Hurricane Harvey: August 24, Update A

Harvey is no longer the little pipsqueak that he was this time yesterday. He definitely grew into a Cat 1 hurricane today, and the NHC have him growing into a category 3 major storm so no time for dilly-dallying now!

He is currently centered at 25.5N, 94.9W and heading NW at 10mph. His central pressure is 967mb, and winds are an estimated 100mph (from a plane), which makes him a weak cat 2 storm (cat 2 range: 96-110 mph). For most of today his winds were around 85mph, meaning he was a mid-size category 1 hurricane (cat 1 range: 74-95mph) - including in the previous advisory from the NHC. So this is a remarkable jump in 2 hours... something seems a little amiss to me - not so much on the wind speed (although I think it may be an over-estimate at this time), but that the NHC put out something so different in such a short time! 

He has actually been fluctuating throughout today and earlier today he did have a clearly visible eye, which indicates his winds got up to around 95+ mph, but since then he did weaken and although he has an eye it is very murky, suggesting he dropped down to around 90 mph winds again:

I think he is actually now fluctuating around 95 mph winds because the eye is not very clear, and also because the convection is not as strong as it was earlier (there are no dark grey parts, which would indicate that he is quite a tall storm):

One thing we agree on is that he is definitely Hurricane Harvey. We can see this if we look at his structure in the circulation (vorticity) maps. At the lowest level of the troposphere (850mb), it is really strong:

There is also a pretty strong signal in the mid-troposphere (500mb):

And because there is a signal at the upper troposphere (200mb), which wasn’t there yesterday, it shows us that he is definitely a hurricane:

Looking at the forecast…

The track: He did shift a little to the right of yesterday’s track, but overall, from the NHC forecast it looks like a southern Texas landfall on Friday night/Sat morning, currently maybe just north of Corpus Christi:

I agree it’s in that general vicinity – maybe a little south of Corpus Christi, but as we are about 24-36 hours out from landfall, I’m going with the NHC track from here on out. It looks like once he has made landfall, he’s going to essentially stall again because high pressure is building up in front and to each side, so he can’t really move too much.

The intensity: The NHC had upgraded his forecast from a cat 1 to a cat 3 earlier today, with him becoming a cat 3 in a few hours. They continue to forecast a cat 3 at landfall. But looking at the ocean and atmosphere conditions, if anything he should show a steady increase in intensity, not a large jump... and it is especially odd, given that his convection is weaker now than earlier today. Let’s look at the data.

First, the ocean sea surface is still around 29-30 deg C in this part of the Gulf. The upper ~70m of the ocean is warmer than 26 deg C, so that also has not changed much in the path that he travelled along today so that isn’t a big reason for any jumps. It would allow him to intensify, but not so quickly. 

And the atmosphere… it looks like he passed through an area of low wind shear earlier today (hence getting stronger) and is now moving into an area of higher wind shear - which is why he is looking a bit weaker; we can see the shear in the satellite images because the clouds are asymmetrical with more on the northeast side. It looks like this shear will remain for a few hours, if not for the rest of tomorrow and through to landfall. This should be a factor in slowing his growth a bit.  

He is also going to start interacting with land soon – his outer bands are already there. Given all of this, I am not as convinced he is going to be a cat 3 at landfall as the NHC, but dear readers, I hate to have to tell you this however I have been wrong before (yes, even me!!! ;-)) and given that he is approaching land, anyone in his path should really listen to their local emergency managers, the NHC official forecast, and those who know what’s what locally.

For those along the coast who are watching sea levels, from the Tides Online website (data from TCOON – the Texas Coastal Ocean Observing Network) you can see that the sea level at the moment is almost 1.5 feet above normal at Galveston Bay and around 1 foot above normal at Corpus Christi and South Padre Island.

<Technical Alert!> Winds move in a counter-clockwise direction around a low pressure system, which is what a Tropical Storm or Hurricane is, so sea level will rise to the north of the center and will drop to the south of the center – just because the winds are pushing the water towards or away from the coast. Conveniently, from Tides Online you can also look at the actual wind measurements along the coast – currently they are around 23 mph... not Tropical Storm strength yet.

Another great source for offshore data in this area is GCOOS– the Gulf of Mexico Coastal Ocean Observing System. This is part of a national network of offshore data and a pretty invaluable network for you if you want to know what shenanigans the storms get up to before they get to you. I went to their data portal (which includes data from many sources) http://data.gcoos.org/fullView.php and clicked on a mooring (a green pin drop) under the ‘green’ part of the storm radar map that is overlaid.  Then I clicked on the type of data I wanted to see on the right side – of course I chose winds. A note that for winds, the data is in m/s, not mph, so you will have to do the conversion (luckily google can do that for you! J). Looking at winds off the southern Texas coast, it currently shows a reading of just over 8.4 m/s, which is around 18mph… so at least in this location at the moment, the winds are not yet tropical storm level:

So to summarize – I agree with the NHC that Hurricane Harvey is heading for southern Texas, near Corpus Christi, but I am not in total agreement with them over the intensity forecast. However, in addition to stocking up on water, wine, ice cream, flashlights, more ice cream etc… Please please pay attention to your local emergency managers! They know what is best locally and have the best available local data. <End Technical Alert!>

Stay safe out there! 

More tomorrow,

J.

Blogs archived at http://jyotikastorms.blogspot.com/
Twitter @JyovianStorm
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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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