Tropical Storm Isaac, Part 1: Moving Through The Eastern Caribbean Sea

[csnavywx]

My take is that if the Euro indeed has the mid-level ridging properly modeled out in time, then solutions further west than even what the Euro (and every other model out there) have currently, are NOT off the table.....It's not like we have a massively digging trough that turns Isaac. And with the ongoing drought feeding back into the seasonal propensity to hold strongly onto ridging across the deep south, I wouldn't overlook the potential of Isaac to head back westward, once/if he reaches the GOM.

Strongly agree here. I was going to post in response to wxmann's post about the WPAC typhoon and mention the differences in ridging to the north of each storm making them incomparable, but you said it best here already. I really think we might see a virtual collapse of the steering currents once he does get near/in the GOM, which would cause all kinds of track forecast problems.

[Superstorm93]

Recon seems to have found the 'center' even further SW. 1005 mbs with no wind just NE of Dominica.

Both Kermit and the USAF plane went through the area. (Displaying both planes here)

I'll wait for a VDM.

[Typhoon Tip]

That was a fascinating discussion for Isaac by TPC. The idea of multiple vortex foci and their having to take a mean of them for their 500pm fix is pretty thrilling. I don't see they have any other choice.

I have been following the satellite fairly closely since yesterday, and it seems pretty clear that a lightly sheared environment helped dislodge a CDO feature. That sort of "decapitated" Isaac. This feature than moved off toward 62/15 or so; the low level circulation appeared to become diffused and elongated when that happened - the UVM associated may have, or be, attempting to core down and couple with the lower levels near 62.5W/15N, to produce a new center there.

Another statement they say is that they admit that a better center might be SE of their fix. There is a smart looking convective mass around 59/14.5N, which could collocate with that statement.

This is a really interesting system to watch evolve.

[HurricaneJosh]

This snippet from 5 p.m. EDT discussion is worth noting:

Even though the pressures were not as low, recon actually found a decent wind shift down around 14.5N, 62W, almost like a trough or lobe extending SW into that area.

I am loving this dual mission data!

Recon seems to have found the center even further SW. 1005 mbs with no wind just NE of Dominca.

Needless to say, if the center is reforming further to the S-- say, even below 15N-- that is going to have big implications Re: land interaction-- i.e., there might be much less than currently shown in the official forecast.

[Dsnowx53]

Strongly agree here. I was going to post in response to wxmann's post about the WPAC typhoon and mention the differences in ridging to the north of each storm making them incomparable, but you said it best here already. I really think we might see a virtual collapse of the steering currents once he does get near/in the GOM, which would cause all kinds of track forecast problems.

And often people say that a stronger system means it goes more poleward. But if there are very weak mid and upper level steering currents, then what does it matter? I could certainly see the weakness in the Atlantic ridge and a little nudging from the transient midwest trough pulling this towards the western FLA Panhandle, but it's not like there is anything particularly impressive in the mid-levels, such as a large trough, that would definitely pull this northward. There is a very reasonable chance that these features will not be enough to pull Issac northward, and that if it stays south and west enough, it will instead feel the effects of the very strong and persistent 594dam ridge in the southern Plains and turn back to the west after initially trying to go north a bit.

[Typhoon Tip]

And often people say that a stronger system means it goes more poleward. But if there are very weak mid and upper level steering currents, then what does it matter? I could certainly see the weakness in the Atlantic ridge and a little nudging from the transient midwest trough pulling this towards the western FLA Panhandle, but it's not like there is anything particularly impressive in the mid-levels, such as a large trough, that would definitely pull this northward. There is a very reasonable chance that these features will not be enough to pull Issac northward, and that if it stays south and west enough, it will instead feel the effects of the very strong and persistent 594dam ridge in the southern Plains and turn back to the west after initially trying to go north a bit.

Is it possible that what people are referring to in their polarward statement, whether they realize or not, is "beta drift" ??

[Superstorm93]

That looks like an ill-defined center north of Dominica.

Was obviously interesting enough for the crew to do that sort of pattern around the "center"

(See image in next post)

[turtlehurricane]

This is the sort of cyclone that could survive Hispaniola if it stays in this state, nice and broad.

[andyhb]

18z GFS is ever-so slightly further east, although basically the same as 12z.

[Amped]

18z GFS doesn't eject the PACNW on day 5 much like the 12z Euro. This could delay the turn NE day 5 and 6.

