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

[toad strangler]

I assume that recon data will be injested for 0z runs. Calibration if you will.

[turtlehurricane]

I summarized my current forecast thoughts on my blog: http://weather.schematical.com/

Basically not taking either side yet on whether this hits Hispaniola and/or Western Cuba and is significantly disrupted, or slides to the south unharmed. Even though I said earlier in this thread I think there's a >50% chance it hits Hispaniola, it really is too early to say that definitively, small changes in the sub-tropical high will make the difference. Hispaniola sometimes isn't a death blow as well. For now this will be a very interesting storm for the Lesser Antilles westwards through at least Hispaniola.

[wxsmwhrms]

It must be difficult for NHC with the only two models showing a strong system in the next 5 days (CMC and NOGAPS) being so unreliable and both showing a strong right turn--out to sea even--whereas the reliable global models keep the storm relatively weak for 5 days, which is possible but seems unlikely given the structure of the storm and the good conditions. No, I'm not predicting it will come to CHS! My money is on a brush with FL and then out to sea or perhaps an OB brush, i.e., the climo favored route. I think the NHC forecast is quite reasonable. I'm just troubled by the lack of model guidance showing the deep system that I think is likely and by the fact that BAM deep, which has been pretty accurate lately, is pretty far right of GFS despite using GFS data.

Please stop. There is so much wrong with this post I don't even know where to start. First off, the global models DO NOT keep the storm weak for 5 days. You cannot look at a global model and just look at the MSLP and say it has a weak system. Both the GFS and ECMWF show a dramatic increase in vorticity in the lower and middle levels, and have actually both become more aggressive with this and better aligned in their vertical structure. That is what is important as far as the track, that they have a deep system.

Secondly, assuming the first few days of the forecast happen, a brush with FL and then passing the OBX no longer becomes a climatological favored track. If it was moving WNW or NW now and going to pass north of the islands, then it would be. But if it makes it to Hispaniola south of it or near the south coast, then that's not a climate favored track.

Finally, please do not talk about the BAMD as a pretty accurate model.

[phil882]

T 3.5 I'd guesstimate with this simple chart (I wish I still had my Dvorak manual to make really accurate measurements), real T numbers always lag though so it's at T2.5 right now according to ADT, which is the way it should be since the maturity of cloud structures tends to precede intensity. Another indicator we might be on our way to a strong TS if trends hold.

Nice diagram. Some of the more recent satellite estimates also incorporate microwave imagery, which is useful since the cloud pattern on Infrared can be deceiving. Take for instance the large burst that occurred over Isaac.

Instead of the giant blow up representing a developing inner core of convection wrapping around the center, it just represents a singular hot tower that fired near the center of circulation. This suggests that the storm is not really organizing, rather the shear vector is enhancing convergence on the down-shear side which produces hot towers that fall apart quickly as they rotate to the up-shear side of the storm.

[EasternUSWX]

Adt up to 39kts 2.7T

[HurricaneFrances04]

18Z GFDL coming in now. Much stronger than the 12Z.

[phil882]

Instead of the giant blow up representing a developing inner core of convection wrapping around the center, it just represents a singular hot tower that fired near the center of circulation. This suggests that the storm is not really organizing, rather the shear vector is enhancing convergence on the down-shear side which produces hot towers that fall apart quickly as they rotate to the up-shear side of the storm.

Indeed, here is a self confirmation of this expectation. The hot tower that went up couldn't sustain itself and has collapsed leaving the low-level circulation devoid of convection. This shows why microwave imagery is a very important tool in determining the intensity of tropical cyclones, because based on the IR presentation alone, you can't tell if a storm is wrapping convection around its center, or if its just producing hot towers that create the appearance of a more organized circulation center than in reality.

[HurricaneJosh]

I like how various models are now starting to show a deep cyclone. All I'm saying is it better miss Shredderola or the kingdom is lost (or in jeopardy of being lost).

[Hurriplane]

18Z GFDL coming in now. Much stronger than the 12Z.

It goes over the mountains as if it never existed.

[turtlehurricane]

Nice diagram. Some of the more recent satellite estimates also incorporate microwave imagery, which is useful since the cloud pattern on Infrared can be deceiving. Take for instance the large burst that occurred over Isaac.

Instead of the giant blow up representing a developing inner core of convection wrapping around the center, it just represents a singular hot tower that fired near the center of circulation. This suggests that the storm is not really organizing, rather the shear vector is enhancing convergence on the down-shear side which produces hot towers that fall apart quickly as they rotate to the up-shear side of the storm.

