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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Leslie:

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This is old news to you guys, but news to me...


A LOW PRESSURE SYSTEM LOCATED NEAR THE ALABAMA COAST CONTINUES TO PRODUCE SHOWERS AND THUNDERSTORMS MAINLY TO THE SOUTH AND SOUTHWEST OF THE CENTER OF CIRCULATION. UPPER-LEVEL WINDS ARE EXPECTED TO BE MARGINALLY CONDUCIVE FOR DEVELOPMENT OF THIS DISTURBANCE OVER THE NEXT COUPLE OF DAYS AS IT MOVES SLOWLY SOUTH-SOUTHWESTWARD.
THIS SYSTEM HAS A MEDIUM CHANCE...50 PERCENT...OF BECOMING A TROPICAL CYCLONE DURING THE NEXT 48 HOURS. AN AIR FORCE RESERVE RECONNAISSANCE AIRCRAFT IS SCHEDULED TO INVESTIGATE THE SYSTEM THURSDAY AFTERNOON...IF NECESSARY.
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90L:

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Quoting spathy:


Really?
Please ...
Dont ... just dont.
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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The cold water surfing should be great along the east coast of Nova Scotia in a few days.Just some waves as the storm passes out to sea, well to our south east and it will probably miss Newfoundland as well.
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Quoting KoritheMan:


Somehow I'm not doubting that.

You shouldn't, it's a whole process being with him.

Quoting weatherh98:
the great major race



Off to the races, will it be Michael or Leslie? (I think Michael)
Member Since: March 16, 2012 Posts: 127 Comments: 7942
Quoting wxchaser97:

We thought Gordon and Kirk would but I think Michael is different. If he continues with what he is doing then a major hurricane is a great possibility.


Agreed.

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Quoting Civicane49:


Can't rule out the possibility for Michael becoming a major hurricane.

Michael is expected to intensify as it is in relatively favorable conditions for the next several days. If the cyclone continues to improve both of its eye and deep convection through tomorrow or so, then it's likely that it would attain major hurricane status. But, we shall see.

We thought Gordon and Kirk would but I think Michael is different. If he continues with what he is doing then a major hurricane is a great possibility.

Quoting weatherh98:


its like talking to a wall he just won listen

I know right, It's like he isn't even there.

Quoting pottery:

Doing good, thanks!
Hot and dry down here for the past week or so.
Ground is still damp enough, but the plants are suffering from 95F days.

Often happens in late August early September..
The ITCZ takes a little rest, before kicking in again in Sept.

Glad to hear everything is good at where you reside, you'll get the relief soon.
Member Since: March 16, 2012 Posts: 127 Comments: 7942
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Member Since: June 20, 2012 Posts: 0 Comments: 2159
the great major race


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Quoting wxchaser97:

Try being with this guy in chat, that's where its worse.


Somehow I'm not doubting that.
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this will be the NHC at 11... "Hurricane Micheal appears to be undergoing under Rapid Intensification. If this continues we will have the season's first major hurricane at the next advisory. Once again we are in awe of nature's power"...yada..yada..yada

Any idea who is writing the next discussion?
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Quoting wxchaser97:

Well he has proved that well, how are you pottery?

Doing good, thanks!
Hot and dry down here for the past week or so.
Ground is still damp enough, but the plants are suffering from 95F days.

Often happens in late August early September..
The ITCZ takes a little rest, before kicking in again in Sept.
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Quoting wxchaser97:

Try being with this guy in chat, that's where its worse.


its like talking to a wall he just won listen
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Quoting wxchaser97:
The eye is really clearing out and stronger convection is wrapping the center.


Can't rule out the possibility for Michael becoming a major hurricane.

Michael is expected to intensify as it is in relatively favorable conditions for the next several days. If the cyclone continues to improve both of its eye and deep convection through tomorrow or so, then it's likely that it would attain major hurricane status. But, we shall see.
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Quoting KoritheMan:


lol, this guy.

Try being with this guy in chat, that's where its worse.
Member Since: March 16, 2012 Posts: 127 Comments: 7942
Quoting TropicalAnalystwx13:

No.


lol, this guy.
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Quoting pottery:
Well, I see we have Michael the MiniCane.
Isn't he a handsome little fellow?
Trying to prove that Big things come in Small packages, or what ??

Well he has proved that well, how are you pottery?
Member Since: March 16, 2012 Posts: 127 Comments: 7942
Well, I see we have Michael the MiniCane.
Isn't he a handsome little fellow?
Trying to prove that Big things come in Small packages, or what ??
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The eye is really clearing out and stronger convection is wrapping the center.
Member Since: March 16, 2012 Posts: 127 Comments: 7942
Quoting KoritheMan:


Yes.

