Monday, August 16, 2021

Tropical Storm Fred, Tropical Depression Grace, and Tropical Depression Eight: 15 August, Update A

Well I guess the 'relaxing' summer holiday season is over; there are three stormy systems to chat about today, plus Bermuda's 'force-field' :-), so I think I'll just jump right in. 

Tropical Storm Fred

Just like Freddie Kruger (today's Famous Fred - there are a lot of them!), our Tropical Storm Fred is back! Hopefully he won't be quite the same nightmare though. 

He is squarely in the Gulf of Mexico and is currently at 27.3N, 85.9W, heading NNW at 9mph towards the Florida panhandle:

Fred is still a relatively weak Tropical Storm, with official winds of 50mph, central pressure of 999mb (TS range: 39-73mph). It looks like he is going over the eastern edge of the Loop Current (deep warm water), which is contributing to convection we now see in the satellite imagery:

Wind shear has actually also decreased, although there is some which is why that convection is mostly to the western side of the storm. Both the warm water and slightly lower wind shear have allowed him to regain some strength. Looking at his vorticity (circulation) maps, at the lowest levels of the troposphere (850mb) there is a pretty good signal:

But at the mid-level of the troposphere (500mb), his circulation is not quite as strong or well developed as I would expect to see in a proper Tropical Storm: 

Based on this, I am not sure about the intensity, but the NHC sent a plane into the system and this is what it is reporting, so we will go with that. The only record of wind speed that I can see is from satellites... 

<Technical Alert!> Satellite Winds: We can collect some wind data from satellites, and a particularly useful one is from the Advanced Scatterometer (ASCAT) sensor on board the EUMETSAT METOP satellite:

The wind 'barbs' and colours indicate the direction and speed. These winds are estimated to be at 10m above sea level and you can clearly see the winds from Tropical Storm Fred as they are circling around the center. The different colours give the wind speed - the colour bar is at the top right of this image. The black wind 'barbs' are where they are not sure of the wind measurements - there may be errors in the data. 

What this data is showing is that when TS Fred was in this location, the strongest wind speed was around 40 knots (there is one wind barb in the purple colour), which equals around 46 mph - so in one area of the storm (as far as we know), the winds are close to the intensity of 50mph. The rest of the storm is generally weaker, with the bulk of the strongest winds (around 30-35knots = 34.5 - 40 mph) in the northeast quadrant of the storm.

The reason the entire map isn't covered in winds and why there are bands of winds interspersed with blank areas is because the satellite is in orbit around a rotating planet (ours... in case you were wondering ;-)) and on each pass it can only collect information over a certain width (called the 'swath' width). As the planet turns, the satellite covers a slightly different region on each pass. 

On each orbit there is an ascending pass and a descending pass. The ascending pass is when the satellite is traveling north towards the north pole, and the descending pass is when the satellite is traveling south towards the south pole - so each one orbit around the planet has an ascending and descending set of data. The ASCAT ascending passes map for the entire world at the time I grabbed the image above looks like this: 

And the ASCAT descending passes map looks like this: 

You can see the different passes of the satellite quite clearly from these two maps. I got this data from NOAA/NESDIS Center for Satellite Applications and Research (STAR). If you check each day, you will see different parts of the world have data or are blank. To get the 'zoomed in' map that I grabbed for Fred, just click on the part of the world you are interested in - super easy! But data will not be available for all parts every day... today, it just happened that the ASCAT winds were available for Fred. <End Technical Alert!>

Back to Fred... he will make landfall within 24 hours (somewhere in that Cone of Uncertainty) so there won't be much room for too much intensification (the NHC think he will be at 60mph at landfall - maybe because of the thundery weather, but he will be moving away from the Loop Current as he gets closer to the coast so it won't get beyond that). However there is a lot of convection at the moment, so he'll be more of a water event (rain and some thundery weather over Florida and Georgia) than a wind event. If you want to check water levels along the coast, there is the NOAA TidesandCurrents website - the <Technical Alert!> in this link gives a guide on how to use it. 

And of course, even though he is a relatively weak TS, your local emergency managers will have the best local information so please listen to them! 

Tropical Depression Grace

Meanwhile, in the Caribbean, we still have Tropical Depression Grace. She is currently at 17.3N, 68.6W, heading W at 15mph. She has slowed down but also got weaker, which is good news for the islands - and especially Haiti! She has been staying on the southern edge of that Cone of Uncertainty and her track has been shifted south in response to this:

Winds are 35mph, central pressure is 1011mb (TD range: 17-38mph). I am not sure she is quite this weak given her strong vorticity (circulation) throughout the lower half of the troposphere - I think she may still be a full fledged Tropical Storm - albeit a weak one. But regardless, she has a lot of convection and, like Fred, water will be a bigger issue than wind:

There is some wind shear which is pushing that convection to the southeast of her center. Her intensity depends of course on her path - if she continues to stay on the southern edge of that Cone (and there is a good possibility of this), she won't interact as much with Hispaniola and will be able to increase in intensity. She is expected to make landfall on that island in the next few hours. Her long term intensity forecast is all over the place because it is difficult to know about the land interaction. 

Tropical Depression Eight

This will be the future Henri. He formed just northeast of Bermuda today - you can see him in the satellite image above, northeast of Grace. I have to say, I am very impressed with Bermuda's force-field - are you all standing on the seawall blowing him away? ;-) He is currently at 33.2N, 62.7W, heading S at 7mph. It looks like he is skirting Bermuda - the convection from satellite imagery is shows this, but his forecast track is also expected to do this:

Winds are currently 30mph, central pressure is high for a storm at 1014mb. He doesn't have too much convection yet, and his vertical circulation structure is not as robust as Grace's (from the vorticity maps above) in the mid-levels of the Troposphere, so I agree with the Tropical Depression status.

And now you know all about them, here are his winds from ASCAT: 

It looks like the strongest are around 20knots = 23mph, so although not as strong as the official estimate, it is still within the Tropical Depression range.

That's all for today methinks. Be safe, be good out there! 

Ciao for now,




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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