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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1634. jipmg
Quoting JLPR:


huh?

Very windy; overcast with a couple of thunderstorms around. Winds from the SE at 44 mph.


oh I thought I had read 80 knots, its actually 80 kilometers
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Quoting Patrap:

Patrap..do you know how many (if any) dropsondes the NOAA G IV will launch?
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1631. JLPR
Quoting jipmg:


OMG they are forecasting hurricane force winds for Thursday XD


huh?

Very windy; overcast with a couple of thunderstorms around. Winds from the SE at 44 mph.
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the i have read all year so far

000
ABNT20 KNHC 141737
TWOAT
TROPICAL WEATHER OUTLOOK
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
200 PM EDT FRI AUG 14 2009

FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO...

SHOWER ACTIVITY HAS BECOME A LITTLE BETTER ORGANIZED IN ASSOCIATION
WITH THE REMNANT LOW OF FORMER TROPICAL DEPRESSION TWO...LOCATED
ABOUT 1225 MILES EAST OF THE LESSER ANTILLES. WHILE UPPER-LEVEL
CONDITIONS ARE ONLY MARGINALLY FAVORABLE FOR DEVELOPMENT...
REGENERATION OF THIS SYSTEM INTO A TROPICAL CYCLONE IS POSSIBLE AS
IT MOVES WESTWARD AT ABOUT 15 MPH OVER THE NEXT COUPLE OF DAYS.
THERE IS A MEDIUM CHANCE...30 TO 50 PERCENT...OF THIS SYSTEM
BECOMING A TROPICAL CYCLONE DURING THE NEXT 48 HOURS.

SHOWER AND THUNDERSTORM ACTIVITY ASSOCIATED A BROAD AREA OF LOW
PRESSURE CENTERED ABOUT 250 MILES WEST-SOUTHWEST OF THE SOUTHERN
CAPE VERDE ISLANDS HAS BECOME A LITTLE LESS ORGANIZED OVER THE PAST
SEVERAL HOURS. HOWEVER...CONDITIONS STILL APPEAR FAVORABLE FOR
FURTHER DEVELOPMENT OF THIS SYSTEM...AND A TROPICAL DEPRESSION
COULD FORM DURING THE NEXT DAY OR TWO AS IT MOVES WESTWARD AT NEAR
15 MPH. THERE IS A HIGH CHANCE...GREATER THAN 50 PERCENT...OF THIS
SYSTEM BECOMING A TROPICAL CYCLONE DURING THE NEXT 48 HOURS.

A TROPICAL WAVE INTERACTING WITH AN UPPER-LEVEL LOW IS PRODUCING AN
AREA OF CLOUDINESS AND SHOWERS FROM JAMAICA AND EASTERN CUBA
NORTHWARD INTO THE BAHAMAS. THIS ACTIVITY IS EXPECTED TO SPREAD
WEST OR WEST-NORTHWESTWARD DURING THE NEXT COUPLE OF DAYS. THERE
ARE NO SIGNS OF ORGANIZATION AND UPPER-LEVEL WINDS ARE NOT
FAVORABLE FOR SIGNIFICANT DEVELOPMENT OF THIS SYSTEM. THERE IS A
LOW CHANCE...LESS THAN 30 PERCENT...OF THIS SYSTEM BECOMING A
TROPICAL CYCLONE DURING THE NEXT 48 HOURS.

SHOWERS AND THUNDERSTORMS OVER THE NORTHEASTERN GULF OF MEXICO ARE
ASSOCIATED WITH A WEAK TROUGH OF LOW PRESSURE. SURFACE PRESSURES
ARE HIGH IN THIS AREA AND THERE ARE NO SIGNS OF ORGANIZATION.
THERE IS A LOW CHANCE...LESS THAN 30 PERCENT...OF THIS SYSTEM
BECOMING A TROPICAL CYCLONE DURING THE NEXT 48 HOURS.

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Quoting ClearwaterSteve:
Kman How are ya. Been almost a year. LOL. How is it in Paradise?

