Leslie near hurricane strength; Son of Isaac (90L) emerges in the Gulf

By: Dr. Jeff Masters , 3:05 PM GMT on September 05, 2012

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Tropical Storm Leslie is growing more organized and is approaching hurricane strength on its slow voyage northwards at 2 mph towards the island of Bermuda. Moderately high wind shear of 15 - 20 knots due to strong upper-level winds out of the northwest continues to keep most of Leslie's heavy thunderstorms pushed to the east side of the storm, but satellite loops show that Leslie now has an impressive blow-up of heavy thunderstorms with cold cloud tops near its center. Leslie's slow forward speed means that the storm is staying over the cold water stirred up by the storm's winds, inhibiting intensification, but the waters underneath Leslie are warm to great depth, making this less of a factor than usual. According to the latest SHIPS model forecast, the shear is expected to fall steadily today, reaching the low category, 5 - 10 knots, by Thursday afternoon. Leslie is over warm ocean waters of 29 - 30°C, and the reduction in shear and warm waters should aid intensification, and potentially allow Leslie to be at Category 2 strength at its closest pass by Bermuda Saturday night and early Sunday morning, as indicated by the official NHC forecast. The latest 11 am EDT NHC wind probability forecast calls for a 48% chance that Leslie will be a Category 2 or stronger hurricane Sunday morning at 8 am EDT. Leslie is a huge storm, and tropical storm-force winds are expected to extend outward from its center 250 miles by Friday. Bermuda is likely to see a 42-hour period of tropical storm-force winds beginning Saturday morning near 2 am AST, and lasting until 8 pm AST Sunday night. The official NHC forecast shows Leslie nearly making a direct hit on Bermuda, and Leslie will be capable of bringing an extended period of hurricane-force winds lasting six or more hours to Bermuda Saturday night through Sunday morning, should a direct hit materialize. NHC is predicting that hurricane-force winds will extend outwards from the center of Leslie by 35 miles on Thursday night, and I expect this will increase to at least 60 miles by early Sunday morning, when Leslie will be making its closest pass by Bermuda.


Figure 1. Morning satellite image of Tropical Storm Leslie. Heavy thunderstorms have built near the center of the storm, and Leslie is near hurricane strength.

Leslie's impact on Canada
Leslie will stay stuck in a weak steering current environment until a strong trough of low pressure approaches the U.S. East Coast on Saturday. The timing of this trough is such that Leslie will be pulled northwards and then north-northeastwards over the weekend. There are still significant differences among the models in the timing and speed of Leslie's track over the weekend, but we can now dismiss the threat of Leslie making a direct hit on New England. The storm is likely to make landfall in Nova Scotia or Newfoundland, though there are significant differences in the models' predictions of the timing of Leslie's arrival in Canada. The GFS model predicts an early Tuesday landfall in Newfoundland, but the ECMWF model is much faster and farther west, predicting a Monday afternoon landfall in Nova Scotia. Large swells from Leslie are pounding the entire Eastern Seaboard, and these waves will increase in size as Leslie grows in strength this week. The Hurricane Hunters are scheduled to make their first flight into Leslie on Thursday afternoon.


Figure 2. Morning radar image of Invest 90L off the coast of the Florida Panhandle.

Son of Isaac: Invest 90L emerges in the Gulf of Mexico
During Tropical Depression Isaac's trek across the center of the U.S. during the Labor Day weekend, the storm was ripped in half. One portion of the storm moved over the Northeast U.S., bringing heavy rains there, and another portion sank southwards over Alabama. You can see this split by studying an animation of the vorticity at 850 mb (the amount of spin at low levels of the atmosphere, near 5,000 feet above sea level) from the University of Wisconsin. This remnant of Isaac, which still maintained some of Isaac's spin, brought heavy rains of 5 - 10 inches that caused flooding problems over portions of Alabama on Tuesday. The storm has now emerged over the Gulf of Mexico near the Florida Panhandle, and was designated Invest 90L this morning by NHC. In their 8 am Tropical Weather Outlook, NHC gave 90L a 20% chance of developing into a tropical depression by Friday morning. According to NHC naming rules, "if the remnant of a tropical cyclone redevelops into a tropical cyclone, it is assigned its original number or name". Since "the remnant" refers to the primary remnant, and 90L does not fit the definition of a "primary remnant", the storm will be given a new name should it develop into a tropical storm, according to information posted on the NHC Facebook page. Esau or Jacob--the names of the sons of the biblical Isaac--would be fitting names for 90L, but the next storm on the list of Atlantic storms is Nadine.

