97L growing more organized, will bring heavy rain to the Lesser Antilles

By: Dr. Jeff Masters , 3:55 PM GMT on July 19, 2009

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The tropical wave near 12N 52W, about 600 miles east of the Lesser Antilles Islands (97 L), has grown more organized this morning as it tracks west at about 20 mph. This wave is surrounded by an area of very dry air from the Sahara Desert, but 97L has been able to steadily moisten a large region of the atmosphere over the past day, insulating itself from the dry air. This moistening process has been aided by sea surface temperatures (SSTs) that have steadily increased from 26.5°C to 27.5°C, plus the presence of only 10 knots of wind shear. The system now has a small area of intense thunderstorms near its center, with some rotation of the cloud pattern evident at mid-levels of the atmosphere. An upper-level outflow channel has opened to the north, and there is evidence that surface spiral bands are beginning to form. This morning's QuikSCAT pass missed 97L, so we don't know what is happening at the surface.

Wind shear is a modest 10 knots over the disturbance, and is forecast to remain in the moderate 10 - 15 knot range through Monday morning. This should allow further development to occur today, and 97L could be approaching tropical depression strength on Monday as it moves through the central Lesser Antilles Islands. Monday night, shear is expected to rise to 20 - 30 knots, thanks to the presence of a trough of low pressure at upper levels of the atmosphere over the eastern Caribbean. Since 97L is a relatively small system, it is very vulnerable to wind shear. This shear may be able to drive some of the dry air west of 97L deep into its interior, significantly disrupting the disturbance. Shear will remain high along 97L's path through Thursday, when the storm should be in the western Caribbean near Cuba. If there is anything left of 97L by then, some development is possible. The National Hurricane Center gave 97L a low (less than 30% chance) of developing into a tropical depression in the next 48 hours in their 8am Tropical Weather Outlook. However, I'd say the odds are now in the medium range (30 - 50%).

None of the computer models are forecasting tropical storm development over the next seven days.


Figure 1. Current satellite image of African wave 97L.

I'll have an update Monday morning. As 97L approaches the islands, you may want to follow local observations there using our wundermap for 97L.

Jeff Masters

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The structure is impressive. Kinda looks like Gustav over Cuba.


Member Since: June 9, 2007 Posts: 4 Comments: 15946
Quoting Patrap:
97L Early Track Guidance



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Moi's opinion?
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Lifting in lattitude, will be contacting shear soon.
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Quoting scottsvb:
You guys are like kids jumping up and down in the back seat of the car... this is just a strong tropical wave... yes its close to TD status.. but right now this has no model support .. (Which means) this will probably get sheared later tonight into Monday
Whether it actually survives any future amount of shear or not, it does have some model support. Please do a little research before making a blanket statement.
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Quoting WeatherStudent:


Excuse me?


Do not pay attention to them! Do you have an opinion on what this storm will do? You are on my favorite bloggers here and I want your opinion.
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97L Early Track Guidance

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Quoting weatherwatcher12:
NOGAPS predicting very favorable shear


Then again its the NOGAPS.
Member Since: June 9, 2007 Posts: 4 Comments: 15946
Quoting Patrap:
Convection is waning,..

Floater 1 - Water Vapor Loop


Yes convection over the approximate center, but in the mean time a rather strong band is developing on the NE side.
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The NHC is covering their rears. They were already wrong in droping the system last night and can't face the fact they were wrong and the system is nearing TD status. 97L will organize tonight and burn them more. What you expected from a government organization, corruption of course. They are inconsistent, labelling 93L with 50% when it neever was close to the organization of 97L.
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655. IKE
Quoting scottsvb:
You guys are like kids jumping up and down in the back seat of the car... this is just a strong tropical wave... yes its close to TD status.. but right now this has no model support .. (Which means) this will probably get sheared later tonight into Monday


So you wouldn't give it the famous yellow circle?
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well ok scott I will go eat my chicken fingers on my finding nemo plate and leave it to the experts lol


RIP 97L because scott thinks so obviously
Again, the "shear" is not going to be a killer factor for 97L - not anytime soon imo. The shear environment, now and in the area directly ahead, is quite favorable for development. The upper trough that is "dissipating" further ahead of 97L will at best knock the tops of the system which is now starting to gather circulation more towards the surface, gaining some organization. The upper level winds from the trough will act to only "delay" further organization as it passes through the windwards into the cCaribbean. 97L will indeed become a TD imo and likely our first named-storm of the season.

And, I am less concerned with what shear will look like in the nCaribbean 4 to 5 days out - too far out to determine what that will look like at that this imo; but, I still suspect it will be more conduscive at that time as well.
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Quoting hunkerdown:
oxymoron


Excuse me?
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You guys are like kids jumping up and down in the back seat of the car... this is just a strong tropical wave... yes its close to TD status.. but right now this has no model support .. (Which means) this will probably get sheared later tonight into Monday
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NOGAPS predicting very favorable shear
Member Since: May 16, 2009 Posts: 1 Comments: 1231
Even though convection is weakening/keeps getting its structure better/moistening air.
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Floater 1 - RGB Color Infrared Loop
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season is waning...
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646. eye
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Quoting Patrap:
Convection is waning,..

