Major Hurricane Igor

[Paranoid]

We've probably seen the end of this particular round of rapid intensification, as i said earlier there's a lot of dry air to the west of Igor. It's nothing that should cause any signficant weakening (unless dry air manages to get into the circulation, possibly via an eyewall replacement) if any at all, but don't think conditions are sufficiently conducive to further RI until Igor clears the dry air in the next day or two. Once it's clear then i think Igor has a fair chance of getting to Cat 5, given that the NHC forecast a peak of 155mph it only needs to exceed that by 1mph to be classified as such. The models are currently useless as none of them have taken into account Igor's current round of RI and as such initialise him two categories too low and with a pressure about 30mb too high. Will be interesting to see what the next runs will make of it.

Any thoughts on this from the more experienced members?

Edited September 12, 2010 by Paranoid

[knocker]

Yes, the eye is starting to take shape and Igor is very nearly a hurricane (currently 70mph sustained winds). The intensity forecast has also been bumped up, calling for major hurricane status in 2-3 days.

I know this is digressing slighly but why must hurricanes have eyes?

[londonsnow]

igor is building towards a cat 5

[Paranoid]

@ Weather Ship: I think it's happens due to updrafts from the thunderstorms in the eyewall rising and then sinking at the centre of the storm. As air sinks it becomes warmer and more stable so water vapour does not condense, leaving a cloud free area that is the eye. Sometimes you can see an eye on rainfall radar but not on satellite, this is where the sinking air is not sufficient to remove clouds from the area and is known as a Central Dense Overcast area (CDO).

That's the general jist of the idea but i'm sure one of the more experienced members can give you a better explaination.

[ZONE 51]

Warm tropical moist air flows inwards, where it builds in a circle and rises, condensing out to create a ring of violent thunderstorms tightly packed forming the eye, this formation is the eyewall of the 'eye' or center of the hurricane, the eye itself is there because thunderstorms of the eyewall release large amount of heat, warming the air and building a higher pressure above, forcing it down inside the eye creating the eye and creating calmer conditions and hardly a breeze, thats the eye.

[The PIT]

I found this interesting http://www.nhc.noaa.gov/text/refresh/MIATCDAT1+shtml/130248.shtml?

AN EXPERIMENTAL PRODUCT FROM UW-CIMSS RUN AT NHC INDICATES THAT THERE IS A 42 PERCENT CHANCE OF SECONDARY EYEWALL FORMATION

WITHIN THE NEXT 12 HOURS...AND A 90 PERCENT CHANCE WITHIN 24 HOURS.

[Iceberg]

Not been around much, so a few general points.

Igor yesterday afternoon was a stunning looking storm, the best we've seen this year(and last) and very pacific in it's nature.

However I don't believe for a second that he was a CAT 4 when they raised him, it would take the winds 6-12 hrs to respond to his better organisation so it probably happened quite a bit after this.

Now however I would say he is a good solid CAT 4, particularly with the stable eye, good CDO and very cold cloud tops circuling the centre, which is indicative of instability and a strong eye wall.

I would like his appearence to tidy up a bit more before he reaches a CAT 5, his inflow and outflow needs to be better.

It's a shame there is no recon for the system yet as atm what we have are educated guesses and Dvorak estimates can be 10-20% out, so if we assume 15% the intensity might be anywhere from 110kts to 150kts.

One thing this system will do is provide a massive boost to the ACE score , a rough estimate is that Igor might well add 40 or so to the years total which would take the ACE total into the very active category, validating many a prediction.

[Somerset Squall]

What a beauty! Unless an eyewall replacement happens (The Pit's post shows it's more likely than not that it won't happen today), Igor could become the first category 5 in the Atlantic since Felix in 2007. Intensity is currently at 130kts which makes Igor a solid cat 4, and just an increase in intensity of 10kts would allow this monster to become a cat 5. Igor has a very distinct eye, embedded in a deep central dense overcast feature. However, I agree with Iceberg that Igor doesn't quite have the appearace of a cat 5 yet- it looks as though there is more convection to the northeast of the eye and Igor would need to become a little more symmetrical before an upgrade would occur.

Edited September 13, 2010 by Somerset Squall

[Iceberg]

I have to admit I am sceptical of an EWRC happening today, looking at microwave imagery the eye looks pretty solid, there doesn't seem to be the tell tale signs and instead the eyewall looks very strong. I think in storms where we see very rapid intensification (and Igor certainly falls into this), the eye wall is inheritantly strong and degredation often occurs before the ERC takes place.

