Major Hurricane Irma

[jojo762]

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

[NEG NAO]

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....

[SnowGoose69]

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.

[wthjnky]

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.

[Random Chaos]

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.

[jojo762]

153kt SFMR, supported by other ~150kt readings in the NW eye wall.

[NJwx85]

Where are these shear forecasts coming from? I see very minimal shear on the GFS until Irma has already made LF in FL and even less on the Euro.

[Random Chaos]

2 minutes ago, jojo762 said:

153kt SFMR, supported by other ~150kt readings in the NW eye wall.

153 was rain flagged. 149kt was not.

[Morris]

2 minutes ago, jojo762 said:

153kt SFMR, supported by other ~150kt readings in the NW eye wall.

This is likely why they've held it at 160 kts. They figure it's likely there and either way, they would only lower it by 5 knots, so not worthwhile to change for messaging and other purposes.

[NJwx85]

5 minutes ago, wthjnky said:

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.

Expect the models to continue to over correct East and West the next few days. It's way to early to determine the exact track near FL, but I like the official track.

[CaWx]

4 minutes ago, SnowGoose69 said:

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.

I guess I didn't realize the shear was suppose to increase that much. In theory how much shear could Irma take on while sitting over 84F+ temps before it weakens? 

[donsutherland1]

CNN reported:

French Interior Minister Gérard Collomb said some of the strongest buildings on the island of 75,000 people had been destroyed, boding ill for weaker structures on St. Martin. 

"In terms of material damage, the four strongest buildings on the island of St. Martin have been destroyed," which meant that weaker structures likely had been damaged or destroyed, he told reporters.

http://www.cnn.com/2017/09/06/americas/hurricane-irma-caribbean-islands/index.html

This highlights the potential danger Miami faces should Irma make landfall just to the city's west at Category 5 strength. Even the most modern buildings would likely be susceptible to varying degrees of damage. In addition, windows would blow out on a widespread basis from others. Other parts of the city would be flooded by the storm surge. 

My guess is that Irma will make landfall as a Category 4 storm (and I think LEK's thinking on strength and track is very realistic at this time), but a Category 5 storm still can't be ruled out, as some of the guidance continues to insist on Irma's intensification ahead of possible Florida landfall. 

Hopefully, Irma will stay offshore to spare Miami and the southeast Florida coastline of the kind of catastrophic damage that is possible from such a powerful storm.
 

[Morris]

2 minutes ago, Random Chaos said:

153 was rain flagged. 149kt was not.

How do you see if it's flagged? I used to remember....

[Latitudedfb]

What are opinions on the wind speed if Irma makes landfall in South Florida near Miami and then travels inland up the state?

What effects would the "ground friction" have on the winds onshore 10 to 15 miles inland?

[Random Chaos]

Just now, Morris said:

How do you see if it's flagged? I used to remember....

Last column, 3 , 5 , 6, or 9:

http://www.nhc.noaa.gov/abouthdobs_2007.shtml

[Dcwx]

FWIW 12z JMA has Irma splitting FL down the middle.

[Random Chaos]

1 minute ago, Latitudedfb said:

What are opinions on the wind speed if Irma makes landfall in South Florida near Miami and then travels inland up the state?

What effects would the "ground friction" have on the winds onshore 10 to 15 miles inland?

Florida is very flat, with good moisture sources off both coasts. Hurricanes have been know to cross E-W with minimal impact to intensity. No idea about a S-N route, though.

[the ghost of leroy]

1 minute ago, Dcwx said:

FWIW 12z JMA has Irma splitting FL down the middle.

Do you know where to find verification scores for the JMA?

[Morris]

2 minutes ago, Random Chaos said:

Last column, 3 , 5 , 6, or 9:

http://www.nhc.noaa.gov/abouthdobs_2007.shtml

Thanks!

[otown]

5 minutes ago, Dcwx said:

FWIW 12z JMA has Irma splitting FL down the middle.

Earlier JMA models had Irma way out west, wouldn't this just show its beginning to catch up to other trends of an easterly path

[Bostonseminole]

2 minutes ago, cmasty1978 said:

Do you know where to find verification scores for the JMA?

not sure those exist, at least not  on the same scale as other models

[JC-CT]

Do you know where to find verification scores for the JMA?

