Sunday, August 16, 2020

Tropical Storm Josephine, Tropical Storm Kyle, and a bit about Derechos: August 15, Update A

We're undergoing a bit of a heat wave in this part of the world. I saw reports of temperatures of 106 deg. F (>41 deg C) in southern California today (and reports of a fire tornado - toasty indeed!). So I thought I'd better share a bit of solid advice I saw on the internet...

Tropical Storm Josephine

Although you may think that Bermuda get-away this week is still on, I'd pack an umbrella or two. Our pal Josephine is definitely struggling under that higher wind shear and I don't think she's really a Tropical Storm anymore, but she's still got a lot of rainy weather she'd like to share. She's officially at 20.1N, 62.6W, heading WNW at 14mph and even though she's forecast to be a Tropical Depression, the track still takes her towards Bermuda.

Her winds are 45mph, central pressure is 1008mb, so she's still a very weak storm officially. She still has a pretty good satellite appearance and quite a bit of convection, but she doesn't have a good vertical structure and her vorticity (circulation) is only strong in the lowest levels of the troposphere. 

Tropical Storm Kyle

As I mentioned yesterday, I don't think they should have named Kyle at all because he's not a Tropical Storm by any stretch of the imagination. He is officially at 40N, 60.4W, heading ENE at a very fast 20mph.  

Officially winds are 45mph, central pressure is 1002mb. But if we look at his vorticity (circulation) structure, we can see that it's an elongated mess (which is the vorticity format of a front, not a Tropical Storm) in the lowest levels of the troposphere (compared to Josephine's circular structure - the vorticity format a Tropical Storm should have): 

And 'Kyle' is clearly even more of a front in the middle levels of the troposphere!...  

So he's definitely NOT a Tropical Storm! This is one of the problems with putting a number on the named storms we think we're going to have at the beginning of a season, isn't it? You have to meet the goals you set. I've always been against the 'numbers' game - just say it'll be busier than usual or not as busy and be done. Mother Nature isn't paying any attention to the numbers that are set, and our forecasting capabilities for the entire season are pretty rubbish.  

Derecho in Iowa and other US States

I've had a couple of queries about 'the hurricane that hit Iowa' a few days ago. Although winds were over a 100mph (and close to 110mph in some places) which is the equivalent of a mid-to-strong cat 2 hurricane, this is not a hurricane, but rather what's called a derecho. There was very little warning as it went through the mid-west states, which included Iowa (worst hit was Cedar Rapids), Illinois and a number of other states. So, what's the difference between a derecho and a hurricane?

<Science Alert!> Derecho: A derecho is essentially a straight line of fast-moving strong winds and thunderstorms - versus the strong winds and very severe thunderstorms that we see in a circular pattern in a tropical cyclone. It is a Spanish word and means 'straight' - to indicate that the strong winds are straight instead of circular as we see in a tornado (which, by the way, is also a Spanish word meaning 'twister'). 

A derecho is classified by winds greater than 57.7mph - versus a tropical cyclone that has winds greater than 39mph. They have to extend over a path at least 250miles long - versus a tropical cyclone which has to have closed circulation but can cover an area over 1000miles in diameter. And they typically move at speeds of 50-70mph - versus a typical tropical cyclone that moves forward at speeds of 9-17mph. 

Here's a great time-step image of the derecho that crossed the northern central US this week to give you an idea of the 'straight line' of storms:

Like a hurricane, the red and orange areas are the really strong thunderstorms with lots of wind, the yellow/orange areas are torrential downpours with 'lighter winds', and then we get towards the green/blue end which is heavy rain etc.  

Derechos occur in central and eastern US, as well as in other parts of the world. In the US, for example, this year in early June one hit Pennsylvania and New Jersey that caused power outages. They are rarer in the west, although again in June of this year part of one hit Colorado. They usually occur sometime between April and August (usually). 

From that image above, you can see the derecho actually growing as it crossed Iowa - from something that was small in length, to something that grew to span the entire length of the state in just 5 hours. They are notoriously difficult to forecast because the understanding of these systems is not really in place - it's definitely an active area of research! <End Science Alert!>

That's all for today. 


Twitter: jyovianstorm

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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