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

[icebreaker5221]

no kidding. i know water vapor imagery only catches the upper levels, but you can already see the impact with convection and water vapor drastically diminishing.

Not only is the land interaction disrupting the convergent moisture feed in the boundary layer, its also disrupting the vorticity structure lee of the topography. CIMSS 850mb vort shows this nicely, with vertical stretching of the vortex column lee of the topography (not dissimilar to lee cyclogenesis) and deflected to the right due to coriolis. The net effect is a "vort min" directly ahead of the cyclone.

edit: Phil, don't you have a post where you talk about this effect in the W Pac?

[phil882]

The more I think about this... this is a very Gustav-esque track forecast. The ridge with Gustav was stronger, but we saw a similar temporary weakness develop when it crossed over Hispaniola. The main difference here is that Isaac is large, while Gustav was a very small TC initially. It was strongly influenced by the lee cyclone that developed off the Hispaniola terrain and redeveloped further south into the Caribbean post-impact. Thats unlikely to happen with Isaac given the large size.

[Derecho!]

http://meteocentre.c...hh2=120&fixhh=1

I see it a bit further west?

Apalachicola? I don't see that at all.

Eh, it's looking at Ls on a low res map - Looking at it again more carefully, probably more like Panama City.

[icebreaker5221]

Not only is the land interaction disrupting the convergent moisture feed in the boundary layer, its also disrupting the vorticity structure lee of the topography. CIMSS 850mb vort shows this nicely, with vertical stretching of the vortex column lee of the topography (not dissimilar to lee cyclogenesis) and deflected to the right due to coriolis. The net effect is a "vort min" directly ahead of the cyclone.

edit: Phil, don't you have a post where you talk about this effect in the W Pac?

The more I think about this... this is a very Gustav-esque track forecast. The ridge with Gustav was stronger, but we saw a similar temporary weakness develop when it crossed over Hispaniola. The main difference here is that Isaac is large, while Gustav was a very small TC initially. It was strongly influenced by the lee cyclone that developed off the Hispaniola terrain and redeveloped further south into the Caribbean post-impact. Thats unlikely to happen with Isaac given the large size.

Actually I'm in full agreement that we won't see an actual lee-cyclone develop, just some generation of cyclonic vorticity that should not develop a closed surface circulation.

[OSUmetstud]

Actually I'm in full agreement that we won't see an actual lee-cyclone develop, just some generation of cyclonic vorticity that should not develop a closed surface circulation.

GFS has a pressure trough extending that way for the next couple days. NAM actually develops a second closed low over there but it's the nam lol.

[phil882]

We have data sharing agreements in place and the obs get sent over the GTS. Here is an example plot showing that the reccon observations made their way into the ECMWF system last night for the 00z cycle (obviously the green dots):

I'm 100% certain that the UKMO at least receive these (and I'd be shocked if they didn't assimilate them).

Thanks for your input... glad to hear we are sharing with everyone

Not only is the land interaction disrupting the convergent moisture feed in the boundary layer, its also disrupting the vorticity structure lee of the topography. CIMSS 850mb vort shows this nicely, with vertical stretching of the vortex column lee of the topography (not dissimilar to lee cyclogenesis) and deflected to the right due to coriolis. The net effect is a "vort min" directly ahead of the cyclone.

edit: Phil, don't you have a post where you talk about this effect in the W Pac?

The thread is here: http://www.americanw...pical-cyclones/

When you get really strong flow over high terrain you often see a vorticity dipole that develops, with the ascending air undergoing vortex squashing, creating anticyclonic vorticity (note the minimum directly S of Hispaniola. However, in the decending air on the other side of the mountain chain, you get vortex stretching, which produces cyclonic vorticity when then propagates downstream with the flow, which is northeasterly on the tip of Haiti.