Long-range radar out of Mobile, Alabama shows a large area of heavy rainfall along the coast due to 90L. The echoes do show some spiral banding behavior, but there is only a slight evidence of rotation to the storm. Infrared satellite loops show that the thunderstorms associated with 90L are not that vigorous and do not have particularly cold cloud tops, and the area covered by the thunderstorms is relatively small. Wind shear is a high 20 - 30 knots over the northern Gulf of Mexico, but is predicted to fall to the moderate range, 10 - 20 knots, by Thursday afternoon. Ocean temperatures in the Gulf have been cooled down considerably by the passage of Hurricane Isaac last week, and are 28 - 28.5°C. This is still plenty warm enough to support formation of a tropical storm, and I expect 90L will increase in organization on Thursday and Friday as it moves slowly south or south-southwest. 90L could become a tropical depression as early as Thursday, though Friday is more likely. A hurricane hunter aircraft is scheduled to investigate 90L on Thursday afternoon. A trough of low pressure and an associated surface cold front will move southeastwards over the northern Gulf of Mexico on Sunday, and this trough should be capable of pulling 90L to the northeast to a landfall along the Florida Panhandle or west coast of Florida on Sunday.


Figure 3. Morning satellite image of Tropical Storm Michael.

Tropical Storm Michael in the Central Atlantic
Tropical Storm Michael has strengthened to 50 mph winds, and appears to have a favorable enough environment to become a hurricane later this week. Satellite loops show that this is a small tropical cyclone, far out over the open Atlantic, and none of the models show that Michael will threaten any land areas.

Elsewhere in the Atlantic
The GFS and ECMWF models are predicting that a new tropical wave due to move off the coast of Africa on Friday will develop into a tropical depression by the middle of next week. It's too early to tell if this system might threaten the Lesser Antilles Islands.

Jeff Masters

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Quoting lobdelse81:

Any possibility that 90L could undergo rapid intensification like Humberto or Alicia?


I'd say like 2%, lol.
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Quoting KoritheMan:


A poleward path toward Florida. The only chance Louisiana has of getting hit again is for it to move farther south than anticipated.

Any possibility that 90L could undergo rapid intensification like Humberto or Alicia?
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0z microwave pass over 90L:



Pitiful.
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Quoting TampaBayfisher:
Thank you, KoritheMan. Is there a ridge to the west blocking it from going that way or is it just late enough for them to curve like Wilma?


A little of both. There is a ridge over Texas (what else is new, lol), but climatology doesn't favor a meandering westward moving system in September. Generally the troughs start to pick it up this time of year, especially in this location.

One thing to watch for in regards to a threat to the northern Gulf Coast is that this ridge advances a little eastward, which could cause it to keep moving southwestward longer than the models are indicating. This could happen with the shortwave over Oklahoma. However, water vapor imagery suggests a mostly zonal flow accompanying the trough, implying that it is weak in nature.
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has a single storm ever been in the gulf three times?
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Thank you, KoritheMan. Is there a ridge to the west blocking it from going that way or is it just late enough for them to curve like Wilma?
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Quoting LostTomorrows:
she's just a monster that eats everything haha.




Middle monster.

(Now I have the inkling to play this game again. It was just that awesome.)
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1017. Grothar
Blob Alert #5

Member Since: July 17, 2009 Posts: 69 Comments: 25395
Every year someone expects a Fujiwara. I remember in 2005 when people were asking if one could happen wih Wilma and Alpha, but Wilma just ripped the kid apart and ate him. Now, Michael is much better defined than Alpha ever was, and can last longer if he gets tangled up in Leslie's circulation, but she will still chip away until he dies if he gets close enough. She has a large and powerful circulation.

