Isaac is strengthening

By: Dr. Jeff Masters , 2:58 PM GMT on August 24, 2012

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Tropical Storm Isaac is strengthening. An Air Force Reserve hurricane hunter aircraft measured surface winds of 60 mph on the east side of the center, about 170 miles south of Santo Domingo in the Dominican Republic, at 8:40 am EDT this morning. Winds at the aircraft's flight level of 5,000 feet were hurricane force, 76 mph. The surface pressure remained fairly high, at 1000 mb. Tropical cyclones have a warm core, and the Hurricane Hunters typically find that a storm's lowest pressure is also where the warmest temperature are. However, this morning's flight found that Isaac was still disorganized, with the storm showing almost no evidence of a warm core. Isaac's warmest temperatures were displaced 75 miles to the west of where the lowest pressure was. There were no signs of an eyewall beginning to build. Infrared and visible satellite loops show that Isaac is somewhat asymmetric, with a large band of intense thunderstorms to the east, separated from the core region. This is interfering with both the storm's low-level inflow and upper-level outflow, but the band appears to be dying out. An analysis of upper level winds from the University of Wisconsin CIMSS shows an upper-level outflow channel well-established to the north, and an intermittent outflow channel to the south.


Figure 1. Evening shot of Tropical Storm Isaac taken on August 23, 2012, by the NOAA Hurricane Hunters.

Isaac's rains
Radar imagery from Puerto Rico shows that Isaac is dumping some very heavy rains to the south and east of the center. Ponce, Puerto Rico had a wind gust of 37 mph this morning as a heavy band of rain moved through, and radar-estimated rainfall amounts are in excess of 7 inches for the region just north of Ponce. Power outages to 2,000 homes have been reported in Puerto Rico this morning. NOAA buoy 42085 offshore from Ponce reported a wind gust of 54 mph near 9 am EDT this morning. Rainfall estimates from microwave satellite instruments suggest that Isaac's heaviest rains are to the south of the center, and that the Dominican Republic and Eastern Haiti will escape the worst of Isaac's rains. Haiti's southwest peninsula and Eastern Cuba should suffer the heaviest rains.


Figure 2. Radar-estimated precipitation from the Puerto Rico radar shows the region near Ponce has received up to 7" of rain as of 10 am EDT August 24, 2012.

Latest model runs for Isaac
The latest set of 00Z and 06Z (8 pm and 2 am EDT) model runs have come into better agreement, thanks to the dropsonde mission by the NOAA jet yesterday afternoon and evening. Isaac should move over Haiti's southwest peninsula and then eastern Cuba, then track along the spine of Cuba before popping off into the Florida Straits on Sunday. A trough of low pressure will then pull Isaac to the northwest, and then north, towards the Central Gulf Coast. Landfall locations range from Mississippi (06Z HWRF model run) to the Florida Panhandle south of Tallahassee (06Z GFDL model run.) It is possible that the trough of low pressure pulling Isaac to the north may not be strong enough to pull Isaac all the way to the northeast and out to sea, and the ECMWF model indicates that Isaac could stall out after landfall over the Tennessee Valley for several days.


Figure 3. Predicted 5-day rainfall total ending at 2 am EDT Wednesday August 29, from Tropical Storm Isaac. Graphics were generated from the 6Z (2 am EDT) August 24, 2012 run of the HWRF model. Image credit: Morris Bender, NOAA/Geophysical Fluid Dynamics Laboratory.