Cmc has them for North America. Not very good, but I find useful as an ensemble member at long range. Useless for this though.
https://weather.gc.ca/verification/monthly_ts_e.html

[Hoosier]

3 minutes ago, Latitudedfb said:

What are opinions on the wind speed if Irma makes landfall in South Florida near Miami and then travels inland up the state?

What effects would the "ground friction" have on the winds onshore 10 to 15 miles inland?

Historical cases would suggest gust potential near/over 100 mph well northward, at least into central Florida if not farther, IF it landfalls as a cat 4-5 and takes a track like you suggest.

[Dcwx]

3 minutes ago, cmasty1978 said:

Do you know where to find verification scores for the JMA?

No idea - mostly post the JMA as a joke. It always gets posted in HECS winter storms too. I do recall it verified better than any other model for one of the major 2010 DC snowstorms but that's probably it's claim to fame. 

[bdgwx]

18 minutes ago, NJwx85 said:

Where are these shear forecasts coming from? I see very minimal shear on the GFS until Irma has already made LF in FL and even less on the Euro.

The shear will start to increase as Irma begins the turn north while interacting with the trough. I'm looking at 950-800mb across 600-400mb for mid level shear and up to 300-200mb for deep layer shear. The question is...will Irma get a brief boost from being placed favorably in the RER of the upper level jet streak before the shear encroaches too close to the core? I like LEK's position of the lowest pressure. It's about as perfect of a location as I can imagine...between land masses, high SSTs, jet dynamics for increased outflow, but early enough that shear *might* not take hold yet. That last item is a big if though.

[Random Chaos]

4 minutes ago, Dcwx said:

No idea - mostly post the JMA as a joke. It always gets posted in HECS winter storms too. I do recall it verified better than any other model for one of the major 2010 DC snowstorms but that's probably it's claim to fame. 

At least no one is posting CRAS - I just checked it - and it makes every outlier model look sane

[Windspeed]

Irma's windfield will continue to expand as it nears the southwestern Bahamas and Florida. I would imagine the size of the hurricane combined with a decrease in surface to 500mb level background pressures as it turns into the weakness will weaken the pressure gradient. The ridge will have weakened/backed its position further east. That basically means Irma should not be able to sustain the absurd wind speeds even if it remains a sub 920mb cyclone or goes through another strengthening phase at a much larger size. Unfortunately it just means a much larger swath of major hurricane force winds. Also, the eyewall, if still intense, will be able to produce upper Cat 4 or borderline 5. This is perhaps semantics, but I am just reiterating that you can bring down the winds 20-30 kts. The hurricane still remains catastrophic in winds and surge. But it will technically weaken aside from structural changes just based on its surrounding atmosphere.

 

[yoda]

00z GFS out to 30... Irma in the SE Bahamas

[Kleimax]

No big changes on GFS out to 24 hrs

[donsutherland1]

Since 1851, there were 15 major hurricanes that formed in August or prior to September 15 and passed within 100 nautical miles of Irma's 11 pm position (19.4°N, 66.8°W). 60% made U.S. landfall.

Areas of Landfall:
Florida: 56%
North Carolina: 22%
South Carolina: 11%
Texas: 11%

Strength at Landfall:
Category 1: 11%
Category 2: 22%
Category 3: 22%
Category 4: 44%
Major: 67%

The Category 4 Storms were: 
Great Miami Hurricane (1926): 125 knots
Lake Okeechobee Hurricane (1928): 125 knots
Donna (1960): 125 knots
Hugo (1989): 120 knots

The spread of the 12z ECMWF ensembles continues to highlight the Florida-South Carolina area as being at highest risk of U.S. landfall. North Carolina is also a landfall site for some of those ensemble members. None make landfall west of the Florida Panhandle. A few take Irma offshore and avoid landfall. 

I still think Florida landfall is somewhat more likely than not and probably at similar strength to the three Category 4 hurricanes that made Florida landfall. Second landfall should occur at Georgia or South Carolina.