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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1174. Levi32
ECMWF certainly keeps the worry wide open for the Gulf of Mexico. I still think it looks unrealistically far west, but the eastern gulf coast has a lot of models pointed at it. I still want to see the 0z runs withe G-IV data in them.
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Quoting StormTracker2K:
Euro is much further north this run.


Hmmmmmmmmmmmm go figure....LOL
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Quoting reedzone:
EURO is out to lunch this afternoon. A Hurricane cannot slam into a ridge, maybe a banana like ridge, but not the side of a ridge.. Should recurve near Florida. I'm holding off on the REED run until I see better data in the models tonight. Off to work.
Looking at the Euro 500mb data, one can see that it does not slam it into a ridge as that is obviously, as you said, impossible. Rather the Euro has consistently forecast a stronger ridge off the coast of Florida to send Isaac farther west.
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What the hell is wrong withe Euro. I mean really from one of the best performing models to one of the worst in one year.

Member Since: October 26, 2011 Posts: 0 Comments: 2651
Quoting floridaboy14:


um this is further west on the euro and um we got a strong hurricane ready to plow lousiana


The Euro seems to have amnesia about how it failed with Debby in June.
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Quoting clwstmchasr:


In the middle:)

I;m sorry...but this is just too good not to laugh at...now everyone ...LAUGH
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1167. hydrus
Maybe Isaac will pull a 1981 Dennis.. WIKI..Dennis began as Cape Verde-type hurricanes typically do. A tropical wave leaving the coast of Africa on August 5 developed into Tropical Storm Dennis on August 7 near Cape Verde. Unlike most such storms, Dennis degenerated to a tropical wave before reaching the Windward Islands on August 12.[31] This wave crossed the Caribbean, passing over Jamaica before reaching the southwestern coast of Cuba on August 15.[32]

Once near Cuba, the wave began rapid organization, restrengthening into a tropical storm. It crossed Cuba, then moved into southern Florida. Over Florida, steering currents weakened and the storm stalled. On August 19, Tropical Storm Dennis reemerged over water, skimming the coasts of the Carolinas before accelerating out to sea. Dennis reached hurricane strength before becoming extratropical on August 22.[32]

Most damage associated with Dennis was from the heavy rainfall caused by its slow passage over Florida, although two tornadoes were also reported within Florida.[32] The highest amount registered was near Homestead, Florida, where 25.56 inches (649 mm) of rain was measured.[7] Agriculture damage in Dade County, Florida was estimated at $15 million (1981 USD). Coastal areas of the Carolinas were also affected by heavy rainfall, with spots seeing over 10 inches (250 mm), as well as minor beach erosion.
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Well if this Euro run is somehow true then Houston, we got a problem.
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Quoting reedzone:
EURO is out to lunch this afternoon. A Hurricane cannot slam into a ridge, maybe a banana like ridge, but not the side of a ridge.. Should recurve near Florida. I'm holding off on the REED run until I see better data in the models tonight. Off to work.


What?


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1164. trey33
Quoting clwstmchasr:


In the middle:)


+1
Member Since: August 16, 2007 Posts: 0 Comments: 499
HH finding a much better defined windfield this afternoon to the east of Isaac
Member Since: September 6, 2007 Posts: 0 Comments: 6697
Quoting LargoFl:
yes ive been to that site, my house is 45 feet above sea level, im not worried about storm surge, more about wind damage etc, thanks

I lived in  Palm Harbor and St Pete for 15 years, I remember watching Dennis Phillips on abc as Charlie moved up the coast and seeing it turn, and Phillips almost yelling on the air that it was turning. And I sat there and went WHEWWWWWWWWWWWWWWWWWW!!!!

That was scary stuff.
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This can't be good.

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Quoting 1900hurricane:
168 hours out on the 12Z ECMWF brings landfall near Sabine Pass.



