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 MississippiWx:
12z GFS Ensembles at 132 hours:



Dang...that's def more west than I expected....
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this thing looks like it wants to decouple... just sayin...
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Quoting NICycloneChaser:


No. Just... no.


Quoting stormtopz1:


thats and old map. the new models come out now for a texas/louisianna landfall.


No. Just... no.

LOL Im not gonna post anything for a while and just enjoy this and get some good belly laughs.
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intresting to see that other people other than me agree with me that the LLCOC is near 15N/14.9N or near that area


I am not surprised that recon failed when I found out what plane it was (AF300) this plane has a history for not working sometimes and for having to fly back home during missions
Member Since: June 13, 2009 Posts: 2 Comments: 12716
Quoting louisianaboy444:


UM lol


How is that funny, fellow Louisianian? I've been watching the Euro for days - it had NOLA in the crosshairs for a number of runs, and now it's showing more of a panhandle hit. Which is east last time I checked. Furthermore, the GFS has shifted west over the past few days. So what am I missing here? I did actually sleep, so maybe I missed something.
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Isaac just has way too much competition right now. God knows how many smaller vertices are embedded with his giant circulation.
Member Since: February 2, 2010 Posts: 0 Comments: 1540
Quoting RitaEvac:
uh oh....seeing low level rotation SW of PR, looking obvious, but we'll see



Hmmm…I'd be surprise to see a center relocation that far north, but it would throw a HUGE wrench into the track if it did. Not much thunderstorm activity there based on IR. Perhaps one of many vortexes?
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Quoting hurricanehanna:

She came right over me...actually got to see the eyewall...it was cool. Just very grateful it did not make landfall as a 4. Did enough damage and spawned lots of tornadoes though.


I bet that was neat. Of course the only hurricane that passed over my head was at 2am. Lol. Couldn't see a thing in Humberto. :)
Member Since: August 24, 2010 Posts: 0 Comments: 253
1613. LargoFl

Category One Hurricane: Sustained Winds 74-95 mph (64-82 kt or 119-153 km/hr).
Very dangerous winds will produce some damage
People, livestock, and pets struck by flying or falling debris could be injured or killed. Older (mainly pre-1994 construction) mobile homes could be destroyed, especially if they are not anchored properly as they tend to shift or roll off their foundations. Newer mobile homes that are anchored properly can sustain damage involving the removal of shingle or metal roof coverings, and loss of vinyl siding, as well as damage to carports, sunrooms, or lanais. Some poorly constructed frame homes can experience major damage, involving loss of the roof covering and damage to gable ends as well as the removal of porch coverings and awnings. Unprotected windows may break if struck by flying debris. Masonry chimneys can be toppled. Well-constructed frame homes could have damage to roof shingles, vinyl siding, soffit panels, and gutters. Failure of aluminum, screened-in, swimming pool enclosures can occur. Some apartment building and shopping center roof coverings could be partially removed. Industrial buildings can lose roofing and siding especially from windward corners, rakes, and eaves. Failures to overhead doors and unprotected windows will be common. Windows in high-rise buildings can be broken by flying debris. Falling and broken glass will pose a significant danger even after the storm. There will be occasional damage to commercial signage, fences, and canopies. Large branches of trees will snap and shallow rooted trees can be toppled. Extensive damage to power lines and poles will likely result in power outages that could last a few to several days. Hurricane Dolly (2008) is an example of a hurricane that brought Category 1 winds and impacts to South Padre Island, Texas.
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1612. angiest
Quoting RitaEvac:
uh oh....seeing low level rotation SW of PR, looking obvious, but we'll see



Nothing is clear on the velocity scan. And at that range you are looking pretty high up.
Member Since: August 26, 2006 Posts: 16 Comments: 4766
Center is just NW of the small red ball of convection. Appearance wise, Isaac is nearly back to where we started this morning with two areas of convection.

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12z GFS Ensembles at 132 hours:

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Quoting stormpetrol:
Link

Look at 14.8N/66.8W and tell me what you see.


Kinda looks like a runaway hot tower near 15 north right around where the COC looks to be. Interesting....
Member Since: August 28, 2006 Posts: 6 Comments: 2899
Quoting cirrocumulus:


Even after all that, the EURO is the Best overall and the other models are trending west now!

With all do respect, over the past two days you post the same thing just different wording. U live on the central gulf coast I assume by what u say in every post. I know you want it to go west and it just might. But please stop reminding me every 20 minutes. Let's all wait and see. I lurk for information because I live in Sarasota. If you want the storm that bad then u can have it. I'll take sunny skies with a nice breeze over Isaac any day. Back to the shadows.
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1606. WoodyFL
Quoting ILikeIke:
1492 lol columbus sailed the ocean blue


And grothar in the boat behind him. j/k you know wer'e kidding.
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Be back later.
Member Since: October 3, 2010 Posts: 40 Comments: 4129
Lol, the ECMWF isn't even a viable option, I don't even know why it's getting much credence om the blog.

Will be more than happy to choke on crow if it verifies though.
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Quoting RitaEvac:
uh oh....seeing low level rotation SW of PR, looking obvious, but we'll see



Seems to be moving slightly north of west, if that is indeed the center.
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Quoting RTSplayer:


That alleged center fix is an optical illusion easily dispelled by the "Base Velocity" tool on the radar.

The CoC is nowhere near that fix, and I mean not even within 2 degrees of it...





The real circulation is off the screen far to the SSW of the red/green split at the edge of the range...


and





your rader are not showing up



when posting NWS raders you have too ues the Standard Version
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Quoting Tazmanian:
i really think Isaac is at 14N and not where the nhc has it


You need to look at the PR radar loop instead of the sat and their call will all make sense.
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Quoting mojofearless:


Actually, the EMCWF shifted slightly east today. It's the GFS that has trended west.