[Superstorm93]

Product: Air Force Vortex Message (URNT12 KNHC)

Transmitted: 22nd day of the month at 22:07Z

Aircraft: Air Force Aircraft (Last 3 digits of the tail number are 307)

Storm Number & Year: 09L in 2012

Storm Name: Isaac (flight in the North Atlantic basin)

Mission Number: 3

Observation Number: 10

A. Time of Center Fix: 22nd day of the month at 21:44:50Z

B. Center Fix Coordinates: 15°44'N 61°25'W (15.7333N 61.4167W)

B. Center Fix Location: 27 miles (44 km) to the SE (132°) from Basse Terre, Guadeloupe (FRA).

C. Minimum Height at Standard Level: 1,470m (4,823ft) at 850mb

D. Estimated (by SFMR or visually) Maximum Surface Wind: 27kts (~ 31.1mph)

E. Location of the Estimated Maximum Surface Wind: 90 nautical miles (104 statute miles) to the SE (131°) of center fix

F. Maximum Flight Level Wind Inbound: From 73° at 13kts (From the ENE at ~ 15.0mph)

G. Location of Maximum Flight Level Wind Inbound: 13 nautical miles (15 statute miles) to the W (278°) of center fix

H. Minimum Sea Level Pressure: 1005mb (29.68 inHg)

I. Maximum Flight Level Temp & Pressure Altitude Outside Eye: 18°C (64°F) at a pressure alt. of 1,525m (5,003ft)

J. Maximum Flight Level Temp & Pressure Altitude Inside Eye: 19°C (66°F) at a pressure alt. of 1,524m (5,000ft)

K. Dewpoint Temp (collected at same location as temp inside eye): 18°C (64°F)

[andyhb]

18z GFS doesn't eject the PACNW on day 5 much like the 12z Euro. This could delay the turn NE day 5 and 6.

Yes, good catch.

[HurricaneJosh]

B. Center Fix Coordinates: 15°44'N 61°25'W (15.7333N 61.4167W)

A few tenths of a degree S of the 5 pm EDT advisory position-- just like Beven indicated might happen in the Discussion.

[PSUBlizzicane2007]

18Z GFS up the west coast of Florida again. It's good to see its shifts becoming smaller... hopefully that trend continues and we can see a better idea of a final solution. It will be interesting to see if the Euro shifts east again at 00Z.

[SN_Lover]

Looks like the dry air to the northeast of the storm has dissipated at least.

http://www.esl.lsu.edu/static/animations/goes/atmospheric/latest_wv_loop.gif

[Amped]

Yes, good catch.

Yes but I was wrong on the effect. The stronger troff in the east will probably pull it north faster then NNE. It won't stall over Georgia like 12z.

[AgeeWx]

18z GFS samples the wave at 500mb off the Aleutians and does nothing with it to break down the 594 dam ridge at the end of the D5 period, thus rendering the aforementioned ridge as the dominant steering component as Isaac enters the GOM. However, the GFS still moves Isaac up the coast of Florida and into the big bend region.

[wxsouth]

This is the sort of cyclone that could survive Hispaniola if it stays in this state, nice and broad.

Agreed. There's no inner core to disrupt and rebuild. Time will tell if it remains this disorganized through the approach to Hispaniola. I find the current structure pretty interesting...there are at least 3 different vorticity centers rotating around a common center, with two of the vort maxes associated with deep convective bursts. I have no clue how this will evolve over the next 12-18 hours.

[David Reimer]

Looks like NHC was spot on with their 5 PM advisory position relative to the new LLC.

[dan88]

Some convection starting to appear North of the Western convection, the NOAA plane had a couple observations with 25/26 mm/hr convection around 15.9N/63.1W with NW flight level winds. This goes along with recent IR images showing multiple new convective cells.

[bigtenfan]

Total total amature here that really appreciates the posts from all of the red tag posters. From reading the latest posts it appears that a direct hit to the SE coast of Florida is looking less and less likely. Is that correct?

[Big O]

18z GFS samples the wave at 500mb off the Aleutians and does nothing with it to break down the 594 dam ridge at the end of the D5 period, thus rendering the aforementioned ridge as the dominant steering component as Isaac enters the GOM. However, the GFS still moves Isaac up the coast of Florida and into the big bend region.

In light of this are you discounting the 18z GFS solution in favor of a more westerly trajectory as LEK and some others have mentioned as a possibility?

[AgeeWx]

Total total amature here that really appreciates the posts from all of the red tag posters. From reading the latest posts it appears that a direct hit to the SE coast of Florida is looking less and less likely. Is that correct?

