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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Quoting Drakoen:
Looks like it's going to be a close call...


close call for what?
Quoting HurricaneSwirl:


would that mean the upper level trough is expected to move out or dissipate?


I will have to go with the model consensus and the shear pattern over the last 24 hrs. there is a ridge building from Nother SA and that should lift the trough northward.

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Looks like it's going to be a close call...
Member Since: October 28, 2006 Posts: 57 Comments: 29714
But it did make a HUGE comeback form last night. Ill give it that.

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Quoting Weather456:
Yep favorable conditions in the Eastern Caribbean during the next 48 hrs



would that mean the upper level trough is expected to move out or dissipate?
Member Since: July 7, 2008 Posts: 2 Comments: 3237
59. JRRP
Link
O o
this ball of convection is impressive
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Welcome back Invest 97L.
Member Since: May 16, 2009 Posts: 0 Comments: 684
Yep favorable conditions in the Eastern Caribbean during the next 48 hrs

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97l has some good upper level divergence/but only modest low level convergence.I do expect shear to weaken in the caribean/by the time it reaches it.The gfs has not been the bes tmodel this year/i think one of the worst to use.
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Thanks Dr. Good afternoon everybody. 97L never gone. It's a fighter. We finally can have Ana in the next 24 hrs. Persistance is the key.
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morning everyone, well here we go guys...this place is going to blow up big time...be aware of the trolls so not to get banned...
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53. JRRP
Quoting weatherwatcher12:

I'm confused. There is no dead one in the Caribbean.

ok
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Quoting sporteguy03:


Gams, 97L hit some favorable conditions an axis of the wave remains only convection goes away if the axis can hit a sweet spot you have what we see now a developing system.


Thank you so much Sporteguy!

I wasn't paying attention last night so didn't realize what had happened!

Guess I will be checking in here off and on this afternoon.

So glad Dr Masters did his update already.
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Quoting Drakoen:
If the NHC gives it a yellow that would repudiate anything the system has done over the past few hours.


wouldn't be surprised LOL
Member Since: July 7, 2008 Posts: 2 Comments: 3237
50. JRRP
Quoting WAHA:
okay, i will give a poll of 97L.

A. td by this evening
B. td by tomorrow
C. td after tommorow
D. no td at all

If td forms,

1. ana by tomorrow
2. ana by tuesday
3. ana after tuesday
4. no ana at all

d
4
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97L was that really organized wave off Africa a few days ago. Interesting to see what its doing.
We're going to see Ana eventually.
NHC moved floater to 97L.
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If the NHC gives it a yellow that would repudiate anything the system has done over the past few hours.
Member Since: October 28, 2006 Posts: 57 Comments: 29714
Quoting Stormchaser2007:
Large decrease in shear...I doubt that it'll lower enough for 97L to reach tropical storm strength.


It is 900 miles away from the island's. IMO I think that it has enough time
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Quoting Stormchaser2007:
Large decrease in shear...I doubt that it'll lower enough for 97L to reach tropical storm strength.



I believe this could become Ana if the current trend continues. The trough is going to be a wildcard it would appear.
Member Since: August 2, 2007 Posts: 19 Comments: 19234

Another question here I thought the MJO was in downward motion in this area any factor with that at all??
Member Since: July 7, 2005 Posts: 0 Comments: 5120
Quoting Stormchaser2007:
Large decrease in shear...I doubt that it'll lower enough for 97L to reach tropical storm strength.



Thats indeed correct as mentioned previously upper conditions look hostile ahead of this system but i think theres a possibility it reaches depression status before it runs into those screaming westerlies.
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Quoting WAHA:
okay, i will give a poll of 97L.

A. td by this evening
B. td by tomorrow
C. td after tommorow
D. no td at all

If td forms,

1. ana by tomorrow
2. ana by tuesday
3. ana after tuesday
4. no ana at all
Quoting WAHA:
okay, i will give a poll of 97L.

A. td by this evening
B. td by tomorrow
C. td after tommorow
D. no td at all

If td forms,

1. ana by tomorrow
2. ana by tuesday
3. ana after tuesday
4. no ana at all


the only possible scenarios i can make out of that is

D4
B1
B4

i think ill go with B4.
Member Since: July 7, 2008 Posts: 2 Comments: 3237
Quoting seflagamma:
WHAT???

Thanks for the update Dr Masters!


What is going on here.. last night I thought you all "RIP" and wrote off 97L, I thought it was down graded ????? I thought that blob nearest the islands was what we are watching?


Now 97L is reborn and could develope in to a TD?


Just goes to show you, never count any system totally out until it is totally out.

Good morning everyone!


