Disturbance 90L close to tropical depression strength; TD 2 may rise again

By: Dr. Jeff Masters , 2:02 PM GMT on August 14, 2009

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A strong tropical wave (90L), with a large circulation and a moderate amount of heavy thunderstorm activity, is a few hundred miles southwest of the Cape Verdes Islands, off the coast of Africa. This morning's QuikSCAT pass shows that 90L has a large circulation, but this circulation is not yet well-formed, as it is elongated along an east-west axis. Satellite imagery from the European METEOSAT satellite show that the heavy thunderstorms associated with 90L are gradually becoming better organized, with some spiral banding evident on the south side of the storm. Satellite intensity estimates from NOAA already put 90L at tropical depression strength with 30 mph winds, but the storm does not yet have a well enough formed circulation to be classified as a depression. Water vapor imagery shows that since 90L is forming several hundred miles south of the dry air of the Saharan Air Layer (SAL), the storm should not be affected by dry air and dust as much as Tropical Depression Two was. However, there is some dry air from the SAL being sucked into the circulation of 90L, and this is retarding its development.

Wind shear is moderate, about 10 - 15 knots, but is forecast to fall to the low range, 5 - 10 knots, later today. Shear is then forecast to remain low for the next five days. Sea Surface Temperatures are about 28°C, and will remain in the 27 - 28°C range the next five days. The combination of low wind shear and sufficiently warm SSTs should allow 90L to develop into a tropical depression by Saturday, and potentially reach hurricane strength by Wednesday. Most of our reliable models strengthen 90L into a hurricane by Wednesday, when the storm is expected to be near the northern Lesser Antilles Islands. One important outlier is the ECMWF model, which has 90L passing to the north of the islands, and eventually recurving out to sea east of Bermuda. The ECMWF was by far the most reliable model for forecasting hurricane tracks last year. However, all of our models are pretty unreliable going out 5 days in advance for systems that have yet to reach tropical depression strength. Once 90L becomes a tropical depression and develops a well-formed circulation, the models will have a better handle on forecasting where it will go.


Figure 1. The remnants of Tropical Depression Two (left side of image) and tropical wave 90L (right side of image).

Tropical Depression Two may come back
Tropical Depression Two died yesterday, but may rise again. Satellite loops of the remnants of the storm show that heavy thunderstorms are again attempting a comeback near the axis of what is now a tropical wave. However, dry air continues to interfere with the development of the thunderstorm activity, and moderately high wind shear of 15 - 20 knots is also inhibiting the process. Wind shear over the remnants of TD 2 is expected to remain in the moderate range, 10 - 20 knots, over the next four days. Sea Surface Temperatures (SSTs) are 27°C, but will warm to 28°C two days from now. There is plenty of dry, stable air from the Saharan Air Layer (SAL) to the north and west that will continue to cause the storm problems. The relatively cool SSTs and dry air mean that any re-development of the storm will be slow to occur. NHC has given the system a medium (30 - 50% chance) of becoming a tropical depression again by Sunday morning. Only one reliable model, the NOGAPS, is predicting regeneration of TD 2. The NOAA Hurricane Hunters are flying research missions into the remnants of TD 2 today and Saturday.

I'll have an update Saturday morning, or earlier, if significant developments occur.

Jeff Masters

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Quoting OSUWXGUY:


Hey Kman-

As of now it's more creating ventilation for TD2. To me looking at the RAMMSDIS loop the ULL has turned more westerly and I don't see it bringing a great deal of shear. The GFS (for what it's worth when handing ULL) shows continued light easterlies over the circulation itself with southerly winds to its north.


Here is the shear tendency map showing that shear has been increasing in the path of Ex TD2.

You can see the pocket of very low shear above the current position that is the core of the ULL itself. If you click on the actual shear map you will see the winds cutting across the path of Ex TD2 and swinging up to the ENE from the flow around the ULL base. Given the weak state of the feature it would not take a lot of adverse wind flow combined with the bone dry conditions to do it in again IMO.

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Canes,

Call me crazy, but I think 90L misses us altogether. (again going with the Hebert theory I mentioned this morning), but if the models are jogging this thing south of said box. I'm inclined to say that this would be more of a Southern Cuba Storm. It's gong to be interesting to see what happens in the next 24-48 hours.

I hope I'm right on this one. Month end is right around the corner, and we had to work during Rita in 2005. I work downtown, and I never understood the meaning of skyscraper until I was on the 30th floor of a building during the outer bands of a hurricane.
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1581. Drakoen
Quoting Vortex95:


what system is that for?


That is where the NOAA jet aircraft is. They are fly towards 02L
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Quoting stormno:
canesrule look at the sat pic and tell me how much dry air is out in 90L PATH...it doesnt look like any intensification will happen in those conditions..im afaid 90L will have a very tough time....Stormno


All the models showing strengthening into a powerful hurricane with light shear. Sorry.
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A ball of flames!

