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

You don't have to prove anything to these idiots.
Member Since: Posts: Comments:
Time: 17:54:00Z
Coordinates: 15.6667N 66.4833W
Acft. Static Air Press: 842.8 mb (~ 24.89 inHg)
Acft. Geopotential Hgt: 1,532 meters (~ 5,026 feet)
Extrap. Sfc. Press: 1004.3 mb (~ 29.66 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 234° at 4 knots (From the SW at ~ 4.6 mph)
Air Temp: 18.0°C* (~ 64.4°F*)
Dew Pt: 18.0°C* (~ 64.4°F*)
Peak (10s) Flt. Lvl. Wind: 4 knots (~ 4.6 mph)
SFMR Peak (10s) Sfc. Wind: -
SFMR Rain Rate: -
Member Since: Posts: Comments:
Quoting Levi32:
Guess who shifted significantly northeast from last night's run, in the short-range at least:


Ehh, maybe a little bit, but not too much.





That would be between these two frame from 00Z. Looks like it may be a little north, but looks like its forecast continuity is more or less intact.
Member Since: Posts: Comments:
I'm getting tired of people bombing on Levi. Let's the guy do his forecasting in peace.
Member Since: August 1, 2011 Posts: 28 Comments: 8027
Quoting VAbeachhurricanes:


no they found 1002.7

Well north of the center...
Not at the center.
1004 Central Pressure.
Member Since: October 3, 2010 Posts: 40 Comments: 4129
Quoting Levi32:


I spend many breaths in each video stressing the amount of uncertainty in a 4-5 day forecast and the different possibilities that are on the table besides my exact track. If people don't know how to listen, that's not really my problem.


They don't listen. They just look at the picture. Keep up the good work.
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Quoting StormTracker2K:
GFDL aims for S FL.
yep its what it has stuck to all along, a very dangerous track it has for florida alright, and..its the same track..the GFS had a couple of days ago, and might come back to it..
Member Since: August 6, 2011 Posts: 4 Comments: 39113
12z GFS ensembles shifted west a bit.

Member Since: November 24, 2005 Posts: 635 Comments: 26652
recon appears to have found the center with a good wind shift here is one obs


Time: 17:53:30Z
Coordinates: 15.65N 66.5167W
Acft. Static Air Press: 842.9 mb (~ 24.89 inHg)
Acft. Geopotential Hgt: 1,530 meters (~ 5,020 feet)
Extrap. Sfc. Press: 1004.4 mb (~ 29.66 inHg)
D-value: -
Flt. Lvl. Wind (30s): From 177° at 3 knots (From the S at ~ 3.4 mph)
Air Temp: 17.0°C* (~ 62.6°F*)
Dew Pt: 17.0°C* (~ 62.6°F*)
Peak (10s) Flt. Lvl. Wind: 4 knots (~ 4.6 mph)
SFMR Peak (10s) Sfc. Wind: -
SFMR Rain Rate: -
Member Since: Posts: Comments:
Quoting LargoFl:
hey ST..your almost in the middle of florida..maybe a tad closer to the east coast..what do YOU expect to feel when issac comes up either coast?


Either way west or east doesn't matter as i can expect heavy rains 5" to 8" with isolated tornadoes and winds in the tropical storm force range.

Member Since: October 26, 2011 Posts: 0 Comments: 2651
Folks we dont even have a solid, real center of circulation yet.  Knowing where this imaginary center that keeps jumping around is going to be in 4-5 days is going to be tough to call.  Today may be the west coast of FL, or 100 miles off shore, and tomorrow after the GIV missions it could move back to the East Coast of FL. 
Member Since: February 28, 2012 Posts: 0 Comments: 66
Quoting Levi32:
Guess who shifted significantly northeast from last night's run, in the short-range at least:


lol. keep doing your thing and don't mind the haters. at least you put reason behind your forecasts.
Member Since: August 6, 2007 Posts: 0 Comments: 693
Quoting Bluestorm5:


That's not dry air, at all.
Link


Thanks for link

Water Vapor

Member Since: August 2, 2010 Posts: 21 Comments: 9821
Center fix at 15.6 N.
Member Since: October 3, 2010 Posts: 40 Comments: 4129
Quoting Bluestorm5:
1004.1 mb is the lowest so far in this recon mission.



no they found 1002.7
Member Since: September 6, 2007 Posts: 0 Comments: 6592
Let me try this again...

Member Since: August 1, 2007 Posts: 0 Comments: 182
Ok, what the hell is NHC smoking?


There's no way in hell the CoC is at 16N unless both the satellite and radar are broken...


