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

[QVectorman]

12z Euro @180 hours. 60-70 kt wind max...Finally something "decent" to use the winds fct during hurricane season on the Euro from weatherunderground.

[hurricaneman]

About a half hour ago, Recon found 2 areas of low pressure, 1 at 16N and 1 at 15N, it looks like there might be a center relocation going on towards the southern vortex

[HurricaneJosh]

About a half hour ago, Recon found 2 areas of low pressure, 1 at 16N and 1 at 15N, it looks like there might be a center relocation going on towards the southern vortex

If that's true, cool. The S one is preferred!

[ALhurricane]

The Euro scenario is very Frederic-ish: weak over the Greater Antilles --> very strong on the C Gulf Coast:

Taking a brief look at the NARR reanalysis for Frederic, there appears to be some similarities in the evolution of the synoptic pattern. Will have to take a closer look at this when I can get on a regular PC.

[MarkSC]

About a half hour ago, Recon found 2 areas of low pressure, 1 at 16N and 1 at 15N, it looks like there might be a center relocation going on towards the southern vortex

Yes, looks like a relocation to the SSE judging by visible.

[icebreaker5221]

Excellent post Will... I completely agree with you, that the main problem is that the mid-level circulation has not been "closed" on the southwestern flank... allowing for the advection of dry air in the mid levels. My only question is that despite this, why is all of the deep convection on both satellite imagery and microwave imagery focused on the southern flank. If there was dry air being ingested from the SW, why is it not resulting in displaced convection to the NE or E of the circulation center?

Thanks Phil!

I found that a bit puzzling as well. It could be that the dry air being advected in is also more unstable. I found this quite a bit looking at PREDICT soundings. Meaning we are getting stronger bursts of convection, but also lots of downdrafts killing convection further north. Just a guess

[EasternUSWX]

This thing is finally getting organized it seems like center may relocate under the deep convective firing up in the middle of the storm. If so maybe tonight we see it finally get interesting as shear decreases as well.

[LakeEffectKing]

This thing is finally getting organized it seems like center may relocate under the deep convective firing up in the middle of the storm. If so maybe tonight we see it finally get interesting as shear decreases as well.

None of the models have any real "interesting" deepening over the next few days....at 20+mph movement, through the usually hostile C. Carribean, we are lucky to have a fairly stacked system.

[Superstorm93]

No wind shift where one would extrapolate it to be based on the first pass.

Might actually be further south.

[Derecho!]

No wind shift where one would extrapolate it to be based on the first pass.

Might actually be further south.

I'm always skeptical of center relocations but based on the pressures on the current pass, they are now a little bit lower on the new "southern center" where the actual windshift is than on the old "northern center" of the last VDM. Actually does seem to be relocating.

[CUmet]

Looking at the WV imagery, one could easily be fooled into thinking that dry air to the NE of the system is causing the biggest woes. However, TPW suggests the dry air wrapping in from the W is a far greater problem in terms of actually affecting the core of the system.

One can also see the advantage of looking at systems from a co-moving (LaGrangian) framework. This is done by simply subtracting the wave's phase speed from the wind field at every point. An earth-relative framework depicts virtually no ingestion of dry air into Isaac (right), while the co-moving framework shows lower TPW air working its way S, SE, and eventually E into the heart of the vortex (left).

00Z GFS analysis (sorry, don't have 12Z yet) with LaGrangian wind vectors depicts the ingestion of dry air nicely. We can see how the LaGrangian wind vectors are nearly parallel to the TPW gradient along the path of the dry air, suggesting significant advection of dry-air. This leads to dry-air entrainment, a large saturation deficit in the core of the storm, and dry convective downdrafts destroying any secondary circulation that is trying to develop. This is what was occurring last night and is still occurring somewhat right now. Note that wind vectors (red) are indicated between the two outer-most closed 700 mb streamlines, as this is a reasonable estimate of the outer-most environment affecting the storm.

You can see that the GFS forecast for 00Z today suggests an entirely different situation. Wind vectors are nearly perpendicular to the TPW gradient (again, within the outer-most closed streamlines) and there is significantly less advection of dry air into the center of circulation. Essentially the system has "closed off" and isolated itself from the dry air. An expected decrease in wind shear is certainly a large contributor as to why this becomes possible. I suspect that the GFS is slightly too quick as to when this will occur (00Z tonight seems a bit too soon to me). But I have a feeling that by tomorrow morning we'll see much less dry air infiltrating Isaac, and we should have a hurricane on our hands.

[TPW imagery courtesy CIMSS, all other products courtesy Montgomery Research Group.]

