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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**We are not even to the peak of the season and you are making bold predictions.**

That's what I thought, along with "Way to make a Cat 6 hit Washington by way of Murphey's Law and hubris"!!

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Quoting NOLALawyer:


We are not even to the peak of the season and you are making bold predictions.

Anyway, it does not matter. Three storms have already impacted the US, and there are damages in the billions. This season has been far from a bust.


Didn't say a bust there Sherlock. I said a strong storm, storms that have hit were not strong by no means
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Quoting ILwthrfan:


I would agree with you if storms were forming further east this year, but most of them have not got going until they have reached the islands, so what are you basing your reasoning on that the rest will season will be harmless?



The overall pattern, first of all, the east coast is pretty much out of the equation with the trough pattern which is normal this time of the year. The gulf might be open to a storm but like i said is very slim,
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Quoting mcluvincane:
Well, looks like the conus has escaped another season without a strong storm impacting them. Just a couple of months left but highly doubt there is anything that could impact the U.S. the remainder of the season, the Atlantic is nothing but fish storms from here on out, the only place that could possibly get a storm would be the gulf states which is very slim.


We are not even to the peak of the season and you are making bold predictions.

Anyway, it does not matter. Three storms have already impacted the US, and there are damages in the billions. This season has been far from a bust.
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1269. 7544
gulf blob looking a little better today but its really going further south than exspected imo looks like shear will lesssen soon has a window to form ?
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Quoting mcluvincane:
Well, looks like the conus has escaped another season without a strong storm impacting them. Just a couple of months left but highly doubt there is anything that could impact the U.S. the remainder of the season, the Atlantic is nothing but fish storms from here on out, the only place that could possibly get a storm would be the gulf states which is very slim.


I would agree with you if storms were forming further east this year, but most of them have not got going until they have reached the islands, so what are you basing your reasoning on that the rest will season will be harmless?
Member Since: February 2, 2010 Posts: 0 Comments: 1456
Quoting mcluvincane:
Well, looks like the conus has escaped another season without a strong storm impacting them. Just a couple of months left but highly doubt there is anything that could impact the U.S. the remainder of the season, the Atlantic is nothing but fish storms from here on out, the only place that could possibly get a storm would be the gulf states which is very slim.


its possible for the US to get hit, it will have to be a very low rider though to escape the arctic "Trough Train"(2012 GeorgiaStormz)
Essentially it would have to be a caribbean storm and then get turned N/NE by a trough to hit the US especially the Eastern gulf coast/Florida, somewhat of a wilma type recurve.

Member Since: February 11, 2012 Posts: 0 Comments: 9720
1266. Grothar
Looks like a new low wants to form closer to the convection. This would move the center quite a bit South before it starts its trek towards Florida.

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1265. KEEPEROFTHEGATE (Mod)
Quoting TomballTXPride:

Ok ma'am. keep it real okay. we like good clean fun on here. keep the rif raf on the other side of the tracks.
or in a bunker
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1264. VR46L
Quoting TomballTXPride:

Ok ma'am. keep it real okay. we like good clean fun on here. keep the rif raf on the other side of the tracks.



HAHA I didnt think humor was allowed .... what did I miss though?
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Quoting RussianWinter:
What developments can we expect from 90L?


Its going to be a Category 6 Hurricane at FSU. :P

In all seriousness, i think we just need to wait and see if it can get to depression/very weak storm status before the impending trough in a day or 2 brings the proverbial "kitchen sink" of shear to 90L, either way, it will drift west, and will need to develop more before it turns back east with the trough and heads to FL.
Its a wait and see, follow organizational trends

Anyway guys, im back out.
Member Since: February 11, 2012 Posts: 0 Comments: 9720
Lots of spin, and stacked for the most part.



Yep


Its under 10-20kts of wind shear, and if it moves east it will hit 40kts.
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Well, looks like the conus has escaped another season without a strong storm impacting them. Just a couple of months left but highly doubt there is anything that could impact the U.S. the remainder of the season, the Atlantic is nothing but fish storms from here on out, the only place that could possibly get a storm would be the gulf states which is very slim.
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Quoting CloudGatherer:
Meanwhile, here's Leslie over the same period. That's one mess of a storm.


Leslie might get downgraded to a Tropical Storm at this rate. Michael looks amazing, I would have thought that it would have been Leslie that would have the Category 3 on her at this point, and Michael the weakening TS a few days ago. Anyways, looks like we will have the possibility of another CV cyclone in a few days. The ECMWF, GFS, and UKMET are all showing a system by 5 days or so. Will have to watch for Nadine, as I am starting to doubt 90L will develop to anything more than a rainmaker for Florida.

Totals are up to 13-7-1, I'm still thinking we will see at least one more major hurricane, we've still got half a season to go. If not in September, then in October the possibility exists. I'm not so sure on a sudden season 'death' like in 2006, which had no storms after Isaac in late September. Insane activity for an El Nino year, who cares about whether they haven't hit the US or not besides Isaac, which was a bad storm, this hurricane season has been historic to me. Proof that the ENSO isn't the deciding factor for seasonal activity.
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What developments can we expect from 90L?
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Quoting bassis:
The other day everyone was ripping the designation of Michael that it was padding the numbers. I would be curious of what those thoughts are now after Michael has become a hurricane


I think people were saying it wasnt really worthy of being a storm yet, and the NHC was doing this thing just to pad the numbers, and they did things like Helene for that as well, but however offline their sentiments were, they didnt quite imply that this former TD would never be a storm, just that it wasnt quite one then and wasnt worthy of its name at that time.

