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Major Hurricane Irma

NJwx85

By NJwx85,
in Tropical Headquarters

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Drz1111

Drz1111

Posted
Posted
2 minutes ago, OUGrad05 said:

Yeah the presentation is definitely down a couple notches from earlier, dry air I suspect combined with ERC.  We'll see how it ramps back up over night.

Come on.  Dry air?  There's a bare bit on the SJU sounding but nothing that explains any dry air ingestion into the core.

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4waresnowcone

4waresnowcone

Posted
Posted
1 minute ago, USCG RS said:

OK.. So this has been bothering me the whole night. Therefore i spent the past three hours working and researching eyewall replacement cycle s (ERC) . I will preference with this: ERC are very poorly understood and research on them is just beginning. However, I found a theory about ERC which not only makes sense, but I was likewise able to extrapolate an answer for which I believe fits why these ERC are doing nothing to weaken Irma.

Terms:

ERC : Eyewall replacement cycle. HBL : Hurricane boundary layer. NRF : Net radial force. PGF : Pressure Gradient Force.

I will also preferance with the fact that I view weather as the earth's immune system and really all weather is is an attempt for the atmosphere to find an equilibrium (much like our immune system is to bring homeostasis to the body).

So let's get to it. ERC, in this theory, are believed to take place due to a set of complicated physics. As a hurricane is born, there are three main components, physics wise, which make up the storm (at least as it pertains to ERC). The first is the HBL which extends approximately 1 km into the atmosphere, from the surface. It interacts with most of the surface friction. The second is the PGF, which essentially forces particles from areas of higher density (pressure) to areas of lower pressure. The last is known as the NRF, which is essentially the force directed towards the center of a rotating mass.

As a hurricane develops, these forces work with each other and eventually, when the storm reaches a certain intensity, an excess of NRF creates a secondary wind maximum approximately 40-60km from the center of the hurricane. This secondary wind maximum is always believed to be because the HBL cannot properly maintain a balance of angular momentum (or the acceleration of particles/mass in a circulation motion), or in other words, the HBL cannot keep the particles separated in such a way that the pressure and buildup of particles remain uniform. This, combined with a positive NRF, creates convergence. This convergence creates an up welling which then leads to a secondary maximum. This secondary maximum, in turn, creates more PGF (the force of a particle over a given area. The higher the PGF, due to more particles in a smaller area, the higher the winds). Eventually, this leads to enough of a convergence that a secondary eyewall is formed and two wind maxima are thus created. As many know, during this time, the overall windspeed of the maximum sustained winds decrease due to the maximum winds being spread out over a larger area and thus the wind speed lowers (or.. The PGF is distributed over a larger area and therefore while the mass remains the same, because force equals mass times acceleration, acceleration is lower and therfore the force or net wind speed decreases). Eventually, the PGF, combined with angular momentum, will coerce the outer eyewall to contract, thus ending the ERC. At this time, the winds will increase as will the overall size of the wind field of the storm, while allowing the hurricane to likewise strengthen.

But the question that's plagues me today is why has Irma skipped this weakening phase of the ERC and likewise why has she been constantly cycling through them and having them take place in a rapid fashion? What I propose is this. Irma has been in essentially a perfect, or lab like environment, for her lifespan. This is what is allowing her to explosively deepen and shatter records. What I likewise propose is this. Irma is showing us that hurricanes create ERC of their own accord. In other words, an ERC has nothing to do with outside influences and is strictly because of the mechanism of the hurricane itself (and the strengthening mechanisms within). Likewise, because ERC are part of the hurricanes overall structure, in theory, it should not hurt the hurricane, but rather, help it. Left to its own devices, this is what Irma is doing. It's part of a necessary strengthening cycle for well developed hurricanes. The HBL, I propose, creates the issue for ERC as within this layer, there is friction. This friction slows the contraction of the secondary eyewall and forces the storm to balance itself out to preserve itself, while it deals with the friction and the effects of it. Because Irma is in a lab like environment, the friction of the ocean is essentially non existant as it is over flat and incredibly warm water. Likewise, there is no sheer or dry air to interfere with the Core and venting mechanisms of Irma. As such, these ERC can begin and complete without any interference and then, once the outer wind maxima are mature, the PGF and angular momentum can allow the outer wind maxima to contract, replacing the old eyewall and thus completing the ERC, in time for it to build and start once again. Therefore, she can bypass any weakening state that the friction within the HBL would cause, as, essentially, there isn't any to speak of.