In the short term, this cyclonic vorticity maxima that is developing in the lee of the Hispaniola mountains will undergo a binary interaction with Isaac, causing it to bend more rightward than what would typically be expected by the steering currents. If the lee cyclonic vorticity becomes strong enough, it could then cause a bend back to the left as the vorticity associated with Isaac moves over Hispaniola. Instead, the GFS keeps Isaac as the dominant disturbance, so we don't really see a major leftward bend afterwards, since the vortex of Isaac remains distinct and thus the flow will quickly reverse over Hispaniola resulting in lee cyclone dissipation.

[Derecho!]

There are 12Z GFS Ensemble members through the Bahamas and up the East coast of Florida now...

[Ground Scouring]

Does anyone think that the lee cyclone may also transfer more angular momentum into the northern semicircle and thereby, despite its size, allow Isaac to organize farther north and more easily once it clears Haiti, especially if it follows the GFS ensembles? In that case, with a good 72 hours over water before SE FL, would it still have a good chance to be a strong TS, if not a minimal hurricane, in the Miami area? (12Z GFS does show about 988 mb there, supporting close to 60 kt.)

[turtlehurricane]

Thanks for your input... glad to hear we are sharing with everyone

The thread is here: http://www.americanw...pical-cyclones/

When you get really strong flow over high terrain you often see a vorticity dipole that develops, with the ascending air undergoing vortex squashing, creating anticyclonic vorticity (note the minimum directly S of Hispaniola. However, in the decending air on the other side of the mountain chain, you get vortex stretching, which produces cyclonic vorticity when then propagates downstream with the flow, which is northeasterly on the tip of Haiti.

In the short term, this cyclonic vorticity maxima that is developing in the lee of the Hispaniola mountains will undergo a binary interaction with Isaac, causing it to bend more rightward than what would typically be expected by the steering currents. If the lee cyclonic vorticity becomes strong enough, it could then cause a bend back to the left as the vorticity associated with Isaac moves over Hispaniola. Instead, the GFS keeps Isaac as the dominant disturbance, so we don't really see a major leftward bend afterwards, since the vortex of Isaac remains distinct and thus the flow will quickly reverse over Hispaniola resulting in lee cyclone dissipation.

Excellent point to bring up, I drew a little schematic of the gyres you're talking about. Seems like both the anticyclone and cyclone would push the storm more west after the cyclone initially deflects the storm north into Hispaniola. There's probably another smaller set of gyres to the west of the island that may convolute things.

[Srain]

MODEL DIAGNOSTIC DISCUSSION

NWS HYDROMETEOROLOGICAL PREDICTION CENTER COLLEGE PARK MD

142 PM EDT FRI AUG 24 2012

VALID AUG 24/1200 UTC THRU AUG 28/0000 UTC

...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR

INGEST...

12Z MODEL EVALUATION INCLUDING THE UKMET AND CANADIAN

INITIALIZATION ERRORS DO NOT APPEAR TO SIGNIFICANTLY IMPACT THE

LARGE SCALE FORECAST.

...TROPICAL STORM ISAAC...

THE TRACK OF ISAAC IN THE 12Z NAM NOW MORE CLOSELY RESEMBLES THE

MAJORITY OF GUIDANCE...WHEREAS EARLIER NAM RUNS HAD BEEN FAR TO

THE RIGHT. IN THIS RUN...HOWEVER...THE STRUCTURE IS MUCH

DIFFERENT. MOST OF THE MODELS INDICATE A BROAD AREA OF SURFACE LOW

PRESSURE...AND PERHAPS A SECOND LOCAL PRESSURE MINIMUM SOUTHWEST

OF THE TRACK CENTER-LINE. THE NAM NOW SHOWS THE MORE SOUTHWESTERN

MINIMUM BECOMING EQUAL TO THE ORIGINAL CENTER...AND THEN BECOMING

DOMINANT AS THE SYSTEM CROSSES CUBA. THIS NOTION IS NOT REFLECTED

IN THE OTHER MODELS...BUT AT LEAST BEARS WATCHING GIVEN THAT THE

SYSTEM IS BROAD...HAS BEEN SLOW TO ORGANIZE...AND WILL BE

INTERACTING WITH THE LARGER ISLANDS AT THE WESTERN END OF THE

ANTILLES.

