ATL: ISAAC - Post-Tropical - Discussion

[ROCK]

San Juan is not a good gauge for center location....you are looking 26k feet in the air....

[KWT]

I'm liking the look of Issac right now, its finally lost that dual convection it had over the previous 24hrs which is a sure fire sign that this system is finally in the position to strengthen.

At a guess motion is around 285-290...but its hard to tell!

[cpdaman]

i think everyone is waiting on next recon for center fix

perhaps 15.8 / 65.5 i dunno

[meriland23]

Honestly, I just hope this thing does not significantly take over E cuba or W haiti.. dem dere mountains gonna rain on my parade

[Vortex]

It appears were about ready for takeoff here-I suspect by overnight steady stengthening will be underway...

[Jimsot]

Thatsa one big storm. Feeder bands over Anguilla all the way down to the ABC islands. It is quite windy here on Anguilla and the seas are just amazing. Not a lot of sleep last night, too noisy!

[wxwatcher1999]

Still hearing talk that there is still multiple vortices

[OuterBanker]

Interesting. JB is sticking to his guns. Admits he could be wrong, but still sticking to a east Fl landfall. Then north along the coast.
Flame on skeptics.

Then he'd better get moving northward a bit (Isaac, not JB that is...), wouldn't you think?

Not really, he's not even to 70 w yet.

I think all of us will be watching for the turn tough.

[cpdaman]

looks like the convection near 15.8 N 65-66 w is still wrapping into toward another circulation to the Sw which is rotating a bit itself, i guess surface features per recon may tighten up first, but besides slowing i haven't seen much with my own eyes that show this thing is consolidating to one center

maybe 15.4 /66 ?

[ColdFusion]

LOCATION...15.6N 65.4W

[cpdaman]

convection continues to expand near center colf fusion mentioned

[Hurricane Andrew]

IN THE SHORT TERM...THE STRUCTURE OF ISAAC SUGGESTS THAT ONLY SLOW
STRENGTHENING IS LIKELY. HOWEVER...THE ENVIRONMENTAL CONDITIONS
ARE FAVORABLE FOR RAPID INTENSIFICATION...WHICH COULD OCCUR IF THE
CYCLONE DEVELOPS A WELL-DEFINED INNER CORE.

http://www.nhc.noaa.gov/text/refresh/MI ... 1453.shtml?

[wxwatcher1999]

So I guess the Nhc is giving the western track a little more of a chance

[ColdFusion]

From the discussion...

THE OVERALL CIRCULATION OF ISAAC IS A LITTLE FARTHER SOUTH THAN 12
HR AGO. HOWEVER...IT SEEMS TO HAVE RESUMED A WESTWARD MOTION AT
ABOUT 13 KT. THE TRACK MODEL GUIDANCE IS IN GOOD AGREEMENT THAT
ISAAC SHOULD MOVE WEST-NORTHWESTWARD FOR THE NEXT 36-48 HR...
STEERED BY THE SUBTROPICAL RIDGE TO THE NORTH. AFTER THAT...THE
STORM SHOULD APPROACH A WEAKNESS OR BREAK IN THE RIDGE OVER FLORIDA
AND THE EASTERN GULF OF MEXICO. THE NEW FORECAST TRACK IS SHIFTED A
LITTLE TO THE WEST OF THE PREVIOUS TRACK BASED ON CURRENT POSITION
AND LIES DOWN THE MIDDLE OF THE GUIDANCE ENVELOPE NEAR THE
CONSENSUS MODELS. IT SHOULD BE NOTED...THOUGH...THAT NEAR THE 96
HR POINT THE GUIDANCE MODELS ARE SPREAD BETWEEN SOUTHEASTERN
FLORIDA AND WESTERN CUBA...AND AT 120 HR THEY ARE BETWEEN
NORTHEASTERN FLORIDA AND THE CENTRAL GULF OF MEXICO. THUS...
SIGNIFICANT UNCERTAINTY REMAINS ABOUT THE THREAT ISAAC POSES TO
FLORIDA.

[Weatherboy1]

IN THE SHORT TERM...THE STRUCTURE OF ISAAC SUGGESTS THAT ONLY SLOW
STRENGTHENING IS LIKELY. HOWEVER...THE ENVIRONMENTAL CONDITIONS
ARE FAVORABLE FOR RAPID INTENSIFICATION...WHICH COULD OCCUR IF THE
CYCLONE DEVELOPS A WELL-DEFINED INNER CORE.

http://www.nhc.noaa.gov/text/refresh/MI ... 1453.shtml?

I picked up on that too. In my opinion, the ONLY inhibiting factor here really is the disorganization of the core. Granted, that's an important one. But if this thing can stack, I think it can pop significantly given the slower speed, lack of sheer, very warm SSTs, and so on. Certainly not a MAJOR hurricane, but it could ramp up to hurricane winds in a short period of time. My opinion as an amateur, as always!

[Houstonia]

Jeff Lindner (HCFCD):

Disorganized tropical storm moving across the eastern Caribbean Sea.


