Tropical Storm Isaac, Part 2: Isaac Moving Into The Gulf Of Mexico-Hurricane Warnings For Morgan City, LA/MS/AL & FL Panhandle To Destin, FL

[andyhb]

What's the minimum pressure?

949 mb.

[SN_Lover]

What's the minimum pressure?

at 850mb it's 950.3mb

[OKpowdah]

12z HWRF simulated IR product:

[Connor McCrorey]

at 850mb it's 950.3mb

Uh, what?

KBYX couldn't be in a better position for us to get a good look at Isaac's low level structure for the time being. The improvement may be slow, but it's happening, and that's all Isaac needs for the time being.

[Mallow]

But we've got other data suggesting barely a hurricane at landfall. I don't really think given all the potential issues at play that making an accurate forecast of landfall strength is possible. I'm not saying ignore it or anything but the strong TS seems to be every bit as likely a cat4 storm at this [time]...

Very much disagree. It's tough to imagine a scenario where Isaac is still a TS in 48 hours. Much easier to imagine a cat4 (though that's not the most likely scenario, either). I noticed in one of your earlier posts you had suggested that the gfs was only showing a ts at landfall... Perhaps, if you took pressure/wind at face value, it would be a strong ts/weak cat 1. But the gfs doesn't have the spatial resolution to get the inner dynamics correct, and as a result will always, without fail, underrepresent the strength of a strong cane. In other words, you can't take gfs pressure at face value for tropical cyclones.

So again, I'd argue that, regardless what the gfdl is saying, a ts at landfall is a very unlikely scenario. If I were to pull probability numbers out of thin air, I'd say something like...

TS: 10%

Cat1: 20%

Cat2: 25%

Cat3: 25%

Cat4: 15%

Cat5: 5%

[PennyForYourThoughts]

Very much disagree. It's tough to imagine a scenario where Isaac is still a TS in 48 hours. Much easier to imagine a cat4 (though that's not the most likely scenario, either). I noticed in one of your earlier posts you had suggested that the gfs was only showing a ts at landfall... Perhaps, if you took pressure/wind at face value, it would be a strong ts/weak cat 1. But the gfs doesn't have the spatial resolution to get the inner dynamics correct, and as a result will always, without fail, underrepresent the strength of a strong cane. In other words, you can't take gfs pressure at face value for tropical cyclones.

So again, I'd argue that, regardless what the gfdl is saying, a ts at landfall is a very unlikely scenario. If I were to pull probability numbers out of thin air, I'd say something like...

TS: 10%

Cat1: 20%

Cat2: 25%

Cat3: 25%

Cat4: 15%

Cat5: 5%

A question if you have time to answer it. AS you say you can't take the gfs pressure at face value...what about the Euro, is it the same with that model? Does it also under represent the pressure?

[Uncle Bobby]

Ship SSE of Key West reported wave height of 6.5m every 6 sec. That's pretty big for a TS, isn't it?

[andyhb]

A question if you have time to answer it. AS you say you can't take the gfs pressure at face value...what about the Euro, is it the same with that model? Does it also under represent the pressure?

The Euro has a higher resolution that the GFS, so it should be able to handle intensity forecasts a bit more proficiently than the GFS.

[OUGrad05]

Very much disagree. It's tough to imagine a scenario where Isaac is still a TS in 48 hours. Much easier to imagine a cat4 (though that's not the most likely scenario, either). I noticed in one of your earlier posts you had suggested that the gfs was only showing a ts at landfall... Perhaps, if you took pressure/wind at face value, it would be a strong ts/weak cat 1. But the gfs doesn't have the spatial resolution to get the inner dynamics correct, and as a result will always, without fail, underrepresent the strength of a strong cane. In other words, you can't take gfs pressure at face value for tropical cyclones.

So again, I'd argue that, regardless what the gfdl is saying, a ts at landfall is a very unlikely scenario. If I were to pull probability numbers out of thin air, I'd say something like...

TS: 10%

Cat1: 20%

Cat2: 25%

Cat3: 25%

Cat4: 15%

Cat5: 5%

Ahh ok. Thank you very much for a response to that. I've noticed the GFS has been wrong a LOT in the past with regard to intensity which is why I posted specifically about the spread between the models. And yes I did take the pressure and wind at face value, something I should probably quit doing with regard to tropical systems based on feedback from you and a friend of mine that works at NWS in FL.

