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Tropical Storm Isaac, Part 1: Moving Through The Eastern Caribbean Sea


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ECMWF is still very weak with Invest 94L... If anything, I think its more likely the models will continue to trend further west with Invest 94L. I am still skeptical about the 500 hPa ridge breakdown as the upper level trough over the Eastern US is fairly weak and lifting out per the GFS. The main contribution towards the breakdown of the ridge is an TUTT feature located to the NE of the Bahamas which is currently located between 50-60W. The GFS expects this feature to build downward into the mid-levels as it breaks down the mid-level ridge.

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Why?

A stronger system would allow for it to turn northwestward more easily, as a stronger system would be able to "feel" any weakness in the ridge, since it would be the path of least resistence for the cyclone. Phil posted images of the Kelvin Wave over 94L, which should be enough for it to spin up into a Tropical Cyclone.

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A stronger system would allow for it to turn northwestward more easily, as a stronger system would be able to "feel" any weakness in the ridge, since it would be the path of least resistence for the cyclone. Phil posted images of the Kelvin Wave over 94L, which should be enough for it to spin up into a Tropical Cyclone.

I didn't know tropical systems had nervous systems.

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As I learend from Irene models suck with the track of a TC before it even forms!!

If Heights build in the east central atlantic early recurve is favored with no troff necessary. It is also very important how far south the storm is when it crosses the winwards.

Edit. Looking at the 12z GFS I would expect the storm to turn north @120hrs near PR.

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I didn't know tropical systems had nervous systems.

The GFS curving it up along the East Coast is due to a Weakness in the Ridge. The weakness generally has lower pressure than the surrounding regions, and is able to influence a stronger Tropical Cyclone, moreso than a weak Tropical Cyclone. The weakness in the ridge on the GFS is what brought 94L up and along the East Coast in the first place.

Note that there is an "L" off the East Coast. This doesn't mean that there is a storm off the east coast, since the pressures are very high to be an area of anomalously low pressure (generally standard pressure is around 1012-1014 mb). But it is the path of least resistence for the cyclone. Low and behold, the Cyclone takes a path along the East Coast where the area of lower pressures in the ridge is.

12zgfs500mbHGHTPMSLtropical168.gif

12zgfs500mbHGHTPMSLtropical300.gif

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A stronger system would allow for it to turn northwestward more easily, as a stronger system would be able to "feel" any weakness in the ridge, since it would be the path of least resistence for the cyclone. Phil posted images of the Kelvin Wave over 94L, which should be enough for it to spin up into a Tropical Cyclone.

While I'm still in support of this system becoming a tropical cyclone over the next couple of days, I don't think this system will rapidly organize. Moreover, the ridge north of the system is very strong and continues to trend stronger. Whether or not the storm "feels" the weakness is often dependent on how strong the vortex is in the mid-levels. However, If the mid-level ridge north of 94L remains strong, then it won't matter how strong 94L gets.

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While I'm still in support of this system becoming a tropical cyclone over the next couple of days, I don't think this system will rapidly organize. Moreover, the ridge north of the system is very strong and continues to trend stronger. Whether or not the storm "feels" the weakness is often dependent on how strong the vortex is in the mid-levels. However, If the mid-level ridge north of 94L remains strong, then it won't matter how strong 94L gets.

Thank you for your input Phil.

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Too bad the CMC tends to spin up spurious cyclones. It develops a cyclone dropping down from the mid-latitudes that clearly influences the track of 94L. It'd have been nice to have non GFS/non Euro global guidance.

UK Met is sort of close to the GFS. If it doesn't get classified until 45º not sure it'd be considered a true CV long tracker, for those interested in semantics. It just looks like it'll get upgraded before that on satellite. I'm told UK Met intensity descriptions are based on manual interpretation of 850 mb vorticity. Someone will correct me if that is wrong.

NEW TROPICAL STORM FORECAST TO DEVELOP AFTER 60 HOURS

FORECAST POSITION AT T+ 60 : 16.3N 45.0W

VERIFYING TIME POSITION STRENGTH TENDENCY

-------------- -------- -------- --------

00UTC 21.08.2012 16.3N 45.0W WEAK

12UTC 21.08.2012 16.2N 48.8W WEAK INTENSIFYING SLIGHTLY

00UTC 22.08.2012 16.0N 51.8W WEAK LITTLE CHANGE

12UTC 22.08.2012 16.0N 53.7W MODERATE INTENSIFYING SLIGHTLY

00UTC 23.08.2012 16.6N 55.5W MODERATE LITTLE CHANGE

12UTC 23.08.2012 17.5N 58.6W MODERATE LITTLE CHANGE

00UTC 24.08.2012 17.7N 61.4W STRONG INTENSIFYING SLIGHTLY

12UTC 24.08.2012 18.7N 64.3W STRONG LITTLE CHANGE

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Some really surprising differences on the 18z GFS.

Yesterday, this was a 100 knot system in the eastern Caribbean. On todays run, it's maybe a 50-60 knot system moving very close to "The Shredder"

Forward motion of 20mph+ could screw yet another system.

It's the 18Z GFS stock not invested. lol

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Some really surprising differences on the 18z GFS.