This, of course, is strictly a theory

You are a saint for the acronyms at the beginning. As a newbie, it really makes my life easier. Thanks for all of the hard work!

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NavarreDon

NavarreDon

Posted
Posted
Can't say that I differ too much from NHC...I think Irma will move a tad quicker initially, but, overall, the only subtle differences is wind speed at LF and a slightly western position at LF.

 

Irma.png.44f25bbd6af991b56c3892acf4ef9c67.png


Thank you! That is not a good look for SE Florida! Just to give some of the posters an idea of Irma's size look at these pics.
bc44b92825f36e31b9f4d1401b9fb308.jpgfcc1cbff9a068019943f6b754c003e8a.jpg


Sent from my iPhone using Tapatalk
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harrisale

harrisale

Posted
Posted
8 minutes ago, LakeEffectKing said:

Can't say that I differ too much from NHC...I think Irma will move a tad quicker initially, but, overall, the only subtle differences is wind speed at LF and a slightly western position at LF.

 

Fully agree with your estimate of position of lowest pressure. I think we'll really see Irma go for the hail mary sub-900mb after the north turn begins, and she pulls just far enough away from Cuba to avoid any effects of land interaction and dry air ingestion. And the water between FL and Cuba is toasty enough to do it.

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Chinook

Chinook

Posted
Posted

In response to an earlier post, the hurricane has been heading at 294 degrees compass direction for the past few hours (according to my estimation by radar). NHC says it is moving 14 knots. It is 275 nautical miles to Grand Turk, at a compass heading of 298 degrees. So, if the hurricane moves at the same compass heading, it will get to Grand Turk in 19.6 hours, if the compass heading is exactly 298 degrees. There is a lot of time in there for the heading of the hurricane to change a bit, one way or the other. So it's not totally certain that it will track exactly to Grand Turk.

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jojo762

jojo762

Posted
Posted
6 minutes ago, Drz1111 said:

Come on.  Dry air?  There's a bare bit on the SJU sounding but nothing that explains any dry air ingestion into the core.

I'd call it more of a "less-moist" inflow into the core type scenario because of interaction with P.R. Likewise, some of the less-than earlier satellite presentation appears to be partially caused by some weird glitch tonight with GOES 16... Its color scheme on the east side of the cyclone is glitchy.

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Hoosier

Hoosier

Posted
Posted

As others have said, the NAM is fairly useless for tropical purposes, but I want to use it in this instance to point something out.  Look how far offshore it has the center and still manages to brush the coast with hurricane force winds.  

Anybody that gets any part of the core/eyewall of Irma is in serious danger, even if "only" the western side.

namconus_mslp_wind_seus_49.thumb.png.560934528f5f0fa23a53524f5500ac95.png

 

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NEG NAO

NEG NAO

Posted
Posted
3 minutes ago, MattPetrulli said:

Wouldn't rule out a 160 MPH hurricane making landfall in South FL Sunday. Waters are just too warm to pass up.

Also can't rule out what the 0Z NAM is showing - an off the coast solution - still within the current forecast "cone"

 

namconus_ref_frzn_seus_49.png

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LakeEffectKing

LakeEffectKing

Posted
Posted
1 minute ago, NJwx85 said:

What's your reasoning for that speed max given your forecasted pressures? They don't seem to correlate well.

I think shear will start to impact it 6-8 hours before.  Plus, I do think it will have a bit larger eye.  I think when it is at 899mb a few hours before, it will probably have winds around 155-160.

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jojo762

jojo762

Posted
Posted 
Hurricane Irma Discussion Number  32
NWS National Hurricane Center Miami FL       AL112017
1100 PM AST Wed Sep 06 2017

Earlier radar observations from the San Juan WSR-88D showed a
concentric eyewall, and observations from an Air Force Hurricane
Hunter aircraft showed a secondary wind maximum over the northwest
quadrant.  These phenomena may be associated with some weakening,
but since the central pressure hasn't risen much the intensity is
held at 160 kt at this time.  Moreover, the concentric eyewall has
become less apparent in recent radar imagery.  Considerable
lightning activity has been noted in the eyewall of Irma, which
research has shown to sometimes be a harbinger of weakening.
Notwithstanding, low vertical wind shear and warm waters along the
forecast track of Irma should allow it to remain a very powerful
hurricane during the next several days.  The official intensity
forecast is similar to the previous one except at day 5 when
stronger shear should cause more weakening.  This forecast is
generally above the intensity model consensus.

