Little change to Isaac, but intensification coming; Joyce forms

By: Dr. Jeff Masters , 3:13 PM GMT on August 23, 2012

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Tropical Storm Isaac is a large and impressive-looking storm on satellite images, but data from the Hurricane Hunters reveal that Isaac remains a minimal-strength tropical storm with 40 mph winds, as it heads westward across the Eastern Caribbean. An Air Force Reserve hurricane hunter aircraft which completed its mission into Isaac at 8 am EDT found top winds at the surface near 40 mph, and highest winds at their 5,000 foot flight level of 47 mph. A NOAA hurricane hunter aircraft flying at 10,000 feet has found top winds of 47 mph at that altitude. The Hurricane Hunters found a broad area of light winds with a central pressure of 1003 mb. The aircraft did not observe an eyewall trying to form, and recent microwave satellite images also show no signs of an eyewall forming. There does not appear to be much in the way of dry air near the core of Isaac, as seen on water vapor satellite loops, which is a big switch from what we've seen previously. Visible satellite loops show that Isaac has a much more symmetric circular cloud pattern, and has developed a Central Dense Overcast (CDO) of high cirrus clouds, the hallmark of an intensifying storm. These clouds have very cold cloud tops, indicating that the updrafts creating them are quite strong. An analysis of upper level winds from the University of Wisconsin CIMSS shows that an upper-level pattern of outflow supportive of significant strengthening has developed this morning, with an upper-level outflow channel now well-established to the north, and a new outflow channel opening to the south. Radar imagery from Puerto Rico shows some weak low-level spiral bands that are slowing intensifying and becoming more organized.


Figure 1. Morning radar image of Isaac taken from the Puerto Rico radar. Isaac's rain bands are weak, but are starting to take on a more spiraling shape.

Intensity forecast for Isaac
Isaac has consistently confounded predictions that it would intensify, but all the potential factors inhibiting intensification seem to have diminished to the point where intensification has to occur. Wind shear is low, 5 - 10 knots, and ocean temperatures are warm, 29°C, and these warm waters extend to great depth, giving the storm a high total heat content to work with. These factors, combined with the favorable upper-level outflow pattern and more symmetric cloud pattern, support intensification, and all of the intensity models except the HWRF model predict intensification of Isaac to a strong tropical storm or Category 1 hurricane by Friday afternoon. The latest 8 am EDT run of the SHIPS model predicts that wind shear will be light to moderate, 5 - 15 knots, for the next five days. The 11 am EDT wind probability forecast from NHC predicted a 34% chance that Isaac will become a hurricane by Friday afternoon, and a 6% chance it will be a Category 2 or stronger hurricane then. By Friday afternoon, Isaac will likely be close enough to Southwest Haiti that the inner core will be disrupted, and the storm will likely be a 45 - 55 mph tropical storm on Saturday and Sunday as it moves over Cuba. Once Isaac pops off the coast of Cuba into the Florida Straits, it will be over very warm waters of 31 - 32°C (88 - 90°F), wind shear will be light to moderate, and the upper-level wind pattern favorable for intensification, with low wind shear due to an upper-level anticyclone over the storm. It will probably take at least 24 hours with the storm's center over water for it to become a hurricane.

Impact of Isaac on the Islands
The south coast of Puerto Rico should see Isaac's heaviest rains and strongest winds beginning near 8 pm EDT tonight, with tropical storm-force winds of 40 - 45 mph potentially affecting the southwest portion of the island. The San Juan airport may be able to stay open during Isaac's passage, but with delays when spiral bands move overhead.

Heavy rains and tropical storm-force winds should arrive on the southern coast of the Dominican Republic late tonight, and the Santo Domingo airport will probably be closed on Friday. Rainfall amounts of 8 - 12 inches will likely affect the Dominican Republic Thursday through Saturday, creating dangerous flash floods and mudslides.

Isaac is potentially a very dangerous storm for Haiti, where 400,000 people still live outside underneath tarps in the wake of the 2010 earthquake. Heavy rains from Isaac will begin on Friday morning in Haiti, and last through Sunday. Rainfall amounts of 8 - 12 inches are possible, which will be capable of causing extreme flooding on the vegetation-denuded slopes of Haiti. It will be a major challenge to keep those Haitians living outside safe, if rainfall amounts of 5 - 10 inches occur.

