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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Here's the thing, if Isaac is heavily struggling to get its act together despite well established outflow, very deep convection, warm water and low shear, the assumption that it will survive passage over the islands and quickly become a hurricane again is highly questionable until Isaac establishes a core. If the current trend of Isaac continues, and Isaac never manages to establish a well developed core during the next 48 hours, Isaac will not be much of a threat to the RNC convention whatsoever accept some street flooding issues from outer bands due to saturated grounds. If Issac stays weak for too long in the Caribbean, it may not even survive passage over Cuba, Cuba could possible cause degeneration to a TD or remnant low lets not forget this. Furthermore if it does survive the passage, I really doubt after crossing it will be any strong than 50 to 60 mph when it gets back over water and that's assuming it survives the passage, it can still take a while over water after striking land for intensification to begin again.

Considering then that it is a large tropical cyclone. We must expect a considerable time of passage needed before restrengthening occurs. Now I realize forecasting intensity can be very poor, so I could be completely wrong here. But based on what I'm seeing, I don't see Isaac even being a significant threat at all right now to the West Coast of Florida unless it strengthens significantly first before impacting the islands.

Because a weak and large disrupted cyclone by land entering the gulf will not only take longer to organize in the gulf, but it will likely go further west than the model consensus if that happens. Yes guys, this would mean far less hype, but just a mere moderate tropical storm making landfall in the central gulf coast cannot be ruled out at all, it's also a very real possible solution.


Although model grouping is tightening and it seems like a good consensus is forming and a far eastern gulf landfall is more likely, the overall outcome is still VERY much in question, and the overall forecast could change very much still even though it hasn't yet.
Am I saying it won't come up into the eastern gulf as a category 1 hurricane? No, All I'm saying the confidence in this happening is deceivingly lower than what you might expect from a tighter model grouping like we have given what I've stated above.


Now, the NHC does well with their forecasts and so I'm sticking close to what they show for now because that's what the best current scenario looks like based on current data, the only difference is right now I think Issac would bend more north in the gulf like the GFS shows rather than just plowing Northwest which doesn't seem that likely to me regardless of how far to the west it makes landfall.

I agree with my local MET Denis Phillips, who is a great tropical forecaster, that the outcome of Issac is and will remain highly unknown really until Saturday because of what is "in the equation" so to speak.
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Quoting presslord:
It takes a great deal of testicular fortitude to do what Levi does...and none whatsoever to take asshat pot shots at him...

I agree with you.
But please, leave me out, eh ?
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MOrning again, all. Not sticking around much today... gotta lot of preps to do for a variety of reasons... I'm still trying to be hopeful about eventual impacts from Isaac, and keeping my eye on JOyce...

Keep the good stuff flowing!

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Also remember that the West Coast of Florida got raked (and flooded) by TS Debby; if the East Gulf track holds true, and we have a Cat 1 or higher, this system will cause major coastal flooding and storm surge along the entire Gulf coast of Florida (wind issues aside).
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Quoting HurricaneDean07:

Let it begin....
NHC should bump this up to 50 Mph at the Intermediate.
I kind of doubt it since the pressure is still relatively high and the inner core is still poorly organized. i think it will strengthen but not right away. Maybe tonight during DMAX.
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FWIW, hurricanealley.net's latest forecast track (NOT official):

Member Since: August 26, 2006 Posts: 16 Comments: 4766
Quoting Beachfoxx:
Hey SJ,

Is that http://www.tropicaltidbits.com?


Thanks BF and great to see ya!

Added to the Quick Links page under General Tropical (purple).
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Quoting Beachfoxx:
Third! Does that make a quorum?


Fourth. Quorum formed...
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Latest GFS wind fields forecast.

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Looks like Isaac's getting retirement this time around ;)

Sarcasm flag: on?
Member Since: October 3, 2010 Posts: 40 Comments: 4129
Quoting Elena85Vet:


Yes the next GFS is at 1pm ET

I expect it to come more in line with the ECMWF and the NHC. It has yet to account for 6 mph decrease in fwd speed, and continued west movement of a weaker storm.

The GFS runs now, 12 pm daily.
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guess I am not out of the woods yet.
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Quoting PensacolaDoug:



I'll second that!


I am definitely noting an uptick of WU comments from Escambia, Santa Rosa, and Okaloosa counties. Interesting to know that there are a lot of us out here.

One Ivan a lifetime is enough.
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Observations>Current>East>Lanyu

Pressure: 983.5mb

Tembin has dropped the pressure at this location by 5mb in just 30mins. WOW!!!!
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Quoting presslord:
It takes a great deal of testicular fortitude to do what Levi does...and none whatsoever to take pot shots at him...