Its got climatology on its side...but i dont know about going that far...i suspect once it gets past a point its going west without recurve, since it is recurving back into the trough from behind, but i doubt it will get that far....but we shall see...
That NOAA G-IV data should help with sampling the ridge, adn the radiosonde network should have the trough covered
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1159. Chiggy
12Z Euro at 168 hrs, 959mb, TX/LA border...!
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Quoting jeffs713:


Hey... we are actually on AT LEAST the Sr. High level. Well, most of us. A few of us are still stuck in preschool.

And for the record, quite a few of us are on here while @ work. And probably a few of those are on here because we don't want to be @ work.


Jeff the Euro has this aimed at you this time.
Member Since: October 26, 2011 Posts: 0 Comments: 2651
Quoting moonlightcowboy:


That's a fairly significant shift.


Too funny,the EURO moves east the GFS goes west....lol,and still it seems everyone knows where Isaac is going. And now the EURO goes back west...lol
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Quoting StormTracker2K:
Euro taking Issac in TX. The biggest key right now is what happens to him when he hits Haiti. Sometimes these system can jump out into the Atlantic. Euro takes issac across the whole country of Cuba and if that happen then Issac is poof for sure.



Nothing is for sure except death and taxes.
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Quoting Dunkman:
At 168 the Euro is hitting Lake Charles, LA. Let me just say I'm not buying it. I think the Euro is way overdoing the ridge in the east to push it that far west.


And yet the Euro is the best overall.
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1154. trey33
Quoting aislinnpaps:


Hope you didn't forget the tarps if you think you'll need roof caulk, and wood strips and nails to nail the tarp down.


It's a flat, commercial grade pvc roof - vulkem caulk will do the trick if I can just get it to dry out for the next few days. Tampa has had so much rain lately.
Member Since: August 16, 2007 Posts: 0 Comments: 499
If the NHC follows the Euro then they will look like a bunch of monkeys especially if this pulls a Debby which seems more likely for issac to take a Debby track.

Member Since: October 26, 2011 Posts: 0 Comments: 2651
EURO is out to lunch this afternoon. A Hurricane cannot slam into a ridge, maybe a banana like ridge, but not the side of a ridge.. Should recurve near Florida. I'm holding off on the REED run until I see better data in the models tonight. Off to work.
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Quoting Torgen:


Some of us actually have to work during the day and can't keep up with the Degrassi Jr High stuff going on.


Hey... we are actually on AT LEAST the Sr. High level. Well, most of us. A few of us are still stuck in preschool.

And for the record, quite a few of us are on here while @ work. And probably a few of those are on here because we don't want to be @ work.
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1149. Thaale
ECMWF 168 hours. Big change west from the 0Z run, which had Isaac 100 miles inland on the MS-AL border:

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Quoting Levi32:


The one on the 22nd had more words.


Dis.......is humor
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1147. luigi18
Quoting CaribbeanWave:
Hello,

I live on St. Croix , USVI and this is the condition right now live from TS Isaac.

http://www.gotostcroix.com/live


nice place what about the rain?
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1146. Levi32
Quoting Grothar:


So when your were talking about the trough coming down. You weren't being condescending, you were just explaining something descending? Now it makes sense.

Just some quick questions.

1. The people who defend you, obviously like you. Does it upset you that they attack other people?

2. Would you be happier if all of this stopped?

3. Do you hope that people understand you are just
trying to provide an explanation of conditions
for them to think about?

4.



Yes to all of the above, especially that last one.
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Quoting TXHuRRicanE:
just got back.... WHERE IS THE CENTER


Anywhere near the center of the storm where you want it to be.
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1144. LargoFl
Quoting wakd3Xn04:


Some of the worst flooding in Miami Dade has actually been during Tropical Storms that moved through slowly and dumped tons of rain. Irene in 99 comes to mind. Lost my truck in that one.
i hope this is a fast mover for sure, we dont need much more rain
Member Since: August 6, 2011 Posts: 4 Comments: 41845
168 hours out on the 12Z ECMWF brings landfall near Sabine Pass.

