Disturbance 91L spins towards Cape Hatteras; Cyclone Aila toll at 180

By: Dr. Jeff Masters , 1:28 PM GMT on May 27, 2009

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An area of disturbed weather, dubbed "91L" by the National Hurricane Center, is centered 120 miles south of Cape Hatteras, North Carolina. While the storm does not have much in the way of heavy thunderstorm activity, it does have a well-developed circulation, and the spin of the system is readily apparent on long range radar animations out of Morehead City, North Carolina. The disturbance is over waters of 25 - 26°C and has wind shear of 10 - 15 knots over it, and these conditions are marginally favorable for some slow development to occur until Thursday afternoon, when the system will begin moving over waters too cold to support tropical cyclone development. The disturbance will track north or north-northeastward at 10 - 15 mph towards North Carolina's Outer Banks today, then get swept northeastwards out to sea on Thursday. It is unlikely that the disturbance has enough time to develop into a tropical depression, but an Air Force hurricane hunter flight is on call to investigate the system this afternoon, if necessary. If the system does develop, the current location of the heaviest thunderstorm activity in a band well removed from the center suggests that 91L would be classified as a subtropical depression. The Outer Banks of North Carolina can expect 20 - 25 mph winds and heavy rain from this system tonight and Thursday morning. In a Special Tropical Weather Outlook issued at 8am EDT this morning, NHC gave 91L a low (less than 30% chance) of developing into a tropical depression.


Figure 1. Latest radar image from the Morehead City, NC radar.

Tropical Cyclone Aila death toll at 180
The year's deadliest tropical cyclone so far, Tropical Cyclone Aila, has killed at least 180 people in India and Bangladesh border region, according to the latest media reports. Aila hit the India/Bangladesh border region on May 25 as a borderline tropical storm/Category 1 hurricane, bringing sustained winds of 65 - 75 mph and a 3 - 4 meter (10 - 13 foot) storm surge to the coast. Aila has left over 150,000 homeless in India and 500,000 in Bangladesh. The cyclone destroyed over 180,000 homes in Bangladesh--a severe blow for a region still recovering from the devastation wrought by Category 4 Tropical Cyclone Sidr of November 2007, which killed 3,500 people. The death toll form Aila will likely go much higher, as over 500 people are still missing. The Bay of Bengal is no stranger to deadly cyclones--fifteen of the world's twenty deadliest tropical cyclones have been Bay of Bengal storms that have hit Bangladesh, India, or Myanmar. The most recent was last year's Cyclone Nargis, which killed 146,000 people in Myanmar.


Figure 2. Satellite image of Aila as it made landfall near the India/Bangladesh border. Image credit: NASA.

Interview tonight on hurricanetrack.com
I'll be doing one of my periodic spiels on Internet radio tonight at 9pm EDT. Tune your browsers to www.hurricanetrack.com and listen in to my interview with host Mark Sudduth. There is also a live chat to participate in. Hurricane season starts Monday!

Jeff Masters

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You guys are right I watched the system last week in the Caribbean, I'll still say these systems maybe more interesting than a 90 or91L that had little chance of developing either. The bottom line is obviously the system in the Caribbean was much more damaging. Sometime you guys sound like lawyers, willing to argue over the mundane, past what is important. My editorial for the night. May the good weather be yours.
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looks to have shifted heading from e se to e ne in last latest frame
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Quoting StormJunkie:
Still 30+ knots of shear and any low forming at that point would be cold-core in nature.

Just to play devil's advocate, is the entire system not moving in the same general direction as at least some of the shear?


Yes but if it was really a tropical system it wouldn't be in that position. Wind shear isn't an inhibiting factor for systems like this. If there was no wind shear, there would be no upper diffluence, and no thunderstorms at all.
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Still 30+ knots of shear and any low forming at that point would be cold-core in nature.

Just to play devil's advocate, is the entire system not moving in the same general direction as at least some of the shear?
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little popper in 91L
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440. beell
well, I'll watch it anyway lol
Night, all.
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439. beell
a persistent trough off the east coast, maybe?

