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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573. 7544
hmmm anyone seen the cmc
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Quoting Tropicsweatherpr:
There is the 12z GFS landfall.



Looks like my backyard in Blue Mountain Beach, Walton County....
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Quoting HoustonTxGal:


I am simply sharing what someone else posted.. I did not ask them to explain the reason they used Katrina, perhaps they just tried to put it into perspective.


Quoting jeffs713:

Rita was slightly larger than Katrina, and Ike was massive once he got into the GOM.


ok thanks..
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Quoting HoustonTxGal:


I am simply sharing what someone else posted.. I did not ask them to explain the reason they used Katrina, perhaps they just tried to put it into perspective.



Largest Atlantic hurricanes by gale diameter

Rank Storm Season Diameter
(mi) (km)
1 Igor 2010 920 1,480
2 Olga 2001 863 1,389
3 Lili 1996 806 1,296
4 Karl 2004 777 1,250
5 Helene 2006 748 1,204
6 Irene 1999 719 1,157
7 Gabrielle 1989 690 1,111
8 Florence 2006 690 1,111
9 Wilma 2005 662 1,065
10 Keith 1988 633 1,018
11. Grace 1991 633 1,018
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Update:

Isaac still having trouble developing a central core, but it is developing quite a torque in it's motion as it continues to move on average on almost a due west course. The estimated position is 15.5N 66.5W. The system has not gained any latitude this morning which further convinces me that the center is going to miss and go well south of Hispaniola and Cuba as well except for it's western tip which will enhance it's potential to strengthen more than projected at this time. Also the projected path will also have to be pushed a little further west with a possible landfall near the MS/AL border or possibly further west.

The trough over the GOM is still digging south and east with the axis being over Alabama this morning depicted on a recent WV loop. By Saturday the trough is forecast to begin to lift out to the NE and be replaced by a building ridge moving in from the 4 corners region of the SW US. Where this ridge sets up will in part determine where the weakness between this ridge and the Bermuda High over the western Atlantic is located over the SE US.

Next update tomorrow about this time.
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Quoting wunderkidcayman:
waiting on HH RECON they are almost in that Channel between DR and PR what it called again


yep



no I think what you are seeing is the NE quad spining around the circulation in WNW-NW fashon hence the reason you are seeing WNW movements
kinda like my washing machine when it's overloaded to one side....see Moms have real life scientific applicable experience.
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Quoting jeffs713:

Rita was slightly larger than Katrina, and Ike was massive once he got into the GOM.


Yeah BIG difference..., Katrina was cat-5 at one stage, Issac a weak TS.. for now!
Member Since: June 26, 2012 Posts: 0 Comments: 297
Quoting AussieStorm:
This says it all really....
Lanyu, Orchid Island, Taiwan.





Latest Obs.... 08/24 00:30 975.5mb


Keep an eye on this weather station, I will get the eye of Tembin.

Goodnight all. Get prepared if your in the cone or at least make a plan. I hope I don't wake up to find TS Isaac is Hurricane Isaac.


How far away are the two storms from each other??
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Just had a 40 mph GUST with blob crossing Turks and Caicos
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looks like Isaac will begin its WNW/NW track once it reaches 70 W until then a west movement will continue, but the NHC should issue a tropical storm watch for jamaica its going to be a close call
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Even Ernesto didn't look this well developed when the storm was at this location...

Isaac
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Quoting TXHuRRicanE:


why doesn everyone compare the size of a strom to katrina's size... wasn't rite larger than she was.. if not i know Alex '10 was... heck even Ike was

Rita was slightly larger than Katrina, and Ike was massive once he got into the GOM.
Member Since: August 3, 2008 Posts: 16 Comments: 5882
Quoting Clearwater1:
My 10 yo daughter, who suffers through my hurricane rambling talk, every year, suggested that if an eye of a hurricane came directly over us, why not just drive around in it? Or keep up with the eye in our car.

I may give that a try someday, but don't worry, I'll leave her home, safe with her sane mother.


