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

[Nikolai]

GFS landfalls due south of NOLA then drifts Isaac west.

[thunderbird12]

It's been fascinating to watch the evolution of the new circulation all evening. The biggest problem with it is that it is so darn small. You could fit most of the circulation and associated deep convection into Miami-Dade county. I don't know what kind of evolution would have to take place to make this the dominant circulation compared to the much broader pre-existing circulation. It may end up as a meso-vortex rotating around the larger circulation.

[Ground Scouring]

Based upon the wind directions gleaned from the reconnaissance data, I think the northern radar vortex is a mesoscale feature within the broad LLC. Such persistent features often occur downwind of an ULL-induced shear axis. The real LLC is south of the eye-like feature due to mid-level shear and some dry air. And because the ULL is stronger and moving more slowly than forecasted, the prospects of more rapid intensification, within the next 24 hours, are in my view lower than some people might anticipate.

[SACRUS]

(posted the wrong maps prior)

The best fuel for Isaac lies ahead, perhaps tues as it approaches the north central gulf coast...

[Superstorm93]

Cross section of the "eye feature"

Looks like an ellipses shape for now

[Amped]

Those bands to the east continue to pump up the northern center. Looking more dominant every frame. Too bad it's moving away from the radar.

[Superstorm93]

Isaac does look improved from earlier.

Surface winds were still rather unimpressive.

[tornadotony]

Based upon the wind directions gleaned from the reconnaissance data, I think the northern radar vortex is a mesoscale feature within the broad LLC. Such persistent features often occur downwind of an ULL-induced shear axis. The real LLC is south of the eye-like feature due to mid-level shear and some dry air. And because the ULL is stronger and moving more slowly than forecasted, the prospects of more rapid intensification, within the next 24 hours, are in my view lower than some people might anticipate.

I wouldn't be so sure about that. That feature is quite deep and has been quite persistent. Additionally, there was no good center fix where the old center was located. If I had to guess, my bet would be that this feature is going to be dominant and that the winds will eventually respond (which perhaps they haven't quite done yet on the leg that recon has flown, though they haven't really gotten close to this feature yet).

[Minnesota Meso]

UKMET just shifted west...MS landfall (painfully slowly) now

I don't have the UKMET yet...how far west did it go?

Edit: maybe this belongs in the banter thread as things seem to be heating up in this thread, it would be nice IMO to have some body start a now cast thread only and how the now cast could effect the track...

[wxmeddler]

I was looking at cross sections in GR2AE with Dual-Pol and what struck me was the amount of grauple above 30 kft or so. Good chance that all that flying around up there is causing all the lightning. Didn't see any good zdr towers, just a generic signature of potent updrafts carrying liquid water above the environmental freezing level.

[Derecho!]

I don't have the UKMET yet...how far west did it go?

From the FL/AL border (several runs in a row) to landfall in MS now.

[Ian]

I wouldn't be so sure about that. That feature is quite deep and has been quite persistent. Additionally, there was no good center fix where the old center was located. If I had to guess, my bet would be that this feature is going to be dominant and that the winds will eventually respond (which perhaps they haven't quite done yet on the leg that recon has flown, though they haven't really gotten close to this feature yet).

The feeder bands that seem to be trying to develop are all arcing to that 'eyelike feature' as well. The radar presentation is mixed tho for now I think.

[FredRed]

Based upon the wind directions gleaned from the reconnaissance data, I think the northern radar vortex is a mesoscale feature within the broad LLC. Such persistent features often occur downwind of an ULL-induced shear axis. The real LLC is south of the eye-like feature due to mid-level shear and some dry air. And because the ULL is stronger and moving more slowly than forecasted, the prospects of more rapid intensification, within the next 24 hours, are in my view lower than some people might anticipate.

Agree. You can observe the southerly shear:

[SACRUS]

I don't have the UKMET yet...how far west did it go?

Edit: maybe this belongs in the banter thread as things seem to be heating up in this thread, it would be nice IMO to have some body start a now cast thread only and how the now cast could effect the track...

00z ukmet, seems to just stall isaac beween NOLA and Gulfport--Mobile

[Superstorm93]

Agree. You can observe the southerly shear:

Anticyclone is still displaced from the storm.

