Thursday, July 03, 2014

July 2: Tropical Storm Arthur, Update A

Tropical Storm King Arthur of the Nights of the Round Storms is officially at 30.6N, 79.1W, heading N at 8mph. And officially, his winds are now 70mph and central pressure is 988mb.

Winds of 70mph makes him a very strong Tropical Storm (TS range: 39 - 73mph)/borderline cat 1 hurricane. I think he is already a hurricane and has been since yesterday, but at least he continues to remain a little hurricane for now (although he is getting stronger). My reasons for knowing that he is a little hurricane (other than just using The Force) are:

1. Since yesterday he has shown signs of trying to develop an eye. An eye will form with winds of 85-90mph and higher (cat 1 range: 74 - 95mph). The fact that it is not there all the time suggests that he is a weak cat 1. You can see it trying to form in this latest visible satellite imagery:

2.  The circulation (vorticity) throughout the troposphere is now stronger than it was yesterday, including in the upper levels (see yesterday's <Science Alert!> about tropospheres), which is a clear signal that he is a hurricane and is actually getting a bit stronger.

3. The dry air and a bit of wind shear is keeping him in check, so he is not a strong hurricane. Although he looks like he has an eye that is surrounded by clouds in the visible image, the clouds are not evenly spread around the storm, as you can see in this infrared satellite imagery:

This shows that there is pretty decent convection all the way around the eye that you and I can see... but that the NHC can't!! Actually... <Minor Rant Alert!> I can't believe the NHC haven't upgraded him!! He's approaching a mid-size cat 1 with winds in the 80-90mph range, not a 'Tropical Storm' with 70mph winds!! How is this storm not a hurricane?? Grrrr. <End Minor Rant Alert!>

<Technical Alert!> Satellite Imagery: I mainly use three sorts of satellite images: visible, water vapour, and infrared. The visible one is obvious… it is what you would see if you took a black and white photo. Best used during daylight hours of course! The water vapor image is also pretty obvious…it shows how much water vapor there is in the atmosphere. Brown areas are dry (think of parched deserts) and any other colour indicates some amount of water vapor. The infrared satellite image is the most interesting though because not only does it show where the storm is, but it also gives us an indication of how strong it is and what sort of weather we have. The colours represent how high the clouds reach into the atmosphere because they are based on the temperature at the top of the cloud (which is what the satellite sees). It gets colder the higher you get in the troposphere (as I mentioned yesterday), so we can tell from cloud top temperature how deep the clouds are and therefore how strong the convection is! The red colours are very big high clouds with the coldest temperatures (other than a dark gray), and blues and whites are lower, warmer clouds. The redder the cloud colour, the more active the convection. My general rule of thumb (having seen these images and lived under them at the same time) is that blue and yellow areas are mostly clouds, with some rain in the yellow areas. But as you get to the orange and red, you get thunderstorms and possible tornadoes (especially in the red/dark gray areas). <End Technical Alert!>

So, it looks like some extremely strong convective activity on the east side of that thing we shall call an 'eye', and some rather strong convective activity on the west side.

Track: I see they have moved the track slightly to the west, bringing it to North Carolina tomorrow night and on Friday, pretty close to the South Carolina border...

so, umm... Wilmington, are you ready? (Gee, I hope no-one just moved there to start a new job or anything... Got those bottle of wine ready? How about the ice cream?) The track may change a little - maybe move slightly further west - but overall it looks reasonable to me. If any of my on-the-ground reporters have photos or reports (I recall some plant damage photos from Wilmington a few years ago), let me know!

At the moment it looks like he will be a cat 1 by the time he gets there because of the dry air and wind shear. The thing to watch out for is if the eye forms and stays fully developed - that's an indication that he's a strong cat 1 and slowly approaching cat 2 intensity.

More tomorrow. Good luck with the preparations out there.
Over here, it is ice-cream o-clock!

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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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