No.
Member Since: July 6, 2010 Posts: 113 Comments: 32025



tenacious like bull
Quoting dearmas:
how is 90 looking??


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Quoting dearmas:
how is 90 looking??


Still looks pretty bad at the moment, but the shear is forecast to decrease.
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Michael looks fantastic! This is what I have been craving all season long, a system that actually looks like a hurricane on satellite.

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Quoting KoritheMan:


lol, isn't it always?


Some more than others lol
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Quoting spathy:


Really?
Please ...
Dont ... just dont.


Huh?
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90L should look much better by morning. Hope FL still needs more rain. Should move pretty quick once it gets caught and starts tracking NE. Nadine? Wouldn't expect more than weak TS.
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how is 90 looking??
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Quoting KoritheMan:


Why root for something that's not gonna happen? lol

Because its fun and we haven't had a cat5 in a while, obviously:)
Member Since: March 16, 2012 Posts: 127 Comments: 7942
Quoting MAweatherboy1:
I really hope Michael becomes a Cat 5... It'll never happen of course, but I'll be rooting for it :)


If that happened then we would all go crazy, 105-115 I see but cat5 would be nuts.
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Quoting TropicalAnalystwx13:

No.


Yes.
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Quoting MAweatherboy1:
I really hope Michael becomes a Cat 5... It'll never happen of course, but I'll be rooting for it :)



Why root for something that's not gonna happen? lol
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Quoting GTcooliebai:
Amazing that this far north storms have undergone rapid intensification this year. Will be an interesting read to see what researchers come up with as to why this area has been favorable this year. I suspect the warm sea surface temperature anomolies in the the North Atlantic have something to do with it and AGW might be part of the reason as well.


I kinda think AGW will make the US hurricane season a joke. The south is supposed to get drier as the planet warms, which will lead to more years like 2010 and 2011. Recurve might become the new normal. But then, Central America and Mexico would also suffer, and I would rather the states take a hurricane than I would third world nations.
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Quoting KoritheMan:


lol, isn't it always?

No.
Member Since: July 6, 2010 Posts: 113 Comments: 32025
Quoting GTcooliebai:
Amazing that this far north storms have undergone rapid intensification this year. Will be an interesting read to see what researchers come up with as to why this area has been favorable this year. I suspect the warm sea surface temperature anomolies in the the North Atlantic have something to do with it and AGW might be part of the reason as well.

it's been a weird year
alberto: was an early bird
beryl: first storm to visit north florida
chris: wat you said
debby: huge unpredictability
ernesto: should have been major
florence:????
gordon: wat you said
helene: was supposed to be gordon
isaac: well, we all know why
joyce: ummmm
kirk: shoulda been a major
leslie: unknown
michael: unknown
nadine-william: ??????????!!!!!!!!!!!!????
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Quoting caneswatch:


Can't go wrong on that prediction. One thing's for sure, September is going to be big for hurricanes this year.


lol, isn't it always?
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Quoting GTcooliebai:
Amazing that this far north storms have undergone rapid intensification this year. Will be an interesting read to see what researchers come up with as to why this area has been favorable this year. I suspect the warm sea surface temperature anomolies in the the North Atlantic have something to do with it and AGW might be part of the reason as well.

The warm SST's help a lot, I think Michael should become a major. But yet that's what we said for Gordon and Kirk.
Member Since: March 16, 2012 Posts: 127 Comments: 7942
Quoting CybrTeddy:


I'm just impressed by the fact that this was supposed to be an inactive, boring El Nino year. Everything pointed to it all through July, then August came..

We've still got the rest of September and October to get through.


What do you expect October to bring?

We're approaching the down side peak of hurricane season. If history is any indication, this should, and I hope this will be a "boring" tropical season and I think that scenario is more likely than not.
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Michael didnt like us teasing him about being small.
Lookk how fast he organized though.
Someone compared him with a small cloud and now he is a great looking hurricane.
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I really hope Michael becomes a Cat 5... It'll never happen of course, but I'll be rooting for it :)

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Quoting caneswatch:


Can't go wrong on that prediction. One thing's for sure, September is going to be big for hurricanes this year.

yep
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Quoting wxchaser97:
I can't wait to see what the NHC says for intensity and their forecast as Michael is intensifying pretty quickly now.
Amazing that this far north storms have undergone rapid intensification this year. Will be an interesting read to see what researchers come up with as to why this area has been favorable this year. I suspect the warm sea surface temperature anomolies in the the North Atlantic have something to do with it and AGW might be part of the reason as well.
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Quoting Civicane49:


I noticed that too.

i will call 2012" year of unpredictability"
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Quoting KoritheMan:


There are some signals that the troughing this year is not as strong as in recent years. I think another 1 - 3 US hits is a good bet.


Can't go wrong on that prediction. One thing's for sure, September is going to be big for hurricanes this year.
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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.