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

[Ed Lizard]

The GFS ensemble mean continues the westward shift and targets New Orleans. It also really stalls the system as well. I'm curious to see the individuals later, as I'm sure a few members might even be well west of New Orleans.

Expected spread, but GEFS generally support op 0Z GFS.

[dan88]

000

URNT12 KNHC 260813

VORTEX DATA MESSAGE AL092012

A. 26/07:33:40Z

B. 22 deg 54 min N

078 deg 39 min W

C. 850 mb 1391 m

D. 43 kt

E. 061 deg 73 nm

F. 128 deg 69 kt

G. 065 deg 81 nm

H. 995 mb

I. 19 C / 1520 m

J. 20 C / 1520 m

K. NA / NA

L. NA

M. NA

N. 12345 / 8

O. 0.02 / 0.5 nm

P. AF301 1809A ISAAC OB 11

MAX FL WIND 69 KT NE QUAD 07:07:30Z

;

[HurricaneJosh]

Pressure's gone down a couple of mb, but it sure ain't bombing out.

Dying to see the 5 am EDT forecast. Very curious how they're going to handle the dramatic model shifts, as well as the basic disagreement between the Euro and the GFS. It's sort of like the Debby conflict-- but with the roles reversed and the stakes way higher.

[phil882]

Not the picture of a healthy tropical storm at the moment.

[QVectorman]

Not the picture of a healthy tropical storm at the moment.

Yeah I was just looking at the loop and man it looks craptastic!

Looking at the models with the lowest cumulative intensity errors so far it looks like the SHFR is doing fairly well and the top performing models don't really do much with it intensity wise over the next 48 hrs. The average max intensity of the top 4 models only max it out at 67 knts at 72 hours.

[LovintheWhiteFluff]

Pushed it up to 65 MPH

DISCUSSION AND 48-HOUR OUTLOOK

------------------------------

AT 500 AM EDT...0900 UTC...THE CENTER OF TROPICAL STORM ISAAC WAS

LOCATED NEAR LATITUDE 23.1 NORTH...LONGITUDE 79.0 WEST. ISAAC IS

MOVING TOWARD THE NORTHWEST NEAR 18 MPH...30 KM/H. A WEST-

NORTHWESTWARD TO NORTHWESTWARD MOTION IS EXPECTED DURING THE NEXT

48 HOURS WITH A GRADUAL DECREASE IN FORWARD SPEED. ON THE FORECAST

TRACK...THE CENTER OF ISAAC IS EXPECTED TO MOVE JUST NORTH OF THE

NORTH COAST OF CUBA THIS MORNING...MOVE NEAR OR OVER THE FLORIDA

KEYS LATER TODAY AND TONIGHT...AND MOVE INTO THE EASTERN GULF OF

MEXICO ON MONDAY.

DATA FROM AN AIR FORCE RESERVE HURRICANE HUNTER AIRCRAFT INDICATE

THAT MAXIMUM SUSTAINED WINDS HAVE INCREASED TO NEAR 65 MPH...100

KM/H...WITH HIGHER GUSTS. SOME ADDITIONAL STRENGTHENING IS FORECAST

DURING THE NEXT 48 HOURS...AND ISAAC IS EXPECTED TO BE AT OR NEAR

HURRICANE STRENGTH WHEN IT REACHES THE FLORIDA KEYS.

TROPICAL-STORM-FORCE WINDS EXTEND OUTWARD UP TO 205 MILES...335 KM

FROM THE CENTER.

THE LATEST MINIMUM CENTRAL PRESSURE REPORTED BY THE AIRCRAFT WAS 995

MB...29.38 INCHES.

[wxsmwhrms]

00Z EC ENS mean continues to be a little west of the op run, suggesting landfall over the MS/AL line. Also looks a bit weaker with the trough developing over the NE CONUS and inducing the weakness. I keep expecting the GFS to start shifting back east, but...

[atownwxwatcher]

00Z EC ENS mean continues to be a little west of the op run, suggesting landfall over the MS/AL line. Also looks a bit weaker with the trough developing over the NE CONUS and inducing the weakness. I keep expecting the GFS to start shifting back east, but...

Not looking that way thus far !

[wxsmwhrms]

Not looking that way thus far !

It's right on top of the previous run through 42 hours, actually started off a hair left and is now the same. 500 mb heights do look a bit lower N of the storm though, closer to the ECMWF, so maybe it will end up a bit right.

[atownwxwatcher]

It's right on top of the previous run through 42 hours, actually started off a hair left and is now the same. 500 mb heights do look a bit lower N of the storm though, closer to the ECMWF, so maybe it will end up a bit right.

Still looks to be heading on a WNW tracking ...

[atownwxwatcher]

Here is 66 hrs...

[wxsmwhrms]

There's no change at all to the track or depth of the system on the 06Z GFS at least up to reaching the coast, 981mb S of New Orleans along the coast at 00Z Wed. The 500 mb heights are a hair lower, though, so maybe there is some room for shifting right in future runs. The EC does have a bit lower heights at 00Z Wed than the GFS, but the ECENS is a bit higher than the op EC - so maybe a track in between the EC and GFS is trending toward the most likely scenario right now - although I am not sure I would put the eastern scenario of the UKMET on total life support until we get a little farther down the road. It's such a close situation, and with the UKMET still farther right and in that camp...but it certainly seems less likely at this point.

[LakeEffectKing]

06z HRWF had a 940ish hit on the Mississippi river delta. Verbatium, the northeastern EW would pass over New Orleans from the ESE.