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1578. jipmg
couple of models taking TD 2 towards SFLA .. its got some impressive convection blowing up right on the center yet again.. lets see if it holds, certainly could though now that the sun is going down.
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Quoting Patrap:
90L is nothing but a little puppy thats wont even reach hurricane strength...latest info out of my office continues to show strong shear affecting 90L AND SST are near 80 degrees.also there is lots od dry air just nw of 90L and this will keep this system from developing...i still say the gom is the hot game in town i will have more to say on this later tonight..please all interests in la miss and alalbama should keep up on the weather for the next 72 hours...Stormno
..Kerry..its aLil early to be sippin Dome Foam as the Game isnt till 7 pm CST


Go Saints!!! we will be watching her in MS tonight Pat I know you will be too!!!
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Quoting stormno:
90L is nothing but a little puppy thats wont even reach hurricane strength...latest info out of my office continues to show strong shear affecting 90L AND SST are near 80 degrees.also there is lots od dry air just nw of 90L and this will keep this system from developing...i still say the gom is the hot game in town i will have more to say on this later tonight..please all interests in la miss and alalbama should keep up on the weather for the next 72 hours...Stormno


Why thank you for that false forecast STORMKAT! ;) .. we know who you really are
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1572. CJ5
Quoting reedzone:
DMIN, that is the reason why both of these systems look unimpressive right now. DMAX might be able to fire up 90L to depression status now that it has a TCFA. Tropical Depression 3 will most likely form by tomorrow.


Generally, there is truth in that but TD2 has diminished in the last two Dmax periods only to build during Dmin.
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1571. Drakoen
NOAA coordinates

13.067N 59.517W
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Quoting stormno:
90L is nothing but a little puppy thats wont even reach hurricane strength...latest info out of my office continues to show strong shear affecting 90L AND SST are near 80 degrees.also there is lots od dry air just nw of 90L and this will keep this system from developing...i still say the gom is the hot game in town i will have more to say on this later tonight..please all interests in la miss and alalbama should keep up on the weather for the next 72 hours...Stormno


Well welcome back STORMTOP. Glad to see the toilet model is still up and running.
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1569. Patrap
90L is nothing but a little puppy thats wont even reach hurricane strength...latest info out of my office continues to show strong shear affecting 90L AND SST are near 80 degrees.also there is lots od dry air just nw of 90L and this will keep this system from developing...i still say the gom is the hot game in town i will have more to say on this later tonight..please all interests in la miss and alalbama should keep up on the weather for the next 72 hours...Stormno
..Kerry..its aLil early to be sippin Dome Foam as the Game isnt till 7 pm CST


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Quoting stormhank:
anyone think 90L will be classified a depression by tomm? and if would possibly be a threat down the road to US?
i think Depression by tomorrow is very possible, track wise wait till it reaches the Antilles.

Quoting ackee:
the centre 90L is moveing SW OR WSW ?


It's moving Westward.
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A small burst of convection starting to bubble to the northwest of the exposed center.

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Quoting stormno:
90L is nothing but a little puppy thats wont even reach hurricane strength...latest info out of my office continues to show strong shear affecting 90L AND SST are near 80 degrees.also there is lots od dry air just nw of 90L and this will keep this system from developing...i still say the gom is the hot game in town i will have more to say on this later tonight..please all interests in la miss and alalbama should keep up on the weather for the next 72 hours...Stormno
i am very doubtful of ur prediction.
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1565. ackee
the centre 90L is moveing SW OR WSW ?
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Quoting WeatherStudent:


Could those adverse conditions affect 90L down the road, KMan? Everything indicates a NO. But what do you have to say?


The existing ULL will be gone by the time 90L gets to where Ex TD2 is presently located but the very dry air is pervasive and does not seem to be in any hurry to leave the Atl.

90L is carrying a lot of moisture on the South side but you can see from the image Drak posted that there is a lot of dry air to the North and NE.

At the very least the atmosphere would not appear to be conducive for any quick development by 90L which might lead to a weaker system making it further W and S before strengthening significantly, assuming it does.

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1563. jipmg
Quoting OSUWXGUY:


Hey Kman-

As of now it's more creating ventilation for TD2. To me looking at the RAMMSDIS loop the ULL has turned more westerly and I don't see it bringing a great deal of shear. The GFS (for what it's worth when handing ULL) shows continued light easterlies over the circulation itself with southerly winds to its north.

that certainly seems possible..
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anyone think 90L will be classified a depression by tomm? and if would possibly be a threat down the road to US?
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1560. Patrap
90L Circulation at 1800UTC ,impressively Large,and Like Dr. Masters says above,it will take some time to consolidate the Low into a Depression. Size DOES Matter in the Atlantic as well.