Impact on Florida, Cuba, Jamaica, and the Bahamas
Florida, Cuba, Jamaica, and the Bahamas are all at risk of receiving flooding rains and high winds from Isaac. The latest set of 00Z (8 pm EDT) and 06Z (2 am EDT) model runs for Isaac are fairly unified for the coming three days, showing a west-northwestward track over Southwest Haiti and into Western Cuba. At the 4 - 5 day forecast period for Sunday and Monday, the models have come into better agreement, and have shifted west some. Our best-performing model, the ECMWF, has now shifted Isaac's path more to the east, but still is the westernmost of the models, predicting a landfall for Isaac near the Alabama/Florida border on Wednesday. While we do still have some models predicting a path up the east coast of Florida, model consensus now favors a path up the west coast of Florida through the Gulf of Mexico. The recent reformation of Isaac's center more to the south supports the idea that Isaac will take a track more to the west through the Gulf of Mexico. Since this now means a final landfall for Isaac in the Florida Panhandle is likely, the storm will probably have an extra day over water, increasing the odds that it will become a Category 2 or stronger hurricane before this final landfall. The NOAA jet is scheduled to fly into the storm this afternoon, to do a large-scale dropsonde mission to aid model forecasts. These missions can improve model forecasts by 10 - 20%, so the model runs that will be available early Friday morning should have increased reliability.

Impact on Tampa, Florida
The Republican National Convention begins on Monday in Tampa, Florida. The latest 11 am EDT wind probability forecast from NHC gives Tampa a 15% chance of receiving tropical storm-force winds and a 1% chance of receiving hurricane-force winds on Monday. The latest model tracks for Isaac suggests that the trough of low pressure pulling the storm to the north will not be strong enough to give Isaac a northeastward component of motion when it crosses Tampa's latitude. Thus, Isaac will have difficulty making a direct hit on Tampa without passing over a considerable amount of land first, making a multi-billion dollar hurricane disaster in Tampa very unlikely. I put the odds of a mass evacuation occurring during the convention at 1%; a limited evacuation of people in the Tampa Bay area living in mobile homes in low-lying areas is probably about 5 - 10 % likely. I have detailed information on Tampa's storm surge vulnerability in a post from last week.


Figure 2. Morning satellite image of Tropical Storm Joyce.

Tropical Storm Joyce forms in the Central Atlantic
The season's tenth named storm of the year, Tropical Storm Joyce, has formed in the Central Atlantic. Joyce's formation on August 23 puts 2012 in a tie for second place with 1995 for earliest formation date of the season's tenth storm. Only 2005 had an earlier appearance of the season's tenth storm, when Tropical Storm Jose formed at 2 pm EDT on August 22. None of the models show that Joyce will be a threat to the Lesser Antilles Islands, but it may be a storm that will affect Bermuda. It is possible that Joyce will complicate the track forecast for Isaac 4 - 5 days from now, when the storms may be close enough together to interact.

Jeff Masters

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Quoting Grothar:


Wow. Some pictures. I heard there was sever damage way inland. Is that true?


Friends of mine lived outside Jackson, in north MS, and they had severe damage, trees down, roof damage, etc.
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723. yoboi
Quoting presslord:


I normally tune out media bashing as shrill...but in this case, you are 100% correct....they did a negligent job telling the full story of that storm...


look at it this way i live in swla and we had rita in 2005 national news was still in nola....rita the forgotten storm....
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this is what i was wondering

Joyce will complicate the track forecast for Isaac 4 -5 days from now, when the storms may be close enough together to interact.

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One thing I think we can all agree on, Isaac is one of the most impressive looking 40-45mph tropical storms EVER.
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Quoting Levi32:
Good afternoon.

Blog update:

Tropical Tidbit for Thursday, August 23rd, with Video


Thanks Levi.

U DA MAN!
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Quoting mississippired:


I live about 80 miles north of Waveland, MS. We had no power or water for a month.


that is far inland. Must have been powerful.
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Quoting MississippiWx:


Yeah, but we are used to it.

This was the scene down our entire coast:



#647
My greatest fear. It would hurt like hell to see that happen here.
Member Since: July 18, 2005 Posts: 9 Comments: 4146
Quoting masonsnana:
You mean Tampa correct?


The NHC had it near Tampa but the forecasted landfall was actually Panama City. The NHC really busted that forecast just like they did with Debby. levi's right those mtns. across Cuba and Haiti can cause irratic shifts in the track.

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Isaac is starting to get his act together...

Member Since: August 1, 2011 Posts: 28 Comments: 8031
715. JLPR2
Now I'm confused as to where the center really is. RECON should eliminate all of these doubts.