So basically NHC is claiming the satellite is down-shifted at least a full degree, and the radar is just malfunctioning, right?
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GFDL aims for S FL.
Member Since: October 26, 2011 Posts: 0 Comments: 2651
Member Since: July 4, 2005 Posts: 204 Comments: 8896
Quoting StormTracker2K:
Issac is very unpredictable. Center now 16N. I guess the NHC was a little quick to adjust the track left.

hey ST..your almost in the middle of florida..maybe a tad closer to the east coast..what do YOU expect to feel when issac comes up either coast?
Member Since: August 6, 2011 Posts: 4 Comments: 39113
Quoting Levi32:
Good afternoon.

Blog update:

Tropical Tidbit for Thursday, August 23rd, with Video


GREAT analysis! Thanks for your time
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1004.1 mb is the lowest so far in this recon mission.

Member Since: August 1, 2011 Posts: 28 Comments: 8027
Quoting Bluestorm5:


That's not dry air, at all.
Link


That shows another nice little spin coming off of Africa.
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901. Elena85Vet
6:05 PM GMT on August 23, 2012
Getting tired of scanning over posts about Levi, pro or con

What ev's..

thx
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900. Levi32
6:05 PM GMT on August 23, 2012
Guess who shifted significantly northeast from last night's run, in the short-range at least:

Member Since: November 24, 2005 Posts: 635 Comments: 26652
899. CaneHunter031472
6:04 PM GMT on August 23, 2012
It seems to me that the GFS has shifted a bit to the west on this run. Any comments?
Link
Member Since: August 1, 2007 Posts: 0 Comments: 182
898. wpb
6:04 PM GMT on August 23, 2012
until and unless a central core forms it will be ditto for some time.the big islands will cut this to shreeds. but a new center could form off the north coast of cuba that could be trouble. 72 hour from now???
Member Since: May 28, 2011 Posts: 0 Comments: 573
897. Bluestorm5
6:04 PM GMT on August 23, 2012
Quoting StormTracker2K:
Issac is very unpredictable. Center now 16N. I guess the NHC was a little quick to adjust the track left.

They don't adjust the track until 5 pm.
Member Since: August 1, 2011 Posts: 28 Comments: 8027
896. GTcooliebai
6:03 PM GMT on August 23, 2012
48 hrs.

Member Since: August 31, 2009 Posts: 0 Comments: 5628
895. ncstorm
6:03 PM GMT on August 23, 2012
Quoting VAbeachhurricanes:
48 euro:

Shift North


:0
Member Since: August 19, 2006 Posts: 13 Comments: 15666
894. DVG
6:03 PM GMT on August 23, 2012
Quoting Joshfsu123:
Just not strengthening... while the cloud pattern has improved, Isaac is still unable to fire convection right over the center... until he does that, he won't strengthen much.


Looking at the RGB, I have the sense that convection will appear shortly. I may be mistaken, but I think it's finally decided on it's center. 15 by 66.5 is where it is if I'm correct.
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893. StormTracker2K
6:03 PM GMT on August 23, 2012
Issac is very unpredictable. Center now 16N. I guess the NHC was a little quick to adjust the track left.

Member Since: October 26, 2011 Posts: 0 Comments: 2651
892. VAbeachhurricanes
6:03 PM GMT on August 23, 2012
48 euro:

Shift North
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891. SouthTXWX
6:03 PM GMT on August 23, 2012
The fact that you can still see downdrafts from the thunderstorms in the N and NE quadrants says that Isaac is still dealing with Dry air, and it looks that he has multiple vortices.  The chances of him becoming a hurricane before crossing Haiti/DR are becomign less with each passing hour that he just spins and spins.
Member Since: February 28, 2012 Posts: 0 Comments: 66
890. hartyparty
6:02 PM GMT on August 23, 2012
Predictions of wind and rain are great for planning, but we need similar predictions for storm surge in the same forecast.
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889. LargoFl
6:02 PM GMT on August 23, 2012
Quoting ncstorm:


If the NHC is uncertain about the track then no one should be calling anyone out on this blog yet about wrong forecasts..What a weekend this will be..
exactly
Member Since: August 6, 2011 Posts: 4 Comments: 39113
888. Bluestorm5
6:02 PM GMT on August 23, 2012
Quoting sunlinepr:
A stream of dry air into Isaac from the NE...