Intensity forecast for Isaac
Isaac has not intensified as much as predicted, and I think that the storm's very large size is partially responsible for that. It's tough to spin up as much atmosphere as Isaac is attempting to do very quickly. Conditions remain favorable for intensification today, with wind shear low, 5 - 10 knots, ocean temperatures warm, 29°C, and dry air mostly mixed out of the storm's core. The large band of intense thunderstorms to the east, separated from the core region, appears to be dying out now, which will help the storm grow more organized. The storm's structure has improved considerably between 9 am - 10 am EDT, with a fairly tight center forming, exposed to view, on the north edge of Isaac's heaviest thunderstorms. A curved band of heavy thunderstorms is now trying to wrap around this center to the northeast, and this band will bring very heavy rains to Haiti and the Dominican Republic this afternoon. I expect that the Hurricane Hunters will observe a partial eyewall in their vortex reports between 2 - 4 pm EDT this afternoon. The storm's large size and disorganized structure suggests that Isaac will be able to intensify only slowly today, and will have top winds of 70 - 75 mph before encountering Southwest Haiti and Eastern Cuba tonight and Saturday. Isaac will likely be a 50 - 60 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. The upper-level wind pattern favorable may be quite favorable for intensification, with low wind shear due to an upper-level anticyclone over the storm--though the models disagree on whether or not this anticyclone will set up directly over Isaac or not. It will probably take at least 24 hours with the storm's center over water for it to become a hurricane. It is possible that Isaac could be approaching Category 3 strength by the time it makes landfall on Tuesday on the Gulf Coast, as suggested by the latest 06Z run of the HWRF model.

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 17% chance of receiving tropical storm-force winds and a 1% chance of receiving hurricane-force winds on Monday. Tampa is in the NHC cone of uncertainty, though near the edge of it. At a minimum, Tampa will receive very heavy rains and wind gusts in excess of 40 mph. Isaac is going to be hard-pressed to bring hurricane-force winds to the city, though, since any path that takes it close to Tampa would keep the storm too close to land for significant intensification to occur. I put the odds of a mass evacuation being ordered for Tampa during the convention at 1%. I have detailed information on Tampa's storm surge vulnerability in a post from last week.

Invest 97L off the coast of Africa
A tropical wave that moved off the coast of Africa on Thursday has been designated Invest 97L by NHC this morning. The storm has a modest amount of spin and heavy thunderstorms, and is under moderate wind shear of 10 - 20 knots. In their 8 am EDT Tropical Weather Outlook, NHC gave 97L a 30% chance of developing by Sunday morning. The 8 am EDT SHIPS model forecast predicts that 97L will track west-northwest over the next few days, and encounter a region of high wind shear associated with an upper-level low on Monday and Tuesday. This low may be capable of tearing the storm apart, as happened to Tropical Storm Joyce. None of the models currently foresee that 97L will affect the Lesser Antilles Islands, but 97L may pass near Bermuda 7 - 8 days from now.



20th anniversary of Hurricane Andrew
Today, August 24, is the 20th anniversary of Hurricane Andrew, which hit South Florida as a Category 5 hurricane with 175 mph winds--one of only three Category 5 hurricanes ever to hit the U.S. With Isaac churning through the Caribbean this week, I didn't have time to prepare a special post on Andrew, but our Hurricane Andrew archive page has links to satellite and radar images, newspaper headlines, and 49 YouTube videos. Here's an additional link for an Andrew damage video shot by wunderblogger/storm chaser Mike Theiss, when he was 14 years old.

Jeff Masters

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Quoting caribnewsman:

Not to the same degree, but it does identify someone as a tourist/foreigner.

The point is, Isaac could very well pass closely to two of the three Cayman Islands just as easily as it could pass very closely to Miami.


And I live on the Northern most of the 3 and am by no means convinced that we are "out of the woods and in the clear".
Thankfully, at this time the Wind fields in Southern quadrants seem much smaller - BUT - the way Isaac has flobbled around, who can tell?
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1760. Grothar
Quoting HurricaneDean07:

Ive known the Drill ever since I learned what the f5 Button truly does. LOL


I think TropicalAnalyst taught me about that button. :)
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1759. WxLogic
Quoting stormchaser19:
Isaac is a bad boy...few miles of my house


DR doesn't have sink holes (at least that I've heard of) but sure looks like the ground decided to collapse in that crossing.
Member Since: August 14, 2008 Posts: 4 Comments: 4881
Quoting jeffs713:
Looking at the recon info on Levi's website (which is awesome, by the way), it looks like Isaac still isn't perfectly stacked. His lowest MSLP is shifted slightly to the north of the flight-level center. The shift isn't much to speak of, so he can still strengthen - it is just an interesting item of note.