UM lol
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uh oh....seeing low level rotation SW of PR, looking obvious, but we'll see

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Quoting HurricaneDean07:

NHC's Center fix was a Lie. It was at 15.6 when NHC updated. Now its at 15.7


Was a lie??
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1594. K8eCane
Quoting SFLWeatherman:
Be back later i'm on minecraft lol see you all at 5!!:)



OMG My son LOVES that game
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Quoting mojofearless:


Actually, the EMCWF shifted slightly east today. It's the GFS that has trended west.


Euro just went to LA/TX border, much further west than it has been
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Quoting Jedkins01:



It's easy to jump to that conclusion, but you can't forecast where a storm will be when it reaches point:X2 based on the current data now for the starting point:X0. Yes a westerly path right now into the Central gulf may seem like its plowing into a ridge instead of the weakness over Florida, but that's based on initial conditions. The models that bring it west believe the greatest weakness will be over the central gulf by then and ridging over Florida instead. The euro isn't showing it plowing through a ridge, if it was then you could throw it because that would be a major and obvious error. But it's not.


I don't want to be confrontational here, but just because you expected this storm to move into Florida doesn't mean it will, I know you know that's true, but sometimes you need to get back to the basics in order to have balanced perspective. The fact is Isaac has about the same change of hitting the Central gulf coast right now as hitting Florida/eastern gulf. Yes there really is that much uncertainty, and the recent model shifts prove that all the more.


However, the proof of why in reality is based on the intensity of Isaac. We don't even know if Issac will even survive right now, yet an eastern gulf landfall is entirely dependent on it reaching hurricane strength or at least a strong tropical storm by the time it impacts the islands. If Isaac stays weak it will continue to go further west because that is how tropical cyclones work. A shallower tropical cyclone will travel farther west, a deeper one will feel the pull to turn north more. That is the biggest variable alone. I don't know about you but I don't have the greatest confidence in Isaac intensifying. The mid level center is continuing to slide southwest and Issac is continuing to become more disorganized regardless of how favorable conditions are for it to strengthen.


Yes I too am very confused as to why Issac isn't organizing as all conventional understanding of meteorology tells you Isaac should at least be steadily strengthening by now if not quickly. However, the fact is, it's not. If Issac stays weak, the eastern gulf path will become the less likely scenario because a weaker storm will be much less likely to turn north into the eastern gulf. However if Issac does finally strengthen, and becomes a hurricane in the Caribbean, it will likely turn northwest, and I would expect models to shift back east. Models or computer systems and the output is attempting to find the most logical solution. Well a weaker storm that has been jogging wsw repeatedly will cause the models to respond west.

You might think they are "out to lunch" but examine the situation further and you'll find it makes more sense as to why they have.


It will be very interesting to see if the ECMWF was right all along, keeping Isaac weak and moving it into the Central or East Central GOM instead of into or very near the Florida peninsula.

If this turns out to verify, undoubtedly an intricate analysis of the data will be undertaken in order to find out what the Euro saw that none of the other models saw. This, whatever it is, will then formulate a tool which could lead to better modeling forecast in the future, particularly for unusually complex systems like Isaac.
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Quoting GoWVU:


Taz: Why do you say that??




i this have that feeling



i think its at 14N
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Quoting Buhdog:


What up river guy... been awhile.

Just to show you how much off of florida the forecast has become...

Link


It has, good to see you once again. Was gone nearly a year, glad to be back.
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Quoting RitaEvac:
I do see where the center could be just SW of PR

I think you're onto something. I definitely see a rotation, and it has to be low level since the mid level rotation is further to the south.
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Quoting Hurricanes101:


um, it hasnt been as consistent as the GFS, the Euro keeps shifting further west, not buying it


Actually, the EMCWF shifted slightly east today. It's the GFS that has trended west.
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Quoting WPBHurricane05:
That was the 2nd time a comment I've posted messed up. Must be my computer. 


Could be a direct effect of Global Warming
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1585. LargoFl
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Quoting uncwhurricane85:


if that were the case joyce would not be forecast to go over bermuda...come on..there is a weakness west of bermuda..almost all models see it


it seemed like a good response for the models shifting west but I'm probably wrong since I'm not really an expert, just an avid weather enthusiast who knows just enough to get into trouble. :0
Member Since: August 28, 2006 Posts: 6 Comments: 2899
1582. GoWVU
Quoting Tazmanian:
i really think Isaac is at 14N and not where the nhc has it


Taz: Why do you say that??
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Quoting nrtiwlnvragn:
Recon may be going to do this part of the synoptic mission:




That alleged center fix is an optical illusion easily dispelled by the "Base Velocity" tool on the radar.

The CoC is nowhere near that fix, and I mean not even within 2 degrees of it...



The real circulation is off the screen far to the SSW of the red/green split at the edge of the range...


and

Oh well, removed the images because they weren't working.
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Quoting TropicalAnalystwx13:

The ECMWF has shown a major hurricane hitting Louisiana for the past three days. How is that inconsistent?
Because it was showing a Panhandle landfall last night and is now showing a TX/LA landfall. Actually the GFS has also been a little inconsistent because it went from the East Coast of FL. and has been trending west now into the Eastern Gulf. At this point we really need some good data from the G-IV Mission.
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Quoting scott39:
The GFS has swung from an East Coast landfall to pensacola. I would call that a significant swing.


OK Im wrong as usual. :)
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1577. LargoFl
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Good afternoon all... Isaac just looks... terrible.
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It seems like Isaac is struggling with dry air slots but not sure where it's coming from as I don't see any ULL's nearby now on the water vapor. Any thoughts on what's causing the ebb and flow of Isaac's convection?
Member Since: August 28, 2006 Posts: 6 Comments: 2899

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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.

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