Personally I would say that Igor ahs probably peaked for the next 24-48 hrs with an EWRC tomorrow and possible some strong intensification Tuesday night/Wed morning.

[Paranoid]

The sun is now up over Igor so we could see some further intensification due to the effects of diurnal heating. It's also nice to be able to observe something like this without it about to bear down on some unfortunate population centre.

[Iceberg]

This is a Modis picture from Igor yesterday when he was probably at his tidyest.

It's also really helpful in understanding the eye dynamics, the CDO is very evident, as it the whispy outflow and the CDO curling from the SW to the NE (the cloud to the NE is notably higher), this is one of the reasons why the NE has the strongest winds and the most pronounced verticle drop down the eye.

The temp differential is the staggering thing though as mentioned above, its a very easy to read understanding of the strength of the eye wall i.e 22C in the carm of the eye and maybe 5C or 7C in the eyewall thanks to the downdrafts.]

The first signs of the inner eye wall collapsing will be the DP rising and a narrowing of the temp differential, with an associated clouding over of the inner eye.

[Iceberg]

A noticeable SW jog on the latest loops, probably just an eye wobble though. Appearence is improving with outflow and inflow.

The movement to the NE should occur at approx 5-6pm today.

[knocker]

Thanks to those who answered my query. I've since realised that there was an article in weather on this very subject by Professor Robert Pearce, Department of Meteorology. University of Reading in 2005.. The article has been revisited by Roger K. Smith andthe latter is available online. I think I'll stick to the explanations on here as the complexities of the dynamics I freely admit are beyond me.

http://www.meteo.physik.uni-muenchen.de/~roger/Publications/Weather_2005_Smith.pdf

[johnholmes]

. I think I'll stick to the explanations on here as the complexities of the dynamics I freely admit are beyond me.

http://www.meteo.phy..._2005_Smith.pdf

me too Fred; seeing those maths signs sends shudders down my spine-with memories of my AMC!

[knocker]

Glad to see I've reminded you of the good old days John.

[AderynCoch]

I know this is digressing slighly but why must hurricanes have eyes?

I'm not sure whether a hurricane (sustained winds of at least 74mph) has to have an eye, but I've never seen one without. I have seen an eye develop at tropical storm strength before, which seems to suggest that the eye is a feature which develops sooner rather than later.

Igor has matured to say the least this past day or two, with a very impressive satellite presentation typical of powerful hurricanes. The NHC have slightly lowered the intensity forecast from 140kt (low-end Category 5) to 135kt (high-end Category 4). Category 5 still possible I suppose, though I don't know of a hurricane reaching that strength so far east...

[AderynCoch]

Igor's structure also looks annular, very little rainbands and a well defined circular shape.

Nothing on the loops which suggests the convective spiral bands are deteriorating. I also think the eye would need to be just that bit larger (another EWRC required).

For what it's worth, study (what little there has been) of annular hurricanes suggests that they tend to achieve annularity over warm-but-not-that-warm waters (26-27C); Igor is trawling through 28-29C waters and should do for the next few days at least.

Edited September 13, 2010 by AderynCoch

[Iceberg]

Igors sw wobble earlier, was less a wobble and more a just south of west track change as can be seen from the below. Also the expect turn north of west now should be taking place, but it is not.....

A few models do not show the North turn quite yet and the effects on where Igor goes is quite different in this small cluster. It could be that Igor is shearing his way through the dry air and the dry air and his size is creating a small ridge of pressure to his north where the weakness should be. Well worth watching over the next few hrs.

Edited September 13, 2010 by Iceberg

[Paranoid]

@ AC: I said that it looked annular, not that it was (although it probably wasn't very clear so my fault). It certainly doesn't look annular at the moment but at the time i said it, Igor had 1 outer rainband. Besides, annular hurricanes are extremely rare in the Atlantic so it's unlikely purely on a statistical front.

Igor probably doesn't have very long before an eyewall replacement starts. NHC estimated at least a 90% chance of an eyewall replacement in 24 hours on the 10am advisory, so Igor probably has another 12-18 hours.

[Iceberg]

As expected still not sign of an ERC. apart from the slight warming of the cloud tops I think Igor is actually looking better and better, outflow is now outstanding in all quads and he has dual inflows.

The path comment below is interesting as they concur that against expectations Igor is still going due west, it will be fun to see how much he turns north overnight, atm there is very little sign of a turn.