Which reminds me...Is Stephanie Abrams still in Key West?
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Quoting oceanspringsMS:


I always buy more wal-mart stock a few days before a predicated GOM, SE Coast landfall

Home Depot or Lowe's might be a better bet.
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1756. Grothar
Quoting Tribucanes:
Just got my first chance to look at Isaac in the last few hours. What the heck happened to him? He looks just horrible. Is he as disorganized as satellite would suggest, or is he better than he looks at the moment?


Oh, Trib, I thought you were talking about me for a minute.
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Even the NWS floater satellite is starting to show a well defined location of central rotation (visible spectrum).

http://www.ssd.noaa.gov/PS/TROP/floaters/09L/flas h-vis-long.html
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1754. LargoFl
....................look how THis has changed since 11 am
Member Since: August 6, 2011 Posts: 4 Comments: 33350
Tampa Bay is right in the center of the eye on the 12Z GFS. Aw hell just throw it out per NHC


CMC precip accum.

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Quoting Grothar:
You guys know the drill.


Ive known the Drill ever since I learned what the f5 Button truly does. LOL
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Quoting seflagamma:


funny! reminds of the that first blogging summer of 2005!
Me too lol, needed a little laugh right now :)
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1750. guygee
Quoting oceanspringsMS:
You weather geeks have more acronyms than we had in the army. :)
A lot of acronyms but no cigar for you. No one beats the DoD for acronyms. On top of that when there is a big change (like firing Gen. Shinseki)there is always a whole new set of acronyms.
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Quoting Surferdude:
Welcome to Florida ...we have been expecting you.

I am jumping on the bandwagon(didn't think he would make it), Isaac is getting serious now. Will be Hurricane Isaac by 2am giving the current intensification...close to 994mb now and moving NW...

Levi's track seems very reasonable. Best dude we have on this blog.


I concur. I think it hits just west of Miami at a Cat 2
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Quick look at the dropsonde data from Gonzo indicates the ridge to the north is a little weaker than yesterday. 500mb heights 10-20 m lower.
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Isaac is a bad boy...few miles of my house
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Quoting Bluestorm5:
Isaac the last 72 hours... really cool.



Link


Very cool...thanks for sharing
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wow next Tue. Aug 28th, will be the 7th Anniversary of the day I signed up here on WU!..
I was "lurking" a little while before I got the nerve to join in the fun.
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Interesting to note that the ECMWF has been right on everything up to this point. We will have to see if that continues.
Member Since: July 6, 2010 Posts: 108 Comments: 30251
Quoting Grothar:


The shift may be very significant as well as intensity. Just saying. I really have no way of knowing for sure.
I agree, it's a bad/bad situation for south Florida. In order for the cyclone to come closer to the mainland, it has to also be more intense as well.
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1742. Thrawst
Quoting StormTracker2K:


Look north of Cuba right now. Lots of action building there almost like it's getting ready for Issac to move in. Very very interesting to say the least and I'm the only to comment on this over the last couple of days.



I was thinking that too.. very good upper divergence in our area. Wait, lemme get this straight...

Has the expected track of Isaac gotten any closer to me (Nassau, Bahamas) ?
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Quoting WPBHurricane05:
I'm not sure you guys are getting the whole thing Re: HPC

THE MODEL CHOICE THIS PERIOD WAS PARTIALLY CONSTRAINED BY THE LATEST NATIONAL HURRICANE CENTER (NHC) FORECAST FOR ISAAC, WHICH RESEMBLED THE 12Z UKMET THROUGH THURSDAY MORNING. A 00Z GFS/00Z ECMWF COMPROMISE WAS USED THEREAFTER, WITH INPUT RECEIVED FROM NHC REGARDING TROPICAL SYSTEMS AT 17Z. THE 06Z/12Z GFS AND 12Z CANADIAN SOLUTIONS HAD TO BE THROWN OUT BY DEFAULT. THIS LED TO REASONABLE CONTINUITY WITH THE MID-MORNING PRESSURES.

The last NHC track is similar to the UKMET, and for consistency purposes they had to show a track similar to the UKMET. Obviously the GFS and CMC are further east and could not be considered when it came to fronts and pressure.

At least thats what I got out of it.