Good call, the way that looked on vis was deceiving. A good reminder to always use microwave to confirm trends on infrared/vis.

Side rant which is not aimed at you: The word hot tower should be thrown out already, it's unnecessary and ambiguous. Deep convection is deep convection, there's no reason to call certain thunderstorms that have overshooting tops "hot towers", they're not particularly hot or anything (slight LHR max, which is not significant relative to the actual warm core of the TC), and what sort of deep convection isn't in the form of a tower? The term was originally meant to describe particularly strong deep convective complexes during TC genesis (prior to tight circulation formation) which have the embryonic warm core vortex, which makes sense in that case but it's still jargon. Then people started using it for every deep thunderstorm in or near the TC inner core since it's a fun word to say.

[phil882]

Side rant which is not aimed at you: The word hot tower should be thrown out already, it's unnecessary and ambiguous. Deep convection is deep convection, there's no reason to call certain thunderstorms that have overshooting tops "hot towers", they're not particularly hot or anything (slight LHR max, which is not significant relative to the actual warm core of the TC), and what sort of deep convection isn't in the form of a tower? The term was originally meant to describe particularly strong deep convective complexes during TC genesis (prior to tight circulation formation) which have the embryonic warm core vortex, which makes sense in that case but it's still jargon. Then people started using it for every deep thunderstorm in or near the TC inner core since it's a fun word to say.

I generally agree with you, both that its definitely overused, but its still a fun word to use

[am19psu]

It must be difficult for NHC with the only two models showing a strong system in the next 5 days (CMC and NOGAPS) being so unreliable and both showing a strong right turn--out to sea even--whereas the reliable global models keep the storm relatively weak for 5 days, which is possible but seems unlikely given the structure of the storm and the good conditions. No, I'm not predicting it will come to CHS! My money is on a brush with FL and then out to sea or perhaps an OB brush, i.e., the climo favored route. I think the NHC forecast is quite reasonable. I'm just troubled by the lack of model guidance showing the deep system that I think is likely and by the fact that BAM deep, which has been pretty accurate lately, is pretty far right of GFS despite using GFS data.

This is a good example of a post that should be made in the banter thread. In the future, this post will be deleted and if it continues, the poster will be restricted.

[toad strangler]

Looks like half of GFS ensemble members split with the Euro op?

[Eyewall2005]

18z GFDL..

[Amped]

Nice diagram. Some of the more recent satellite estimates also incorporate microwave imagery, which is useful since the cloud pattern on Infrared can be deceiving. Take for instance the large burst that occurred over Isaac.

Instead of the giant blow up representing a developing inner core of convection wrapping around the center, it just represents a singular hot tower that fired near the center of circulation. This suggests that the storm is not really organizing, rather the shear vector is enhancing convergence on the down-shear side which produces hot towers that fall apart quickly as they rotate to the up-shear side of the storm.

I agree. IR satellite just doesn't tell the whole story. It captures cloud debris not clouds. The current shear would not effect a hurricane that had a stronger mid level circulation. Isaac is still to weak at the mid levels to hold onto it's convection and wrap in the outer bands, much like Earnesto was.

[DopplerWx]

What are the implications on the track (near term) if she strengthens a decent bit tonight during dmax?

[icebreaker5221]

Saw a good amount of discussion about the ECMWF ensemble, but didn't see it posted anywhere:

[thewxmann]

All model tracks aside, people need to realize that though there is a high chance of this impacting Hispaniola and Cuba, it's not a death blow. It could mean Isaac will never become a major hurricane, but as far as I know Hispaniola alone has never actually killed a TC.

[Eyewall2005]

00z models clustered around south florida

[wxsmwhrms]

I like how various models are now starting to show a deep cyclone. All i'm saying is it better miss Shredderola or the kingdom is lost (or in jeopardy of being lost).

Yes, definitely a notable trend. And after looking at some more stuff I want to say in response to my own earlier post that after looking at a few more things, I am less surprised by the NHC forecast being as far to the right as it is. The UK and the GFS are definitely along and even to the right of the current forecast at days 4-5, and while I have a definite bias based on long experience of leaning toward the ECMWF, I can see why they are hesitant to go as far left as it would imply. There is still a fair amount of uncertainty as to when exactly the more poleward motion will begin, and obviously you will be hoping it doesn't start until it's well past The Island that Shall Not be Named.