[Ground Scouring]

One thing I have noticed is that the mid-level circulation associated with Isaac seems to be moving faster than projected due to a binary interaction with the ULL to its southwest. The convergent UL flow in which Isaac is situated, along with the strong shear axis, does not favor strengthening and is in fact similar to that of TS Bonnie (2008), which was in a similar environment as it approached FL. The GFS-based models have not handled the depth of the ULL well and have forecasted that it would move out to the NNW by now; in fact, due to the deeper-than-expected ridge, it has shifted S, placing Isaac in a rather unfavorable position. CIMSS analysis indicates shear has actually increased over Isaac over the last few hours, as the outflow boundaries in the southern semicircle attest. In my view, unless things change (which, given the synoptic pattern, is not likely for another 24 hours), the potential for the intensity forecasts to bust, perhaps significantly, is elevated and will increase further. I think that the models, excepting the dynamically based intensity models, have been far too slow to account for the changes in the initial environment, which is worrisome; for if the forecasts bust, the NHC will be unfairly blamed and complacency will set in.

[Frank]

Interesting that the NAM/UKMET keep Isaac at a relatively weak stage as he makes landfall while other global models are much more bullish with his intensity.

[atownwxwatcher]

One thing I have noticed is that the mid-level circulation associated with Isaac seems to be moving faster than projected due to a binary interaction with the ULL to its southwest. The convergent UL flow in which Isaac is situated, along with the strong shear axis, does not favor strengthening and is in fact similar to that of TS Bonnie (2008), which was in a similar environment as it approached FL. The GFS-based models have not handled the depth of the ULL well and have forecasted that it would move out to the NNW by now; in fact, due to the deeper-than-expected ridge, it has shifted S, placing Isaac in a rather unfavorable position. CIMSS analysis indicates shear has actually increased over Isaac over the last few hours, as the outflow boundaries in the southern semicircle attest. In my view, unless things change (which, given the synoptic pattern, is not likely for another 24 hours), the potential for the intensity forecasts to bust, perhaps significantly, is elevated and will increase further. I think that the models, excepting the dynamically based intensity models, have been far too slow to account for the changes in the initial environment, which is worrisome; for if the forecasts bust, the NHC will be unfairly blamed and complacency will set in.

Shear tendency from 03 Z actually shows that the shear has decreased around Isaac...

However to its west there is a increasing area..

[Superstorm93]

Interesting that the NAM/UKMET keep Isaac at a relatively weak stage as he makes landfall while other global models are much more bullish with his intensity.

Gonna ignore the fact that you said the NAM, but the UKMET does not have the resolution of the other models.

Or at least that version doesn't.

[CUmet]

One thing I have noticed is that the mid-level circulation associated with Isaac seems to be moving faster than projected due to a binary interaction with the ULL to its southwest. The convergent UL flow in which Isaac is situated, along with the strong shear axis, does not favor strengthening and is in fact similar to that of TS Bonnie (2008), which was in a similar environment as it approached FL. The GFS-based models have not handled the depth of the ULL well and have forecasted that it would move out to the NNW by now; in fact, due to the deeper-than-expected ridge, it has shifted S, placing Isaac in a rather unfavorable position. CIMSS analysis indicates shear has actually increased over Isaac over the last few hours, as the outflow boundaries in the southern semicircle attest. In my view, unless things change (which, given the synoptic pattern, is not likely for another 24 hours), the potential for the intensity forecasts to bust, perhaps significantly, is elevated and will increase further. I think that the models, excepting the dynamically based intensity models, have been far too slow to account for the changes in the initial environment, which is worrisome; for if the forecasts bust, the NHC will be unfairly blamed and complacency will set in.

First of all, the upper level flow is quite divergent, as the SHIPS and the CIMSS analyses would attest. Second, the upper level low was never forecast to move NNW, and in fact was forecasted to cut off and move towards the WSW, which is pretty much exactly what it's doing...

[Minnesota Meso]

00z ukmet, seems to just stall isaac beween NOLA and Gulfport--Mobile

What about the H5 Heights? that may give us a clue as to what the Euro will do?

[SnowGoose69]

One thing I have noticed is that the mid-level circulation associated with Isaac seems to be moving faster than projected due to a binary interaction with the ULL to its southwest. The convergent UL flow in which Isaac is situated, along with the strong shear axis, does not favor strengthening and is in fact similar to that of TS Bonnie (2008), which was in a similar environment as it approached FL. The GFS-based models have not handled the depth of the ULL well and have forecasted that it would move out to the NNW by now; in fact, due to the deeper-than-expected ridge, it has shifted S, placing Isaac in a rather unfavorable position. CIMSS analysis indicates shear has actually increased over Isaac over the last few hours, as the outflow boundaries in the southern semicircle attest. In my view, unless things change (which, given the synoptic pattern, is not likely for another 24 hours), the potential for the intensity forecasts to bust, perhaps significantly, is elevated and will increase further. I think that the models, excepting the dynamically based intensity models, have been far too slow to account for the changes in the initial environment, which is worrisome; for if the forecasts bust, the NHC will be unfairly blamed and complacency will set in.