Is the HWRF typically more accurate then GFS with regard to modeling the inner dynamics and thereby strength of tropical cyclones?

It's looking like the ULL will have less negative impact on the storm then I thought when I went to bed last night, what are your thoughts? I seen a bit of back forth both here and on a friends blog.

[Superstorm93]

While the HWRF and ECM are a bit concerning, but when taking into consideration what has happened over the past day, I think Isaac will continue to gradually intensify for the next 36 hours. Thereafter, we may see a slightly quicker ramp up until landfall. Right now, I'd stick with 90-100 knots at landfall.

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[Derecho!]

Ship SSE of Key West reported wave height of 6.5m every 6 sec. That's pretty big for a TS, isn't it?

Wave height is a function of wind speed, wind duration, and importantly, fetch - the distance the winds are blowing over. You'll see some bodacious waves because Isaac is so wide, the fetch is very large.

[dan88]

850mb extrap from recon down to 992.8mb

[Indystorm]

Ship SSE of Key West reported wave height of 6.5m every 6 sec. That's pretty big for a TS, isn't it?

Winds of 50-60 mph can produce 20 foot waves on Lake Michigan in our late fall/early winter storms so I guess this is on target.

[OKpowdah]

T numbers have been improving notably this morning and afternoon. CDO is becoming more symmetric near the center

----- Current Analysis -----
Date : 26 AUG 2012 Time : 174500 UTC
Lat : 24:15:30 N Lon : 81:26:31 W

CI# /Pressure/ Vmax
3.4 / 985.6mb/ 53.0kt

Final T# Adj T# Raw T#
3.4 3.7 4.0

I'm confused as to what you're referring to. Your initial post said "at 850mb it is 950.3mb" which makes no physical sense. Perhaps you could clarify what you meant?

One minute in this thread and I already have a headache.

[CUmet]

Interesting to note the accelerating outflow out of the convection in the NW quad, and some cirrus starting to emanate out of the south quad. The moderate southwesterly shear that was present yesterday and early today continues to diminish. This is evident also looking at the radar...the south quad is gradually filling in with more and stronger returns.

[DaculaWeather]

I saw the ship report of 21 feet with a 6 second dominate wave period and 25kt winds. 208 miles west of Naples the waves are about 8 feet with a dominate wave period of 8 seconds.

[Mallow]

Ahh ok. Thank you very much for a response to that. I've noticed the GFS has been wrong a LOT in the past with regard to intensity which is why I posted specifically about the spread between the models. And yes I did take the pressure and wind at face value, something I should probably quit doing with regard to tropical systems based on feedback from you and a friend of mine that works at NWS in FL.

Is the HWRF typically more accurate then GFS with regard to modeling the inner dynamics and thereby strength of tropical cyclones?

It's looking like the ULL will have less negative impact on the storm then I thought when I went to bed last night, what are your thoughts? I seen a bit of back forth both here and on a friends blog.

The HWRF is generally thought of as having a bias to make storms too strong, so although what it shows is possible, it's unlikely at this time.

A question if you have time to answer it. AS you say you can't take the gfs pressure at face value...what about the Euro, is it the same with that model? Does it also under represent the pressure?

The Euro has better resolution and generally seems to do pretty well with intensity, comparatively. But of course, intensity forecasts are extremely tough, regardless, so I'd hesitate to say you should always use any given model.

I wonder if these kinds of conversations might be better suited in the banter thread...

[PSUBlizzicane2007]

I'm having trouble rationalising in my mind the differences between the GFS and Euro models. Ever since last night's 18Z run, I have expected the GFS and the Euro to come back to better agreement...but it hasn't happened. Even though the 00Z Euro shifted west to a roughly Orange Beach landfall, the 12Z models showed the GFS go slightly to the west while the Euro maintained itself, if not an ever so slight shift closer to Pensacola. Of course the UKMET is in better agreement with the Euro, but trying to look at how the two/three models are developing and progressing the mid latitude weakness is quite frustrating, especially when the models are doing the complete opposite of their previous stereotypes.

Hopefully a clearer picture is formed by 00Z tonight.... but it seems like the best bet is for the entire coast from Morgan City to Destin needs to prepare as if the system will make landfall there.

[battlebrick]

dropsonde 992 mb 6 kts at surface.