[wxsmwhrms]

All model tracks aside, people need to realize that though there is a high chance of this impacting Hispaniola and Cuba, it's not a death blow. It could mean Isaac will never become a major hurricane, but as far as I know Hispaniola alone has never actually killed a TC.

Absolutely. As I believe was discussed earlier in this thread, many notable TCs that went on to have significant impacts in the US impacted one or both.

[thewxmann]

Looking at the vorticity maps, I believe that the differences in the ECM and GFS may be attributed to how they handle the vorticity of Isaac upon interaction with Hispaniola. It appears as if the GFS keeps the main vortex dominant, whereas the ECM develops a lee cyclone (as explained by Phil earlier) and takes that to be dominant.

[SnowGoose69]

00z models clustered around south florida

Thats likely great news for anyone in Florida, if the storm took that sort of track it would likely at worst be a strong tropical storm upon arrival, there's a heck of alot of land trajectory over Hispaniola and Cuba there.

[HurricaneJosh]

Thats likely great news for anyone in Florida, if the storm took that sort of track it would likely at worst be a strong tropical storm upon arrival, there's a heck of alot of land trajectory over Hispaniola and Cuba there.

Not necessarily. David 1979 took the Shredderola-to-Miami route and arrived with winds of 85 kt. No, it wan't the Cat 5 it had been, but it was still a significant hurricane.

As wxsmwhrms and others have pointed out, there are plenty of examples of post-Shredderola cyclones hitting the USA hard.

[SnowGoose69]

Not necessarily. David 1979 took the Shredderola-to-Miami route and arrived with winds of 85 kt. No, it wan't the Cat 5 it had been, but it was still a significant hurricane.

As wxsmwhrms and others have pointed out, there are plenty of examples of post-Shredderola cyclone hitting the USA hard.

Georges also had massive land interaction down there, it took almost exactly the worst track for a storm to take as far as it becoming strong nearly crossing all of the DR and a good part of Cuba.

[wxsmwhrms]

Not necessarily. David 1979 took the Shredderola-to-Miami route and arrived with winds of 85 kt. No, it wan't the Cat 5 it had been, but it was still a significant hurricane.

As wxsmwhrms and others have pointed out, there are plenty of examples of post-Shredderola cyclone hitting the USA hard.

Thanks Josh for posting this. We go through this every year it seems. Nobody is going to argue that Hispaniola is a positive for tropical cyclones (LOL) but there are plenty of examples of systems becoming potent systems, and even major hurricanes, after significant interaction with Hispaniola. It depends on what the structure was like before arrival, how much it interacts with island, and the environment and time over water after. The models right now seem to think that the environment will be pretty favorable after passing Hispaniola.

[Gwhizkids]

Looks like half of GFS ensemble members split with the Euro op?

The operational track is the State of Florida's worst nightmare. Three major urban areas on the right side of the track.

[wxsmwhrms]

The operational track is the State of Florida's worst nightmare. Three major urban areas on the right side of the track.

Yes, and it somehow manages to avoid the worst of both Hisp and Cuba, LOL.

[Superstorm93]

Still in the structural development stages. Looks to be trying to expand a weak area of centralized convection with some disorganized banding.

ULAC still appears to be slightly displaced to the west of the LLC.

Not expecting any major changes in intensity for the next 12 hours. Still has to vertically align and maintain deep convection.

SHIPS RI index is a bit high though:

Prob of RI for 25 kt RI threshold=    52% is   4.1 times the sample mean(12.8%)
Prob of RI for 30 kt RI threshold=    32% is   3.8 times the sample mean( 8.4%)
Prob of RI for 35 kt RI threshold=    19% is   3.8 times the sample mean( 5.0%)

[icebreaker5221]

Still in the structural development stages. Looks to be trying to expand a weak area of centralized convection with some disorganized banding.

ULAC still appears to be slightly displaced to the west of the LLC.

Not expecting any major changes in intensity for the next 12 hours. Still has to vertically align and maintain deep convection.

SHIPS RI index is a bit high though:

Prob of RI for 25 kt RI threshold= 52% is 4.1 times the sample mean(12.8%)
Prob of RI for 30 kt RI threshold= 32% is 3.8 times the sample mean( 8.4%)
Prob of RI for 35 kt RI threshold= 19% is 3.8 times the sample mean( 5.0%)

Yeah, meaning that if we had, at this point, an organized 35 kt system, the prospects would be fairly good for RI. But the RI parameter doesn't know about mis-aligned vortices.