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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1224. kwgirl
Good morning. While watching GMA this morning I saw this blurb "Son of Isaac in GOM" with the blob circled and "Nadine?" next to it. Whoever wrote the headline needs to know it's a girl. LOL Everyone have a good day.
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Quoting washingtonian115:
If Micheal becomes a cat 5 at that altitude then I should expect a cat 6 or 7 in the MDR right?.


Wash being a DOOMCASTER!! ;p haha Maybe someday though?! I always reckon would be WPac if ever one got that strong though
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Quoting Tropicsweatherpr:
Things starting to take shape in the West Africa area.

Very interesting.
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Things starting to take shape in the West Africa area.

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1217. GetReal


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1216. VR46L
Same thing happened last September Both Katia and Maria were "fish" LOL
Member Since: March 1, 2012 Posts: 0 Comments: 6842
If Micheal becomes a cat 5 at that altitude then I should expect a cat 6 or 7 in the MDR right?.
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Quoting VR46L:


LOL .... But that thing is going to be around for half the month I suspect
Yeah.I predicted that Leslie will be around for three weeks.
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ATCF holding Michael at 100 knots:

AL, 13, 2012090612, , BEST, 0, 299N, 414W, 100, 964, HU, 64, NEQ, 15, 15, 10, 10, 1015, 150, 10, 0, 0, L, 0, , 0, 0, MICHAEL, D,
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1211. VR46L
Quoting TomballTXPride:

way to crap in the punch bowl on this lovely happy morning. haha. j/k. yeah forgot about our friends on the other side of the pond. Hopefully this thing poofs out after today.


LOL .... But that thing is going to be around for half the month I suspect
Member Since: March 1, 2012 Posts: 0 Comments: 6842
Quoting TomballTXPride:

future Nadine maybe???


Is it still on track to head to Florida?
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Quoting Bobbyweather:
If Michael manages to become a Category 5 hurricane by this afternoon, it will be one of the fastest storms to strengthen from a tropical storm to a Category 5 hurricane.

WILMA : 60, 65, 75, 130, 150, 160 (peak)
FELIX : 60, 65, 85, 90, 115, 150 (peak)
Michael : 60, 65, 95, , ,
(wind speeds in kt)


You can forget Cat 5, in spite of it's small size.

It does not have a favorable enough upper environment.

Plus, it looks like Michael actually peaked 2 or 3 hours ago.
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What's with the BLOB in the gulf?
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Quoting weatherh98:


Isaac actually sifted dry air out but it wasnt a major

Wilma was the last... I think.... My memory sucks with Greek storms


Isaac didn't shift out that dry air until after he pretty much made landfall. I Irene struggled with dry air, Alex struggled with dry air. Gustav managed to only stay at a CAT 2 after "core disruption" across the entire Gulf of Mexico. Even Katrina and Rita both managed to find ways to significantly weaken only when nearing land...
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1204. VR46L
Quoting washingtonian115:
Just horrid.Good for Bermuda as they do not need a strong storm coming for them.


Yes very true ... still wonder when she will become extropical .... Kinda selfish of me but would not like her bearing down on me as a extrop storm
Member Since: March 1, 2012 Posts: 0 Comments: 6842
Storms approaching the USA naturally have dry air filtered into them unless they're cross Flordida from west to east or east to west. Coming towards the coast messes up the western quadrant of the storm. This is partially why Irene was a Cat 1 and not Cat 3.
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Dr. Masters was right about Isaac tar-balls. Just don't know for certain if the source is the BP spill or the extent.

This excerpt was something, he didn't predict, and likely neither did anyone else: "...Hurricane Isaac washed ashore tens of thousands of dead "swamp rats" in the Gulf."

Tar Balls, Oily Pelicans Found in Isaac's Wake

Sorry if someone already posted this. Don't have time to read back through the blog this morning.
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1200. VR46L
Quoting TomballTXPride:

She sure is doing the walk of shame. It is the year of the man alright. And boy does that Michael look sexy in rainbow. Wow.