That being said, I mused at the possibility that if he gets close enough and is strong enough, he might just latch onto her like a clingfish and let her spin take him for a ride, but she might just hurl him somewhere cold and far away or something.

I also think that his rapid intensification was aided by Leslie's sheer size; she's the buffer that's been shielding him from nearly every hostile thing thrown at him by the environment, she's just a monster that eats everything haha.
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Quoting TampaBayfisher:
Hypothetically, if something were to develop in the Gulf, which direction would be more likely from the conditions?


A poleward path toward Florida. The only chance Louisiana has of getting hit again is for it to move farther south than anticipated.
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Hypothetically, if something were to develop in the Gulf, which direction would be more likely from the conditions?
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1013. bappit
Some definitions from the Glossary of NHC Terms that might be worth noting, not that these topics are currently an issue.

Present Movement:
The best estimate of the movement of the center of a tropical cyclone at a given time and given position. This estimate does not reflect the short-period, small scale oscillations of the cyclone center.

Relocated:
A term used in an advisory to indicate that a vector drawn from the preceding advisory position to the latest known position is not necessarily a reasonable representation of the cyclone's movement.

Edit: another definition to note.

Direct Hit:
A close approach of a tropical cyclone to a particular location. For locations on the left-hand side of a tropical cyclone's track (looking in the direction of motion), a direct hit occurs when the cyclone passes to within a distance equal to the cyclone's radius of maximum wind. For locations on the right-hand side of the track, a direct hit occurs when the cyclone passes to within a distance equal to twice the radius of maximum wind. Compare indirect hit, strike.


Indirect Hit:
Generally refers to locations that do not experience a direct hit from a tropical cyclone, but do experience hurricane force winds (either sustained or gusts) or tides of at least 4 feet above normal.
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Quoting Tropicsweatherpr:


Is timing once again for this potential development. If the ridge builds back after Leslie and Michael are gone,then it may pose a threat to the NE Caribbean but if the ridge doesn't build fast enough, then it goes on a sharp recurve mode.


Good night 97. Oh boy,I replied to my own post,things happen.
Member Since: April 29, 2009 Posts: 75 Comments: 14055
1011. Shluggo
looks like Leslie could be forming a giant eye, although this may take a couple of days.
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Quoting wxchaser97:

Wow, looking good already. It should have a good environment for strengthening and this is the next long range threat.


Is timing once again for this potential development. If the ridge builds back after Leslie and Michael are gone,then it may pose a threat to the NE Caribbean but if the ridge doesn't build fast enough, then it goes on a sharp recurve mode.

Good night 97.
Member Since: April 29, 2009 Posts: 75 Comments: 14055
I have to go to bed, good night. It will be interesting to see what Leslie and Michael look like in the morning.
Member Since: March 16, 2012 Posts: 127 Comments: 7928
Quoting captainktainer:
Leslie and Michael are about 2000 km apart. What do people think about the possibility that they'll get another 500 km closer and start a Fujiwhara effect, and potentially throw Leslie's course further south and west?

I doubt a Fujiwara effect would happen and I don't think they will get close enough to do so anyway.
Member Since: March 16, 2012 Posts: 127 Comments: 7928
Derived from NHC_ATCF data for HurricaneLeslie for 6Sept12amGMT
All times in GMT. BDA is Bermuda
The southernmost dot on the connected lines is Leslie's position starting its 6th day as a TropicalStorm
The southernmost dot on the longest line is where TS.Leslie's became HurricaneLeslie, and its most recently reported position

The longest line is a straightline projection through TS.Leslie's 2 most recent positions to it's closest approach to Bermuda
5Sept.12am: TS.Leslie had been headed for passage 238miles(383kilometres)SWest of Bermuda
5Sept.06am: TS.Leslie had been for passage 88miles(141kilometres)WSWest of Bermuda
5Sept.12pm: TS.Leslie had been headed for passage 108miles(174kilometres)East of Bermuda
5Sept.06pm: H.Leslie had been headed for passage 290miles(467kilometres)ESE of Bermuda
6Sept.12am: H.Leslie was heading for passage 331miles(532kilometres)ESEast of Bermuda