THE 12Z GFS CAME IN WITH A TRACK SIMILAR TO THE OFFICIAL

TRACK...BUT IS ABOUT 90 NM TO THE RIGHT BY DAY 3. THE

CANADIAN...INCLUDING ITS 12Z RUN...WAS SIMILAR...BEGINNING TO

DRIFT RIGHT OF THE OFFICIAL TRACK AFTER 26/00Z. STILL...THESE

SOLUTIONS FALL TO THE RIGHT OF CONSENSUS...AND WE NOTE THAT THE

12Z UKMET DEPICTS A TRACK THAT IS VERY CLOSE TO THE ECMWF AND THE

OFFICIAL TRACK. PLEASE REFER TO THE NATIONAL HURRICANE CENTER FOR

UPDATES ON THE OFFICIAL FORECAST TRACK/STRENGTH AND FOR ADDITIONAL

INFORMATION.

[wxsmwhrms]

So the UK shifted left and is slower, and the GFS/Canadian shifted right and is faster. There is no doubt that the UK has a substantially stronger ridge than the GFS does in the day 3-6 period, on the order of 30-40 meters.

[wxsmwhrms]

MODEL DIAGNOSTIC DISCUSSION

NWS HYDROMETEOROLOGICAL PREDICTION CENTER COLLEGE PARK MD

142 PM EDT FRI AUG 24 2012

VALID AUG 24/1200 UTC THRU AUG 28/0000 UTC

...SEE NOUS42 KWNO (ADMNFD) FOR THE STATUS OF THE UPPER AIR

INGEST...

12Z MODEL EVALUATION INCLUDING THE UKMET AND CANADIAN

INITIALIZATION ERRORS DO NOT APPEAR TO SIGNIFICANTLY IMPACT THE

LARGE SCALE FORECAST.

...TROPICAL STORM ISAAC...

THE TRACK OF ISAAC IN THE 12Z NAM NOW MORE CLOSELY RESEMBLES THE

MAJORITY OF GUIDANCE...WHEREAS EARLIER NAM RUNS HAD BEEN FAR TO

THE RIGHT. IN THIS RUN...HOWEVER...THE STRUCTURE IS MUCH

DIFFERENT. MOST OF THE MODELS INDICATE A BROAD AREA OF SURFACE LOW

PRESSURE...AND PERHAPS A SECOND LOCAL PRESSURE MINIMUM SOUTHWEST

OF THE TRACK CENTER-LINE. THE NAM NOW SHOWS THE MORE SOUTHWESTERN

MINIMUM BECOMING EQUAL TO THE ORIGINAL CENTER...AND THEN BECOMING

DOMINANT AS THE SYSTEM CROSSES CUBA. THIS NOTION IS NOT REFLECTED

IN THE OTHER MODELS...BUT AT LEAST BEARS WATCHING GIVEN THAT THE

SYSTEM IS BROAD...HAS BEEN SLOW TO ORGANIZE...AND WILL BE

INTERACTING WITH THE LARGER ISLANDS AT THE WESTERN END OF THE

ANTILLES.

THE 12Z GFS CAME IN WITH A TRACK SIMILAR TO THE OFFICIAL

TRACK...BUT IS ABOUT 90 NM TO THE RIGHT BY DAY 3. THE

CANADIAN...INCLUDING ITS 12Z RUN...WAS SIMILAR...BEGINNING TO

DRIFT RIGHT OF THE OFFICIAL TRACK AFTER 26/00Z. STILL...THESE

SOLUTIONS FALL TO THE RIGHT OF CONSENSUS...AND WE NOTE THAT THE

12Z UKMET DEPICTS A TRACK THAT IS VERY CLOSE TO THE ECMWF AND THE

OFFICIAL TRACK. PLEASE REFER TO THE NATIONAL HURRICANE CENTER FOR

UPDATES ON THE OFFICIAL FORECAST TRACK/STRENGTH AND FOR ADDITIONAL

INFORMATION.

