Invest 91L more organized, but has little time to develop

By: Dr. Jeff Masters , 2:31 PM GMT on April 22, 2011

Share this Blog
2
+

Hurricane season is more than a month away, but we have a tropical disturbance (91L) typical of what one might see in June or November. 91L is spinning over the waters a few hundred miles south of Bermuda, and has improved considerably in organization since yesterday, thanks to a drop in wind shear. The latest SHIPS model output is showing shear of 40 - 55 knots over 91L, but shear analyses from the University of Wisconsin CIMSS group is showing lower shear values of 20 - 30 knots over the main circulation center and to 91L's north, where the heaviest thunderstorms are. The system has a warm core at low levels, but a trough of low pressure lies over the storm at upper levels, and this trough is pumping cold, dry air into 91L, making it not completely tropical. One characteristic of subtropical systems like 91L is the presence of the main band of heavy thunderstorms removed several hundred miles from the circulation center, and 91L fits that description. 91L has two centers of circulation competing to be dominant, and this competition is slowing the storm's development. The storm was headed north at 5 - 10 mph early this morning, but that motion has halted, and 91L appears to be moving more south-southwesterly now, away from Bermuda. Sea surface temperatures are 23°C, which are very cold for a tropical storm to form in, but could support development of a subtropical storm.

As 91L moves south today, shear will steadily rise, and the storm likely has only until Friday night before shear grows too high to permit development. NHC is giving 91L a 20% chance of developing into a subtropical or tropical depression, which is a reasonable forecast. There has been only been one named April storm in the Atlantic since 1851, Tropical Storm Ana of 2003. The formation of a tropical disturbance at this location this time of year is unusual, but is not a harbinger of a active season ahead. Had this been going on in the Caribbean, that would be a different story.


Figure 1. Morning satellite image of the Atlantic tropical disturbance 91L. Note the two centers of circulation competing to be dominant. I expect the northern center will become dominant.

Jeff Masters

Reader Comments

Comments will take a few seconds to appear.

Post Your Comments

Please sign in to post comments.

or Join

Not only will you be able to leave comments on this blog, but you'll also have the ability to upload and share your photos in our Wunder Photos section.

Display: 0, 50, 100, 200 Sort: Newest First - Order Posted

Viewing: 105 - 55

Page: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14Blog Index

.
Member Since: Posts: Comments:
Member Since: Posts: Comments:
101. j2008
I don't get this, it should at least be a subtropical-depresion based off of the way it looks combined with its wind speed of 30-35 mph.
Member Since: Posts: Comments:

Click on the Avatar,delete the image and upload another.Click the Primary Portrait thingee.
Member Since: Posts: Comments:
Quoting AussieStorm:
How do i change my avatar


Go to Wunderphotos, view a photo, and check "primary portrait" in the options.
Member Since: Posts: Comments:
Quoting cchsweatherman:


Even still, without gale force winds (equivalent to tropical/subtropical storm force winds), you cannot have a naming of a subtropical/tropical system.


You can have a classification though....and it would have had a name yesterday when it had gales. The point is recognizing it as subtropical. I'm fine with whatever criteria the NHC uses as long as they're consistent, but it is times like this when I would like to know why they think it isn't subtropical, because based on the written criteria, it is.
Member Since: Posts: Comments:
How do i change my avatar
Member Since: Posts: Comments:
Quoting cchsweatherman:


Even still, without gale force winds (equivalent to tropical/subtropical storm force winds), you cannot have a naming of a subtropical/tropical system.


Erm, you can have a depression with winds above 25mph?
Member Since: Posts: Comments:
Quoting RandomText:


It has an Upper Level cyclone instead of an anticyclone.

Right now 91L doesn't meet the technical definition of a tropical low, and wouldn't meet it even if it had 100mph winds.


Tropical? Heck no. Subtropical? They always have upper lows.
Member Since: Posts: Comments:
There is a decent chance Florida will receive some rain from 91L...And it looks like there might be another surge of moisture heading towards Northern South America and the Antilles...
Member Since: September 27, 2007 Posts: 1 Comments: 20547
Quoting Levi32:


Many forget that cloud tops with subtropical systems rarely get colder than what 91L has right now.