Kinda like jumping up as the elevator crashes down....Lol....sounds like a plan! ;)
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Quoting TXHuRRicanE:


why doesn everyone compare the size of a strom to katrina's size... wasn't rite larger than she was.. if not i know Alex '10 was... heck even Ike was


I am simply sharing what someone else posted.. I did not ask them to explain the reason they used Katrina, perhaps they just tried to put it into perspective.
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HWRF starting to run....
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EVERE WEATHER STATEMENT
NATIONAL WEATHER SERVICE JACKSONVILLE FL
1255 PM EDT THU AUG 23 2012

FLC083-231715-
/O.CON.KJAX.SV.W.0205.000000T0000Z-120823T1715Z/
MARION FL-
1255 PM EDT THU AUG 23 2012

...A SEVERE THUNDERSTORM WARNING REMAINS IN EFFECT UNTIL 115 PM EDT
FOR SOUTHEASTERN MARION COUNTY...

AT 1255 PM EDT...NATIONAL WEATHER SERVICE METEOROLOGISTS CONTINUED TO
TRACK A SEVERE THUNDERSTORM CAPABLE OF PRODUCING DAMAGING WINDS IN
EXCESS OF 60 MPH. THIS STORM WAS LOCATED NEAR SUMMERFIELD...OR 8
MILES NORTHWEST OF THE VILLAGES...MOVING NORTHEAST AT 15 MPH.

OTHER LOCATIONS IN THE WARNING INCLUDE BUT ARE NOT LIMITED TO LAKE
WEIR...WEIRSDALE...SILVER SPRINGS SHORES...OCKLAWAHA...MOSS BLUFF...
LYNNE AND JUNIPER SPRINGS.

PRECAUTIONARY/PREPAREDNESS ACTIONS...

DOPPLER RADAR HAS INDICATED SOME WEAK ROTATION WITHIN THIS STORM.
WHILE NOT IMMEDIATELY LIKELY...A TORNADO MAY STILL DEVELOP. IF A
TORNADO IS SPOTTED...ACT QUICKLY AND MOVE TO A PLACE OF SAFETY IN A
STURDY STRUCTURE...SUCH AS A SMALL INTERIOR ROOM.

REPORT SEVERE WEATHER OR DAMAGE TO THE NEAREST LAW ENFORCEMENT AGENCY
OR YOUR COUNTY EMERGENCY MANAGEMENT.

&&

LAT...LON 2900 8224 2932 8174 2912 8165 2895 8198
2896 8220
TIME...MOT...LOC 1655Z 237DEG 13KT 2903 8204

$$

NELSON
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visible images close up indicates Isaac is moving due west along 15N but the core is begining to consolidate
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Quoting Seflhurricane:
it left from biloxi it just did not report any info until it cleared the Miami,fl Area thats why you see that


Someone just posted it and said 1:30 is wheels up.

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Quoting HoustonTxGal:


Someone posted this last night.... thought I would share again


" Isaac is about 509 miles from N to S and about 850 miles in its widest area from E to W."



Hurricane Katrina was 415 miles in diameter


why does everyone compare the size of a strom to katrina's size... wasn't rita larger than she was.. if not i know Alex '10 was... heck even Ike was
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Quoting StormJunkie:


Actually, I this isn't the upper air mission. It is a C-130 so they will be going in to the storm for center fixes. It just left from Miami instead of from the Islands so it is in route to the storm now.


It could be an upper air mission. HRD is showing a C130 flight in tandem with the G-IV this afternoon.