Yes, the UK is almost on top of the 00Z EC through 60 hrs, then at 72 hours it is a little left and actually has a slightly stronger ridge than the EC, but by the time it makes landfall is a little right of the EC.

[Tropopause_Fold]

euro scrapes the northern coast of cuba and is then near/just south of KEYW by 60hrs

[dan88]

Recon is descending now to mission level.

[peribonca]

any radar links from the DR? The center should be getting close to there and within range.

[andyhb]

Euro at 72 hrs looks to be well west of Fort Myers, and intensifying quickly.

[Tropopause_Fold]

euro basically goes due NW from KEYW toward KMSY

[andyhb]

euro basically goes due NW from KEYW toward KMSY

Really bombing in the Gulf by 96.

[phil882]

any radar links from the DR? The center should be getting close to there and within range.

http://www.onamet.gov.do/?s=web&p=1078

[Tropopause_Fold]

euro basically goes due NW from KEYW toward KMSY

looks like it slows SE of Louisiana and then heads north...nearing landfall in mobile bay region by 120

[atownwxwatcher]

Really bombing in the Gulf by 96.

Definitely SW of the GFS

[andyhb]

Ends up being a bit west of the 00z verbatim, and well separated from the GFS.

[Srain]

Hour 120: Euro

[Derecho!]

A couple of 70 kt FL obs...

[gkrangers]

A couple of 70 kt FL obs...

Probably going to see some nice wind obs with that convection wrapping around the northern part of the center.

[Dunkman]

I guess the prudent thing to do is split the difference between the Euro and GFS which ends up with the same track most including the NHC have. Should be an interesting few days with them already over 150 miles apart at 72 hours.

[Dsnowx53]

It's still way too early to be assuming any consensus, or that model consensus necessarily means much at this juncture. There are lots of features in the medium range that still need to be ironed out, and the handling of those features will change every run. Strength of the ridging, land influence, and now the upper level low in CO that is causing troughing east of the ridge. The latter feature has just recently been picked up and models might waver a lot on its influence on Issac. It definitely makes sense that the GFS would make this feature a bigger influence considering its propensity to dig troughs, whereas the Euro and Ukie have a stronger ridge and presumably a weaker trough.

Since the trough is not a large scale feature like the ridging is, it still wouldnt surprise me if models backed off on the trough's influence and Issac turns back west again. As others have said, that feature will need to be watched closely.

[ALhurricane]

So the Euro actually shifted slightly west compared to the big east shift on the GFS. If I was NHC I would leave the track as is for now in respect for the UKMET/ECMWF.

[wxsmwhrms]

Ends up being a bit west of the 00z verbatim, and well separated from the GFS.

Yeah, it is a bit left than the previous run. I know the EC had that weird 12Z run yesterday when it went to LCH, but otherwise the last several runs have been fairly targeted in on somewhere from SE LA to MOB, and the EC ENS has been very consistent with a mean SE LA landfall. The GFS has become extremely inconsistent the last several runs, and I have a leeriness born of long experience with the GFS not showing ridges being strong enough. The fact that EC/UK are in pretty darn good agreement would make me want to lean that way, probably most closely to the UK assuming the EC is probably a bit too far left.

[Connor McCrorey]

183030 1654N 07050W 8428 01472 9985 +170 +170 167034 036 041 007 01

998.5 mb extrapolated, with no wind shift yet.

Edit - 184130 1647N 07123W 8429 01446 9948 +180 +180 161004 006 017 001 05

There's the center, 994.8 mb. Assuming they dropped a sonde, I'm looking forward to see what it reports!