Thursday, July 06, 2017

The Atlantic Blob aka Tropical Depression Four: July 5, Update A

Hey up friends… there’s a bit of a blob way out there in the Atlantic. It was looking scruffy but it has put on some new clothes today and is looking a little spiffy now so they have called it Tropical Depression Four.  It is currently at ~13N, 40W and is heading NW. Winds are currently at 30mph, but I think this is a gross underestimate and in my humble but not quiet opinion, it should be the next named Tropical Storm… never underestimate the Don. Tropical Storm Don that is… ;-)  

You can see from the satellite imagery that the convection (rainfall) has definitely improved:

How do we know that the convection has improved? Well, methinks that this is a delightful moment for a…. (drum roll) <Technical AND Forecasting Alert!>. A double bill… lucky times are here! ;-)

<Technical AND Forecasting Alert!> Satellite Imagery: I mainly use three sorts of satellite images: visible, water vapour (spelled with a ‘u’ of course ;-), and infrared. To access these, go to this NOAA website: Click on the storm of interest (you can look at storms anywhere in the world from here), and then I usually look at the 'Animated GIF with Lat/Long' column (on the right). But you can have a play and click on the rest of the images because it's fun! :-)

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! ;-) To see the center of circulation of the storm I usually look at the visible satellite imagery - never the infrared satellite imagery unless it is a very well defined storm.

The water vapor image is also pretty obvious…it shows how much water vapor there is in the atmosphere. I think the NHC ran out of paint this year – these images were much easier to see last year (and for many years before then) when they were colour-coded. They used to show brown areas which were dry (think of parched deserts) and any other colour indicated some amount of water vapor, with green being a lot. However, now it looks like shades of gray on this site… didn’t I already have a little rant on using the colour gray?? BUT, you can still look at a colour version of this on a different NHC page: Scroll down and find the basin of interest, and click on water vapor (the last link under each list). I like the animated GIF for the ‘movie’ version. Don’t use the Adobe Flash version.   

The infrared satellite image is the most interesting and useful 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. (To get to these, click on the link marked 'AVN' (or IR AVN) on either of the two pages above) 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 (you should know all about the troposphere by now – I mentioned it at least two weeks ago!), 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). As I said, there is no point looking at infrared imagery to find the center for weak systems. The clouds are a bit too messy. <End Technical and Forecasting Alert!>

The reason why TD Four was struggling earlier was because it was caught under the Saharan Air Layer (SAL) as it moved westwards across the Atlantic:

A layer of dry and dusty air from the Sahara Desert (ingeniously going by the name the 'Saharan Air Layer' (SAL)) tends to inhibit storms from developing.

Hmm… I do believe there is time for (drum roll) ANOTHER <Science and Forecasting Alert!>! J Squeal! J (go on, grab another glass of wine… I am!).

<Science and Forecasting Alert!>The dust that comes off northern Africa quite often travels westward across the Atlantic towards the Caribbean and Florida, amongst other places. This is called the Saharan Air Layer. It has two major impacts. The good one (especially good if you want a social life and are writing a hurricane blog during an active season) is that it tends to suppress tropical storms, in particular it impacts their ability to get their convection groove on. The bad impact is that, amazingly, it carries with it microbes and stuff (technical term for ‘stuff’ ;-)) across thousands of miles, and these have been known to result in an increase of certain health issues along the western side of the Atlantic – especially respiratory illnesses. 

In the satellite image above, the red and yellow bits are the dust levels. That red indicates areas where you’ll need an industrial strength vacuum cleaner to suck up all the dust, and the yellow areas are places where a feather duster used daily might work ;-).  You can see TD4 to the west of a large area of red. This image is from the University of Wisconsin CMISS page, which is an excellent website for this sort of handy information! Those of you in the Caribbean know about this, because a red layer of dust appears on cars when there is a strong SAL (I saw it myself a couple of weeks ago – it’s just like every day in LA!). 

You can find the SAL map from this fantastic University of Wisconsin website:, should you want to have a look for yourselves (and I'm sure you do! :-)). Click on the 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 other things) you will see 'Saharan Air Layer Analysis'. Click on the Saharan Air Layer and you will see the map above. Easy peasy! Now you are all experts in the SAL! J <End Science and Forecasting Alert!>

I’ll keep an eye on this one, but I don’t think it will get too strong:  this storm is heading NW and although it has moved away from the bulk of the SAL, there is some more ahead of it which may inhibit it from too much development (along with a spot of wind shear).  

More tomorrow… or maybe not if he doesn’t develop any further. You know how it goes. J

Ciao (I'm practicing my Italian for the future TS Don),

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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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