Copy&paste 29.885n67.781w, 31.729n66.233w, 32.382n62.8w, 30.6278n60.166w, 32.281n64.887w-bda-32.368n64.647w, 24.8n62.5w-25.0n62.6w, 25.0n62.6w-25.1n62.7w, 25.1n62.7w-25.3n62.8w, 25.3n62.8w-25.6n62.8w, 25.6n62.8w-25.8n62.7w, 25.8n62.7w-26.1n62.5w, 25.8n62.7w-29.887n59.866w, 32.368n64.647w-29.887n59.866w into the GreatCircleMapper for a larger map and other information
The previous mapping for comparison
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Quoting Tropicsweatherpr:


Look at how that area is with plenty of convection and it has not emerged West Africa.


Wow, looking good already. It should have a good environment for strengthening and this is the next long range threat.
Member Since: March 16, 2012 Posts: 127 Comments: 7928
1005. Walshy
Quoting captainktainer:
Leslie and Michael are about 2000 km apart. What do people think about the possibility that they'll get another 500 km closer and start a Fujiwhara effect, and potentially throw Leslie's course further south and west?


Keep dreaming.
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Quoting captainktainer:
Leslie and Michael are about 2000 km apart. What do people think about the possibility that they'll get another 500 km closer and start a Fujiwhara effect, and potentially throw Leslie's course further south and west?


Michael is too small to be significant.
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1003. FMDawg
Quoting spathy:
This is a cruel joke!


I am directly under the y in Myers and not a drop.
And I am not kidding I pulled brown dry weeds today.


It's been pouring at 75 & Daniels for the last hour.
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1002. FMDawg
Quoting spathy:
This is a cruel joke!


I am directly under the y in Myers and not a drop.
And I am not kidding I pulled brown dry weeds today.
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1001. pottery
Quoting Tropicsweatherpr:


Look at how that area is with plenty of convection and it has not emerged West Africa.


Yeah, that's a healthy looking one there.
Need to see if it will maintain it's convection over the water, as it comes off.

There is very little SAL. That should help it.
But a fair amount of dry air, which won't.....
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Leslie and Michael are about 2000 km apart. What do people think about the possibility that they'll get another 500 km closer and start a Fujiwhara effect, and potentially throw Leslie's course further south and west?
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Quoting wxchaser97:

The SSD site can only go out so far, I would've included it.


Look at how that area is with plenty of convection and it has not emerged West Africa.

Member Since: April 29, 2009 Posts: 75 Comments: 14055
Go look at history of weather,repeats itself,there is nothing new under the sun.
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Quoting Tropicsweatherpr:


Maybe you can include possible CV development from the convection that is now in West Africa.

The SSD site can only go out so far, I would've included it.
Member Since: March 16, 2012 Posts: 127 Comments: 7928
Quoting NoloContendere:
Where on earth do you get your "information"?




From AGW proponents? If the south becomes warmer, that indicates a mean ridge over those locations. I can't definitively say that AGW is the cause of the current drought pattern across Texas and southern plains, but a pattern like this is to be expected in a warming world.

The better question is, do you have anything to contradict anything I've said?
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Quoting wxchaser97:
The big picture:


Maybe you can include possible CV development from the convection that is now in West Africa.
Member Since: April 29, 2009 Posts: 75 Comments: 14055
Quoting spathy:
Happy days.
The weather Gods must have liked my forced prose.



I'm happy for you spathy. I felt the same way when the clouds opened up here earlier after no rain for a week. Fingers crossed that we can keep the rainy season going (more or less) through most of September.
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Quoting trHUrrIXC5MMX:


there is no way Leslie and Michael have 75 mph winds.... why the NHC has been soooo conservative this year??
They have a big time work doing the post analysis on this storm, Isaac, Ernesto, Beryl and some other storms including that Invest in May

Leslie= fine with intensity & forecast
Michael= Needs to be higher(85-90mph)
Isaac= Fine with intensity
Ernesto= Cat2 based on ground reports
Beryl= Should remain the same

All in my opinion
Member Since: March 16, 2012 Posts: 127 Comments: 7928
Quoting JLPR2:


Who would have thought that Michael, at some point, would look better than Leslie?