Irma continues on a west-northwestward heading at around 14 kt.  A
ridge of high pressure over the western Atlantic should maintain
this course for the next 48 to 72 hours.  Later in the forecast
period a shortwave trough moving into the southeastern United
States is likely to turn the hurricane northwestward to
north-northwestward.  There has been a slight eastward shift in
some of the track guidance models, but since the models could shift
back to the west it is prudent to make little change to the
official forecast at this time.  In any event, users are reminded
not to focus on the exact track since the average NHC track
errors at days 4 and 5 are about 175 and 225 miles, respectively.

KEY MESSAGES:

1. Irma is a potentially catastrophic category 5 hurricane and will
bring life-threatening wind, storm surge, and rainfall hazards to
Puerto Rico tonight, the northern coast of Hispaniola Thursday, and
the Turks and Caicos and southeastern and central Bahamas Thursday
and Friday.

2. Hurricane watches are in effect for the northwestern Bahamas and
much of Cuba.  Irma is likely to bring dangerous wind, storm surge,
and rainfall to portions of these areas on Friday and Saturday.

3. The threat of direct hurricane impacts in Florida over the
weekend and early next week has increased.  Hurricane watches will
likely be issued for portions of the Florida Keys and the Florida
peninsula on Thursday.


FORECAST POSITIONS AND MAX WINDS

INIT  07/0300Z 19.4N  66.8W  160 KT 185 MPH
 12H  07/1200Z 20.3N  68.8W  155 KT 180 MPH
 24H  08/0000Z 21.3N  71.7W  150 KT 175 MPH
 36H  08/1200Z 22.1N  74.1W  145 KT 165 MPH
 48H  09/0000Z 22.7N  76.3W  140 KT 160 MPH
 72H  10/0000Z 24.0N  79.3W  135 KT 155 MPH
 96H  11/0000Z 27.4N  80.3W  120 KT 140 MPH
120H  12/0000Z 32.0N  81.0W   95 KT 110 MPH
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NEG NAO

NEG NAO

Posted
Posted
5 minutes ago, Blue_Ridge_Escarpment said:

That's well west of previous NAM runs. Look for it to correct even further 

there is no solid proof that it will correct even further - too far out to make that assumption in my opinion ...............too any variables upstream that may change the track of IRMA possibly further east - anyone up and down the east coast is not out of the woods yet IMHO....

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SnowGoose69

SnowGoose69

Posted
Posted
8 minutes ago, CaWx said:

I was thinking this as well. I'm still confused why NHC, models, most mets are forecasting a weakening to Cat 4? If conditions remain ideal (low wind shear), what is causing Irma to lower to CAT 4. I could understand some land interaction, but Irma should stay away from cuba, I don't think the tiny islands of Turks or Bahamas would weaken it that much. The water is so so warm, I could see it actually remaining near 155+ just based on ssts alone. 

The shear increases pretty significantly after 90 hours and more so after 100.  I could see a scenario where if it stayed say 50-100 miles off FL and never made landfall til SC that it could be down to a Cat 2.  If it comes straight into MIA though I would be shocked if it was under 125mph at this point...to me thats the low end possibility.

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wthjnky

wthjnky

Posted
Posted

By looking at the Models It appears that if this track holds then the Center of the storm will pass about 30 miles from the coast of Jacksonville. I have some friends there and I was really wondering what the winds would be like that distance from the eye.

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Random Chaos

Random Chaos

Posted
Posted
9 minutes ago, CaWx said:

I was thinking this as well. I'm still confused why NHC, models, most mets are forecasting a weakening to Cat 4? If conditions remain ideal (low wind shear), what is causing Irma to lower to CAT 4. I could understand some land interaction, but Irma should stay away from cuba, I don't think the tiny islands of Turks or Bahamas would weaken it that much. The water is so so warm, I could see it actually remaining near 155+ just based on ssts alone. 

A large part of the reasoning is due to intensity model guidance - they can't go any higher, so the "average" of max and weakening intensity forecasts is a slight weakening, which means cat 4. Basically, it has nowhere higher to go, so at some point the reasoning is "it has to weaken eventually".

every past cat5 storm has had similar weakening forecasts.

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