ATL: ISAAC - Post-Tropical - Discussion

[wxwatcher1999]

I guess what I'm asking is what is causing this to turn north and not go into the gom?

[msbee]

Careful down there, Gusty!

[WeatherOrKnot]

Recon is on it's way. Left at 11:30am (est).

[Sanibel]

Could be my worst nightmare unfolding. I think this one might break the rule and zone close to track many days out. Sanibel has had many historic storms that swept surge across the island in its history. I don't like the looks of this one. Pray the "If you are in the crosshairs 5 days out you are safe" rule holds. Looks like somebody's going to get this one.

[SouthFLTropics]

Recon is on it's way. Left at 11:30am (est).

I'm assuming they are flying out of St. Croix and not Biloxi...Would be a long flight from Biloxi.

SFT

[artist]

I guess what I'm asking is what is causing this to turn north and not go into the gom?

cool/cold fronts that are going to be pushing south thus the timing of that is what will help determine exactly where it might go as well as if it becomes a hurricane then they tend to be pulled poleward. Hope that helps a little.

edited to add the strength of the front will also be a determining factor.

[WeatherOrKnot]

Recon is on it's way. Left at 11:30am (est).

I'm assuming they are flying out of St. Croix and not Biloxi...Would be a long flight from Biloxi.

SFT

Yes, St. Croix. 2.5 hour flight.

[Aric Dunn]

its passing very slightly north of this buoy right now. wsw wind.

http://www.ndbc.noaa.gov/station_page.php?station=41040

[Gustywind]

Careful down there, Gusty!

You're right. I have made some preparations in case of a usual. Be aware too Msbee, we never know consequentlu you know better than me how the things can turn quickly in the deep tropics. So let's monitor extremely carefully the situation.

[robbielyn]

looking at visible looks like its dragging the storm behind it like its hold its hand could have an impact on this whole thing. although its looking like its trying to let go upper plume of clouds you'll see wht i am talking about. maybe will absorb it?

[redneck51]

http://www.playcofi.net/weather

You forgot an 'a' in your link.

Here's the (working) link: http://www.playacofi.net/weather/index.html

Nice site, btw!

[tolakram]

live loop: http://wwwghcc.msfc.nasa.gov/cgi-bin/ge ... umframes=5

latest image

[Wthrman13]

Ah, so like a "cap" that inhibits convection from growing. And "meatballs" must be the same thing as "hot towers?"

Correct....although a "hot tower" refers to a CB while "meatball" is a color infrared depiction of anvil top blow-off.

I'm not a fan of "hot tower" usage, btw; "popping" CBs near the center of an LLC aren't always or even usually in a better moisture plume (i.e., higher atmospheric heat content); they're simply located in the area where the top of the atmosphere ("King Cap") is unraveling. IOW, newly rambunctious CBs are the result of a changing condition, not the cause of it.

I'm not understanding a lot of what you're saying, so let me respectfully ponder some of your theories.

Cap usually refers to a temperature or moisture difference that caps the atmosphere. In this case the dry air being entrained into the system might be considered the cap. it would be great if a pro could comment on this. Here is a nice overview of the way I understand the cap: http://www.stormtrack.org/library/forecast/cap.htm Is this what you see as the primary issue? If so, then it seems to me cap is another way to describe dry air entrainment, or a dry air layer near 94L.

As far as I understand it the height of storms in the tropics has everything to do with latitude rather than a special feature of tropical storms. http://www.windows2universe.org/earth/A ... itude.html

The term Hot Tower is described here: http://en.wikipedia.org/wiki/Hot_tower I think identifying hot towers near the center of circulation might indicate strengthening, but I'm not sure if it has any value in a developing storm like 94L. The only research I see if hot towers in eyewalls, not in developing systems. In any case the research points to hot towers providing a buffer to the dry air. Has there been any updated research on this?

Finally, numerous pro mets, but not the NHC that I can recall, have commented on the forward speed of the low level circulation being a hindrance to development. I don't think it was a coincidence that Ernesto finally developed after the forward speed slowed, but I can't find anything to either support or refute this.

I, personally, think it's terrific to theorize what might be happening, but I also think these theories need to be stated as such, in addition to the standard s2k disclaimer.

Personal Forecast Disclaimer:
The posts in this forum are NOT official forecast and should not be used as such. They are just the opinion of the poster and may or may not be backed by sound meteorological data. They are NOT endorsed by any professional institution or storm2k.org. For official information, please refer to the NHC and NWS products.