Thoughts on the old td2?


Hi there. Paradise has been hot and dry LOL. Well below average rainfall this summer which is our rainy season and heat indices of 115F many days.

I will stick with my take on Ex TD2 from last night which was that it would not regenerate and enter the Caribben as either a TD or worse. Having said that, it is doing the best it can to serve me some crow LOL.

Anyway it makes for interesting blogging. We will see how it plays out but if it does regenerate where it goes will depend on how far W it gets before reorganizing and strengthening, assuming both happen.

No point speculating with all the variables out there now.
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1628. Drakoen
GFS 18z shows 90L continuing to be lopsided until Saturday afternoon.
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1627. Patrap
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Good afternoon, I can help but continue to notice extraordinary similarities between 90L and Dean 2007.
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Quoting kmanislander:


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.



One things for sure... The southerlies are close to TD2's center. The link still show easterlies at the upper levels right over the circulation with southerlies just to the north. As you say, for a small system the shear being this close is dicey.

Have a good one Kman...I'm sure we'll get plenty of opportunities to discuss next week!

Link
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The ensemble is trending east. This morning it was from the Yucatan to Nova Scotia.
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1623. jipmg
Quoting JRRP:
accuweather is forecasting TS conditions for PR
Link


OMG they are forecasting hurricane force winds for Thursday XD
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Quoting JRRP:
accuweather is forecasting TS conditions for PR
Link


Are they forecasting hurricane conditions for NYC next Thursday, yet? I know Bastardi is going nuts.
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Quoting Patrap:
extreme still laughing about those models the spaghetti noodles no brains...i think they are batting this year 0 for 60 if im not mistaken lol...DO YOURSELF A FAVOR STOP LISTENING TO THE MODELS AND DO YOUR OWN HOMEWORK..Stormno

Not to be a prude Kerry,but didnt you and yer,er,..office forecast a Great weekend yesterday for the Gulf Coast and for us not to worry over the Lil Wave entering from the east?

Not be retentive,but I have that quote,can you and the Boys clarify..?

OMG!!! you crack me up Pat...
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Quoting WeatherStudent:


Evening Taz.



hello
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1618. JRRP
accuweather is forecasting TS conditions for PR
Link
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1617. Patrap
extreme still laughing about those models the spaghetti noodles no brains...i think they are batting this year 0 for 60 if im not mistaken lol...DO YOURSELF A FAVOR STOP LISTENING TO THE MODELS AND DO YOUR OWN HOMEWORK..Stormno

Not to be a prude Kerry,but didnt you and yer,er,..office forecast a Great weekend yesterday for the Gulf Coast and for us not to worry over the Lil Wave entering from the east?

Not be retentive,but I have that quote,can you and the Boys clarify..?

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Quoting jipmg:


I think he is talking about the bit of convection that blew up on the north side of the circulation just to the east of the ball of thunderstorms


OK, when he said "northwest of the exposed center" I thought he meant that.
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im out for now, ill be back later.

have a good evening!

-CanesRule1.
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It's called moisture shield Stormno, also the storm will be protected by an anticyclone so it can't get sheared. The system is soo large, it should create a shield of moisture to protect it from any dry air suffocating it. Past storms like this one have been known to do this.. Does someone have an image of Pre-Dean, a water vapor image please...
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Kman How are ya. Been almost a year. LOL. How is it in Paradise?

Thoughts on the old td2?
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Quoting KATRINABILOXIGIRL:
hope so for you too...if so enjoy, enjoy, enjoy...pre season or not a Saints game is a Saints game....
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1609. jipmg
Quoting SavannahStorm:


"Small" is a relative term. It is small compared to the entire circulation of 90L, but that ball of convection is bigger than TD2 ever was.