Member Since: August 2, 2006 Posts: 47 Comments: 11694
Quoting Chiggy:
12Z Euro at 144hrs, 970mb moving NW, perhaps even WEST of NOLA..

yeah..uh...no...don't like this one. Toss it out and start over
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If Torgen had no idea of the friendly relationship between Gro and Levi I could easily see him missing the intended humor with what's gone on here today. Dr. Phil needs to start blogging here, and then he'd need a psychotherapist. :) Lets cut Torgen some slack, he just didn't know the friendship between the two.
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Quoting Levi32:
12z GFS ensembles shifted west a bit.



Id say Florida is pretty well covered,you can't even see it anymore....LOL
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Quoting zoomiami:


Depends on the type of storm: wind or water, and how long it stays over you.

Katrina: cat 1 -- moving medium -- 4-5 hours of lashing winds, tons of rain (which makes the rain go sideways and other strange directions)

Wilma: ??? min cat 2 when it hit? Not so much water, but an entire night of winds.

We don't flood that badly here in Dade County, only in certain low lying areas, so it the water is usually down within 24 hrs. Power outs -- different story.


Some of the worst flooding in Miami Dade has actually been during Tropical Storms that moved through slowly and dumped tons of rain. Irene in 99 comes to mind. Lost my truck in that one.
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Quoting TXHuRRicanE:
just got back.... WHERE IS THE CENTER


Some people would tell you it's up near the Outer Banks, other's would have you believe it's in the South Atlantic.

Realistically, somewhere between 15.5 and 16N.
Member Since: August 10, 2010 Posts: 2 Comments: 1971
Quoting TXHuRRicanE:
just got back.... WHERE IS THE CENTER

so far I've counted 15 in the past 2 hours

sarcasm flag: ON
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Euro taking Issac in TX. The biggest key right now is what happens to him when he hits Haiti. Sometimes these system can jump out into the Atlantic. Euro takes issac across the whole country of Cuba and if that happen then Issac is poof for sure.

Member Since: October 26, 2011 Posts: 0 Comments: 2651
1135. hulakai
Looking at the big picture http://www.ssd.noaa.gov/goes/east/tatl/flash-avn.h tml
I and J seem to be attached to TUTT at the hip. I still looks like he is going West and J is moving NW. Could TUTT be keeping I down there? (which might also explain the SW to NE tilt)
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Quoting TXHuRRicanE:
just got back.... WHERE IS THE CENTER



there
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Quoting Levi32:
Guess who shifted significantly northeast from last night's run, in the short-range at least:



Ummmmmmmmmmmmmmmm might that be the EURO Levi?
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1132. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 41845
So the EURO sticks Isaac under the Texas ridge. Starting to have Debby flashbacks...
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Quoting TXHuRRicanE:
just got back.... WHERE IS THE CENTER


haha that's a loaded queston. Depends on who you ask..
Member Since: August 18, 2008 Posts: 0 Comments: 474
I see the latest gfs has shifted a little south on Cuba so it is starting to get in line with the Ero on its path this could mean the storm will traverse the underside of Cuba
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Quoting Torgen:


Some of us actually have to work during the day and can't keep up with the Degrassi Jr High stuff going on.


#1 if you work during the day then why are you on here now?

#2 for me, I am working from home today, but I work during the day and I knew he was joking
Member Since: March 10, 2010 Posts: 1 Comments: 7874
Good afternoon everyone, I go for a few hours and Isaac is looking better per recon. I know he still has some issues but he is slowly getting more organized.


Don't forget Joyce even though she is having her own problems.
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just got back.... WHERE IS THE CENTER
Member Since: June 11, 2012 Posts: 0 Comments: 51
I'll be glad when this storm hits somewhere just to stop all the damn bickering, holy crap
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Yesterday's 12Z EURO: "Yeah, I should probably nudge east a bit.

00Z EURO: "Yeah those other guys are probably on to something. East we go."

Today's 12Z: "Nah, screw that, I'm going to Texas."
Member Since: August 10, 2010 Posts: 2 Comments: 1971

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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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