(PS: I wasnt on their side in the argument, im just trying to point out some reasoning to make a fair argument)
Member Since: February 11, 2012 Posts: 0 Comments: 9720
1255. icmoore
Quoting TomballTXPride:
icmoore i c u r hitting the ! button. Well done, Sir.


Some things don't belong on this blog...you can drop the sir that would be my husband :)
Member Since: July 18, 2005 Posts: 9 Comments: 4146
Quoting CloudGatherer:
Meanwhile, here's Leslie over the same period. That's one mess of a storm.


Station 41049
NDBC
Location: 27.500N 63W
Date: Thu, 06 Sep 2012 12:50:00 UTC
Winds: ENE (60°) at 42.7 kt gusting to 54.4 kt
Significant Wave Height: 27.2 ft

Dominant Wave Period: 11 sec
Mean Wave Direction: E (99°)
Atmospheric Pressure: 29.56 in and falling
Air Temperature: 76.8 F
Dew Point: 71.4 F
Water Temperature: 81.3 F


That's about 85 miles from the last Center fix.

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Oh my..
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1250. GetReal
Quoting FLWeatherFreak91:
Blog slowing way down this morning despite the fact there is a cat three in the atlantic lol. Hey largo, are you getting any of the storms moving off the gulf right now? Looks like I'll finally get some rain


School is in session...
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1249. GetReal
Station KIPN
Federal Aviation Administration
Location: 28.085N 87.986W
Date: Thu, 06 Sep 2012 13:15:00 UTC
Winds: SSW (210) at 22.9 kt
Air Temperature: 78.8 F
Dew Point: 75.2 F
Visibility: 8.7 nmi


Station 42363
Shell Oil
Location: 28.160N 89.22W
Date: Thu, 06 Sep 2012 13:15:00 UTC
Winds: NNE (20) at 23.3 kt gusting to 23.3 kt
Atmospheric Pressure: 29.91 in
Air Temperature: 84.0 F
Dew Point: 72.0 F


Station KMIS
Federal Aviation Administration
Location: 29.296N 88.842W
Date: Thu, 06 Sep 2012 13:15:00 UTC
Winds: E (80) at 8.9 kt
Air Temperature: 82.4 F
Dew Point: 75.2 F
Visibility: 7.0 nmi


Station KGRY
Federal Aviation Administration
Location: 27.625N 90.441W
Date: Thu, 06 Sep 2012 13:20:00 UTC
Winds: NNW (330) at 8.0 kt gusting to 13.0 kt
Air Temperature: 84.2 F
Dew Point: 77.0 F
Visibility: 8.7 nmi


Offshore reporting stations ( the limited number reporting) indicate that there is likely a closed low level circulation with 90L... The highest winds reported were 23kts.
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1247. Melagoo
Michael looks like Andrew little but full of energy ...

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1244. MahFL
Quoting tater5500:
What's with the BLOB in the gulf?

Shear.
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Mike the Major





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90L
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Meanwhile, here's Leslie over the same period. That's one mess of a storm.
Member Since: August 18, 2009 Posts: 0 Comments: 460
1239. SLU
Quoting TomballTXPride:

small, compact, tight...the real mccoy.


for sure!
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This is Michael's intensity, as calculated by UW-CIMSS. That's about as vertical a rise as I've ever seen it produce. It's holding it steady at 107kts/123mph for now.

But SAB is showing a weaker storm that's in process of weakening further 06/1145 UTC 29.9N 41.3W T4.5/5.0 MICHAEL

Best guess is that the 11am advisory holds steady at 100kts.

Member Since: August 18, 2009 Posts: 0 Comments: 460
Blog slowing way down this morning despite the fact there is a cat three in the atlantic lol. Hey largo, are you getting any of the storms moving off the gulf right now? Looks like I'll finally get some rain
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Local Tampa Bay TV mets addressed 90L last night. Dennis Philip for one stated that he didn't think it would be come a td or ts, but def. a rainmaker for FL. Headed to FL this weekend. I think he said the word blob. So, there you have it.
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1232. SLU
This is almost as good as it gets. A "supercane" putting on a show in no man's land.

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Quoting Tropicsweatherpr:
Things starting to take shape in the West Africa area.

This one could be trouble if it doesn't hitch a ride with a trough.
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1230. bassis
The other day everyone was ripping the designation of Michael that it was padding the numbers. I would be curious of what those thoughts are now after Michael has become a hurricane
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1229. VR46L
Quoting TomballTXPride:

Yeah but nothing compared to my girl Julia. This one knew how to walk the red carpet.





Now that is a prefect storm ...
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Quoting Tropicsweatherpr:
Things starting to take shape in the West Africa area.


Further south too than its predecessors. We best keep an eye on this in case it decides to pull a Georges or Hugo.
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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?.


Although it has not happened in the modern records, it must be possible.

Camille made landfall as a Category 5 at about 30.5N as a 165kts storm, which is 30kts above minimum for Cat 5.


As far as I can tell, Camille is the farthest north that an Atlantic storm has traveled as a cat 5 (in modern records,) and it did so easily, and would have gone a lot further if it hadn't run into land...
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Quoting TomballTXPride:

Yeah but nothing compared to my girl Julia. This one knew how to walk the red carpet.




Julia sure was cooking, lol!!!
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Shear coming back to Atlantic with trough

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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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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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