Thursday, July 08, 2021

Tropical Storm Elsa: 8 July, Update A

I know I said I wasn't going to post any more on Elsa, but she's still hanging on as a pesky Tropical Storm and the NHC have actually bumped up her intensity even though she is over land... and I thought you might like to know what kind of magic this is (and if you don't, then that's ok, you can skip to the end of this... but you'll be wondering all night long what you missed, so you might as well read on. Grin.).

But first, she's at 36.3N, 78.3W, heading NE at a rapid 21mph and is currently near the North Carolina/Virginia border:

They just increased her intensity from 45mph to 50mph, which means she's still a weak Tropical Storm (TS range: 39-73mph), central pressure 1006mb. The speed was increased because some wind sensors detected speeds of just under 50mph, but she's not expected to get much stronger. 

Why is she getting stronger when she isn't over warm water? This increase is actually a result of what is going on in the atmosphere, not the ocean...

<Science Alert!> Vorticity: I haven't mentioned this yet this season, but now is as good a time as any to bring this piece of lovely jargon out for a spin. As you know, a storm has 'circulation', and it's pretty obvious what that is, right? It simply means that a storm is going around a central point, like a carousel. Vorticity, is essentially the way we measure the amount of circulation that a storm has. It's a very useful tool and I've used it for ages - next to a corkscrew for opening yummy wine bottles, it's my favourite (with a 'u') tool during hurricane season. ;-)

Although satellite imagery is one piece of the puzzle, the biggest clue about what sort of storm we have is the vorticity and what it looks like in different levels of the troposphere (see the Science Alert! here for a reminder of troposphere), because that gives us a glimpse into the structure of the storm. 

All types of stormy weather have a recognizable vorticity signal in the troposphere. Like a fingerprint, you can figure out what sort of storm system you have if you know what and where the vorticity is. The vorticity for low pressure fronts looks different compared to tropical storms. For low pressure fronts, the vorticity stretches out in a long line. For proper, grown-up, tropical cyclones, the vorticity is confined and generally circular. 

You can also tell how strong a tropical storm is depending on how strong the vorticity is and how high into the troposphere that signal can be seen. A Tropical Storm ALWAYS has a vorticity signal that reaches the middle of the troposphere (around 500mb) because this indicates that there is some deep convection (aka big thundery clouds). A Hurricane ALWAYS has a vorticity that reaches the upper level of the troposphere (around 200mb) because this indicates even strong convection and activity<End Science Alert!> 

Let's have a look at the vorticity maps for Elsa, shall we?

<Technical Alert!> Vorticity Maps:

Here is the vorticity map for 850 mb (almost the lowest level of the troposphere): 

You can see the signal of TS Elsa - the red (almost white) splodge (technical term ;-)), very conveniently almost covered by a Tropical Storm symbol. In this map, Green areas represent very mild vorticity, yellow is a little stronger, orange is fairly decently strong, red is very strong, and white is really super-duper strong! 

Now let's look a little higher in the troposphere. Here's the map for 500mb (the middle level of the troposphere):

You can still see the vorticity signal for Elsa, but it is a little weaker than the lowest level of the troposphere. Also, the center of the red/white splodge from the 850mb map is slightly offset from this orange/red splodge which indicates that the vortex isn't quite in alignment. This is frequently (but not always) associated with wind shear. 

And here's the map for 200mb (the upper levels of the troposphere):

In this upper level map, you don't see any signal for Elsa. That's because she's not a hurricane, or even close to being a hurricane - there is no upper level circulation. However, you can see a line of stronger vorticity that is fairly close to her - on the west side. That front is essentially giving her a little boost. So it's all in the atmosphere for this one. 

These amazing maps are produced by the University of Wisconsin-Madison's Cooperative Institute for Meteorological Satellite Studies - and a jolly good job they do too! To see them for yourselves in the future, click on the link above. Scroll down to the second map on the page (Regional Real Time Products) and click on the area of the world you are interested in. For the examples above, I clicked over the North Atlantic. From the drop-down menu, chose 'Winds & Analyses'. In the top panel, you'll see a whole array of buttons - including the 850mb, 500mb, and 200mb Vorticity. Clicking on those will give you maps like the ones above. This is one of my favourite websites and such a great resource for data! You can even go back in 3-hour chunks of time and see the vorticity for a storm evolve or move around. It'll provide you with hours of fun and entertainment - all from the comfort of your own living room! :-) <End Technical Alert!>

Because she has been over land, her convection has decreased quite a bit: 

There is still quite a bit of rain, but less of that very thundery/tornado like weather she had earlier in her life. Tomorrow in New England... 

So hold on to your Piglet! (and listen to your emergency managers because they have the best information for your local region). 

Toodle pip!




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 local 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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