You are so right. Levi is extremely good... He'll adjust. He'll be a star... He already is.
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Quoting CajunCrawfishhunter:
Lafayette Louisiana keeping a watchful eye out!!
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Does everyone know the EAST form the WEST?
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Quoting sunlinepr:


Those are really good news... But I heard of what they call an "indirect loss of life", a woman preparing for the storm, felt down a 2nd floor...


Yes I read that too. Glad y'all didn't get a heavy hit.
Member Since: August 24, 2010 Posts: 0 Comments: 253
Quoting StormJunkie:

Correct, my understanding was that the Gulfstream jet would be running a mission today?

Yes, it is today.

000
NOUS42 KNHC 231503
REPRPD
WEATHER RECONNAISSANCE FLIGHTS
CARCAH, NATIONAL HURRICANE CENTER, MIAMI, FL.
1100 AM EDT THU 23 AUGUST 2012
SUBJECT: TROPICAL CYCLONE PLAN OF THE DAY (TCPOD)
VALID 24/1100Z TO 25/1100Z AUGUST 2012
TCPOD NUMBER.....12-096

I. ATLANTIC REQUIREMENTS
1. TROPICAL STORM ISAAC
FLIGHT ONE --TEAL 73 FLIGHT TWO --NOAA 49--
A. 24/1800Z,25/0000Z A. 25/0000Z
B. AFXXX 1209A ISAAC B. NOAA9 1309A ISAAC
C. 24/1630Z C. 24/1730Z
D. 17.5N 71.2W D. NA
E. 24/1730Z TO 25/0000Z E. NA
F. SFC TO 10,000 FT F. 41,000 TO 45,000 FT

FLIGHT THREE --TEAL 71-- FLIGHT FOUR --NOAA 49--
A. 25/0600Z,1200Z A. 25/1200Z
B. AFXXX 1409A ISAAC B. NOAA9 1509A ISAAC
C. 25/0400Z C. 25/0530Z
D. 18.6N 73.5W D. NA
E. 25/0530Z TO 25/1200Z E. NA
F. SFC TO 10,000 FT F. 41,000 TO 45,000 FT

2. SUCCEEDING DAY OUTLOOK: CONTINUE 6-HRLY FIXES.
A G-IV MISSION DEPARTING 25/1730Z AND 26/0530Z.
Member Since: August 28, 2006 Posts: 6 Comments: 2899
Ok, "jeff713" and "SherwoodSpirit"... respectfully, I am not interested in conducting a spelling bee on this site nor engaging in petty irrelevant bickering. However I do look forward to discussing the weather together with all of you. Although I will be the first to admit that I'm an amateur, I have been keenly interested in meteorology since I was a kid. I believe we can all participate freely on this forum and I will reiterate my forecast of a hurricane hitting between Gulport and Panama City with Pensacola being the likeliest to be hit. Let's continue the dialog, my new friends! :)
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Quoting hurricanejunky:
Looking at the latest model runs for GFS and ECMWF, it looks like the NHC is favoring the ECMWF again. Maybe ECMWF will improve its 2012 reliability this time around? Think they'll regret moving away from the GFS at this point? It didn't work out too well for TS Debby. GFS still shows Isaac skirting the FL West Coast and entering the state near the Big Bend area which can ill afford more rain. Isn't the next GFS model run at 1pm?


Yes the next GFS is at 1pm ET

I expect it to come more in line with the ECMWF and the NHC. It has yet to account for 6 mph decrease in fwd speed, and continued west movement of a weaker storm.
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Quoting AussieStorm:

Not just twice but maybe even three times if the GFS is correct. Check out this link....


Potential hell! And since, as far as I can recall - even the Eastern Hemisphere has Tropics, I will still watch Tembin (even though I do live in Cayman).
Would be interesting to know how many bloggers may be from that Pac area or, indeed how many bloggers may have business interests or relatives in that overall region.
But I do understand the current "emphasis" on Isaac
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Quoting pensacolastorm:


Far too similar to Ivan, also an "I" storm.


Been saying that for a couple of days.

'Cane Trivia time!! Did you know that "I" hurricane names have been retired more than any other letter in the alphabet except for "C"?
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Third! Does that make a quorum?
Quoting PensacolaDoug:



I'll second that!
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What's the best site to see the new GFS model?
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Quoting clwstmchasr:


Agree. Reading dr. M's blog he thinks the impact on Tampa will be minimal.


WAY too early to predict severity of impacts anywhere in the US right now. Watching and waiting north of Tampa here. I personally hope it keeps trending west, but so far still a relatively decent spread on the models. And the reliable GFS continues to put him right on the coast.
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Quoting AussieStorm:


I understand that, but even a few lines at the bottom could of been enough. Not to mention a Cat 4 that is going to loop around and make landfall again then loop around and hit again, I think that could help with people widening there tropical storm knowledge. It's not just the ATL that has it's season right now.