That's not dry air, at all.
Link
Member Since: August 1, 2011 Posts: 28 Comments: 8027
887. chevycanes
6:02 PM GMT on August 23, 2012
Quoting WDEmobmet:


I find it dangerous to "STICK TO YOUR GUNS" when people make decisions based on your his forecast

no one should make decisions based on what anyone on here says except for Dr. Masters.

it's their own problem if they are gonna listen to some poster on a blog instead of their local weather service or the NHC.
Member Since: August 6, 2007 Posts: 0 Comments: 693
886. GTcooliebai
6:02 PM GMT on August 23, 2012
Quoting ncstorm:


If the NHC is uncertain about the track then no one should be calling anyone out on this blog yet about wrong forecasts..What a weekend this will be..
Thank You I completely agree, the bashings need to stop! I find uncertainties in tracks is what makes it fun to track these storms. Of course you have to be careful what you wish for.
Member Since: August 31, 2009 Posts: 0 Comments: 5628
885. GetReal
6:02 PM GMT on August 23, 2012
Member Since: July 4, 2005 Posts: 204 Comments: 8896
884. Twinkster
6:02 PM GMT on August 23, 2012
Levi I just have to say I appreciate your forecasts and while I am not as far east as you in where I think Isaac will be going I do think ECMWF and GFS are too far west. I have Isaac making landfall on the extreme eastern big bend of florida area
Member Since: June 7, 2007 Posts: 4 Comments: 962
883. sporteguy03
6:01 PM GMT on August 23, 2012
Quoting Levi32:


I spend many breaths in each video stressing the amount of uncertainty in a 4-5 day forecast and the different possibilities that are on the table besides my exact track. If people don't know how to listen, that's not really my problem.


FWIW,
Levi I am sure many on here respect your forecasts and also take into account what the NHC provides as well. If someone ignores the products of the NHC that is their choice and theirs alone. I see no reason to hold you accountable for a forecast that you provide on a blog.
Member Since: July 7, 2005 Posts: 0 Comments: 5350
882. LargoFl
6:01 PM GMT on August 23, 2012
Quoting WDEmobmet:


I find it dangerous to "STICK TO YOUR GUNS" when people make decisions based on your his forecast
anyone can do what they want..but..if your not listening, not only to him but ALL the mets including the NHC..ALL are saying Prepare NOW and watch and wait and see this weekend, where exactly issac goes..there can be no complaining about who said what and when..the only thing that can be done now is..Prepare..period
Member Since: August 6, 2011 Posts: 4 Comments: 39113
881. WDEmobmet
6:01 PM GMT on August 23, 2012
Quoting Levi32:


I spend many breaths in each video stressing the amount of uncertainty in a 4-5 day forecast and the different possibilities that are on the table besides my exact track. If some people don't know how to listen, that's not really my problem.


OK fair enough... I wasnt trying to start a heated debate. I just wanted to help, sorry if you thought I was attacking you, simply wasnt the case.
Member Since: February 3, 2010 Posts: 0 Comments: 755
880. SFLWeatherman
6:01 PM GMT on August 23, 2012
Center is around 10.7N to 30N lol
Quoting Bluestorm5:
Center is around 15.8 N
Member Since: May 23, 2012 Posts: 0 Comments: 4671
879. 954FtLCane
6:01 PM GMT on August 23, 2012
Thanks Levi. Your tidbits vids are so full of information. Your a natural!
Member Since: September 30, 2008 Posts: 0 Comments: 108
878. aislinnpaps
6:01 PM GMT on August 23, 2012
Just curious, anyone know the statistics for how many forecast paths turn out to be correct a week in advance?
Member Since: August 22, 2009 Posts: 0 Comments: 3141
877. floridaboy14
6:01 PM GMT on August 23, 2012
come on people. im not buying that wnw motion. satalite shows a due west track
Member Since: July 25, 2011 Posts: 0 Comments: 1102
876. mcluvincane
6:00 PM GMT on August 23, 2012
Got to get off of here for a bit and come back after the levi bull crap
Member Since: June 2, 2010 Posts: 0 Comments: 1339
875. jonelu
6:00 PM GMT on August 23, 2012
Quoting Levi32:


Well given my current track obviously I think some slight shifts in the models back towards the east are possible, but we'll see how the G-IV runs go. Nopefully NRT will be around tonight to tell us whether the 0z runs actually got the data.

Very sound reasoning. Now question. Do you think that Isaacs size and relative weakness will make interaction with the mountain less of a factor. Seems to me a deeper low in more likely to be jerked around.
Member Since: October 31, 2007 Posts: 2 Comments: 884
874. wn1995
5:59 PM GMT on August 23, 2012
No wind shift yet on recon... interesting. Also no sfc winds of even 40 mph yet. Other than that cat 3 wind reading ;)
Member Since: July 17, 2011 Posts: 1 Comments: 207

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About

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