Gams, 97L hit some favorable conditions an axis of the wave remains only convection goes away if the axis can hit a sweet spot you have what we see now a developing system.
Member Since: July 7, 2005 Posts: 0 Comments: 5120
nice spinning in the low levels, possibly at the surface

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Quoting WAHA:
okay, i will give a poll of 97L.

A. td by this evening
B. td by tomorrow
C. td after tommorow
D. no td at all

If td forms,

1. ana by tomorrow
2. ana by tuesday
3. ana after tuesday
4. no ana at all


do the poll a bit later lol, my poll is still going on lol. it just confuses people, I didnt start the poll so 4 other polls could be created lol
Models have been really, really, off this year unlike last year. Especially the GFS.
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Large decrease in shear...I doubt that it'll lower enough for 97L to reach tropical storm strength.

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Quoting JRRP:
where is the dead zone??

I'm confused. There is no dead one in the Caribbean.
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Thanks Dr......So folks, "if" 97L is able to reach depression or TS status before it hits some sheer, what are the chances of the TUTT retreating, or, formation of an anti-cyclone to lessen the sheer?
Member Since: August 8, 2006 Posts: 0 Comments: 8269
Quoting BurnedAfterPosting:
That being said, shear in this kind of scenerio is extremely hard to predict

I have seen Dr M many times have to change his tune on shear from one day to the next.


The NHC did it between 2am and 8 am this morning.
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34. WAHA
okay, i will give a poll of 97L.

A. td by this evening
B. td by tomorrow
C. td after tommorow
D. no td at all

If td forms,

1. ana by tomorrow
2. ana by tuesday
3. ana after tuesday
4. no ana at all
Member Since: Posts: Comments:
Its getting better organized convection wise with some outflow but obs out there only show a wind shift nothing closed.As of yet i see nothing on the TCPOD.
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Quoting sporteguy03:
What is more interesting according to Dr.M's post is that none of the models are developing this?? We have seen that before right??


I have heard there is an old adage that says if all the models develop a system or none of them do, look out

Quoting Drakoen:
Jeff Masters goes with B LOL!
Yeap... he joined the voting so I guess those who voted A like me might be in trouble.
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Hopefully we can get some microwave images out soon. Really wanna know what going on at the surface.
Member Since: October 28, 2006 Posts: 57 Comments: 29714
Well if you use the gfs,the shear will not be to favorable i agree,but i am gona use the hwrf and gfdl once they come out.I don't like using the gfs and nam.
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Dr. M...thank you for taking time to update your blog today..enjoy your Sunday.
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27. JRRP
where is the dead zone??
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What is more interesting according to Dr.M's post is that none of the models are developing this?? We have seen that before right??
Member Since: July 7, 2005 Posts: 0 Comments: 5120
That being said, shear in this kind of scenerio is extremely hard to predict

I have seen Dr M many times have to change his tune on shear from one day to the next.
Someone called it last night. Hmmm.. who was it (think, think, think)
Ahh yes! Kudos to reedzone Post #705 5:50 PM EDT, yesterday.
Quoting reedzone:
Well 97L still has very good structure with one bubble of convection that is very slowly growing. Like I said, I'm giving it 14 more hours (basically till Sunday morning) to regenerate. It's moving into a moist environment and shear is decreasing where it's tracking.
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WHAT???

Thanks for the update Dr Masters!


What is going on here.. last night I thought you all "RIP" and wrote off 97L, I thought it was down graded ????? I thought that blob nearest the islands was what we are watching?


Now 97L is reborn and could develope in to a TD?


Just goes to show you, never count any system totally out until it is totally out.

Good morning everyone!
Member Since: Posts: Comments:
Quoting BurnedAfterPosting:


yea but he feels shear will not be favorable after tomorrow, so if it gets to a named storm it better do it quick

Well. You know how these things fluctuate. Maybe the forecast will change.
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Like I said earlier.... Eastern Carrib is usually a moderately hostile place for weak or developing systems.
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Thanks Dr. Masters
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18. IKE
Quoting Drakoen:
Jeff Masters goes with B LOL!


I noticed that too.

This could be a real problem if the predicted shear doesn't hang around.

A voice of reason...thanks doc.

Take that Dr. Lyons and TWC.
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Quoting Weather456:
Well dr. Masters cleared up everything. Now seems we may actually have orange code by 2pm.


yea but he feels shear will not be favorable after tomorrow, so if it gets to a named storm it better do it quick
Quoting BurnedAfterPosting:
Dr M says shear will not be favorable though


Right. The GFS shear forecast has come into the marginal/unfavorable range. Still time to watch before it gets into that region.
Member Since: October 28, 2006 Posts: 57 Comments: 29714
Dr.Masters,
Thank you for the lunch-time update enjoy your Sunday.
Member Since: July 7, 2005 Posts: 0 Comments: 5120

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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.