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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1934. DDR
Quoting Weather456:
this weekend is crucial

I hope you are prepared,is there a hurricane shelter near your house?
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GFDL 18Z run: Well, it's on the Wunderground graphics - the blue line :)
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Just in case you missed it earlier, here is the update that I wrote on the CCHS Weather Center tropical update.

CCHS Weather Center
Tracking The Tropics


Former Tropical Depression 2

Over the past few days, the National Hurricane Center had been tracking a strong tropical wave that came off Africa across the Atlantic which became Tropical Depression 2. Yesterday, the NHC dropped the tropical depression status as the disturbance had lost all thunderstorm and shower action as dry air got entrained into the system and upper level winds were too fast to allow for any showers or thunderstorms to develop.

Well, it seems Mother Nature's throwing yet another curveball as showers and thunderstorms have really fired and become organized. Its possible that this could be coming back to life and the NHC continues to monitor its progress. They are still running computer models on this system since they believe it could make a comeback and based upon the computer models, this system still needs to be watched since this could possibly become a threat down the road to people in the Northern Caribbean islands, the Bahamas, and quite possibly Florida down the road.

Invest 90L

This is the system thats really garnered much attention from the NHC, Weather Channel, and local NWS offices and news stations. Late Tuesday evening, a very impressive strong tropical wave rolled off Africa and out into the Atlantic. In the past couple days, this has become a massive system spanning nearly 600 miles with a well-defined spin.

But at this time, it really hasn't been getting going because the upper level winds are so fast they've blown the thunderstorms off the circulation center which has in turn mostly exposed the circulation to the elements. In order for tropical systems to be born and to survive, they need thunderstorms to protect their circulation. Think of it like your camping in the woods. When its a cold or rainy night, you need a "tent" to protect you from those elements. If you don't have that protection, you won't be able to survive the elements. Thats pretty much the same case with these systems.

In the coming days, the computer models show these upper level winds relaxing which will allow the thunderstorms to move back over the circulation and get this going. Now, reverting back to my opening remark on this system, the main reason why this has become the spotlight in the tropics is because the computer models the NHC and all us forecasters use to try and forecast these storms all suggest this could really become a major threat as most all the models show this impacting the Puerto Rico area as a major hurricane. Now, beyond that it remains uncertain.

The Bottomline

Even though the computer models are coming into good agreement showing these systems becoming possible threats, its too soon to become excited or concerned (depending upon your perspective with storms). Keep in mind what history has taught us: Mother Nature plays by her own rules and can change her mind whenever she wants. Computer models are not gods, but rather just guidance showing possible scenarios based upon the conditions at the time. When conditions change, so do the models. My best advice at this time is to just stay informed on the situation and just make sure you have your hurricane preparedness plans in place just in the case the computer models happen to come true. Its always better to be safe and not have a storm impact your area than to be caught unprepared.
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1931. ackee
Quoting Chiggy007:
18z gfdl is out and it is futher south than its 12Z run!!
can show a link please
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Quoting stormpetrol:
I think ex td2 is on the way to strong comeback.

I think it is too

Quoting nrtiwlnvragn:
000
URNT10 KNHC 142303
97779 23040 60232 75600 88400 14032 79//2 /6971
RMK AF303 WXWXA 090814195909303 OB 05
SWS = 17 KTS
;


Looks like AF303 is headed to St. Croix, pre deployment.
That report is from 23.2N 75.6W.

what please tell more
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18z gfdl is out and it is futher south than its 12Z run!!
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1928. Patrap
Quoting nrtiwlnvragn:
000
URNT10 KNHC 142303
97779 23040 60232 75600 88400 14032 79//2 /6971
RMK AF303 WXWXA 090814195909303 OB 05
SWS = 17 KTS
;


Looks like AF303 is headed to St. Croix, pre deployment.
That report is from 23.2N 75.6W.



Looks Like it for sure. Featured Blogger LRandyB is a HH and he may have some word on that in His Blog I bet
Member Since: July 3, 2005 Posts: 424 Comments: 128261
Did recon go out today? If so, what have they found?
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this weekend is crucial
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Hey (((( Ann ))) Great to see you!!
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Quoting WeatherStudent:


Go with both ears and get the African jumbo rings in the middle of your lobes.Seriously though, you do know how to run models right?
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1923. DDR
Quoting Weather456:


Yea, enough to cause flash floods. Omar of last year was more of a wind event. So basically this maybe the 3rd year in a row if indirect impacts.

Lets hope it stays so...but as you know feeder band rains can also be devastating .
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1922. IMA
1909. DestinJeff Don't bother -- it won't sell since there's no threat!