Floater 1 - Water Vapor Loop


structure better organized however and DMIN is soon, so not too big of a deal
Quoting hunkerdown:
oxymoron


Lol

Member Since: June 9, 2007 Posts: 4 Comments: 15946
Convection is waning,..

Floater 1 - Water Vapor Loop
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Lets remember the NHC goes off the GFS shear maps

GFS is forecasting increased shear tonight and for the next few days, than favorable after that


shear not favorable by the GFS = low chance of development
Quoting CaneWarning:
WeatherStudent, in your expert opinion, what do you think 97L will do???
oxymoron
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640. eye
The wave that rolled off Africa a couple days ago looks descent, though it appears attached to the ITCZ.
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WeatherStudent, in your expert opinion, what do you think 97L will do???
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K... back... well seems NHC is moving with extreme care as I suspected hehe. I guess I can't blame them.

I do expect for 8PM tonight to have this system be declared as a medium (30% to 50%) by NHC... but by that time and due to the current organization that I'm seeing (upper outflow, good low inflow, and TSTM build up around the center) and if it keeps it up... I won't be surprised that it could be a TD before they even get into the High probability area.

20+ shear location will be critical for this system... but I believe it might be able to survive it as it has been attempting to stay shallow enough to keep itself on the southern end of the strong shear until conditions become a bit more favorable.
Member Since: August 14, 2008 Posts: 4 Comments: 5015
Quoting WeatherStudent:
They're taking it the wrong way, Drak. ROFL.


What?
Quoting jm1975fl:
Can anyone tell me anything about the BAMM model. That one has it dangerously close to South Florida where I live. Thanks


Models Explained
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They're taking it the wrong way, Drak. ROFL.
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Quoting jm1975fl:
Can anyone tell me anything about the BAMM model. That one has it dangerously close to South Florida where I live. Thanks
See model summaries here. This link is posted on the tropical/hurricane link on weatherunderground (top left of the page).
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Now that I think of it, NHC is right on the money with this

Low chance of development in the next 48 hours when you consider shear will increase tonight

Again if the shear forecast changes, than they will increase the chances for development.
Quoting weatherwatcher12:
Very moist out there:


YES, move west and away ULL. Give this system the room space that it desperately desires right now so that it could ignite and blossom.
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Technical Attachment

AN OVERVIEW OF NHC PREDICTION MODELS


BAM - The Beta and Advection Model



The Beta and Advection Model is a baroclinic-dynamical track prediction model. It produces a forecast track by following a trajectory in the vertically averaged horizontal wind starting at the current storm location out to 120 hours. The trajectory is corrected to account for the variation of the Coriolis force with latitude, the so-called Beta effect. (Beta is the Greek letter frequently used in meteorological equations to represent the change in the Coriolis parameter with latitude.)

The figure shows how the conservation of absolute vorticity results in the formation of anticyclonic relative vorticity in the northeast quadrant of the storm, and the formation of cyclonic relative vorticity in the southwest quadrant of the storm: Diagram of absolute vorticity advection and relative vorticity formation in the vicinity of a tropical cyclone.. The result adds a component of motion to the northwest to the storm's trajectory.

Three versions of the BAM model are run with shallow (850-700 mb), medium (850-400 mb), and deep (850-200 mb) layers. All three versions of the model are run operationally four times per day.

Reference: Marks, D. G., 1992: The beta and advection model for hurricane track forecasting. NOAA Tech. Memo. NWS NMC- 70, 89 pp.
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97L still dragging Dry Air into its core..or center



Floater 1 - Water Vapor Loop
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Can anyone tell me anything about the BAMM model. That one has it dangerously close to South Florida where I live. Thanks
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Winds up to 30 mph pressure down 1012
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That tstalled front over the south will steer it east.
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Very moist out there:
Member Since: May 16, 2009 Posts: 1 Comments: 1231
Attention all bloggers, please go to your settings and change your "names" and add "ster" to the end your current name :)
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Quoting plywoodstatenative:
Weatherstudent, we are not being given access to that link you posted.


I haven't posted any links lately.
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622. eddye
plywood im there
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Quoting Stormchaser2007:
Central convection looks to be deteriorating quickly. Do not say its due to DMIN. This is expected as the day goes on. Although it should regain convection tonight.


Convection strengthening in the band around it though. Not uncommon for early systems to have weaker central convection.
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Floater 1 - AVN Color Infrared Loop
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Quoting RufusBaker:
I will have to say that if it goes into the gulf it will go east towards central FL


You mean you want it to.
Member Since: June 9, 2007 Posts: 4 Comments: 15946
Weatherstudent, we are not being given access to that link you posted.
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Quoting Patrap:
97L Early Track Guidance



Are you concerned about this one going up towards you in Nola, Patkster?
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Quoting Stormchaser2007:
Central convection looks to be deteriorating quickly. Do not say its due to DMIN. This is expected as the day goes on. Although it should regain convection tonight.


structure continues to improve though
I will have to say that if it goes into the gulf it will go east towards central FL
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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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