is a light drizzle, and the orange areas are heavy rain with some thunder, does this IR satellite image look like it’s strong enough to be approaching hurricane strength?

He looks remarkably similar to yesterday’s image (which was similar to the day before). At the most, I would continue to place him as a mid-sized Tropical Storm.

3. If we look at the lower level convergence and upper level divergence (remember those from earlier in the season? – if not, send me a note and I’ll find my ‘should-be-award-winning-if-there-was-one-for-this’ fabulous explanation for you ;-)), we see that the lower level convergence is strong enough for a tropical storm, but the upper level divergence is really wishy-washy.

The biggest inhibitor continues to be the dry air. Wind shear dropped a little since yesterday, but it’s still enough so wind shear continues to have a bit of an effect on this storm.

Although I’m not buying the intensity thing, I think the track is generally good. He will pass to the west of Bermuda and shouldn’t  really affect any land too much. At the rate he is going, he’ll be passing Bermuda during the day tomorrow instead of overnight.

Before I go, I thought I’d mention that there was a big non-tropical storm in Alaska today. Chris in Washington DC thought I would be interested in knowing such a thing was happening (yes indeed! Thanks!). This is not a tropical storm of course, but it was big enough to evacuate people and create some NOAA storm quick-look pages, which are normally used during a tropical storm. The reason for the evacuations and the storm pages is because there is some rather large storm surge. Here are the National Ocean Service observations of this from the coastal water level instrument in Nome, Alaska (provided via

The top plot shows the water level. The blue line in the top figure is the usual predicted water level along the coast, the red line is the observed water level along the coast, and the green line is the difference between the two… i.e. the storm surge. You can see that the water levels along the coast at Nome were 10ft above normal, and the surge was 9ft!! That is HUGE. And that’s just from a ‘normal’ winter storm I think.  The second plot shows the winds, which went up to about 42 knots. The third plot shows the drop in pressure as the storm passed by… down to 970mb! That’s less than some tropical storms!  The last figure shows the air and water temperatures, in deg F… hmm, I see temperatures of 17 deg F, How toasty is that? Practially bikini-wearing weather, right? Ha. So not only was the storm surge high, but the water was cold (just over 32 deg F when the surge was the highest).

That’s it for today. Time for a lovely nap now. More tomorrow though!

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.

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