The gale-force winds can come and go. I am talking more of a general classification of a subtropical cyclone, which the NHC won't do because for some reason the thing isn't warm-core to them yet.


Even still, without gale force winds (equivalent to tropical/subtropical storm force winds), you cannot have a naming of a subtropical/tropical system.
Member Since: Posts: Comments:
Quoting AussieStorm:

Better hope August isn't busy weather wise.



Late Aug is the worry weeks into Sept.

We dont worry about that as we cant control it.

Ive been to Rising Tide here 3 times and well that date has worked well every year since 06.

www.risingtidenola.com
Member Since: Posts: Comments:
Quoting Patrap:
We just kicking the idea around for now aussie,,but that would be a fine Keynote Speaker fo sho.

Better hope August isn't busy weather wise.
Member Since: Posts: Comments:
Here's a bigger picture.

Member Since: Posts: Comments:
Quoting Patrap:
We just kicking the idea around for now aussie,,but that would be a fine Keynote Speaker fo sho.


It sounds like a very nice idea if it could actually happen.
Member Since: Posts: Comments:
Quoting CyclonicVoyage:


Well, A. It would already have a name and B. There wouldn't be two centers in a 100mph hurricane.

Dual centers is what saved the FLA peninsula a crushing blow from Ernesto.

So no matter 90L wasn't named,the forecasted mid-May caribbean storm will be named Cindy!
Member Since: Posts: Comments:
We just kicking the idea around for now aussie,,but that would be a fine Keynote Speaker fo sho.
Member Since: Posts: Comments:
Quoting cchsweatherman:


If you read the TWO from the NHC, they clearly state that the winds with the non-tropical low have decreased below gale force, which means that winds are no longer at tropical/subtropical storm force. I agree very much with the NHC not naming this system to be honest given this information and given that the system has modest convection at best at this time.


Many forget that cloud tops with subtropical systems rarely get colder than what 91L has right now.

The gale-force winds can come and go. I am talking more of a general classification of a subtropical cyclone, which the NHC won't do because for some reason the thing isn't warm-core to them yet.
Member Since: Posts: Comments:
Quoting cchsweatherman:


Very nice imagery there which clearly shows the northern circulation becoming more dominant and high pressures building over the disturbance forcing the system towards the southwest.


I am to the Southwest in the Turks and Caicos... We may have good surf for our Easter Monday Kite flying on the North Facing Beaches.
Member Since: Posts: Comments:
XX/INV/91L
MARK
25.25N/64.24W
Member Since: Posts: Comments:
Member Since: Posts: Comments:
If it's going to be classified, I'd expect another TWO at 2pm EDT to up the chances, they're not going to go from 20% to a depression or storm. If we get nothing at that time, 91L ain't getting a name.
Member Since: Posts: Comments:
If it's going to be classified, I'd expect another TWO at 2pm EDT to up the chances, they're not going to go from 20% to a depression or storm. If we get nothing at that time, 91L ain't getting a name.
Member Since: Posts: Comments:
Quoting Jax82:
The main center is going to win.



Very nice imagery there which clearly shows the northern circulation becoming more dominant and high pressures building over the disturbance forcing the system towards the southwest.
Member Since: Posts: Comments:
Quoting Patrap:
We are tossing around the idea of a wunderground members conference come August,,time and place TBD.

If interested in attending or have input as to the format and all,,wu mail me.

what will it be about, will Dr J involved?
Member Since: Posts: Comments:
Quoting Levi32:
91L is beginning its move back to the southwest now, and that means that the window of opportunity is already closing, and fast. With a single center, this more than meets the written criteria to be named, and did so also when it had a single center yesterday. Right now as it has developed two vortices, it will need to get rid of the southern one to become a solid candidate again.

But of course, we all know that yellow on the TWO means the NHC will never name it.