And, uhm, the "forecaster" might want to watch out, department stores really aren't kind to boys they find watching girls trying on clothes!
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1920. Patrap
Member Since: July 3, 2005 Posts: 424 Comments: 128261
we may this may have TD 3 now but not jumping the gune


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000
URNT10 KNHC 142303
97779 23040 60232 75600 88400 14032 79//2 /6971
RMK AF303 WXWXA 090814195909303 OB 05
SWS = 17 KTS
;


Looks like AF303 is headed to St. Croix, pre deployment.
That report is from 23.2N 75.6W.
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Keep a close eye on the gulf situation
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Two problems: people who say "this is much ado about nothing", and "THIS IS GOING TO BE A MAJOR HURRICANE AND HIT SOUTH FLORDIA, THEN GO INTO THE GULF". I'm sorry but I don't see how you can know with any amount of certainty whether either of these two statements are or will become true. Down casters seem to say that since conditions are as they are, they will remain the same in the future, and wish casters seem to say that conditions to support "my" BIG hurricane "will" be in place in the future. Don't know how they will be in place, but just know they will. I believe both sides of the spectrum miss the whole "OBSERVATION" part. These two types of "extremists" are the ones who will constantly bicker back and forth with each other; however, this is a weather blog, so I suppose that you can speculate if you want to. On the other hand, I wouldn’t want to speculate because unfortunately there are people who are less educated about the weather than some of you on here. I would hate to see one of those people read the wrong post….
I also think for people who say they aren’t “MODEL WORSHIPERS”, they are really reading a lot into the models this evening, and over the past few days.
Member Since: June 1, 2009 Posts: 0 Comments: 107
I think ex td2 is on the way to strong comeback.
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Quoting Patrap:
90L Has Disappeared from the NOAA RAAMB Page


Very interesting.
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1912. mkmand
IMO both of these systems should be TDs right now.
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1911. JRRP
Quoting Weather456:
5 Days - GFDL


like GFS
Member Since: August 16, 2007 Posts: 0 Comments: 5799
All i can say is its much ado about nothnig and when its all said and done with, im gonna be on here saying i told you so.
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Quoting bajelayman2:


Was katrina not also 'expected' for SF originally, then skipped a bit South into the Gulf?
Katrina passed over Miami and sw through the Keys and then up through the Gulf
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Quoting Patrap:
GFS 18Z run



Not good Pat ....not good
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Currently, I am in the live chat room of XtremeHurricanes.com (via the Live Hurricane Webcam Link)

I will be available for the next hour to answer questions or just chat.

To enter the chat room, you do not need a password...all you need is a username.
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AREA FORECAST DISCUSSION
NATIONAL WEATHER SERVICE SAN JUAN PR
314 PM AST FRI AUG 14 2009

CONVECTION ASSOCIATED WITH TD2 IS ATTEMPTING A COMEBACK AROUND
THE REMNANT LOW PRES. NHC/HPC NOON COORDINATION CALL KEEPS THIS
SYSTEM MOVING JUST NORTH OF THE AREA ON MONDAY AS A OPEN WAVE.
HOWEVER...THERE IS LITTLE SKILL IN PREDICTING STORM STRUCTURE
THREE DAYS IN ADVANCE AND SYSTEMS THIS SMALL ARE SUBJECT TO RAPID
FLUCTUATIONS IN INTENSITY EITHER UP OR DOWN. WITH THIS IN MIND WE
SHOULD ALL KEEP A CLOSE EYE FOR ANY UNEXPECTED DEVELOPMENT. TPW
IMAGERY SHOWS PWATS AROUND 2.6 INCHES OR AROUND 145% OF NORMAL IN
THAT AREA. IF THE SYSTEM IS ABLE TO MAINTAIN ITS CONVECTION OVR
THE WEEKEND IT COULD PRESENT SOME PROBLEMS TO PR AND THE USVI.

NEXT AREA OF LOW PRES IS A COUPLE OF HUNDRED MILES WSW OF THE CAPE
VERDE ISLANDS. MICROWAVE AND CONVENTIONAL SATELLITE IMAGERY SHOWS
THAT THE SYSTEM IS BEING SHEARED WITH THE CENTER TO THE EAST OF
THE CONVECTION. VIS IMAGERY ALSO SHOWS SAHARAN AIR BEING SUCKED
INTO THE CIRCULATION WHICH IS RETARDING ITS DEVELOPMENT. GLOBAL
MODELS CONTINUE TO LIKE THIS STORM FOR FURTHER DEVELOPMENT AND NHC
IS GIVING IT A HIGH CHANCE OF BECOMING A TROPICAL CYCLONE IN THE
NEXT 48 HRS. GFS MODEL HAS BEEN REMARKABLY CONSISTENT OVER THE
LAST THREE DAYS TAKING THIS SYSTEM OVR PUERTO RICO ON THU. 12Z
GFDL HAS SHIFTED ITS TRACK ABT 2 DEGS SOUTH FROM ITS PREVIOUS RUN
AND IS ALSO FASTER BRINGING IT IN LINE WITH THE GFS. 12Z NOGAPS
AND CMC GLOBAL HAVE ALSO SHIFTED TO THE LEFT CLOSER TO PR AND THE
NEW 12Z ECWMF ALTHOUGH IS STILL SHOWS RECURVATURE HAS TRENDED TO
LEFT TOO. EVEN THOUGH THE MODELS HAVE BEEN CONSISTENT ON A TRACK
CLOSE TO PR...USERS ARE REMINDED THAT TRACK FCSTS SIX DAYS OUT
ARE SUBJECT TO LARGE ERRORS. TYPICAL ERRORS THIS FAR OUT ARE IN
THE ORDER OF 350 NM. UNTIL THIS SYSTEM GETS A LOW LVL CENTER WELL
ESTABLISHED DO NOT EXPECT THE MODELS TO GET A GOOD HANDLE ON IT.

