ATL: ISAAC - Post-Tropical - Discussion

[Nimbus]

Looked like a little stair step to the NW and a little slower forward motion might just be temporary though. Next convective pulse should drop the pressure below 1000 MB's so its time to check our credit scores.

Still not sure what the 850-200 MB steering will look like as this gets west of PR.

[Gustywind]

GUSTY -
and all on the islands - stay safe and check in when you can.

Thanks a lot , looks like very busy 24h-48 h if this thing continues to approach dangerously the Leewards area. Anyway, i will keep your informed because of we have friends ( HUC, Ouragans, ouragan and Tanguy ) from the butterfly island who will i hope share you what could happen here.

[ConvergenceZone]

12z Best Track up to Tropical Storm Issac

AL, 09, 2012082118, , BEST, 0, 153N, 532W, 35, 1005, TS

Wow, for some reason I thought they would stay on the real conservative side, but
the close proxmity of the islands could have made a difference....
There's still so much of the tropics that I've yet to understand.

I'm only 1/4 through working on my degree in Meteorology, so I'm assuming by the time I've
got my degree, I'll understand the tropics much more. This board helps though!...

[wxman76]

Greetings all! I am new here but I am a professional met. I am keeping a close "I" on Isaac to be, especially since I live near Tampa! First i am going to take a stab at why the Easterlies have been so brutal this year. I think the key to this season is in the Indian Monsoon. The erraticness of good Cape Verde waves this season is directly related to the erraticness of the Indian monsoon this year. And i think the erraticness of the Monsoon is the key to the strong Easterlies. The reason the Monsoon occurs is the Semi-permanant high pressure cell moves north in the warm season. same as our Bermuda High. This year it hasn't moved as far north. Because of that it tightens the pressure gradient between the equator and the high. That's my best guess. If anyone knows better let me know.

The Euro is not that far fetched. Here's why the CPC 6-10 day anomaly shows as it's 4th closest analog 2005 just b4 Katrina. If this storm stays weak it may well verify but. Also making a showing is 1954 which would be Edna up the East coast. Given the fact this hasn't fallen apart given the unfavorable SAL and it is fighting back i doubt this will die. in fact if you look at the season so far storms can't crank till they are west of 65 degrees. so my thought is it holds its own and slowly strengthens but once it gets to favorable conditions this will blow up. I like a track over Florida, just not sure which coast yet. Those are my thoughts.


This post is NOT an official forecast and should not be used as such. It is just the opinion of the poster and may or may not be backed by sound meteorological data. It is NOT endorsed by any professional institution including storm2k.org For Official Information please refer to the NHC and NWS products.

[floridasun78]

i be looking at cone next few days because i live in mobile home and i work for homedepot that place will rocking as Tropical Storm Issac get closer

[SunnyThoughts]

Wxman76, WElCOME!!!!! We love it when new mets join and give us their expertise. ESPECIALLY ones from Florida (wink wink) j/k We all look forward to your insight as times goes by with this system as well as ones in the future.

[Weatherboy1]

you all still think south fl could be in cone by thur?

I think there's a very good chance of that, yes. Barring significant shifts by ALL of the models west overnight, or a significant slowdown in TD9's movement, I think the Keys will be in the cone as of 11 p.m. tonight or 5 a.m. tomorrow, with the southern tip of FL in it by tomorrow evening. My two cents!

[scogor]

Welcome, wxman76! You're just up the road from several of us storm2k regulars from the Sarasota area and we'll look forward to your insight. Are you able to publicly or privately let us know if you're one of the mets that appears on our local media?

[Windsurfer_NYC]

ISAAC, AL, L, , , , , 09, 2012, TS, S, 2012081700, 9999999999, , , , , , WARNING, 4, AL092012

[Wthrman13]

I did not refer to any cap "above 200mb" or specify the tropopause inversion; the cap ("King") in question is the one which inhibits convection from reaching and exhausting at 200mb (i.e., that which keeps cloud tops generally lower and warmer, rather than, say, -75C). Depending upon the location and time of season, the actual height of this cap could widely vary.

The inability to exhaust above the tops of nearby convection, or the layer of subsidence generated by ITCZ/South American equatorial blow-off, will prevent a tropical system from intensifying.

TD9 is demonstrating the dominance of this factor today: last night, the cap loosened, and a big meatball blew up; this morning, well before the diurnal minima, the cap re-asserted itself and convection waned considerably to the point that the LLC is now exposed.

-- Why did this happen? Most people will fret over various surface factors such as dry air and so on; and much head-scratching ensues as systems inexplicably croak despite entering areas of better surface conditions for intensification.

I believe it's an entirely high altitude phenomena caused by gravity waves and similar perturbations along the tropopause which upset stability variables. Since these triggering phenomena are mainly invisible, there is an unconscious tendency to assume they do not exist, or at least not pay them due mind. (There have been scattered studies of high altitude import, but not enough for a general knowledge to percolate throughout the community of tropical devotees.)

Then I must confess as to not having any idea what you are referring to. Can you point to a study that discusses this capping layer around 200 mb, distinct from the tropopause (which does indeed usually exist higher up than 200 mb in the tropics)? If it exists, it can't have gone unnoticed. The lapse rates near 200 mb do change quite a bit due to effects from antecedent convection, etc., and this of course can have an impact on subsequent deep convection. Earlier in this thread, however, you made the following claim "I say: this is all about lapse rates to 200mb. As in: it is the determinant factor necessary for intensification of any tropical system up through Hurr to Major." That's a pretty strong claim, and I don't know of any tropical meteorologist who would agree with it. There really is no single determining factor, but rather many different ones from the sea surface temperatures to the environmental relative humidity through a deep layer, to the tropospheric lapse rates, to deep-layer wind shear that all contribute, and some are more dominant than others in different situations. If there were one determining factor, TC forecasting (especially of intensity) wouldn't be so darn difficult.