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1559. Drakoen
Quoting Chiggy007:
Circulation just little south that 12.5, around 12.1N, but yeah in that general vicinity!
Also, NHC 18Z surface maps show WSW movement with 90L - someone juts posted that graphic on here...


No. 12.5N 29.3W
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Quoting Vortex95:
its around 9:10 pm where 90l is right?
its 8:14
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Quoting kmanislander:
Ex TD2 is starting to be affected by the ULL to the N that is digging down on top of it. From the WV loop you can see the moisture being pulled off to the NNE by the flow around the base of the ULL and there is NE wind flow in the path of the feature as a result that will also likely create some SW shear faily soon.

The combination of this and the dry air appears to be playing havoc with every comeback attempt by the system.

WV Loop


Hey Kman-

As of now it's more creating ventilation for TD2. To me looking at the RAMMSDIS loop the ULL has turned more westerly and I don't see it bringing a great deal of shear. The GFS (for what it's worth when handing ULL) shows continued light easterlies over the circulation itself with southerly winds to its north.
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Quoting canesrule1:
i think we will have 03L when we all wake up tomorrow.


I agree.
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SHIPS 18Z model for 02L keeps shear below 15 knots for the next 5 days, most of the time below 10 knots.
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TD2

90L

AOI

AOI
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Circulation just little south that 12.5, around 12.1N, but yeah in that general vicinity!
Also, NHC 18Z surface maps show WSW movement with 90L - someone juts posted that graphic on here...
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1548. Drakoen
Dry air from the NE:
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Quoting Drakoen:


12.5N 29.3W It is exposed



Dry air from the northeast

Not surprising they love the GFDL

The GFDL *is* one of the two most complex models (along with the HWRF), and it definitely proved itself last year. It is one of the ones I look at the most for track and intensity guidance, personally.
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90l is a very large wave that will take some time to develop a closed low level circulation due to this size. I believe it will be upgraded to TD3 sometime tomorrow and when it gets a litte further west will explode into a formidable hurricane. With an anticylone forecasted to move in tandem with it, it will also encounter warmer SST's and a moister atmosphere as it moves west. All recipes for rapid intensification.
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Also, I never believed SHIPS intensity - just like HWRF most times it's over doing it.. :)
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1543. Patrap
Well.. the Saints are a NFC team, and the Texans are an AFC team, so I can guess I can cheer for both, until they play each other. Then I have to go with the local team. (I have yet to go to NOLA, although that may change this year)

If you do,look us up..and we can jaw-jack the season over,..er Refreshments here.
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Dude...LLC circulation is huge - not tight enough and yes you are right LLC lacks convection too...it is so strange that since this came off Africa it never had any bursts of convection in its center...
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Quoting weatherboykris:


*Smacks Head*

Dry air is colored orange on the WV loop...by definition, it's not dry if it's not orange.


No, the black areas are also dry. Orange is drier than black, but black is dry air.
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SHIPS has 90L's RI index for 25kt wind increase in 24 hours at 2.4 times the sample mean, so no RI in the near term is likely, even though the levels are bit elevated.
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1539. Patrap

Looks like it is trying to consolidate its circulation, and the winds immediately to the west of the COC are stronger than they were on the last one you posted.



Sounds right to Me Jeff,shes Big and its gonna take some time to get that circulation a moving around that CoC,but time is on her side.
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Quoting Vortex95:


Dry air doesn't have to be organge to be dry air.


*Smacks Head*

Dry air is colored orange on the WV loop...by definition, it's not dry if it's not orange.
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Quoting Patrap:

BR>
Your pick in Women is much better than your NFL Pick,,LOL
Well.. the Saints are a NFC team, and the Texans are an AFC team, so I can guess I can cheer for both, until they play each other. Then I have to go with the local team. (I have yet to go to NOLA, although that may change this year)

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1536. Drakoen
Quoting weatherboykris:


? It isn't exposed on the last shots I saw...sun just set over the storm tho, we're stuck with IR for now.


12.5N 29.3W It is exposed

Quoting Twinkster:


there is no dry air near 90L

http://www.ssd.noaa.gov/goes/east/catl/wv-l.jpg


Dry air from the northeast
Quoting nrtiwlnvragn:
18Z surface map, expects SW motion with 90L



Not surprising they love the GFDL
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DMIN, that is the reason why both of these systems look unimpressive right now. DMAX might be able to fire up 90L to depression status now that it has a TCFA. Tropical Depression 3 will most likely form by tomorrow.
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Ex TD2 is starting to be affected by the ULL to the N that is digging down on top of it. From the WV loop you can see the moisture being pulled off to the NNE by the flow around the base of the ULL and there is NE wind flow in the path of the feature as a result that will also likely create some SW shear faily soon.

The combination of this and the dry air appears to be playing havoc with every comeback attempt by the system.

WV Loop
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About JeffMasters

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