Tropical Storm Erika: August 27, Update A

Deary me, I turn my back for one moment and Tropical Storm Erika tries to make a run for it. I flew through the night so I'm a bit weary, but I thought I'd better have a look at what was going on with our storm-du-jour before things got too blurry. :-)  

Hmm. It looks like the effects of warm surface water, deeper warm ocean water, plus the lack of dry air slightly outweighed the impacts of wind shear and crossing the islands. She is now firmly in the Caribbean, officially at 16.6N, 65.3W and heading W at 17mph. Current winds are 45mph, central pressure is 1008mb. 

'Luckily' her bark is bigger than her bite at the moment. She does look quite large and fierce with all that convection (as you can see in her infrared satellite imagery):

and there are areas of red (and gray), which means heavy thunderstorms with some of the islands, including the VIs are getting bunches'o'rain! But I am not surprised by this convective activity because she continues to move through an area of humid air (the brown dry air is quite far from her center - you can see it to her north and east, in the Atlantic, in this water vapor satellite imagery):

and she is also moving over sea surface waters with really warm temperatures of 29-30 deg C with the upper 100-125m warmer than 26 deg C. I expect her convection to continue while she is in the Caribbean.

She is growing in size which you can also see in the above satellite images... but that is her 'bark', not her bite. How about her circulation? The NHC currently have her winds at 45mph, which makes her a very weak Tropical Storm (TS range: 39-73mph). I think this is a slight underestimate. Her circulation (vorticity) is very well developed in the lower half of the troposphere and I think her winds are actually at least 60mph, possibly higher (oh, if only we had good measurements all over the ocean how much easier it would be to know!). She is not a hurricane (not yet anyway) as there is no real circulation signal in the upper troposphere. 

<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' and 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" (in an effort to sound intelligent, you can see that I continue to use only the most technical terms here ;-)) on a map of vorticity. The stronger the storm, the more cohesive the "splodge" at all levels of the troposphere. 

What does Erika look like at different levels of the troposphere? 

Here are the vorticity maps in the lower levels of the troposphere at 850mb:

and 700mb:

Here is the vorticity map for the middle of the troposphere at 500mb:

You can see her strong vorticity quite clearly as that deep round red "splodge" at these three levels  of the troposphere. The 500mb is the middle of the troposphere. Looking higher into the troposphere, we don't see the same round splodge (which we would otherwise see if she was a hurricane): 

Although there is some vorticity up there, as you can see with the green/yellow patches, it is not very well defined so it's not really the structure of a hurricane. It is more like a 'front' up there. You can look at these maps for yourself at the University of Wisconsin site I mentioned a few days ago: http://tropic.ssec.wisc.edu/tropic.php. Click on the winds and analysis for the area you are interested in, and look at the 850mb, 700mb, 500mb, and 200mb maps. <End Science Alert!>

Despite the increase in convection, she is still undergoing some wind shear which is why she isn't very strong at the moment (certainly not as strong as you would think just looking at her convection). You can see this in the vorticity maps above as well because the splodges don't all line up in the same geographical area from one level of the troposphere to the next. 

Regarding her track... I don't think the location they officially have is actually her center of circulation. In looking at her visible satellite imagery (the best place to 'see' her center), although it is murky because it is night-time, I just don't see a center of circulation as far west as 65.3W... do you? 

It looks to me like she is centered somewhere closer to 64.5W than 65.3W. We'll have a better idea by daylight. But, this may mean her forecast track is also a wee bit off. 

I know I said that the NHC forecast has been really good at ~2 days out. In this case, it may be a bit wonky though if they don't have the correct center location. They do say that there is more uncertainty in their 3-5 day forecast than usual for this storm. However, if I were in that white cone of uncertainty, I would definitely be ready or getting ready! (...have you got your wine, ice cream, and water supplies?).

The NHC are forecasting a WNW-NW turn in the next few hours. If she doesn't get too much stronger, I think there is still a chance she will continue on a more westward track for tomorrow. If she does get stronger, I can see that WNW turn happening sooner. 

I'll be back tomorrow. Good luck out there... and stay safe my friends!
Toodles for now,
J.

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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.