ECM on the 12Z showed the system further south and west than GFS in the hear future and much slower as it does not engage with the weakness shown by GFS.

THE STRUCTURE OF IGOR HAS CHANGED LITTLE OVER THE PAST FEW HOURS.

THE EYE REMAINS VERY DISTINCT IN IR AND VISIBLE IMAGERY...AND

ALTHOUGH THE TOPS SURROUNDING THE EYE HAVE WARMED A BIT...DVORAK

CLASSIFICATIONS STILL SUPPORT AN INTENSITY OF 130 KT. IGOR WILL

REMAIN IN A FAVORABLE ENVIRONMENT FOR MAINTAINING ITS CURRENT

INTENSITY FOR THE NEXT COUPLE OF DAYS...AND THE OFFICIAL FORECAST

DOES NOT SHOW ANY WEAKENING UNTIL AFTER 48 HOURS. WHILE A 1640 UTC

AMSR-E PASS DOES NOT SHOW ANY EVIDENCE OF A SECONDARY EYEWALL

YET...IT IS POSSIBLE THAT AN EYEWALL REPLACEMENT CYCLE WILL OCCUR

IN THE FIRST COUPLE OF DAYS...WHICH COULD RESULT IN SOME INTENSITY

FLUCTUATIONS NOT REPRESENTED IN THE OFFICIAL FORECAST. BEYOND 36

HOURS...ALL OF THE INTENSITY GUIDANCE SUGGEST THAT SLOW WEAKENING

SHOULD BEGIN...ALTHOUGH THERE ARE NO OVERLY NEGATIVE FACTORS UNTIL

THE VERY END OF THE PERIOD WHEN THE VERTICAL SHEAR MAY INCREASE.

THE OFFICIAL FORECAST SHOWS A GRADUAL WEAKENING AT DAYS 3 THROUGH 5

BUT IS HIGHER THAN MOST OF THE GUIDANCE THROUGH THE PERIOD.

THE INITIAL MOTION IS STILL DUE WEST...270 DEGREES...AT 9 KNOTS.

ALL OF THE GUIDANCE...EXCEPT THE UKMET...INSISTS ON A TURN TOWARD

THE WEST-NORTHWEST IN THE NEXT 12 HOURS AS IGOR BEGINS TO MOVE INTO

A LARGE WEAKNESS IN THE SUBTROPICAL RIDGE. HOWEVER...DUE TO THE

INITIAL POSITION AND MOTION...THE OFFICIAL FORECAST HAS AGAIN BEEN

ADJUSTED A LITTLE TO THE LEFT OF THE PREVIOUS ADVISORY THROUGH 48

HOURS.

[Paranoid]

How long until (if ever) Igor comes into range of Recon?

[mountain shadow]

I'm guessing Bermundians hope it keeps going a bit further west before making the turn.

This full disc sat picture from the Met Office is a thing of beauty. On looking at thw three times I would say it's a little more ragged now than this morning.

http://www.metoffice.gov.uk/satpics/namerica_disc.html

[Cookie]

[Iceberg]

As expected still not sign of an ERC. apart from the slight warming of the cloud tops I think Igor is actually looking better and better, outflow is now outstanding in all quads and he has dual inflows.

The path comment below is interesting as they concur that against expectations Igor is still going due west, it will be fun to see how much he turns north overnight, atm there is very little sign of a turn.

ECM on the 12Z showed the system further south and west than GFS in the hear future and much slower as it does not engage with the weakness shown by GFS.

Still no northward movement overnight, despite it being forecasted to start 12 hrs ago !.

All cudos to the ECM and Meto. The question continues though, when will Igor turn norhtwards ?, it has pretty much missed the initial weakness engagement now(hence the longer range GFS for the UK is very different).

Igor is looking a bit starved atm, the very slow speed is having two impacts, firstly SST's under the storm will be depressed reducing convection (some hurricanes have really suffered from this). The second factor is the dry air, Igor needs alot of moisture nad is draining everything it can, but this probably isn't enough to sustain a strong CAT 4.

Re an EWRC I still disagree with NHC the eye is still pretty stable in the latest images I can find, the earliest we are looking at an ERC IMO is later this afternoon.

[Cal]

Here's a nice motion pic of Igor.

http://cimss.ssec.wisc.edu/goes/blog/wp-content/uploads/2010/09/100913_g15_igor_vis_anim.gif