RESIDENTS OF PR AND THE USVI ARE ENCOURAGED TO MONITOR LATEST
FORECASTS FROM THE NATIONAL HURRICANE CENTER OVER THE WEEKEND AND
GO OVER THEIR HURRICANE DISASTER PLANS IN CASE THE THREAT FROM
THESE TWO TROPICAL SYSTEMS BECOMES MORE REAL.

They also mention the left shifts...but this could change alot when it becomes more organized...sooooooooooooooo...*sighs*
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Based on the models here is a general idea of where the storm may be heading.

CMC, GFS, GFDL and HWRF are clustered between 16N and 19N on day 5. The median would be 17.5N which would bring the storm near Antigua on August 20th. That would potentially set up a Puerto Rico impact on the 21st.

GFDL & HWRF are showing a CAT 3 by the time it reaches the islands.

The centerline seems to be about where the GFS predicts the storm to go. I have basically superimposed the GFS track on top of the Cone of Uncertainty, about the middle of the road.

This could potentially be a Florida storm, and is seems nearly certain Ana will travel through "Herberts Box".



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Quoting DDR:

Hey 456
I know The islands could use rain but hopefully not like that.
Did you get alot of rain from dean?


Yea, enough to cause flash floods. Omar of last year was more of a wind event. So basically this maybe the 3rd year in a row if indirect impacts.
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1902. Patrap
90L Has Disappeared from the NOAA RAAMB Page
Member Since: July 3, 2005 Posts: 424 Comments: 128261
according to this Link convection is getting better around the COC
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Quoting IKE:
That GFDL 18Z is showing the COC at near 16N and 63W in 5 days...that's 150-200 miles southeast of PR.

don't think that is the 18z run.

look on the fsu site at the 12z run and it has it in the same exact spot.

same with the HWRF that he just posted.

the 18z runs have not come out on those models yet. it normally takes a while after the GFS completes.
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to answer all questions in 1

The intensity models are in such agreement, and the forecasted conditions ahead, 90L can easily attain hurricane status. As for major hurricane status, not sure at this point.

Those two images are the 12Z GFDL and HWRF models out 126 hrs. The tracks differ slightly with the HWRF slower and a bit north. This is the trend I'm seeing. slower=more pull north, faster=more westward motion. Same as the ECMWF.
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Look Out! There now appear to be some small spiraling vorticies that can be seen on visible satellite from just north of Cayman Islands to another one just off the northern central coast of Cuba within this large area of disturbed weather shaded in yellow. ....hmmmm, maybe somthing startin' up.
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Quoting Tracker09:
90 l is getting better organized!!1 this is gonna be a big one, i haave a bad feeling in my stomach!!!!! ewww i feel bad for s fl! since all of the ensemble models take it there, well most of them
Been watching these things for many years.Something about this one(90L) is very scary.
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1895. Grothar
TO: SavannahStorm

I did enjoy the view. Thank you. Where do you find these sites? I have seen you post others and never seem to find them. It is a spectacular sight. Do you have information on how strong the building high pressure is going to be; or how far down it is projected?
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Well crap those models make me want to review my hurricane plan just in case.
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how strong is the trouf suppose to be next week
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Quoting WeatherStudent:


I'm floored, you shouldn't have.


You do have a point. I just wish he would start using his schooling and give us some synops-not to laugh at but the more informed or knowledgeable mets on here could help out instead of bashing.The gangster look is out of our control though.
More power to ya WS
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Quoting IKE:


I have a feeling the cloud tops are going to explode...maybe not tonight, but sometime this weekend.



Once it gets over those 28C waters I could easily see -80C.
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1889. DDR
Quoting Weather456:
5 Days


Hey 456
I know The islands could use rain but hopefully not like that.
Did you get alot of rain from dean?
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Quoting IKE:
That GFDL 18Z is showing the COC at near 16N and 63W in 5 days...that's 150-200 miles southeast of PR.


If that is the case it would really have to turn to hit the east coast of FL.
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1886. IKE
Quoting CaneWarning:


That would make me think its going into the gulf.


Same here.
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Quoting Vortex95:
1852. may I ask why the circle 456?


The center is located somewhere in that circle. Dont pay attention to it though, that's how I normall decode those microwave images
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Quoting Weather456:
5 days HWRF - they differ in track but agree that 90L will be something to watch



WTF?? I see 948 mb and that's not even close to the center! That's freakin crazy.
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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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