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Thanks Levi, I was feeling better with the shift to the west,,ugh
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thx Levi
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Quoting Levi32:
Good afternoon.

Blog update:

Tropical Tidbit for Thursday, August 23rd, with Video


Nice well reasoned forecast...I don't necessarily agree with it but any forecast with solid synoptic reasoning is a great addition and learning experience for everyone. Thanks!
Member Since: February 6, 2010 Posts: 0 Comments: 214
Quoting GTcooliebai:
Latest HWRF has Miami on this run:



Has it as a stronger storm that some of the other models... stronger = more north and east path.

Definitely an option if Isaac ever got his act together... but an outside one, imo. Still favor the west coast of Florida to the Panhandle.
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Quoting Grothar:


Wow. Some pictures. I heard there was sever damage way inland. Is that true?


I live 60 miles inland and any house that had trees around it had trees in or on the house. Some got lucky of course and just had trees fall in the yard. We had 100mph+ sustained winds here for a time. It was nuts, Gro!
Member Since: July 15, 2006 Posts: 17 Comments: 10284
Should be making the WNW turn soon.


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Quoting StormTracker2K:


Great blog today Levi. That weakness is very evident today and infact more so than yesterday.

Here's that weakness. Folks remeber Charley was supposed to hit Panama City but instead took a hard right into Port Charlotte,FL.

Woah! a little bi-polar are we, not bashing Levi for the East C of FL? good to see the other you!
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Quoting Grothar:


Wow. Some pictures. I heard there was sever damage way inland. Is that true?


I live about 80 miles north of Waveland, MS. We had no power or water for a month.
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HWRF riding up the spine of FL.
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704. HCW
Difference in advisory changes so far end points. Ignore the 7pm stamp on the image. Thanks


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Dry air doesn't seems to be an issue...

Member Since: August 1, 2011 Posts: 28 Comments: 8031
Quoting Levi32:
Good afternoon.

Blog update:

Tropical Tidbit for Thursday, August 23rd, with Video


Good stuff, Levi! Thanks
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Quoting StormTracker2K:


Great blog today Levi. That weakness is very evident today and infact more so than yesterday.

Here's that weakness. Folks remeber Charley was supposed to hit Panama City but instead took a hard right into Port Charlotte,FL.

You mean Tampa correct?
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Quoting Wxouttacontrol:
Following website chock full of graphics, forecast models, sat. images/loops, recon info, etc.

Link
Thank you SO MUCH!
Member Since: August 1, 2011 Posts: 28 Comments: 8031
699. JLPR2
Quoting RTSplayer:


Water Spout.

No way that's hurricane winds...


Hurricane conditions in Mona. Suspect reading are just that. XD
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Quoting trey33:


Like Ivan?


The thought of another "Ivan" makes my stomach churn and brings back too many nightmare memories. I saw/did things after that storm that I hope I'll never have to see/do again.
Please people, if you are in the path of the storm, don't wait until the last minute to leave - and if you DO leave, make sure you take your photographs, pets, and important papers.
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Quoting junie1:
long range radar from PR shows the center really nice http://radar.weather.gov/ridge/radar.php?product=N 0Z&rid=JUA&loop=yes

Member Since: August 2, 2006 Posts: 46 Comments: 11669
G-IV about to head out shortly.
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14.9/66.3 Or so. Tell me what you see there, when looking at the loop.
Member Since: August 3, 2008 Posts: 16 Comments: 5886
Quoting Levi32:
Good afternoon.

Blog update:

Tropical Tidbit for Thursday, August 23rd, with Video


Great blog today Levi. That weakness is very evident today and infact more so than yesterday.

Here's that weakness. Folks remember Charley was supposed to hit Panama City but instead took a hard right into Port Charlotte,FL.

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Quoting angiest:


Probably most of the regulars here are aware of that. But to the general public, I think they only know of what happened in NOLA. I don't think most people have any awareness of the fact that, taking out the deaths in New Orleans, Katrina was still the deadliest US hurricane in several decades, and that most (I think) of those deaths were in MS. The responsibility for that lack of education rests directly on the major media outlets, who provided only scant coverage of the rest of what Katrina did. (I am in no way diminishing what Katrina did to New Orleans, nor am I attempting to cheapen the loss of life there.)