Discussion:

USAF plane overnight investigating Isaac has found multiple centers rotating around a large mean circulation…a disorganized tropical system. Convection continues to struggle to consolidate and maintain itself and this is likely due to dry air shown in the Leeward Island soundings in the mid to upper levels occasionally entering into the fragile inner core of the system. This along with the large size and the multiple centers has prevented Isaac from much intensification. Isaac continues toward the W, but has slowed in the last several hours.

Track:

I have spent a lot amount of time this morning reviewing the various global models and their handling of the upstream features over the US that will ultimately determine the final track and landfall of Isaac on the US coast. For the next 24-48 hours it appears the system will continue to move toward the W to WNW on the south side of the sub-tropical high over the central Atlantic. This will take the system toward the southern coast of Haiti, but if Isaac does not starting gaining some latitude it is very possible the system will pass south of Haiti.

In the longer term (after 48 hours) the guidance spread increases with the ECMWF on the left (west) edge of the clustering and the CMC on the right (east) edge of the clustering. The ECMWF has been shifting eastward in its last few runs while the CMC has been shifting westward and the GFS has been holding strong for the past several runs with very little shifting. It is interesting that at the 120-144 hr points the GFS shows a classic recurve over Isaac over the SE US while the CMC and ECMWF drive the system inland and do not show a hard recurve.

The differences are in the handling of the troughing that currently extends from the NE US into the Gulf of Mexico. This trough will begin to lift over the next 24 hours, but a weakness or break in the ridge over the SW Atlantic and a building high over the southern plains is left behind over the SE US. This weakness appears to be enough to induce a NW turn of Isaac near the eastern or central Cuban coast. At this point the weakness begins to fill some as a zonal flow briefly develops over the US. However a strong shortwave digs down from Alaska which helps pump up (build) the southern plains ridge resulting in troughing digging across the SE US toward early next week in a weak omega pattern. This pattern allows Isaac to be captured or tugged by the trough and brought NNW and then N across the eastern Gulf toward the FL panhandle. Each of the global models has the same solution, but each differ some on the intensity and amplification of the southern plains ridging and the resultant downstream trough. The ECMWF has a weaker trough which allows Isaac to move further west while the GFS has a deeper trough and curves the system nearly up the FL west coast and then into SC. The GFS has been very consistent on its track and the ECMWF has been slowly trending toward its solution over the past two model runs although the ECMWF ensembles are well west of the operational track toward the LA coast.

There is also the potential for short term center relocations to the south of the current position as supported by the aircraft fixes, and this could have some longer term track implications.

With all that said, the consensus is still in very good agreement through the Day 4-5 period with the system track toward the FL Keys and then NW along the west coast of FL.

Intensity:

Isaac is continuing to suffer from dry air intruding into the inner core and the lack of a well defined center. While the potential has been and continues to be there for intensification, the internal dynamics of the inner core continue to be lacking which is preventing organized deep convection. We need a well defined center to take shape and to flush the dry air out of the inner areas of the system allowing deep convection to develop and maintain itself. Another factor is the large size of the system as larger systems tend to take time to consolidate. Isaac has a well defined outflow pattern established with high level cirrus expanding outward in all directions helping to vent the circulation. When the inner core finally does consolidate then the system should begin to intensify given warm waters and good upper level outflow. The next item of concern with intensity will be how much the system interacts with the islands of Hispaniola and Cuba. While the track takes the system across western Haiti and eastern Cuba, I am starting to consider that the system may pass south of both of these land areas and over the water which would have the potential to produce some significant intensity differences compared to the current forecast. Due to the uncertainty of the consolidation of the inner core and how long it is going to take and the possibility of land interaction will follow along with the NHC on the intensity, but if Isaac stays a little more to the south it could be stronger in the middle and latter part of the forecast. Also there is data suggesting Isaac is currently suffering from the passage of an upper level Kelvin wave which is helping to induce unfavorable conditions across this part of the basin in the face of otherwise favorable conditions. Isaac should come out from under the influence of this feature in the next 48 hours or so.

[Frank2]

Per a Tweet one hour ago from NOAA 42:

#NOAA42 is heading south on its last pass through #Isaac. Bumpiest ride we had all day as we went through some convection near the center.

that's sometimes a sign of strengthening...

[GTStorm]

Question for anyone....do the 12z models get incorporated into the 11:00 AM advisories? I know I can't see them until 11:30 AM, but when does the NHC get the data?

[tolakram]

Question for anyone....do the 12z models get incorporated into the 11:00 AM advisories? I know I can't see them until 11:30 AM, but when does the NHC get the data?

No. Models take a while to run, a lot of data. The 12Z models started at 8AM EDT, we only start getting the output around 11:30AM.

Someone please correct me if I'm wrong.

[sponger]

Interesting. JB is sticking to his guns. Admits he could be wrong, but still sticking to a east Fl landfall. Then north along the coast.

Let hope not. We are saturated here in North Fl with another inch or two on the way today. The NHC has been amazing so I have stuck with them since Katrina. IT WOULD BE NICE TO NUDGE THIS TO THE WEST AND PLOW THIS THING INTO THE CENTRAL PLAINS WHERE THEY NEED THE RAIN.