Hurricane Bertha: August 4, Update A

I'm going to jump right in today...

The NHC upgraded TS Bertha to Hurricane Bertha this morning. I think they overestimated her intensity and although she is still officially a hurricane, I have to say, I don't think she's been one for most of the day and certainly doesn't look like one now...

Currently her wind speed is 75mph, so she's barely a cat 1 (cat 1 range: 74-95mph). Central pressure is 1001mb. Although it looked like she was going to hang out in conditions that would allow her to grow a bit, she moved rather quickly over colder water! So although the wind shear is low and she's in an area where there is moisture in the atmosphere, we see the importance of the ocean at play... the sea surface temperature is around 27-28 deg C but there is hardly any warm water with depth; water warmer than 26 deg C is only in the upper ~25m. This would account for her scraggy appearance and structure.

She is apparently centered at 31.3N, 73.1W heading NNE 20mph. I really think she's a Tropical Storm and barely got to hurricane strength today. Of course she will continue to deteriorate as she moves north.

<Science Alert!> Although satellite images are one piece of the information puzzle, the biggest clue about what sort of storm we have is in the circulation (or vorticity as this is supposed to be all sciency stuff ;-)) and what that looks like at different levels of the troposphere - this gives us a glimpse into the structure of the storm. A tropical storm will have a well-defined circular "splodge" (yes, you can expect only the most highly technical jargon to be used here! ;-)) on a map of vorticity. The stronger the storm, the more cohesive the "splodge" at all levels of the troposphere.

Luckily for you, I have a Tale of Three Storms to show you this! :-) 'Hurricane Bertha' in the Atlantic, and both Hurricane Iselle and Tropical Storm Julio in the E. Pacific. Earlier today I grabbed the vorticity maps for the Atlantic and E. Pacific. At that time, Bertha was a cat 1 storm officially with winds of 80mph, Iselle was a cat 4 and had winds of 140mph (she is still a cat 4, but with winds of 135mph, still heading to Hawaii) and Julio was just a baby with winds of 45mph (he now has winds of 60mph, and is also heading in the Hawaii direction!).

We can see the how good the structure of these storms are throughout the troposphere by looking at the circulation/vorticity maps for four levels of the troposphere.

The lowest level, pretty darn tootin' close to the surface of the planet (850mb)...
Atlantic (Bertha, the really dark red splodge just to the right of the Bahamas):

E. Pacific (Iselle is the red 'splodge' on the left - actually, the center is white which indicates that the vorticity is really really strong - and Julio is the one on the right):

A bit higher (o.k., 700mb if you want to be all technical ;-))...
Atlantic (Bertha, the  dark red splodge next to the Bahamas):

E. Pacific (Iselle is the red/white 'splodge' in the center and Julio is the red one on the right):

The middle bit of the troposphere (500mb)...
Atlantic (Bertha, the not-so-dark-red-as-the-lower-levels-of-the-troposphere splodge next to the Bahamas, which indicates that although there is some vorticity, it is weakening as you get higher into the troposphere):

E. Pacific (Iselle is the dark red 'splodge' in the center and Julio is the one on the right):

And the upper troposphere (250mb)...
Atlantic (Bertha... no longer a splodge next to the Bahamas! which indicates no vorticity in the upper troposphere):

E. Pacific (Iselle is the round red 'splodge' in the center and Julio is no longer red splodge on the right):

For a hurricane, we always see a 'splodge' at all levels of the troposphere and they always line up on the same spot. The darker red the splodge is at the 200mb level, the stronger the storm. For a Tropical Storm, the vorticity generally stops somewhere in the middle troposphere (around 500mb), so both TS Julio and 'Hurricane' Bertha were really Tropical Storms, whereas Hurricane Iselle is most definitely a strong hurricane!

And thus endeth a Tale of Three Storms. :-) Let me know if you have any questions! :-) (All of these vorticity maps are from the University of Wisconsin... I'll show you how to get to them tomorrow). <End Science Alert!>

Before I zip off, I ought to mention this historic day... 100 years ago today (4th August, 1914) Britain and hence the Commonwealth jumped into The Great War, also known as World War I... or the largest family squabble in history (and you thought 'Dynasty' was bad?). Amazing times with many courageous stories!

Toodle pip!
J.

Blogs archived at http://jyotikastorms.blogspot.com/
Twitter @JyovianStorm
-------------------------------
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.
-----------------------------