I normally tune out media bashing as shrill...but in this case, you are 100% correct....they did a negligent job telling the full story of that storm...
Member Since: August 13, 2007 Posts: 0 Comments: 10492
Quoting angiest:


Probably most of the regulars here are aware of that. But to the general public, I think they only know of what happened in NOLA. I don't think most people have any awareness of the fact that, taking out the deaths in New Orleans, Katrina was still the deadliest US hurricane in several decades, and that most (I think) of those deaths were in MS. The responsibility for that lack of education rests directly on the major media outlets, who provided only scant coverage of the rest of what Katrina did. (I am in no way diminishing what Katrina did to New Orleans, nor am I attempting to cheapen the loss of life there.)

Exactly. But I think you said it better than I apparently.. thanks.
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long range radar from PR shows the center really nice http://radar.weather.gov/ridge/radar.php?product=N 0Z&rid=JUA&loop=yes
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Latest HWRF has Miami on this run:

Member Since: August 31, 2009 Posts: 0 Comments: 5628
Quoting coffeecrusader:
Thanks for the update Levi. Curious why you are still holding on to an east coast of Florida path where the NHC and most of the models are trending towards AL/FL border? This is not criticism, just curious.


I've explained my concerns about the need for an eastward adjustment in the last several blog posts. If the models remain west after the G-IV data tonight, then I will likely have to join them, but the 4-5 day forecast still has lots of room for adjustment.
Member Since: November 24, 2005 Posts: 635 Comments: 26661
Quoting Levi32:
12z CMC is over the spine of the Florida peninsula now.


Levi, do you think this "westward trend" continues or do you think it shifts back east a bit?
Member Since: March 10, 2010 Posts: 1 Comments: 7826
Quoting Levi32:
Good afternoon.

Blog update:

Tropical Tidbit for Thursday, August 23rd, with Video


Give you credit Levi,right or wrong you are sticking by your guns when so many other people are screaming gulf storm. Joe B is in your camp also. It will be very interesting to see how this all finally unfolds.
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Ok, so if it does continue to shift West, where could this thing end up making landfall? I know it's not certain, but could this thing head to LA? I know the West side of the storm isn't as bad as the East but if we see any of it, could we be looking at hours of rain? I almost flooded out 2 weekends ago with about 3 hours of heavy downpour.

Thanks!
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Quoting Levi32:
Good afternoon.

Blog update:

Tropical Tidbit for Thursday, August 23rd, with Video
Stickin with your guns East of FL Levi, i like it.  You have reasoning behind your forecast unlike others that just follow models every run.  We shall see what happens :)
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Following website chock full of graphics, forecast models, sat. images/loops, recon info, etc.

Link
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Quoting MississippiWx:


Yeah, but we are used to it.

This was the scene down our entire coast:



Wow. Some pictures. I heard there was sever damage way inland. Is that true?
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Quoting GetReal:
700-850mb



500-850mb


There is no doubt that Isaac is going to go much more further west, until it reaches the NW Caribbean.


Yep.
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12z CMC is over the spine of the Florida peninsula now.
Member Since: November 24, 2005 Posts: 635 Comments: 26661
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Quoting GOLSUTIGERS:


I wish they would have seen Mississippi after it, it was horrible. I will never forget going there and see the damage. I will not lessen the destruction of Louisiana but I was in both areas and to think someone could open thier mouth on this blog (a weather blog) and say that is unbelievable.



I honestly never seen those photos of MS... media only really covered what happend in LA.. looks horrible
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Thanks for the update Levi. Curious why you are still holding on to an east coast of Florida path where the NHC and most of the models are trending towards AL/FL border? This is not criticism, just curious.
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677. 7544
hwrf and cmc and gfdl more est to south fl hmmmmm
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Quoting MississippiWx:


Yeah, but we are used to it.

This was the scene down our entire coast:



Probably most of the regulars here are aware of that. But to the general public, I think they only know of what happened in NOLA. I don't think most people have any awareness of the fact that, taking out the deaths in New Orleans, Katrina was still the deadliest US hurricane in several decades, and that most (I think) of those deaths were in MS. The responsibility for that lack of education rests directly on the major media outlets, who provided only scant coverage of the rest of what Katrina did. (I am in no way diminishing what Katrina did to New Orleans, nor am I attempting to cheapen the loss of life there.)
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Quoting LargoFl:
ok thanks, most of the mets are saying that high to the east is going to say where issac goes,and come sunday..brother its influence is towards the east coast of florida,this just may ride up and around it..i sure hope the east coasters arent writting this one off..this could very well go up the east coast of florida too
GFDL and CMC still take this up the East Coast so no one is out of the woods yet.
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HH plane starting to lower from 26,000 feet.
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Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.