It has looked better pretty much all along. Leslie has been walking on the edge of its own disaster.
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The big picture:
Member Since: March 16, 2012 Posts: 127 Comments: 7928
Quoting RTSplayer:
Why SS scale stinks:



vs



and



there is no way Leslie and Michael have 75 mph winds.... why the NHC has been soooo conservative this year??
They have a big time work doing the post analysis on this storm, Isaac, Ernesto, Beryl and some other storms including that Invest in May
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Quoting ScottLincoln:


Yes, without a doubt.

Absolutely !
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Quoting NoloContendere:
The alleged 1/2 degree over 20 years is having an effect?

Actually about 0.5-0.6 degrees celsius since 1900, over half of which occurred since the 1970s. Averaged globally... that is a tremendous, non-trivial amount of energy.

And yes, without a doubt.
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Quoting pottery:

Yeah, they are.
But with Les. tracking North, and Michael heading east of that, will they come together?????

Its possible later that they merge.
Member Since: March 16, 2012 Posts: 127 Comments: 7928
983. SLU
000
WTNT43 KNHC 060240
TCDAT3

HURRICANE MICHAEL DISCUSSION NUMBER 11
NWS NATIONAL HURRICANE CENTER MIAMI FL AL132012
1100 PM AST WED SEP 05 2012

AN EYE HAS BECOME EVIDENT IN INFRARED IMAGES...AND SUBJECTIVE AND
OBJECTIVE DVORAK INTENSITY ESTIMATES RANGE FROM 55 TO 77 KT. THE
CURRENT INTENSITY IS SET...PROBABLY CONSERVATIVELY...TO 65 KT...

MAKING MICHAEL THE SEVENTH HURRICANE OF THE ATLANTIC SEASON.

IMO ... 55kt satellite estimates of a system as impressive as Michael are obviously underestimated. They should have gone with the higher estimates instead of blending the data and coming up with 65kts for a system with a clearing, circular eye and cold cloud tops forming a ring around the center. I'm sure a recon would find at least 85 - 95mph winds in a system like Michael.

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Quoting RTSplayer:
Why SS scale stinks:



vs



yeah micheal clearly is near a 100 mph hurricane....not 75 like leslie, shes a mess and they are saying micheal is the same as leslie...get ta stepn'
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Edited out.....

My brain is not working tonight......
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Why SS scale stinks:



vs



and

Member Since: January 25, 2012 Posts: 33 Comments: 1520
Quoting KEEPEROFTHEGATE:
some popcorn pop ups looks to run along to the north maybe they spread out overnight into the max some passing tropical showers

More than likely the mountains (2000'-3000') along the north coast will get rain.
They generally do.
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Quoting pottery:
Will Leslie and Michael get together?
Will they have a Stormy relationship?
Will they spawn a Monster?
Will it have One Eye? Or two?

Stay tuned, for more Intrigue and Unpredictable nonsense, as "The Season" progresses......

The first line got me off track... but anyway these are two complex and interesting storms.
Member Since: March 16, 2012 Posts: 127 Comments: 7928
I see Bermuda is now forecast to solidly be on the "clean" side of Leslie. The 5:00 pm track would have still put Bermuda in the western eyewall but if the 11:00 pm path pans out, Bermuda will only get tropical storm force winds. Obviously, this will change again many times, but the trend is promising.
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Will Leslie and Michael get together?
Will they have a Stormy relationship?
Will they spawn a Monster?
Will it have One Eye? Or two?

Stay tuned, for more Intrigue and Unpredictable nonsense, as "The Season" progresses......
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The alleged 1/2 degree over 20 years is having an efect? LOL

Quoting GTcooliebai:
Sorry I brought AGW into the discussion, but can't ignore the possible effects it may be having on the hurricane season, and that is as far as I will go.
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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.