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waiting on HH RECON they are almost in that Channel between DR and PR what it called again

Quoting GetReal:
700-850mb



500-850mb


There is no doubt that Isaac is going to go much more further west, until it reaches the NW Caribbean.

yep

Quoting Hurricanes101:


steering currents do not stay constant, I think we are already starting to see the WNW movement now


no I think what you are seeing is the NE quad spining around the circulation in WNW-NW fashon hence the reason you are seeing WNW movements
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Any guesses on strength for 5pm advisory. I'm going with 50 mph. 60mph by 11pm.
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548. 7544
hes not looking so good at this hour hes not suppose to turn untill after 11 pm tonight so hes still going to go west most of today and maybe even might be slowing down in speed
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Quoting StormJunkie:


Actually, I this isn't the upper air mission. It is a C-130 so they will be going in to the storm for center fixes. It just left from Miami instead of from the Islands so it is in route to the storm now.
What time is the Gulfstream IV mission suppose to take off Junkie?
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546. TXCWC
Quoting GainesvilleGator:
There is still a lot of uncertainty with Isaac. The models can easily change depending on the intensification of Isaac. Land interation and the lack of land interation can cause sizable model shifts. I wouldn't get caught up too much with what the models are saying right now because they change.


When EURO and GFS generally agree within a 7 day time frame one should always pay attention. Does not mean things will not change as you said - but definetly would not ignore when 2 top models are saying similar thing.
Member Since: May 18, 2012 Posts: 0 Comments: 520
Quoting StormJunkie:


Actually, I this isn't the upper air mission. It is a C-130 so they will be going in to the storm for center fixes. It just left from Miami instead of from the Islands so it is in route to the storm now.
it left from biloxi it just did not report any info until it cleared the Miami,fl Area thats why you see that
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Quoting sunlinepr:
Tembin....Taiwan.... is going to get hammered...




Wow!
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Quoting coffeecrusader:
Isaac is closer to an open wave than a hurricane right now. Don't know if he will ever be able to develop a strong core to intensify.


Open wave? Def a TS
Member Since: June 26, 2012 Posts: 0 Comments: 297
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Quoting Bluestorm5:
Is recon plane going into Isaac or landing at St. Croix?



Hopefully to Isaac...I don't think they'd be gathering observations if they are only going to land in St. Croix. Why even bother?
Member Since: June 29, 2009 Posts: 13 Comments: 5313
Quoting sunlinepr:
Taiwan.... is going to get hammered...


Watch this loop.
Link

HUUUUUUUGGGGGGEEEEEEEE



TYPHOON 16W (BOLAVEN), LOCATED APPROXIMATELY 500 NM SOUTHEAST OF
KADENA AB, OKINAWA, JAPAN, HAS TRACKED WEST-NORTHWESTWARD AT 09 KNOTS
OVER THE PAST SIX HOURS. MAXIMUM SIGNIFICANT WAVE HEIGHT AT 231200Z
IS 40 FEET.
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Quoting MississippiWx:
The fact that the GFS has Isaac stalling over the Panhandle sends a red flag to me that there must not be much of a trough/weakness to the north. Add in the shift to the west on the 12z run, I'm a little more concerned at a fairly strong hurricane headed to the Panhandle of FL. With a final landfall still 5+ days out, plenty of things can change.
another thing to note is the GFS may be picking up that since ISAAC has its center disorganized it initialized it as a weaker system moving further west than previous forecast , so it can shift back east
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Quoting 1900hurricane:

That storm is massive, even by WPac standards.


Someone posted this last night.... thought I would share again


" Isaac is about 509 miles from N to S and about 850 miles in its widest area from E to W."



Hurricane Katrina was 415 miles in diameter
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Quoting TopWave:
Whats up with the NAM? Model is way east
NAM =/= tropical model
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Is recon plane going into Isaac or landing at St. Croix?

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Quoting cchsweatherman:


That is a plane performing a dropsonde mission of the atmosphere around the storm. It's probably picking up those winds in the sheared thunderstorms just off the northern coast of the Dominican Republic from what I can tell.