I see your point. One line about the landfall and potential loop, I'm fine with that. We're all here for knowledge. Thx Aussie
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141. 7544
gfs more est this run
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No one in the Tampa Bay area should by no means begin to play the "Tampa shields are up" or it's not coming here game. That would be very irresponsible.
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Quoting moonlightcowboy:
SFC MAP 1016mb now all the way into Houston from western Cuba as the ecoast trof lifts out to the neast. A weak Isaac will miss this weakness and it will take the next one coming from across the central plains to turn him substantially - that is if Isaac gains strength. In the present it's still a weak tropical storm heading due west under the high pressure.

Isaac will miss Hispaniola, imo, and Jamaica should be prepping for a potential Cat 1 cane.


We are on opposite sides on this one MLC, not because of my vast knowledge (lol); but because I can not bring myself to go against the consistency of the consensus for days on end. Kudos on sticking to your guns. That said, I have a feeling that the G-IV mission will work all of that out for us.
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Quoting A4Guy:
All the Gulf Casters will be struttin around like male peacocks. I wouldn't get set on a track until tomorrow morning, after the NOAA jsets take air amples and feed the data into the models....which has been the game plan all along. Can't really get too certain more than thre days out. We all know this, but the blog goes through the "Florida" "Texas" "Gulf Coast" "Tampa" battle with every storm.


I was nearly killed by Katrina... But I have to tell you, knowing this WU Community so well, I laughed out loud when I read the male peacock quote. Just got my goat.
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Also. According to Kermit's Recon data.... Isaac's Center fix has been jumping around a bit...
One center fix put it near 14 N. But it jumped north on the last fix-pass, Still interesting to see.
Member Since: October 3, 2010 Posts: 40 Comments: 4129
136. wxmod
Africa dust and pollution. MODIS today

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Quoting gulfshoresAL:
My condo was not livable for over a year after Ivan. I remember all the blue tarps in Pensacola when flying out. We don't want a storm near us at all!



I'll second that!
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Hey SJ,

Is that http://www.tropicaltidbits.com?
Quoting StormJunkie:
Can someone please provide me a link to Levi's page?
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It takes a great deal of testicular fortitude to do what Levi does...and none whatsoever to take pot shots at him...
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131. FOREX
Quoting 69Viking:


Everyone has to make their own call. I pretty much stay for a Cat 3/4 or lower but I have friends I can go stay with who live away from the coast in newer homes built to the new codes. Just make sure you're prepared to board up an South and East facing windows, with the angle of approach and circulation direction those will be the areas of your Apt. most at risk.


Thank you.
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Hey Viking!

Long time no see.... the concern I have is if Issac hits us the ground is so saturated that we may see trees topple over, we've had soooo much rain!

Heck the bay is stained with the tannins and is orange/brown now. I was hoping for the rest of the season to be sunshine and no canes.... Guess we are going to have to keep a close eye on Issac.



Quoting 69Viking:


Everyone has to make their own call. I pretty much stay for a Cat 3/4 or lower but I have friends I can go stay with who live away from the coast in newer homes built to the new codes. Just make sure you're prepared to board up an South and East facing windows, with the angle of approach and circulation direction those will be the areas of your Apt. most at risk.
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Can someone please provide me a link to Levi's page?
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Quoting AAPLTrader:
LEVI has to eat crow today. For 3 days he predicted a path EAST of Florida.


It is mind boggling that some people consistently care more about that than the actual storm at hand and the people to be affected
Member Since: August 4, 2011 Posts: 0 Comments: 631
Quoting jonelu:

I agree...and the fact that the cone by the professionals still includes alot of real estate on the east side of FL...its premature to start calling for crow consumption.


sadly it is how it works on here...you have respected bloggers, most who are studying to have a job in the field of meteorology or who already do...you have some of us that have a healthy interest in it and some really good knowledge and you have those who do not know squat and only come on here to point out who is wrong and cause drama.

Those in the 3rd category are of no use to this blog and therefore should be ignored and IMO should be banned from the blog due to their only purpose being to start drama.
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126. DDR
Most areas in North western Trinidad areas have received 4+inches of rain from 4-8am this morning.
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SFC MAP 1016mb now all the way into Houston from western Cuba as the ecoast trof lifts out to the neast. A weak Isaac will miss this weakness and it will take the next one coming from across the central plains to turn him substantially - that is if Isaac gains strength. In the present it's still a weak tropical storm heading due west under the high pressure.

Isaac will miss Hispaniola, imo, and Jamaica should be prepping for a potential Cat 1 cane.
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Levi's site is very informative. I find it useful in understanding the dynamics involved in forecasting.
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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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