If you read the TWO from the NHC, they clearly state that the winds with the non-tropical low have decreased below gale force, which means that winds are no longer at tropical/subtropical storm force. I agree very much with the NHC not naming this system to be honest given this information and given that the system has modest convection at best at this time.
Member Since: Posts: Comments:
XX/INV/91L

Member Since: Posts: Comments:
Quoting PolishHurrMaster:

Oczywiście,że jestem!(Of course I am!) And a question for you-can you speak Polish as good as I speak English?


No, but I can speak Greek better than you.
Member Since: Posts: Comments:
Quoting Patrap:
We are tossing around the idea of a wunderground members conference come August,,time and place TBD.

If interested in attending or have input as to the format and all,,wu mail me.
Good Idea Pat.
Member Since: Posts: Comments:
Quoting PolishHurrMaster:

So I'll give you a merithorical situation:tropical system in August,gusting to 100mph,heading for Florida.But has two competing centers.Would you name it?


Well, A. It would already have a name and B. There wouldn't be two centers in a 100mph hurricane.

Dual centers is what saved the FLA peninsula a crushing blow from Ernesto.
Member Since: January 30, 2010 Posts: 0 Comments: 3259
Quoting IKE:

And don't call me Shirley.
lol..Surely you cant be serious....I am serious....And dont call me Shirley..:)..........Oh, look, another squall line...What a big surprise.....Yawn.
Member Since: September 27, 2007 Posts: 1 Comments: 20547
I have to admit it looks sub-tropical enough. Must be something with winds. Maybe length of survival?
Member Since: Posts: Comments:
Member Since: Posts: Comments:
Quoting Jax82:
The main center is going to win.



It's building a lot of convection on the west side for the first time, shear must be a good bit lower at the minute. I really don't think this thing's gonna be given a name though.
Member Since: Posts: Comments:
We are tossing around the idea of a wunderground members conference come August,,time and place TBD.

If interested in attending or have input as to the format and all,,wu mail me.
Member Since: Posts: Comments:
Quoting Jax82:
The main center is going to win.


Is 91L moving towards the south? If so, its time is running out.
Member Since: Posts: Comments:
The main center is going to win.

Member Since: Posts: Comments:
Excellent answers,,....

Ike it be.

Almost 1000 days ago as well.


Member Since: Posts: Comments:
Quoting BahaHurican:
Only bad thing about rain on Monday is that Easter Monday is the unofficial start of the beach season here in the Bahamas, and a lot of beach parties and cookouts would get rained out. This is not a good thing....


Here in TCI we have our Kite flying contests on da beaches on Easter Monday... This could get interesting.

http://www.turkscaicosluxuryvillas.com/blog/activ ities/turks-and-caicos-kite-flying-competition/

Member Since: Posts: Comments:
Quoting Patrap:
What was the last Hurricane to Strike the US Mainland?

I think it was Hurricane Ike.
Member Since: Posts: Comments:
Quoting Patrap:
What was the last Hurricane to Strike the US Mainland?


2008: Ike.
Member Since: Posts: Comments:
Quoting Patrap:
What was the last Hurricane to Strike the US Mainland?


Ike, I believe.
Member Since: Posts: Comments:
Quoting Jedkins01:
ughhhhh its bugging me to the core as to why this not being named a subtropical storm.....

Oh well, they have access to information about it that I don't. There must be something it lacks, even though to the eye it should easily be named.


It shouldn't be surprising, the nhc does this every year...we had invests with 60%+ chance of getting designated but yet the nhc wouldn't name them. Im not hating on them, but they probably should make the guidelines they use a bit more clear.
Member Since: Posts: Comments:
What was the last Hurricane to Strike the US Mainland?
Member Since: Posts: Comments:

Viewing: 105 - 55

Page: 1 | 2 | 3 | 4 | 5 | 6 | 7 | 8 | 9 | 10 | 11 | 12 | 13 | 14Blog Index

Top of Page

About

Jeff co-founded the Weather Underground in 1995 while working on his Ph.D. He flew with the NOAA Hurricane Hunters from 1986-1990.

Local Weather

Mostly Cloudy
77 °F
Mostly Cloudy