[Geos]

Ship SSE of Key West reported wave height of 6.5m every 6 sec. That's pretty big for a TS, isn't it?

Winds of 50-60 mph can produce 20 foot waves on Lake Michigan in our late fall/early winter storms so I guess this is on target.

6.5m = 21feet. That sounds respectable for a storm with winds in the middle of the tropical storm scale. Yeah the Great Lakes gets those type of wave heights often enough between October and April.

[phil882]

1) These types of posts are probably more well suited for the banter thread. This thread is mostly for detailed explanations of the various areas of the forecast.

Yes. I'm leaving this up as an example, but additional questions and comments about the storm similar to this will probably be moved to the banter thread, so we can keep this thread clear for good meteorological discussion.

[David Reimer]

Not much change with the 18Z early-cycle guidance.

[HurricaneJosh]

Interesting to note the accelerating outflow out of the convection in the NW quad, and some cirrus starting to emanate out of the south quad. The moderate southwesterly shear that was present yesterday and early today continues to diminish. This is evident also looking at the radar...the south quad is gradually filling in with more and stronger returns.

CU, not to put you on the spot, but what is your take Re: the grand disagreement between the GFS and the Euro? Do you prefer one camp over the other, and what is your hunch Re: the eventual landfall point? (Don't worry-- I'm not even gonna ask about intensity. )

[Indystorm]

And the 18z intensity predictions have not changed much as well with many in the Category 1 to 2 range.

[CoastalWx]

I won't post the images..let people see it for themselves...but you can see the differences in the Euro vs GFS between 500-300mb and why the models do what they do when you look at the Wunderground maps. I use that as an example because it's a similar interface if you want to compare mid and upper levels. The Euro has heights ever so lower in the deep south with less of a branching ridge as compared to the GFS. I know it may not sound like much to the casual enthusiast, but in the tropical world...it means everything.

[PSUBlizzicane2007]

I won't post the images..let people see it for themselves...but you can see the differences in the Euro vs GFS between 500-300mb and why the models do what they do when you look at the Wunderground maps. I use that as an example because it's a similar interface if you want to compare mid and upper levels. The Euro has heights ever so lower in the deep south with less of a branching ridge as compared to the GFS. I know it may not sound like much to the casual enthusiast, but in the tropical world...it means everything.

Thanks for posting this... I keep forgetting about the wunderground maps. I was going off Euro/GFS plots with very erratic, difficult to read isopleths.

I still find it so weird that the two models are reverse of what they typically show. Does anyone have a link to or know what the latest full enKF experimental GFS shows?

[Sunny and Warm]

I won't post the images..let people see it for themselves...but you can see the differences in the Euro vs GFS between 500-300mb and why the models do what they do when you look at the Wunderground maps. I use that as an example because it's a similar interface if you want to compare mid and upper levels. The Euro has heights ever so lower in the deep south with less of a branching ridge as compared to the GFS. I know it may not sound like much to the casual enthusiast, but in the tropical world...it means everything.

Isn't that the opposite of what both models' tendencies are? GFS weaker with ridges versus the Euro. Given your experience (open to any Met here), which model do you believe is more correct in understanding the 500-300mb level for Isaac over the next 72 hours?

[CoastalWx]

Isn't that the opposite of what both models' tendencies are? GFS weaker with ridges versus the Euro. Given your experience (open to any Met here), which model do you believe is more correct in understanding the 500-300mb level for Isaac over the next 72 hours?

It is sort of the opposite...but models are models and they don't always have to behave the way we think they may. It's possible the euro and its resolution may be picking up the tendency for lower heights down that way, but that is just a WAG. I'm just getting back from being away, so perhaps someone else who has looked at this a lot more in detail can answer.

[PSUBlizzicane2007]

Isn't that the opposite of what both models' tendencies are? GFS weaker with ridges versus the Euro. Given your experience (open to any Met here), which model do you believe is more correct in understanding the 500-300mb level for Isaac over the next 72 hours?

Since the UKMET is more in line with the Euro, I would lean to the right side of the middle of the cone for now... (~eastern MS). Until we see more reliable models in line with the GFS solution, I feel the need to take it with a grain of salt being *that* far west.

[CoastalWx]

Euro ensembles at hr 66 have Isaac at the mouth of the MS delta in LA.