Dont ya know its my favourite form of sat imagery... Michael looks real hot in proper sat unenhanced and just really natural a real stud of a cane ....

Member Since: March 1, 2012 Posts: 0 Comments: 6842
Quoting wxchaser97:

Well obviously since its rapid intensification, the first major.


Go to school

Because I have to
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Quoting ILwthrfan:
I find quite strange that for the last 7 years now any storm that nears land within the Caribbean or United States has suffered greatly from dry air. I think the only land falling majors that we have had was Ivan at a CAT 4 in Cuba and then I think we have had a couple of storms that RI at the last minutes as they were making landfall into Mexico. I'll have to find the exact storms, but it certainly seems to be outside the normal distributions of what you would consider normal. Far outside it...


Isaac actually sifted dry air out but it wasnt a major

Wilma was the last... I think.... My memory sucks with Greek storms
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Quoting weatherh98:
lol rapid intensification tends to do this

Well obviously since its rapid intensification, the first major.
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okay i just woke up and im shocked to see michael as a major wow..i went to bed when it was minimal hurricane and i woke up watching the weather channel and i though i was dreaming when he said michael was a major hurricane!! this is why i love studying hurricanes there so unpredictable at times.
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Quoting washingtonian115:
Micheal is a major?!!?? Whaaaaat?.


Totally!! I wasn't expecting that at all when i clicked on just now! LOL And son of Isaacs projected path, oh dear...What a mess...


Hey, well, if we don't get any other majors, maybe Michael will indeed be the Bad monster storm of the season, if just out of cat and wind speed :D Even if he is a wee lil' cutey LOL

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I find quite strange that for the last 7 years now any storm that nears land within the Caribbean or United States has suffered greatly from dry air. I think the only land falling majors that we have had was Ivan at a CAT 4 in Cuba and then I think we have had a couple of storms that RI at the last minutes as they were making landfall into Mexico. I'll have to find the exact storms, but it certainly seems to be outside the normal distributions of what you would consider normal. Far outside it...
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Quoting GeorgiaStormz:
I got back from swim practice and went to www.nhc.noaa.gov, put my cursor over michael and dropped a couple 4-letter words..

1 Major!!
Perfect eye.
lol rapid intensification tends to do this
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Got to go to school, have a great day everyone. I really wonder what I will come home to with Michael.
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Quoting TomballTXPride:
Sportsguy03 you have your coffee yet. You completely missed the sarcasm Bud. I know you were up late last night watching the giants go down at home but now it's blogging time and time to get with it!
I missed the sarcasm as well. Pouring more coffee now!
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Quoting VR46L:


All the lady storms this year have not impressed me

Ugly Leslie



and in rainbow

Just horrid.Good for Bermuda as they do not need a strong storm coming for them.
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Quoting sporteguy03:


So Michael is not a hurricane? Definitely not a wasted name. It is what it is and is a hurricane.
Agree. Whether it's a land-falling storm or only affects maritime interest, if a tropical storm system meets the requirements of a hurricane, it is a hurricane.
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1187. VR46L
Quoting washingtonian115:
Leslie is just ugly to look at...


All the lady storms this year have not impressed me

Ugly Leslie



and in rainbow

Member Since: March 1, 2012 Posts: 0 Comments: 6842
I think this is a busted NHC track forecast.



No way Michael turns west, because it's pinned between the ridge and the trough, both of which are pushing it NE to E.




I seriously doubt Leslie makes it's timeline either, because it doesn't look like this trough is going to be enough to break it free from this pattern either.

Jammed in there nice and tight.



If it intensifies as expected:

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Leslie is just ugly to look at...
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1182. Michfan
At least Michael didn't make me regret having my name on the list this year.
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1181. yqt1001
Quoting sporteguy03:


So Michael is not a hurricane? Definitely not a wasted name. It is what it is and is a hurricane.