Interesting thoughts. I would be interested to see how much dry air is being entrained close to the TC from the northeast above 700 mb, and what/how much of a role that is playing in comparison to the low-level downdrafts (although the two may not be independent). Ryan Torn's EnKF analysis shows the environmental winds (black line) being from the ENE above 600 mb, but from the north in a storm-relative sense. Also note the somewhat larger magnitude of shear between 850-350 mb compared to 850-200 mb.

[Superstorm93]

No surprise there's nothing on radar.

Technically, this could slide even further SW based on those extremely weak winds with the same pressure as the "new center"

EDIT: Kermit just took off from Barbados. Really loaded flight. Shame this doesn't even have a well defined circulation.

[Weathergun]

The 12z Euro ensemble mean, is on par with the operational, with the track towards North-Central Gulf coast:

[thewxmann]

Also interesting to note, the ECMWF has recently shifted right on Typhoon Bolaven in the WPAC, towards the GFS et. al. camp in that basin. Going back to Isaac, even if the physics of the ECM ends up being right, I would still expect it to shift right towards the GFS camp if the over-depiction of ridging in the WPAC coincides with the over-depiction of ridging on this side of the world. And given the ECM's performance with Debby, it may well be that the ECM is too strong with the ridging when this cuts across Hispaniola.

[Hurriplane]

Moving even faster...per 5pm

000

WTNT44 KNHC 222039

TCDAT4

TROPICAL STORM ISAAC DISCUSSION NUMBER 7

NWS NATIONAL HURRICANE CENTER MIAMI FL AL092012

500 PM AST WED AUG 22 2012

SATELLITE...AIRCRAFT...RADAR...AND SURFACE OBSERVATIONS INDICATE

THAT ISAAC HAS A BROAD CENTRAL AREA OF LIGHT WINDS WITH MULTIPLE

VORTICITY MAXIMA EMBEDDED. THE LOWEST PRESSURES ARE IN AN AREA

NEAR GUADELOUPE...WHICH IS USED AS THE CURRENT CENTER SINCE IT HAS

SOME CONTINUITY WITH THE CENTER PREVIOUSLY BEING TRACKED.

HOWEVER...THE MEAN CENTER IS LIKELY TO THE SOUTHEAST OF THE

ADVISORY POSITION. SOME ADJUSTMENT OF THE CENTER POSITION MAY BE

NECESSARY WHEN THE CENTRAL CORE AGAIN CONSOLIDATES. THE INITIAL

INTENSITY IS HELD AT 40 KT...ALTHOUGH THE HURRICANE HUNTER AIRCRAFT

DATA SUGGESTS THIS COULD BE GENEROUS.

THE INITIAL MOTION IS NOW A SOMEWHAT UNCERTAIN 280/19. OTHER THAN

THE CENTER POSITION ISSUES...THERE IS NO SIGNIFICANT CHANGE TO THE

FORECAST REASONING DURING THE FIRST 48 HR OF THE TRACK FORECAST. THE

EAST-WEST ORIENTED SUBTROPICAL RIDGE TO THE NORTH OF ISAAC ALONG

30N LATITUDE IS FORECAST BY ALL OF THE GLOBAL MODELS TO REMAIN

INTACT ACROSS FLORIDA AND THE SOUTHEASTERN U.S. DURING THAT TIME.

THIS EARLY PART OF THE FORECAST TRACK IS SHIFTED A LITTLE TO THE

SOUTH OF THE PREVIOUS FORECAST BASED ON THE OBSERVED POSITION AND

MOTION. AFTER 48 HR...THE RIDGE IS EXPECTED TO WEAKEN AS A

MID/UPPER-LEVEL TROUGH MOVES ONTO THE EASTERN UNITED STATES. THE

GLOBAL MODELS HAVE COME INTO BETTER AGREEMENT SINCE THE LAST

ADVISORY...WITH THE NOGAPS AND CANADIAN MODELS SHIFTING TO THE WEST

AND THE ECMWF TO THE NORTH. HOWEVER...THE ECMWF CONTINUES TO SHOW

A STRONGER RIDGE...AND THUS SHOWS A MORE WESTERLY MOTION THAN THE

OTHER MODELS. THE 72-120 HR PORTION OF THE FORECAST TRACK IS

SIMILAR TO THE PREVIOUS FORECAST AND LIES NEAR THE CONSENSUS

MODELS. THERE REMAINS CONSIDERABLE UNCERTAINTY AS TO WHICH PORTIONS

OF FLORIDA...IF ANY...COULD BE AFFECTED BY ISAAC.