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The GFS fizzles the blob of precipitation as the shortwave trough lifts out by 72 hours and taking away the upper diffluence. That's exactly what will happen. It will go poof like the Jamaican blob last week that looked just as threatening as this.
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There will be many impulses of lower pressure riding along the subtropical jet... these will be very small tight areas of low pressure..but not organized systems. These impulses will be from divergence in the atmosphere in the subtropical jet. Alot of uplifting and areas of lower pressure will develop and race eastward along this jet. Shear wont be much of a problem cause it will ride in tandom with the stream. If 1 of these impulses holds together..it will bring alot of rain in a short period of time..and thunderstorms with possible isolated tornados if any impulse makes it across florida.
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The formation of tropical cyclones is the topic of extensive ongoing research and is still not fully understood. While six factors appear to be generally necessary, tropical cyclones may occasionally form without meeting all of the conditions
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Quoting beell:
things would look a little better off the east coast, SJ-fwiw.


Still 30+ knots of shear and any low forming at that point would be cold-core in nature.
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434. beell
I mean, what would it take to begin the formation of a sub-tropical system?

A stalled cold front with a surface low?
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Giving the GOM blob a 10% chance of development is optimistic at best. I don't think it will develop at all because of:
- No surface feature
- Its caused by upper-level diffluence
- It has 50+ knots of shear over it
- Mother nature enjoys getting WUbloggers into a tizzy over nothing near the start of the season
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Where is Steve McQueen and the fire extinguishers. Look out for the blob from Texas.
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431. beell
things would look a little better off the east coast, SJ-fwiw.
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Quoting StormJunkie:

I just don't want everyone to think this is an out of the blue development that will get people on edge.


I hear you Levi, but are you saying there is 0% it becomes 92 either in the Gulf or off the E coast?


Yes I am. "never say never" does not apply here lol :P
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Levi is right...it has only a 10% chance or less of becoming a tropical system.... that is of doing something before reaching florida.. shear isnt really the problem...problem is a combination of things.
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shear is high and increasing
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I just don't want everyone to think this is an out of the blue development that will get people on edge.


I hear you Levi, but are you saying there is 0% it becomes 92 either in the Gulf or off the E coast?
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Quoting beell:
diffluence or divergence, Levi?


Sorry diffluence, I mix up those terms by accident.

Divergence is very similar, it is the spreading out of air caused by an air stream speeding up, or air pulling apart in opposite directions. Diffluence is "fanning out" of the air flow.
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424. beell
diffluence or divergence, Levi?
or PVA from the shortwave?
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Quoting StormJunkie:
I agree with you Levi. Just saying it looks suspect...And a 6.8mb drop over a 2hr time frame seemed a little high. A shame no wind data comes out of that giant rig...


Yeah that's really cool. I'm not saying it's not an area of low pressure. Every thunderstorm on the planet is a mini area of low pressure, and when they gang together like this in a complex they can cause significant pressure falls like the bouy shows.

I just don't want everyone to think this is an out of the blue development that will get people on edge.
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Here is exactly what's causing the "blob" over the GOM.

Take a big long gander at this image:



Look near and south of the blob, see the wind vectors pointing towards the ESE? Now look north and west of the blob, you'll see the vectors pointing towards the NE due to the approaching upper trough. You can see how the air stream "fans out" in this area.

This is called upper-level diffluence. You can translate that into "fanning out of air in the upper levels". When you think about that, if air is spreading out up there, then new air needs to come in to replace it. The only place it can come from is below, so air at the lower levels rises up to replace the air spreading out at the top. And we all know that when air rises it condenses into clouds and if strong enough, thunderstorms, such as the ones over the GOM right now.