Obvious sarcasm..should be obvious. :P

Just trying to get another poke in at the "Josecasters" for Michael.
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Quoting washingtonian115:
I can't even believe he pulled one on us!.Impressive storm and beautiful to.To believe people called him pathetic a few days ago.

Thats what these higher latitude storms have been doing.
Still he is a really good storm going out to sea.
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Quoting yqt1001:
Ugh another name wasted by the NHC. Come on guys, Michael is just another Jose.

Not that we wouldve known it wouldve beaten leslie to mh status 2 days ago...


So Michael is not a hurricane? Definitely not a wasted name. It is what it is and is a hurricane.
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1178. yqt1001
No doubt, it has been since Ophelia that we've had a storm this good looking in the ATL.



Took a long time for our first major though!
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NHC is now down-playing 90L with 8am T.W.O.

SHOWER ACTIVITY ASSOCIATED WITH A BROAD AREA OF LOW PRESSURE LOCATED
OVER THE NORTH-CENTRAL GULF OF MEXICO HAS NOT BECOME ANY BETTER
ORGANIZED SINCE YESTERDAY...AND IF THIS TREND CONTINUES...THE
RECONNAISSANCE MISSION SCHEDULED FOR LATER TODAY WILL BE CANCELLED.
THIS WEATHER SYSTEM IS DRIFTING SOUTHWARD AND STILL HAS THE
POTENTIAL FOR SOME DEVELOPMENT DURING THE NEXT DAY OR SO BEFORE
ENVIRONMENTAL CONDITIONS BECOME UNFAVORABLE. THIS SYSTEM HAS A
MEDIUM CHANCE...40 PERCENT...OF BECOMING A TROPICAL CYCLONE DURING
THE NEXT 48 HOURS.
Member Since: May 31, 2007 Posts: 0 Comments: 1200
The 2012 Atlantic hurricane season has been quite interesting, for 13 storms, we had only 1 Major, and a bunch of General storms.

Turns out the 13th storm was not the unlucky one after all.
Member Since: February 11, 2012 Posts: 0 Comments: 9721
1174. LargoFl
BULLETIN - EAS ACTIVATION REQUESTED
SEVERE THUNDERSTORM WARNING
NATIONAL WEATHER SERVICE BALTIMORE MD/WASHINGTON DC
740 AM EDT THU SEP 6 2012

THE NATIONAL WEATHER SERVICE IN STERLING VIRGINIA HAS ISSUED A

* SEVERE THUNDERSTORM WARNING FOR...
EASTERN ORANGE COUNTY IN CENTRAL VIRGINIA...
WESTERN CITY OF FREDERICKSBURG IN CENTRAL VIRGINIA...
SOUTHEASTERN CULPEPER COUNTY IN NORTHERN VIRGINIA...
NORTHERN SPOTSYLVANIA COUNTY IN CENTRAL VIRGINIA...
WESTERN STAFFORD COUNTY IN NORTHERN VIRGINIA...

* UNTIL 830 AM EDT

* AT 735 AM EDT...A SEVERE THUNDERSTORM WAS DETECTED 25 MILES WEST OF
MASSAPONAX...OR 15 MILES SOUTH OF CULPEPER...AND WAS MOVING EAST AT
20 MPH. THIS STORM IS CAPABLE OF PRODUCING HAIL UP TO THE SIZE OF
HALF DOLLARS AND DAMAGING WINDS IN EXCESS OF 60 MPH.

* LOCATIONS IMPACTED INCLUDE...
MASSAPONAX...

THIS IS A DANGEROUS STORM. IF YOU ARE IN ITS PATH...PREPARE
IMMEDIATELY FOR DAMAGING WIND GUSTS...LARGE HAIL AND FREQUENT CLOUD
TO GROUND LIGHTNING. MOVE INDOORS TO A STURDY BUILDING AND STAY AWAY
FROM WINDOWS.

LAT...LON 3839 7748 3816 7748 3819 7804 3834 7801
TIME...MOT...LOC 1140Z 269DEG 17KT 3826 7793

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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.