WATER VAPOR IMAGERY AND MICROWAVE TOTAL PRECIPITABLE WATER DATA

CONTINUE TO SHOW DRY AIR TO THE NORTHEAST OF ISAAC...AND A LACK OF

CONVECTION IN THIS AREA SUGGESTS SOME OF THIS AIR IS ENTRAINING

INTO THE CYCLONE AND DISRUPTING THE CONVECTIVE STRUCTURE. GIVEN

THE CURRENT STRUCTURE...THE NEW INTENSITY FORECAST CALLS FOR LITTLE

CHANGE IN STRENGTH FOR THE NEXT 12 HR...FOLLOWED BY INTENSIFICATION

INTO A HURRICANE BY 48 HR SIMILAR TO THE PREVIOUS ADVISORY. AFTER

48 HR...THE MAIN FACTOR CONTROLLING THE INTENSITY IS THE AMOUNT OF

LAND INTERACTION. THIS LOW-CONFIDENCE PART OF THE INTENSITY

FORECAST IS SIMILAR TO THE PREVIOUS ADVISORY BUT ABOVE THE INTENSITY

CONSENSUS.

FORECAST POSITIONS AND MAX WINDS

INIT 22/2100Z 16.0N 61.2W 40 KT 45 MPH

12H 23/0600Z 16.3N 63.6W 40 KT 45 MPH

24H 23/1800Z 16.7N 66.7W 50 KT 60 MPH

36H 24/0600Z 17.2N 69.4W 60 KT 70 MPH

48H 24/1800Z 17.9N 71.6W 70 KT 80 MPH...INLAND

72H 25/1800Z 20.5N 75.5W 60 KT 70 MPH...INLAND

96H 26/1800Z 23.5N 79.5W 60 KT 70 MPH...OVER WATER

120H 27/1800Z 26.5N 82.5W 70 KT 80 MPH

$$

FORECASTER BEVEN

[HurricaneJosh]

Kind of interesting-- and it speaks to the confusion we were having in this thread Re: the center location:

THE LOWEST PRESSURES ARE IN AN AREA NEAR GUADELOUPE...WHICH IS USED AS THE CURRENT CENTER SINCE IT HAS SOME CONTINUITY WITH THE CENTER PREVIOUSLY BEING TRACKED. HOWEVER...THE MEAN CENTER IS LIKELY TO THE SOUTHEAST OF THE ADVISORY POSITION. SOME ADJUSTMENT OF THE CENTER POSITION MAY BE NECESSARY WHEN THE CENTRAL CORE AGAIN CONSOLIDATES.

[AgeeWx]

I find it very interesting that the 5pm NHC update mentioned the "... CONSIDERABLE UNCERTAINTY AS TO WHICH PORTIONS

OF FLORIDA...IF ANY...COULD BE AFFECTED BY ISAAC". I still don't think the models can get a hold of the subtropical ridging but it sounds like the experts in Miami think it may hold and, in effect, steer Isaac into the GOM.

[Ground Scouring]

For what it’s worth, the overall short-term (24-hour) movement since TD formation yesterday has consistently been south of the NHC track. Given the consistency of the GFS ensembles, which through three and a half days now follow the Euro, and given the apparent likelihood of more significant intensification after 24 hours, I believe the system might well skirt the southern half of the NHC cone and instead pass over or just south of the Barahona Peninsula by day three...and thence pass over the less mountainous SW jutting leg of Haiti. As we have seen in previous systems, an intensifying system might well serve to pump up the heights at 500 mb...particularly in this case due to factors such as strong low-level inflow and a reforming center favoring greater convection due to strengthening convergence as shear relaxes in about 24 hours (as the slowing motion of the TC already suggests). Thus I am willing to side with the ECMWF and GFS ensembles here and favor less land interaction with Hispaniola and Cuba initially.

[IsentropicLift]

I apologize if this has been already posted, but this is an excellent product which shows that Isaac is still be affected by shear on the eastern side, but conditions are improving to the west.

[HurricaneJosh]

I find it very interesting that the 5pm NHC update mentioned the "... CONSIDERABLE UNCERTAINTY AS TO WHICH PORTIONS

OF FLORIDA...IF ANY...COULD BE AFFECTED BY ISAAC". I still don't think the models can get a hold of the subtropical ridging but it sounds like the experts in Miami think it may hold and, in effect, steer Isaac into the GOM.

That's an interesting point. Perhaps we are reading between the lines, but it is interesting that FL impact is not treated as a given. (On the other hand, it's probably a best-practice for a forecaster to avoid treating any future track/impact as a given.)

[wxsmwhrms]

The 12z Euro ensemble mean, is on par with the operational, with the track towards North-Central Gulf coast:

If you look at the 24 hours before that, the EC ensemble mean low is actually SW of the operational low with an extension in the trough toward that direction. Can anyone with access to the spaghetti plots comment on how they look? It is obviously hard to guess just by looking at the mean, but it would seem like there might be more members on the SW side than the NE just based on the mean.