[dan11295]

New burst of convection past couple of hours, dont think the centers are re-aligned yet so may not stick (same as the last burst).

[EasternUSWX]

The center looks to be coming into Miami's radar. Am i correct on this?

[dan11295]

The center looks to be coming into Miami's radar. Am i correct on this?

Center is within short range Key West radar actually. Can see spotty convection on the S side of the circulation now, still has a ways to moisten up though. 8 am Advisory is up, no change but appears to be west of 5 am forecast track.

[Weathergun]

The 6z GFDL has shift west to SE LA coast. But not more than a minimal hurricane, before landfall:

[EasternUSWX]

Center is within short range Key West radar actually. Can see spotty convection on the S side of the circulation now, still has a ways to moisten up though. 8 am Advisory is up, no change but appears to be west of 5 am forecast track.

I see it there for sure. Se side of radar. Looks like convection is increasing too.

[CUmet]

I think a better explanation is mainly that the mid-level and low-level centers are once again not vertically aligned. The low-level vortex is feeling the effects of binary interaction with the other low level vortex over the Caribbean, while the mid-level vortex is more influenced by the ese/wnw oriented mid-level ridge. There is also moderate southerly shear over Isaac currently.

While I was pretty optimistic about Isaac's future given the favorable upper level pattern depicted by both the 12z GFS and 12z ECMWF, there is a fly in the ointment. There is a pretty strong upper level low located around 22N 87W. This feature is moving very slowly and will quickly start to disrupt Isaac's outflow if it doesn't start moving eastward. Thus, I don't think we will see much development over the next 12-24 hours given both the center decoupling and this unfavorable position of the ULL. The models ultimately have this same ULL enhancing outflow in the 48-72 hour period, but this will depend on if it can move out of the way in the short term.

The ULL should, and is, moving out of the way due to the interaction with Isaac's diabatic heating and outflow. This heating and outflow is eroding the upper trough, allowing it to eventually cut off and become smaller in scale. Although often times upper troughs do indeed impart shear and restrict outflow, the cutoff nature and the smaller size of this trough would argue that this is more likely to be a positive trough interaction. It is something that could "go wrong", however.

Isaac is currently under moderate southwesterly shear, but I expect this to gradually weaken over the next day as the upper low becomes cut off and upper level winds turn more southeasterly and strengthen over Isaac. This coupled with the lack of inner core organization suggest that intensification should be slow and gradual over the next 24 hours.

[Weathergun]

The ULL backing SW into the Yucatan, might actually help pull Issac more westward. While the shear and dry intrusion caused ULL, will keep intensity in check, over next day or so:

[eekuasepinniW]

[ams30721us]

Starting to raise concerns for the coast of LA now.

12z early Guidance coming in and shows a tightly clustered western shift.

]

[dan11295]

Radar is interesting, heaviest precip close to the center is not around the actual center based on the base velocity but displaced a bit to the NE. That said slow organization continues.

[Ed Lizard]

Why is that?

Isaac's Storm, about the 1900 Galveston Hurricane that killed 6,000 to 10,000 people is in my reading room. Isaac Cline was head of the Galveston NWS office in 1900. His wife died in the storm.

6Z GEFS look more MS/AL, on quick visial inspection.

[Cory]

No idea how well the WRF Rapid Refresh handles tropical systems, but the past few runs forecast a due west movement through this evening http://tinyurl.com/9b4cqjo (Java loop)

[dan11295]

is issac organisiing slower than expected?

De-coupling of MLC/LLC along with some southerly shear have slowly development a tad (i.e I dont think it will be a hurricane when it crosses the Keys, but do not think it will be major factor in landfall intensity. If models are wrong re: shear and outflow features in 48-72 hours, that will be a bigger factor.

[jasons]

No idea how well the WRF Rapid Refresh handles tropical systems, but the past few runs forecast a due west movement through this evening http://tinyurl.com/9b4cqjo (Java loop)

Seems to be the case looking at GrLevel2 out of Key West. It's well south of the Keys and is trekking due west or maybe a tick north of Due West. It looks like it will clear EYW well to the south.

I'm breathing a sigh of relief as I have lots of junk in storage on Big Pine Key. Last night the NHC track was directly overhead. This will miss well to the south of there.

[EasternUSWX]

Seems to be the case looking at GrLevel2 out of Key West. It's well south of the Keys and is trekking due west or maybe a tick north of Due West. It looks like it will clear EYW well to the south.

I'm breathing a sigh of relief as I have lots of junk in storage on Big Pine Key. Last night the NHC track was directly overhead. This will miss well to the south of there.

Hard to tell but looks like it slowed down a bit too.

[jasons]

Maybe a pro-met can chime in, but looking at Isaac's path the last few days, and his path of jumping north towards Haiti and then hugging the north coast of Cuba, could this be a case of the frictional effects on the winds 'pulling' the center towards the coast(s)?

A certain un-named met talked about this at length in his column years about when Isidore would not 'let go' of the Yucatan. The accelerating wind over water would hit land, slow down, and act to tug on the core and pull it inward back to the coast. Isaac's path resembles a weaker version of this Idisore-like land effect. Now Isaac is significantly weaker, so it may not hold water in this case, but is there any validity to it, or can this trend be solely attributed to the mid-upper steering currents?

[OSUmetstud]

Given the visible and IR presentation I think Isaac is going to start an intensification trend soon.