Actually, I this isn't the upper air mission. It is a C-130 so they will be going in to the storm for center fixes. It just left from Miami instead of from the Islands so it is in route to the storm now.
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Quoting A4Guy:
what is with the ATCF images not updating after the 12Z early cycle? Yesterday,they never updated after that, and today they are not updating either.

personally...i am very confused by the time designaitons. I know Z is "zulu time" - which is 4 hrs ahead of ET...but I can never figure out when the runs really update, and the diffs between the early and late runs. probably explained onthe NHC site, but I have been alittle lazy about looking. :)


Today ATCF has been keeping up to the forecast cycles. Eary/Late is explained in the NHC Annual Verification Report:


Numerous objective forecast aids (guidance models) are available to help the
NHC in the preparation of official track and intensity forecasts. Guidance models are
characterized as either early or late, depending on whether or not they are available to the
forecaster during the forecast cycle. For example, consider the 1200 UTC (12Z) forecast
cycle, which begins with the 12Z synoptic time and ends with the release of an official
forecast at 15Z. The 12Z run of the National Weather Service/Global Forecast System
(GFS) model is not complete and available to the forecaster until about 16Z, or about an
hour after the NHC forecast is released. Consequently, the 12Z GFS would be
considered a late model since it could not be used to prepare the 12Z official forecast.
This report focuses on the verification of early models.
Multi-layer dynamical models are generally, if not always, late models.
Fortunately, a technique exists to take the most recent available run of a late model and
adjust its forecast to apply to the current synoptic time and initial conditions. In the
example above, forecast data for hours 6-12 h from the previous (06Z) run of the GFS
would be smoothed and then adjusted, or shifted, such that the 6-h forecast (valid at 12Z)
would match the observed 12Z position and intensity of the tropical cyclone. The
adjustment process creates an “early” version of the GFS model for the 12Z forecast
cycle that is based on the most current available guidance. The adjusted versions of the
late models are known, mostly for historical reasons, as interpolated models. The
adjustment algorithm is invoked as long as the most recent available late model is not
more than 12 h old, e.g., a 00Z late model could be used to form an interpolated model
for the subsequent 06Z or 12Z forecast cycles, but not for the subsequent 18Z cycle.
When the technique to create an early model from a late model was first developed, forecast output from
the late models was available only at 12 h (or longer) intervals. In order to shift the late model’s forecasts
forward by 6 hours, it was necessary to first interpolate between the 12 h forecast values of the late model –
hence the designation “interpolated”.
Verification procedures here make no distinction between 6 h and 12 h interpolated
models.
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Isaac is closer to an open wave than a hurricane right now. Don't know if he will ever be able to develop a strong core to intensify.
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Whats up with the NAM? Model is way east
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REPOST
The Latest
(click to enlarge)



Member Since: June 29, 2009 Posts: 13 Comments: 5313
Quoting GetReal:
700-850mb



500-850mb


There is no doubt that Isaac is going to go much more further west, until it reaches the NW Caribbean.

Correct. NHC's assumption of going WNW and NW to Haiti relies on the fact Issac reaching hurricane status near Haiti. IF that doesn't happen and then more WEST he goes!
Member Since: June 26, 2012 Posts: 0 Comments: 297
Quoting DocBen:
Are Isaac and Joyce close enough to interact with the tracks? Can't recall the name of the 'dance' but the one where they tend to go counterclockwise to each other. Would push joyce north and Isaac more west (keeping south)
It's called a Fujiwhara Interaction which causes two storms in close proximity to each other to orbit around a point. Link

Actually I discussed this scenario a couple days ago and have not at all discount this from happening. 1950 Hurricane's Dog and Easy had an Fujiwhara interaction and the locations seem to be similar with the predictions being made by the various computer models. If that were to occur it would bring Isaac more towards the East and Joyce towards the NW.
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I think the LLCOC is near 14.8N 66W
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Quoting GetReal:
700-850mb



500-850mb


There is no doubt that Isaac is going to go much more further west, until it reaches the NW Caribbean.


steering currents do not stay constant, I think we are already starting to see the WNW movement now
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Quoting sunlinepr:

That storm is massive, even by WPac standards.

Member Since: August 2, 2006 Posts: 46 Comments: 11668

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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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