Having said that, my gut instinct given the trends in the short term and the trends to the right during that time, and the fact that the EC has overdone the ridging makes me think that it is a bit too far left in the longer term. But the GFS is probably too far right given its own tendency to create too strong of weaknesses and the fact its own ensembles suggest a farther west track closer to the EC. A compromise in the 5-7 day period is probably how I would go (real shocker there, LOL).

[HurricaneJosh]

If you look at the 24 hours before that, the EC ensemble mean low is actually SW of the operational low with an extension in the trough toward that direction. Can anyone with access to the spaghetti plots comment on how they look? It is obviously hard to guess just by looking at the mean, but it would seem like there might be more members on the SW side than the NE just based on the mean.

Having said that, my gut instinct given the trends in the short term and the trends to the right during that time, and the fact that the EC has overdone the ridging makes me think that it is a bit too far left in the longer term. But the GFS is probably too far right given its own tendency to create too strong of weaknesses and the fact its own ensembles suggest a farther west track closer to the EC. A compromise in the 5-7 day period is probably how I would go (real shocker there, LOL).

Makes sense. So you're thinking extreme-E Gulf into FL's Big Bend?

[Ground Scouring]

For what it’s worth, the overall short-term (24-hour) movement since TD formation yesterday has consistently been south of the NHC track. Given the consistency of the GFS ensembles, which through three and a half days now follow the Euro, and given the apparent likelihood of more significant intensification after 24 hours, I believe the system might well skirt the southern half of the NHC cone and instead pass over or just south of the Barahona Peninsula by day three...and thence pass over the less mountainous SW jutting leg of Haiti. As we have seen in previous systems, an intensifying system might well serve to pump up the heights at 500 mb...particularly in this case due to factors such as strong low-level inflow and a reforming center favoring greater convection due to strengthening convergence as shear relaxes in about 24 hours (as the slowing motion of the TC already suggests). Thus I am willing to side with the ECMWF and GFS ensembles here and favor less land interaction with Hispaniola and Cuba initially.

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

THIS EARLY PART OF THE FORECAST TRACK IS SHIFTED A LITTLE TO THE

SOUTH OF THE PREVIOUS FORECAST BASED ON THE OBSERVED POSITION AND

MOTION.

[wxsmwhrms]

Kind of interesting-- and it speaks to the confusion we were having in this thread Re: the center location:

THE LOWEST PRESSURES ARE IN AN AREA NEAR GUADELOUPE...WHICH IS USED AS THE CURRENT CENTER SINCE IT HAS SOME CONTINUITY WITH THE CENTER PREVIOUSLY BEING TRACKED. HOWEVER...THE MEAN CENTER IS LIKELY TO THE SOUTHEAST OF THE ADVISORY POSITION. SOME ADJUSTMENT OF THE CENTER POSITION MAY BE NECESSARY WHEN THE CENTRAL CORE AGAIN CONSOLIDATES.

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.

[Alfoman]

Tell me if I'm wrong, but according to the data from the newly departed Kermit mission, the pressure is 1001 mb just off of Barbados!?

[wxsmwhrms]

Makes sense. So you're thinking extreme-E Gulf into FL's Big Bend?

Probably pretty close to the GEFS track if somebody made me make a forecast.

[LakeEffectKing]

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.

[Superstorm93]

12z ECMWF EPS Control is even further west than the OP

Looks like a central Louisiana landfall.

Very intense system given this is the control.

EDIT: Really looking huge. Outflow seems to be expanding in all directions despite the ULAC being displaced.

Probably the most impressive looking 45 mph tropical storm (on satellite) I have seen in the Atlantic in years.

[icebreaker5221]

Interesting thoughts. I would be interested to see how much dry air is being entrained close to the TC from the northeast above 700 mb, and what/how much of a role that is playing in comparison to the low-level downdrafts (although the two may not be independent). Ryan Torn's EnKF analysis shows the environmental winds (black line) being from the ENE above 600 mb, but from the north in a storm-relative sense. Also note the somewhat larger magnitude of shear between 850-350 mb compared to 850-200 mb.

Very interesting! From those products, one could certainly make the argument that the dry air north of the system is causing more trouble than the dry air to the west. In reality, it's almost certainly some combination of the two.

[Dsnowx53]

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.

Definitely agreed with this. If Issac can stay far enough south, it might even feel the effects of the "death ridge" near the southern Plains, and turn much more westerly towards Texas, even. The fact that all the data seems to stall Issac once it hits land is indicative of the lack of a steering flow. There is a weak trough that enters the midwest, which combined with a potential weakness in the Atlantic ridge that Issac could run into once it hits the Gulf, that could turn him northward towards the western Florida Panhandle. But if that is not enough, Issac could bend well back to the west, and hopefully relieve some drought conditions in the southern Plains.

I can see a Track "A" scenario with Issac approaching the western FLA Panhandle area, and a track "B" scenario of it bending much further west. Not sure I see an in-between scenario at this time.