I wouldn't go that far. This is the science of trying to predict the unpredictable, nobody will ever be perfect. I would suggest that if you live anywhere in the Southeastern part of the United States that you remain diligent in your observations of the storm(s) as each day progresses and make sure you have your ducks in a row should any storm be forecast to make landfall in your area.

[David Reimer]

Total total amature here that really appreciates the posts from all of the red tag posters. From reading the latest posts it appears that a direct hit to the SE coast of Florida is looking less and less likely. Is that correct?

At this point, nothing can be ruled out. Until a well defined low level circulation and inner core develops, we'll likely continue seeing these jumps in model guidance. Florida and the Southeast US are definitely still in the game.

[AgeeWx]

In light of this are you discounting the 18z GFS solution?

No, just an attempt to verbalize my analysis of what I saw from the model run. I had 'assumed' that if the ridge had remained in place as previously forecast to that it would lead Isaac into the GOM and not up the eastern seaboard.

[thewxmann]

Strongly agree here. I was going to post in response to wxmann's post about the WPAC typhoon and mention the differences in ridging to the north of each storm making them incomparable, but you said it best here already. I really think we might see a virtual collapse of the steering currents once he does get near/in the GOM, which would cause all kinds of track forecast problems.

My reference to the WPAC typhoon was to shed some light on some possible biases that may account for the ECMWF being the outlier. It is a different synoptic setup, but the idea that the ECMWF may be overdoing the ridge, is similar.

wxsmwhrms may have the right idea in taking the middle solution right now. The ECM has ridging too strong, but the GFS may be too gung ho in taking the vorticity straight through Hispaniola with less impediment. However, if the vortex is strongest in the mid levels because of the detrimental low-level windfield/environment, then the GFS may be right after all. I am leaning towards Haiti -> clip eastern Cuba -> S FL/Keys -> Panama City scenario at this time.

[HurricaneJosh]

Since we've been talking about a possible landfall in the Big Bend area of FL...

As has been mentioned, there haven't been a lot of impacts in this region. Probably the most important modern-era landfall between Tampa Metro and Apalachicola is Easy 1950, which crossed the coast near Cedar Key with an intensity of 958 mb/105 kt. The slow movement and double loop in the track meant Cedar Key experienced hurricane conditions for many, many hours:

[peribonca]

Here in Christiansted (St Croix, USVI) we had some thunder an hour ago and the breezes have been kicking up. Doesn't look like we'll get through the eye this time (unlike Irene) since a. there is no eye and b. the center is going to pass 75miles south of us.

[Avdave]

Here in Christiansted (St Croix, USVI) we had some thunder an hour ago and the breezes have been kicking up. Doesn't look like we'll get through the eye this time (unlike Irene) since a. there is no eye and b. the center is going to pass 75miles south of us.

Glad you posted and was wondering how you were faring down there.

http://www.gotostcroix.com/live

[wxsmwhrms]

My reference to the WPAC typhoon was to shed some light on some possible biases that may account for the ECMWF being the outlier. It is a different synoptic setup, but the idea that the ECMWF may be overdoing the ridge, is similar.

wxsmwhrms may have the right idea in taking the middle solution right now. The ECM has ridging too strong, but the GFS may be too gung ho in taking the vorticity straight through Hispaniola with less impediment. However, if the vortex is strongest in the mid levels because of the detrimental low-level windfield/environment, then the GFS may be right after all. I am leaning towards Haiti -> clip eastern Cuba -> S FL/Keys -> Panama City scenario at this time.

That's pretty much where I would be right now too, although given the more southerly formation right now of the center and looking at the new GFS I would guess the center has a shot to pretty much miss Haiti. Even though they were not major changes, the 18Z GFS continues the GFS trend of being slightly stronger with the ridging, which does result in a somewhat faster and farther west solution than previous runs.

I would point out that I completely agree with LEK that given that this motion is all driven by ridging and associated weaknesses rather than any kind of significant trough, a farther west motion is certainly not off the table. Another point in favor of this: I know each storm is different and you can only use climatology so far, but I can name lots of storms that moved W-WNW and interacted with Hisp/Cuba and then continued to move WNW to eventual landfall in TX or central Gulf Coast; the number of storms on that track that then moved into the Big Bend of FL is (I think looking at the historical plots) much smaller. I don't think this storm can go into TX given the strong upper high forecast to develop just west of there, but the lack of a strong trough or steering mechanism other than ridges makes a more specific forecast beyond a few days difficult (even if the general idea is more clear).