I think most people who have any understanding of weather are fully aware that we're basically still stuck in 1985 when it comes to predicting intensity of tropical systems, we still see storms forecast to become Cat 4s stay Cat1s or 2s or systems forecast to be 85 mph 24 hours out wind up at 140 mph. I have only seen stats for forecasting track and NHC has improved significantly the last 20 years, to me it seems there has been almost no improvement the last 20 years though when it comes to predicting the intensity of these things beyond 24 hours.

[phil882]

Isaac is still experiencing southerly shear at this point. The global models are having difficulty with the forecast position of the upper level low. It was expected to rapidly move southwestward and be out of Isaac's hair, but instead has remained nearly stationary near the Yucatan Peninsula. This is resulting in the mid-level center (observed on radar) and the low-level center to be increasingly decoupled from one another. At this point, the low-level circulation is weakening and becoming elongated as the low-level vortex gravitates towards the down-shear convection that is rapidly moving off to the northwest. The main point is that while the center may redevelop further to the northwest, it will still be getting sheared and the storm won't organize further until this shear decreases. We really need that upper level low to drop far enough southward for the shear to finally lessen over the storm. Its eventually suppose to help the system develop and outflow channel, and that could still happen, but its been a major thorn in the side for this system so far.

[thewxmann]

Isaac is still experiencing southerly shear at this point. The global models are having difficulty with the forecast position of the upper level low. It was expected to rapidly move southwestward and be out of Isaac's hair, but instead has remained nearly stationary near the Yucatan Peninsula. This is resulting in the mid-level center (observed on radar) and the low-level center to be increasingly decoupled from one another. At this point, the low-level circulation is weakening and becoming elongated as the low-level vortex gravitates towards the down-shear convection that is rapidly moving off to the northwest. The main point is that while the center may redevelop further to the northwest, it will still be getting sheared and the storm won't organize further. We really need that upper level low to drop far enough southward for the shear to finally lessen over the storm. Its eventually suppose to help the system develop and outflow channel, and that could still happen, but its been a major thorn in the side for this system so far.

If I had a penny for every time an ULL was forecasted to move out of the way, and didn't... I'd be rich now. It happens all the time.

[Amped]

I think most people who have any understanding of weather are fully aware that we're basically still stuck in 1985 when it comes to predicting intensity of tropical systems, we still see storms forecast to become Cat 4s stay Cat1s or 2s or systems forecast to be 85 mph 24 hours out wind up at 140 mph. I have only seen stats for forecasting track and NHC has improved significantly the last 20 years, to me it seems there has been almost no improvement the last 20 years though when it comes to predicting the intensity of these things beyond 24 hours.

Agreed +1. We have better observations but prediction are a matter of rough. The last 2 years they've mostly been weaker then expected.

[Superstorm93]

00z HWRF still has a southern LA solution for now.

[FredRed]

First of all, the upper level flow is quite divergent, as the SHIPS and the CIMSS analyses would attest. Second, the upper level low was never forecast to move NNW, and in fact was forecasted to cut off and move towards the WSW, which is pretty much exactly what it's doing...

hmmm.. maybe not. Upper low looks to be weakening with a strong southwesterly flow into the Isaac. If the upper low continues south maybe some of that southwesterly shear will become turn more easterly...

[phil882]

First of all, the upper level flow is quite divergent, as the SHIPS and the CIMSS analyses would attest. Second, the upper level low was never forecast to move NNW, and in fact was forecasted to cut off and move towards the WSW, which is pretty much exactly what it's doing...

Its moving much slower to the WSW than expected though. This is a pretty poor 18 hour forecast of an upper level feature.

[OKpowdah]

Shear tendency from 03 Z actually shows that the shear has decreased around Isaac...

However to its west there is a increasing area..

Keep in mind, this is a Eulerian way of analysis ... the shear has decreased in the area where Isaac is currently located ... this will generally be the case as wherever Isaac goes, its immediate upper level conditions will be influenced by the induced divergent anticyclone, generally decreasing shear below the antecedent environmental level.

Must think in a Lagrangian frame of reference ... how has the shear environment changed over time, following the path of Isaac? ... In this case, the upper level environment has in fact become more sheared over Isaac.

[FredRed]

Looks like a 60 mile relocation to the northwest from the last fix position. Maybe the low level circulation is catching up with the mid-level.