On a more general note, there are numerous reasons why these convective bursts in developing TCs might be inhibited, and surface conditions are only one factor. Your posts make it sound like the tropical meteorology community only pays attention to surface conditions. They don't. There are numerous studies on the effects of gravity waves on convection/etc, much more than "scattered".

[SouthDadeFish]

SSMI got the core a little earlier today:

37 GHz Color:



85 GHz Color:

[Blown Away]

Last frames you can see convection starting to cover LLC again, so Isaac is flexing his tiny convective muscle. Still moving WNW to NW IMO.
http://www.ssd.noaa.gov/PS/TROP/floater ... short.html

[cycloneye]

The updated 18z Best Track adds the I word. The official advisory is at 5 PM.

AL, 09, 2012082118, , BEST, 0, 153N, 532W, 35, 1005, TS, 34, NEQ, 40, 0, 0, 0, 1011, 250, 25, 0, 0, L, 0, , 0, 0, ISAAC,

ftp://ftp.nhc.noaa.gov/atcf/tcweb/inves ... 012.invest

[wxman76]

Another reason I think Isaac is pulsing is competing convection. Both yesterday and today convection has fired to the south from diurnal reasons. There is only X amount of energy available and if it competes it has less energy. At night once these diurnal storms die off so does the competition hence the flaring into a "meatball" last night. Love these technical terms. Once the warm core processes really get cranking you have so much vertical motion that the air around the storm sinks "capping" the convection squishing the competition. Again i think even if this thing strengthens slowly it will make the surrounding environment more favorable, mixing out the remaining SAL hence decreasing the stability around it. Yesterday at this time Invest 94L started it's overnight flare and it seems to be happening again.

Also remember the ECMWF GFS and other models are baroclinic (isotherms intersect isobars, advection fronts etc) models. They can't handle a barotropic atmosphere (constant temperature) which tropical storms form in. So if the storm remains weak it isn't that barotropic and the models don't do bad, but if this ramps up the baraclinic models can't get a handle on it so you need to correct for them. Hence the discussion of a stronger storm tracking further north and east. A weaker storm farther south and west. that would be the correct correction for this. note the ECMWF is weaker than the GFS. and the CMC is so far north and east because it bombs the storm out.

This post is NOT an official forecast and should not be used as such. It is just the opinion of the poster and may or may not be backed by sound meteorological data. It is NOT endorsed by any professional institution including storm2k.org For Official Information please refer to the NHC and NWS products.

[adam0983]

Does a slower moving storm mean more impacts for Florida? I am seeing the that the Euro is showing a weaker faster moving storm that gets into the Gulf of Mexico. The GFS model shows a stronger slower moving storm heading toward Southern Florida.

[ROCK]

that and NE shear is shearing off the convection before it has time to build and form a CDO...

[Bocadude85]

Does a slower moving storm mean more impacts for Florida? I am seeing the that the Euro is showing a weaker faster moving storm that gets into the Gulf of Mexico. The GFS model shows a stronger slower moving storm heading toward Southern Florida.

The GFS doesnt have this system moving slower... the 12z GFS shows TD9 over SW Fla at 156hrs.

[SunnyThoughts]

Diurnal max tonight should be interesting. If I can stay awake long enough to see its affect on Isaac..I'm afraid we are all in for quite a few late nights with this one!

[Wthrman13]

Also remember the ECMWF GFS and other models are baroclinic (isotherms intersect isobars, advection fronts etc) models. They can't handle a barotropic atmosphere (constant temperature) which tropical storms form in. So if the storm remains weak it isn't that barotropic and the models don't do bad, but if this ramps up the baraclinic models can't get a handle on it so you need to correct for them. Hence the discussion of a stronger storm tracking further north and east. A weaker storm farther south and west. that would be the correct correction for this. note the ECMWF is weaker than the GFS. and the CMC is so far north and east because it bombs the storm out.

The ECMWF, GFS, and other similar models are "primitive equation" models, not baroclinic models. The primitive equations contain both baroclinic and barotropic solutions, so in principle these models can handle both situations. There are certain simplified models that were popular in the early years of numerical weather prediction that *only* handle barotropic conditions (which is mathematically and physically simpler than baroclinic conditions), but these are outdated and I'm unaware of any model that *only* handles baroclinic conditions, since barotropic conditions are really a special case of baroclinic conditions (when the surfaces of constant density and pressure coincide). Whether these models do well or not in one regime or another is another question, and has a lot to do with how good the physical parameterizations for convection, surface interactions, radiation, and the like are handled, as well as the specific way a given model solves the equations. There is no special correction I'm aware of when a storm "ramps up" in these models, and I'd argue that the models actually do *better* with these systems once they are strong enough to be well-resolved in the model.

EDIT: The word "primitive" in "primitive equation" doesn't refer to any sort of inferiority or simplicity in these equations, but rather to the fact that they are a very general form of the equations of motion of the atmosphere from which other, more simplified and specialized equations that apply to specific situations can be derived. When writing a numerical weather prediction model, you pretty much always want to start from the primitive equations so that you can cover as many situations as possible...

[adam0983]

The Key to Isacc will be how far west will Isacc get when it feels the weakness. A slower moving storm could miss Florida completely if the storm gets so strong and goes over the Bahamas and out to sea.