These thunderstorms have formed into a mesoscale convective complex (MCC), in which they work together to support new thunderstorm growth, and hence can survive much longer than typical thunderstorms. They also have lots of upper-air support at the moment. It's not tropical, it won't develop, and aside from possibly bring rain to Florida it's not a problem. Again look at the maps, you will never see a real tropical low with deep convection over it under 60 knots of shear. It will never happen.
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I agree with you Levi. Just saying it looks suspect...And a 6.8mb drop over a 2hr time frame seemed a little high. A shame no wind data comes out of that giant rig...
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one thing for sure wont be seeing any thing here any time soon nic cold front by the way

this is some in you see in a winter time set up

Link
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418. 7544
yeap what ever it does now if it devolpes or not and it holds on it could bring alot of rain to fla in 48 hours stay tuned

just a note it has rain in miami every day for 11 striaght days so far going for a record
Member Since: May 6, 2007 Posts: 0 Comments: 6874
417. Skyepony (Mod)
91L got a good quikscat pass. Not a nice closed low. High Res shows it better..The ships & buoys haven't shown a closed low either.

I saw the question earlier why hadn't this been named ..if it had been a cane it would still be a depression, so why not name this? Once it's a cane & weakens it holds tight to the surface circulation. That's why you may see a naked swirl still a depression that is weaker & has less rain than a midlevel circulation putting on a show trying to work to the surface.
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Quoting StormJunkie:
Evening JF

I agree, thing looks suspect...Although my better judgment thinks any development is unlikely...

00z GFS has it off the E coast of Fl in 42 hrs from what I can tell???


And do you see a low there SJ? Nope, just a blob of precipitation on the model. It can represent an area of low pressure because thunderstorms do cause low pressure, but it is not a closed low pressure system with a closed isobar around it, and it's not tropical.
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AOI/**/**
MARK
26.6N/95.5W
MOVEMENT
SE









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It's a rain blob, nothing more. If it continues to survive as it advances eastward then Florida may be in for yet another soaking, but this isn't a threat for tropical development in any way shape or form. Remember the blob that looked just like this near Jamaica? It was caused by the exact same thing, and it fizzled just like it was supposed to, and it was never a threat for development.

See post 402.
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Evening JF

I agree, thing looks suspect...Although my better judgment thinks any development is unlikely...

00z GFS has it off the E coast of Fl in 42 hrs from what I can tell???
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Quoting Weather456:
Despite having the necessary conditions for TS genesis, 91L never took advantage



Some systems never do.

This is an enigma within tropical weather forecasting that has yet to be resolved.

Hopefully it will one day.
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Despite having the necessary conditions for TS genesis, 91L never took advantage. A good example of the necessary but not sufficient conditions of cyclognesis.

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true but not in all"quadrents",lol...
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406. 7544
Quoting JFLORIDA:
Thing in the gom is looking a little better than it probably should be.


hmm maybe the new gfs run in a hour will tell us more . it does look pretty strange as its growing all day
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Western Pacific has been rather quiet, but like ours, they peak around September.
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Winter is seeing true outflow. It's not just a hurricane thing. All thunderstorms transport warm air to upper levels and it has to spread outward. When you see a mature afternoon thunderstorm, the cirrus streaming out from the top of it could actually be called its outflow. Now the MCC in the gulf is only going to have outflow on the east side because the winds aloft are blowing so strong that the cirrus will only move towards the east.

That said, this is yet another blob that is in no way tropical, and is in no way surface-based. It is being caused by upper diffluence ahead of the advancing upper trough and associated surface front.

Besides guys, I don't think you will ever see a tropical low with that much deep convection directly over it in 50-60 knots of shear. That just will never happen.
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AOI #1

AOI #2
Member Since: October 1, 2007 Posts: 81 Comments: 26516
I'm just glad the blob is moving away from Texas
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Japan Meteorological Agency
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Quoting stillwaiting:
winter:seeing things there's no anti cyclone over the area,there's a high SE of Cuba....it may move into a area of moderate-low sheer if it moves about parallel w/tampa or north,even thats a big if,IMO


The outflow winter is seeing is the shear pattern. Behind the blob to the west the shear is from the west southwest to southwest. In front of the blob to the east the shear is west northwest to northwest.
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winter:seeing things there's no anti cyclone over the area,there's a high SE of Cuba....it may move into a area of moderate-low sheer if it moves about parallel w/tampa or north,even thats a big if,IMO
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386.
bl!
:)
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Couple of 40-45 knt barbs... on a bad pass...
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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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