In a nutshell it's preventing the quick shift in track. Now if the next cycle continues to show it, then it will be considered. I'll tip my hat to you now and bow if you show it again.
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Quoting Tribucanes:
Just got my first chance to look at Isaac in the last few hours. What the heck happened to him? He looks just horrible. Is he as disorganized as satellite would suggest, or is he better than he looks at the moment?


Not quite sure what you mean. This storm's central pressure has dropped 6 millibars today; it's finally set up a temperature differential within its core, as it's warmed substantially; and it's been wrapping convection around the center of its circulation. I've never seen it look as good as it does right now.
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305degrees heading is about exactly WNW, which is about a 1N to 2W ratio in movement, though it will be distorted somewhat by the curvature of the Earth.


Sine 35 = 0.57


So for every 1W it's 0.57N


So if it held it's present heading, it would cross 75W at about 18.4N...
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KeyWestwx,  here is the offical link to the European model animated.

http://www.ecmwf.int/products/forecasts/d/animate /catalog/products/forecasts/medium/deterministic/m sl_uv850_z500%21Wind%20850%20and%20mslp%2172%21Nor th%20America%21pop%21od%21oper%21public_plots%2120 05101800%21%21%21step/
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1737. Grothar
Quoting seflagamma:


funny! reminds of the that first blogging summer of 2005!


I was amazed by you guys back then.
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1736. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 33350
1735. Grothar
Quoting WPBHurricane05:
I'm not sure you guys are getting the whole thing Re: HPC

THE MODEL CHOICE THIS PERIOD WAS PARTIALLY CONSTRAINED BY THE LATEST NATIONAL HURRICANE CENTER (NHC) FORECAST FOR ISAAC, WHICH RESEMBLED THE 12Z UKMET THROUGH THURSDAY MORNING. A 00Z GFS/00Z ECMWF COMPROMISE WAS USED THEREAFTER, WITH INPUT RECEIVED FROM NHC REGARDING TROPICAL SYSTEMS AT 17Z. THE 06Z/12Z GFS AND 12Z CANADIAN SOLUTIONS HAD TO BE THROWN OUT BY DEFAULT. THIS LED TO REASONABLE CONTINUITY WITH THE MID-MORNING PRESSURES.

The last NHC track is similar to the UKMET, and for consistency purposes they had to show a track similar to the UKMET. Obviously the GFS and CMC are further east and could not be considered when it came to fronts and pressure.

At least thats what I got out of it.



That is a pretty good analysis
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Quoting Grothar:
You guys know the drill.



Command R :)
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Quoting Grothar:
You guys know the drill.



funny! reminds of the that first blogging summer of 2005!
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Looking at the recon info on Levi's website (which is awesome, by the way), it looks like Isaac still isn't perfectly stacked. His lowest MSLP is shifted slightly to the north of the flight-level center. The shift isn't much to speak of, so he can still strengthen - it is just an interesting item of note.
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Quoting leelee75k:
waiting to hear what Max Mayfield say at 5pm on Channel 10 news. He's my weather god! I do as Max says!


I do as no one says. Just Saying.
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Quoting seflagamma:


that is the perfect thing to say.. we don't want panic but we want people to be prepared.
Thanks for posting that here.

I am in Broward County.


I always buy more wal-mart stock a few days before a predicated GOM, SE Coast landfall
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Quoting WxLogic:


Likewise... I expect that for the first 48 hours for the track to closer to the current consensus up to S FL and then a track closer to FL W coast. In which a bending would be noticed W of Tampa and finally ending in the KTLH area.

A guess we'll see in a couple minutes. Also Isaac still moving NW.


Look north of Cuba right now. Lots of action building there almost like it's getting ready for Issac to move in. Very very interesting to say the least and I'm the only to comment on this over the last couple of days.

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1727. Grothar
Quoting WxLogic:


Likewise... I expect that for the first 48 hours for the track to closer to the current consensus up to S FL and then a track closer to FL W coast. In which a bending would be noticed W of Tampa and finally ending in the KTLH area.

A guess we'll see in a couple minutes. Also Isaac still moving NW.