I didn't see anyone else respond to this, so let me take a crack at it. First, a "cap" colloquially and usually refers to a layer of air just above the planetary boundary layer that exhibits a stable lapse rate in temperatures. That is, it's a low level feature, usually around 2-4 km above the ground. It can be an actual temperature inversion (where the temperature actually increases with height), but it doesn't have to be, just so long as the drop in temperature with height is slower than what a rising parcel from the boundary layer below would experience as it attempts to rise through the "cap". In such a case, the parcel's temperature will end up being cooler at some point than the surrounding air, and will therefore become negatively buoyant and decelerate, possibly reversing direction and sinking again. This is why such a layer is called a cap, since it inhibits the upward progress of rising air parcels from below.

Shuriken is using a lot of nonstandard terminology which makes it difficult to evaluate the meteorological merits of what he's saying, but when he's referring to "King Cap" above 200 mb, this is nothing more than the tropopause inversion. It's *not* the top of the atmosphere, only the boundary between the possibly unstable troposphere below, and the very stable stratosphere. The height of the tropopause is typically higher in the tropics than in the midlatitudes, because the troposphere is overall warmer and less dense there, leading to a deeper troposphere than the colder midlatitudes. It serves as another "cap" for deep convection (read, thunderstorms) because as the air parcels rising in a thunderstorm updraft encounter the tropopause, they quickly become cooler than the very warm air in the tropopause, and become negatively buoyant. A sufficiently intense updraft, however, has such momentum that it can "punch" it's way quite a distance above the tropopause before the negative buoyancy stops it and causes it to sink again (the so-called "overshooting top"). This is why thunderstorm anvils form (and the cirrus shield of tropical cyclones), as the rising air eventually settles at its level of neutral buoyancy and spreads out horizontally.

Shuriken is right than when the lapse rates below the tropopause are large (at least I think this is what he was getting at), the troposphere is more unstable to deep convection and thunderstorm updrafts (including "hot towers") will be more intense. This leads to stronger tropical cyclones, all other things being equal.

One final thing: technically, moisture content has nothing to do with the "cap" per se, although in general a capping inversion is drier than the layers above and below it. The dry air doesn't cap the rising convection as much as dilute it, robbing it of it's buoyancy and thus, upward acceleration. This happens throughout the depth of the troposphere, however, whenever the surroundings of the rising cumulus are subsaturated (which is nearly always the case). The drier the air, the more the dilution of the thunderstorm updraft, and the weaker the updrafts become, which is why dry air is so detrimental to developing TC's.

Hope that helps clarify some things!

[WeatherOrKnot]

IF it follows the new GFS which is close to the EURO and rides up the west coast of Florida...what could we expect on the east coast of south florida (WPB area)?

[tolakram]

Hope that helps clarify some things!

It does, thanks for the explanations.

[tropicwatch]

I guess what I'm asking is what is causing this to turn north and not go into the gom?

The models that far out are some what questionable. Here are the current steering currents, which makes me think the track might need to be adjusted a little further south.



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Personal Forecast Disclaimer:
The posts in this forum are NOT official forecast and should not be used as such. They are just the opinion of the poster and may or may not be backed by sound meteorological data. They are NOT endorsed by any professional institution or storm2k.org. For official information, please refer to the NHC and NWS products.

[pricetag56]

so the strength will determine the future track? what are the chances of it becoming a hurricane in 48 hrs like the nhc says?

[Portastorm]

IF it follows the new GFS which is close to the EURO and rides up the west coast of Florida...what could we expect on the east coast of south florida (WPB area)?

That would all depend on the strength/scope of the system as it would approach Florida.

[ozonepete]

Hope that helps clarify some things!

It does, thanks for the explanations.

Yes, thanks so much wthrman13. I wanted to say something myself but didn't have time. We really need to keep the terminology correct here, especially things like the cap or capping inversion; otherwise too many people get confused.

[Javlin]

One final thing: technically, moisture content has nothing to do with the "cap" per se, although in general a capping inversion is drier than the layers above and below it. The dry air doesn't cap the rising convection as much as dilute it, robbing it of it's buoyancy and thus, upward acceleration. This happens throughout the depth of the troposphere, however, whenever the surroundings of the rising cumulus are subsaturated (which is nearly always the case). The drier the air, the more the dilution of the thunderstorm updraft, and the weaker the updrafts become, which is why dry air is so detrimental to developing TC's.


So if the air around the updraft is a wet sponge the acc.continues hence the surrounding enviroment is already saturated?is that correct?