Sunday, August 27, 2017

Tropical Storm Harvey and the Florida Blobette: August 26, Update A

Hurricane Harvey and the Florida Blobette today means no rest for the ice cream eaters of the world. (thanks John S.)

Hurricane Harvey
I’m sure you saw footage and reports of the aftermath of the initial impact from Hurricane Harvey today… unfortunately there were at least two deaths, but remarkably that is all that has been reported (at least at this point) even though the destruction is quite widespread. A huge kudos to the Emergency Mangers and local officials and to those who evacuated in time or sheltered following their directions. And, having been in a post-storm location (as I know many of you have), I have to say that everyone who goes in to help turn the lights back on and put the chair back on its legs after any sort of disaster are truly amazing people – thank you!

But alas, Harvey is not done yet. He is at 29.3N, 97.4W and is essentially stationary as expected. His central pressure is 998mb and officially, his winds are estimated to be a mere 45mph, which makes him a very weak Tropical Storm (TS range: 39 – 73mph).  
His convection did decrease quickly and steadily during the day:
However, although the convection has decreased on the western side so there is little to no rain over the areas that were really hit yesterday (which is a good thing), you can see that there is a large area of red in this infrared satellite image over eastern Texas, including the Houston/Galveston area, and in Louisiana now which indicates they are experiencing strong thunderstorms and tornadoes. The waters in the Gulf just off Galveston are the warmest surface waters in the Gulf – over 31 deg C – so he has some energy to pull from there.

I am not sure I would really classify him as a Tropical Storm quite yet though. From the strong vorticity (circulation) at the lowest level (850mb),
mid-level (500mb),
and upper level of the troposphere (200mb),
I would actually classify him as a weak cat 1 hurricane because his circulation is very good at the lower levels of the troposphere, but more importantly because he still has that circulation (vorticity) at the upper levels of the troposphere. Until this upper level signal goes away, Harvey has shown us he is still a storm we shouldn’t underestimate. This means he can intensify if he is over the Gulf or interacting with it because that physical structure is still in place (hence the blooming convection over eastern Texas). It is not unusual for a strong storm like this to remain at hurricane strength for over 24 hours after landfall if it is relatively close to a warm water source. We have seen this a number of times in Florida as a cat 3 hits one side of the state and then works its way across and is still a cat 1 on the other side.

There is a lot of uncertainty in his track, with the NHC thinking he will remain near the Texas coast until Tuesday:

The reason for the uncertainty is because he is surrounded by high pressure on all sides and cannot move. But the longer he stays there, the more the pressure fields around him change and the more uncertain the forecast becomes. I can see the high pressure that is holding him in place, but unfortunately, due to funding or other issues, the pressure maps I used to have access to in the old days are now no longer being produced, so I am blind on the track front. Grrr. The NHC think there is a chance that he will remain on land, but they acknowledge that there is also a possibility that he will move back into the Gulf… the resources are not available to figure this one out to more than 1-2 days... from the NHC this evening:  
By the end of the forecast period, the system should be
far enough inland so that Harvey will again weaken.  An alternative
scenario is that Harvey could re-intensify if the center emerges
over the Gulf.

From Tides Online, most places along the north Texas/Louisiana coast are currently showing around 2ft of sea level above normal. The big exception is at the top of Galveston Bay (apologies, I don’t know the exact geography!), at the tide gauge called Manchester (on the Tides Online website), which has 7ft (!!) above normal!! It looks like the water is getting pushed in from the Bay to this nexus point, so this is to be expected:
I think that’s it for Harvey for today.

Florida Blobette
You can see this one as the area of circulation (red blobette) in the lowest level of the troposphere over Florida in the 850mb vorticity map above. It looks like there may be the start of a signal in the mid-troposphere (500mb) but it isn’t quite there yet. It looks like Florida got a bunch of rain and thundery weather today as well, in connection to this Blobette:
It is moving across the state and will emerge onto the Atlantic side tomorrow (27th) and that is when it will most likely have a chance to pull its socks up because it will be over a warm sea surface (temperatures of 29 deg C) and also over the Florida Current/Gulf Stream system, where the upper ~100m of the water column are warmer than 26 deg C. However, there is also quite a lot of wind shear in this area, so it will have a difficult time growing too much.

The next name is Irma.

Night for now!

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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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