I think he is talking about the bit of convection that blew up on the north side of the circulation just to the east of the ball of thunderstorms
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Quoting jipmg:


yes I have to disagree with Stormno as well... there is alot of moisture assosiated with 90L, and the system is large in size to go down so quickly, I think once it gets some convection blowing up over the center it will get going
trust me once this gets going it will rapidly intensify, as models are predicting.
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Quoting Patrap:
Hiya Tazaroo,good to see yas dude



hi pat
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Quoting Patrap:
Im hoping my Neighbor gives me His Tickets tonight Biloxi,..as he dosent usually go to the Pre-Season Games.


Seems the Atlantic pre-season is just about over as well,..Wink,wink
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Time to head out of here for the evening...it is Friday night and all... Hope everyone has a fun night whatever you get into!

Final Thoughts:

Deep layer easterly flow is fairly well established south of 20N and should provide low shear to former TD2 over the coming days. The faster forward speed sometimes makes it difficult for storms to intensify, but the warmer waters/higher ocean heat content should help later tomorrow into Sunday, and I think TD2 will become a tropical storm in the next 48 hours (probably later in that period)


90L is a massive beast that has had trouble today with very strong easterly/northeasterly shear and some dryer air being entrained by its expansive circulation from the northeast. When you pull air from as far away as it does, some is destined to be dryer/more stable.

For what it's worth, the upper level easterlies are supposed to subside a bit (and the shear tendency currently is starting to show this). I think that 12 hours of low shear would be all it takes for this powerful circulation to create a storm. I could see this happening as soon as late tonight or sometime tomorrow during the day.

Long term tracks for both systems put them in position to affect land, and they will need to be carefully watched over the coming days. The tight clustering of models for 90L suggest a track to near Puerto Rico as highly likely, with tons of possible tracks after that point. TD2 will likely head W or WNW north of Puerto Rico, and then possibly toward the Bahamas. Needless to say, the oceans are warm in this area and shear is forecast to be low...so if TD2 can remain a factor over the next 36 hours it will be a threat.

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Quoting jasoniscoolman10:
SHIPS 18Z model for 02L keeps shear below 15 knots for the next 5 days, most of the time below 10 knots not good at all. with water temp going up and up by 45 west to 75 west.
precisely, could result into rapid intensification.
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this hit ! and he will go a way



and move on
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Quoting reedzone:
A small burst of convection starting to bubble to the northwest of the exposed center.



"Small" is a relative term. It is small compared to the entire circulation of 90L, but that ball of convection is bigger than TD2 ever was.
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1601. Patrap
Hiya Tazaroo,good to see yas dude
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Quoting Drakoen:


The trough is forecasted to retrograde to the west and then north with favorable conditions in 24-48 hours.


That's a long time to wait for a struggling low but this one seems determined to become something.
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for some reason, I'm a little concerned about this little wave/Depression/Tropical Storm that has its eyes on the Gulf Coast Area...

ALL EYES in the TX/LA/MS/AL area should be on the gulf through the day tomorrow...
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1597. Patrap
Im hoping my Neighbor gives me His Tickets tonight Biloxi,..as he dosent usually go to the Pre-Season Games.


Seems the Atlantic pre-season is just about over as well,..Wink,wink
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the downcasters are out today i see i think will be seeing a lot more of them in the comeing days
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1595. jipmg
Quoting canesrule1:
it would not have a red circle if that was the case.


yes I have to disagree with Stormno as well... there is alot of moisture assosiated with 90L, and the system is large in size to go down so quickly, I think once it gets some convection blowing up over the center it will get going
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Quoting BatTracker:
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.
not calling u crazy, cuz no one knows where its going just yet, lol, keep on waiting.
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1591. Drakoen
Quoting kmanislander:


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.



The trough is forecasted to retrograde to the west and then north with favorable conditions in 24-48 hours.
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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
it would not have a red circle if that was the case.
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This is the beach web cam from cape Verde so it's definitely night over 90L:

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Quoting extreme236:


Well welcome back STORMTOP. Glad to see the toilet model is still up and running.

LOL.
I wonder if it was flushed this time... helps to clear the result.
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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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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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