Yesterday, this was a 100 knot system in the eastern Caribbean. On todays run, it's maybe a 50-60 knot system moving very close to "The Shredder"

Forward motion of 20mph+ could screw yet another system.

That is not surprising. Uh uh. Nothing close to what major windshield wiping can do to the mind.

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Death by land interaction. Disorganized vort mess over Cuba by 180 hours.

I'd like to see if this continues into the other runs.

Quite the difference from the Major Fish that everyone was complaining about three days ago. :whistle:

This is why (as I am sure you well know) all the hyperventilating about track and even strength beyond several days is ridiculous, and this is the time of year when it reaches its climax, and then everyone remembers again all the systems that they have seen over the years that have been shown as fish and ended up in the Caribbean, and the many more that did the vice versa.

A quick look at satellite and the models and what is going on - my key points:

* Even though the op ECMWF does not have much of a system, the ECENS mean (all I have) seems to imply that there are a number of members that do have a robust system that lie anywhere from just north of Hispaniola down into the eastern Caribbean out about 5-7 days. This combined with the fact that is has had a number of bullish runs would make me think that some development is still pretty likely based on the EC suite over the last few days.

* This is supported by the fact that all of the other globals, including the UK, are relatively bullish with some development. The strong turning on satellite currently and lack of any obvious shear in the models would seem to support eventual development as well.

* The TUTT low near 25N, 55W could be a concern if the center consolidates around 15N and the TUTT doesn't get out of the way as the GFS shows it does.

* The strong ridging in all of the models over the central Atlantic and the trends toward less rapid development and a more westerly motion would imply that at a minimum the Antilles will be under a risk from whatever comes from 94L.

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FWIW, Death by land interaction. Disorganized vort mess over Cuba by 180 hours.

I'd like to see if this continues into the other runs. Somewhat similar to the ECMWF, but farther north.

Quite the difference from the Major Fish that everyone was complaining about three days ago. :whistle:

We have to remember at this range, track errors can be large. Those islands are not all that wide. All it has to do is miss by a little, and BOOM this thing could be going crazy over water!

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^^ WRT the last two posts, I agree 100%.

Once this system actually forms a decent/sustainable circulation, the globals are trivial. Still a ton up in the air about this one. Heck, just a few days ago this looked like east of Bermuda fish food. Now we have models showing this affecting the Greater Antilles while moving WNW. Should be an interesting system regardless of track.

18z SHIP/LGEM are still pretty bullish.

atlsfc72_latestBW.gif

----------------------------------------------------------------

18z coordinates had this at 13N, 32W (roughly)

Might have something consolidating much farther north if my eyes aren't too out of whack.

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This is why (as I am sure you well know) all the hyperventilating about track and even strength beyond several days is ridiculous, and this is the time of year when it reaches its climax, and then everyone remembers again all the systems that they have seen over the years that have been shown as fish and ended up in the Caribbean, and the many more that did the vice versa.

A quick look at satellite and the models and what is going on - my key points:

* Even though the op ECMWF does not have much of a system, the ECENS mean (all I have) seems to imply that there are a number of members that do have a robust system that lie anywhere from just north of Hispaniola down into the eastern Caribbean out about 5-7 days. This combined with the fact that is has had a number of bullish runs would make me think that some development is still pretty likely based on the EC suite over the last few days.

* This is supported by the fact that all of the other globals, including the UK, are relatively bullish with some development. The strong turning on satellite currently and lack of any obvious shear in the models would seem to support eventual development as well.

* The TUTT low near 25N, 55W could be a concern if the center consolidates around 15N and the TUTT doesn't get out of the way as the GFS shows it does.

* The strong ridging in all of the models over the central Atlantic and the trends toward less rapid development and a more westerly motion would imply that at a minimum the Antilles will be under a risk from whatever comes from 94L.

I agree with everything in this post. The track beyond 120 is highly uncertain at this point, and the key feature I think that will play a key role is the TUTT low you have mentioned.

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This is why (as I am sure you well know) all the hyperventilating about track and even strength beyond several days is ridiculous, and this is the time of year when it reaches its climax, and then everyone remembers again all the systems that they have seen over the years that have been shown as fish and ended up in the Caribbean, and the many more that did the vice versa.

A quick look at satellite and the models and what is going on - my key points:

* Even though the op ECMWF does not have much of a system, the ECENS mean (all I have) seems to imply that there are a number of members that do have a robust system that lie anywhere from just north of Hispaniola down into the eastern Caribbean out about 5-7 days. This combined with the fact that is has had a number of bullish runs would make me think that some development is still pretty likely based on the EC suite over the last few days.

* This is supported by the fact that all of the other globals, including the UK, are relatively bullish with some development. The strong turning on satellite currently and lack of any obvious shear in the models would seem to support eventual development as well.

* The TUTT low near 25N, 55W could be a concern if the center consolidates around 15N and the TUTT doesn't get out of the way as the GFS shows it does.

* The strong ridging in all of the models over the central Atlantic and the trends toward less rapid development and a more westerly motion would imply that at a minimum the Antilles will be under a risk from whatever comes from 94L.

Great post-- an informative bird's-eye view of the situation. You're right-- we do get really stuck on the run-to-run details rather than looking at the broadstrokes and broad trends.

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