The shift may be very significant as well as intensity. Just saying. I really have no way of knowing for sure.
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Whats up WU, Isaac looking grim for SE Florida than yesterday. The Euro is not having a good handle on the storm because its lack of a dominant LLC and little vertical stacking. I also noticing dry air falling apart as the outflow is just too strong to disrupt it. Thus a eastern shift is likely in the next advisory. Convection was concentrated to the LLC's SW however those bug thunderstorms have wane and new convection is firing right over the COC. MLC is slightly displace and by tonight it should be vertically stacked which could put it over Cat.1 status. It will go over a thin strip of Hait and emerge over water which could help it become a cane if it didnt make it tonight. Before going over Cuba Saturday evening. Emerging on Sunday where it should slow down a bit (10-15mph) giving it the almost the whole Bahamas to strengthen. Possibly rapidly strengthen
into a Major Cane coming to Southern Florida. Always Stay tuned as yesterday SE Florida was becoming less likely and know its more likely and could be way stronger than a TS. For all who lives in S. Florida start preparing tonight and make precautions Saturday I notice some south Florida isnt taking it serious which is bothering me right now. LOL its the 20 year anniversary of Hurricane Andrew man talk about IRONY.
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Really cool pronounced trochoidal oscillation with Bolaven in the WPAC.

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Quoting leelee75k:
waiting to hear what Max Mayfield say at 5pm on Channel 10 news. He's my weather god! I do as Max says!


be sure and let us know what he says, i will be at work until 7pm
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KeyWestwx,  here is the offical link to the European model animated




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Quoting WPBHurricane05:
I'm not sure you guys are getting the whole thing Re: HPC

THE MODEL CHOICE THIS PERIOD WAS PARTIALLY CONSTRAINED BY THE LATEST NATIONAL HURRICANE CENTER (NHC) FORECAST FOR ISAAC, WHICH RESEMBLED THE 12Z UKMET THROUGH THURSDAY MORNING. A 00Z GFS/00Z ECMWF COMPROMISE WAS USED THEREAFTER, WITH INPUT RECEIVED FROM NHC REGARDING TROPICAL SYSTEMS AT 17Z. THE 06Z/12Z GFS AND 12Z CANADIAN SOLUTIONS HAD TO BE THROWN OUT BY DEFAULT. THIS LED TO REASONABLE CONTINUITY WITH THE MID-MORNING PRESSURES.

The last NHC track is similar to the UKMET, and for consistency purposes they had to show a track similar to the UKMET. Obviously the GFS and CMC are further east and could not be considered when it came to fronts and pressure.

At least thats what I got out of it.


Me too.
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Quoting WxGeekVA:


My friend was there earlier this year during 92L:



I wish I can visit NHC one day...
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Just got my first chance to look at Isaac in the last few hours. What the heck happened to him? He looks just horrible. Is he as disorganized as satellite would suggest, or is he better than he looks at the moment?
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1719. LargoFl
Member Since: August 6, 2011 Posts: 4 Comments: 33350
Quoting charlottefl:
If Isaac stays on it's current heading it's going to spend very little time over land...


and .......??? (I know what is gonna happen.
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1717. Grothar
You guys know the drill.

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1716. WxLogic
Quoting Grothar:
I expect a significant shift to the East with all the models at the next advisory.


Likewise... I expect that for the first 48 hours for the track to closer to the current consensus up to S FL and then a track closer to FL W coast. In which a bending would be noticed W of Tampa and finally ending in the KTLH area.

A guess we'll see in a couple minutes. Also Isaac still moving NW.
Member Since: August 14, 2008 Posts: 4 Comments: 4881
1715. guygee
Quoting Levi32:
Holy cow! GOES-14 brought out of storage and running 1-min data until October, currently focused on Isaac!
Very cool, good catch, thanks. They seem to have a glitch in the cartographic projection but that is still an awesome view.
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Isaac the last 72 hours... really cool.



Link
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Quoting trHUrrIXC5MMX:
nice TWC video where Brian Norcross and Knabb talk inside the NHC Hurricane Hurricane Operations room
check it out here!!!


My friend was there earlier this year during 92L:

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1712. LargoFl
.....7-day for tampa bay..they keep changing it
Member Since: August 6, 2011 Posts: 4 Comments: 33350
waiting to hear what Max Mayfield say at 5pm on Channel 10 news. He's my weather god! I do as Max says!
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About JeffMasters

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.