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 KYhomeboy:
Convection beginning to refire near the center. I don't understand why people jump on the "it's dead" bandwagon the second heavy convection subsides. Developing systems typically do through day to day phases/ pulses like this. The shear will be this invests biggest enemy for now.


I have seen systems have the top's sheared completely off, leaving only a surface circulation. This can then fire up again when shear subsides.

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Thats the main issue many seem not to understand without a well defind surface circulation theres nothing to support deep thunderstorm activity.
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Quoting KYhomeboy:
Convection beginning to refire near the center. I don't understand why people jump on the "it's dead" bandwagon the second heavy convection subsides. Developing systems typically do through day to day phases/ pulses like this. The shear will be this invests biggest enemy for now.


I believe that's Taz's job.. It might regenerate enough before it hits shear.
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1362. Patrap
Early-cycle track guidance AL97

Member Since: July 3, 2005 Posts: 426 Comments: 129093
Quoting missmaxi:
Does anyone think that this might come towards Miami?


we dont even know if it will survive the night
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Does anyone think that this might come towards Miami?
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Quoting FloridaTigers:
Koritheman, you're never on AIM :\


Are you Zappa?
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Quoting StormW:
I'm not to fond of the 12Z run.

PSU e-Wall STEERING LAYERS FORECAST



allrighty, i have been looking for the shear map and you posted it! thanks!
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.
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I don't think anyone is saying that we're going to have Ana in the next 12-24 hours, but 97L isn't going away anytime soon either. The little shear in the immediate area to speak of won't kill it. The approaching winds from the trough won't be as prominent either if the trough continues to lift out. The westerly flow is still very visible and dominant which is where 97L is and trying to get a good llc going. The easterly flow from the trough is not as strong as it was, dissipating and is lifting - it won't kill it. 97L, if it stays on a westerly course, as all steering suggests, it'll come in under most of the resistance from the trough anyway. It may not get to TD status until it gets past the islands, but writing it off - at this time isn't going to happen.
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Not surprised...looking at cloud tops, still a strong westerly component that the invest is moving towards. It has not even come close to hitting the shear region yet and it is just starting to get closer. Once it nears the Lesser Antillies the storm goes away but I think it has caused chaos in the region opening hte door for future storms.
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1353. Patrap

Floater 1 - RGB Color Infrared Loop
Member Since: July 3, 2005 Posts: 426 Comments: 129093
Convection beginning to refire near the center. I don't understand why people jump on the "it's dead" bandwagon the second heavy convection subsides. Developing systems typically do through day to day phases/ pulses like this. The shear will be this invests biggest enemy for now.
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Quoting Patrap:
Negative..I didnt save any in my P-bucket


Thanks anyway!
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Quoting Drakoen:
NHC is going to keep development chances at <30%


As they should.
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Quoting reedzone:


Look on the shear map near Costa Rica.. Notice the arrows circulating in a clockwise fashion, that is an anticyclonic flow, and I think the reason why models forecasted low shear was maybe they noticed the Anticyclone.


interesting. just more factors in the possible development of 97L.. very complex situation. im just gonna lay back and see what it does. thanks for the help :)
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1348. Patrap
Negative..I didnt save any in my P-bucket
Member Since: July 3, 2005 Posts: 426 Comments: 129093
The HWRF and GFDL dissipate the storm less then 1 day... hmmm.
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Koritheman, you're never on AIM :\
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1345. Drakoen
I give it only a 30% chance of surviving. In favor of the Upper trough ripping it limb from limb
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KOG - always the smart guy!
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Can someone tell me what the source is of these magically shear maps? Are they just model data or are they based off of soundings lauched over water? If thye are just model data or estimated shear maps then why are we all so quick to think shear is def going to decrease?
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Hi Pat:

Can you take either one of those images & post one from yesterday at the same time?
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1341. KEEPEROFTHEGATE (Mod)
229

WHXX04 KWBC 192322

CHGQLM

ATTENTION...NATIONAL HURRICANE CENTER



NCEP COUPLED GFDL HURRICANE MODEL FORECAST MADE FOR



TROPICAL DEPRESSION INVEST 97L



INITIAL TIME 18Z JUL 19



DISCLAIMER ... THIS INFORMATION IS PROVIDED AS GUIDANCE. IT

REQUIRES INTERPRETATION BY HURRICANE SPECIALISTS AND SHOULD

NOT BE CONSIDERED AS A FINAL PRODUCT. PLEASE SEE THE TPC/NHC

OFFICIAL FORECAST.





FORECAST STORM POSITION



HOUR LATITUDE LONGITUDE HEADING/SPEED(KT)



0 11.9 52.2 270./21.0



STORM DISSIPATED AT 0 HRS AT THE ABOVE PSN


here it is a second time just in case you don't see it the first
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Quoting stormdude77:
the 18Z GFDL dissipates the system after 0 hours (immediately)....


well its been 0 ours and its still alive.. XD

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1339. Patrap
Vis AL97


AVN
Member Since: July 3, 2005 Posts: 426 Comments: 129093
Quoting stormdude77:
the 18Z GFDL dissipates the system after 0 hours (immediately)....

The GFDL is really that reliable until they have a LLC. Isn't it?
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The wave is about to get blasted during D-Min. Watch the WV loop and the reaction of the outflow clouds to the NNW of the surface low.

In my opinion, it's survival is in question if it is inable to fire up convection before that shear is over the bulk of the storm.

Wait and see! This should be interesting!
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ATTENTION...NATIONAL HURRICANE CENTER

NCEP COUPLED GFDL HURRICANE MODEL FORECAST MADE FOR

TROPICAL DEPRESSION INVEST 97L

INITIAL TIME 18Z JUL 19

DISCLAIMER ... THIS INFORMATION IS PROVIDED AS GUIDANCE. IT
REQUIRES INTERPRETATION BY HURRICANE SPECIALISTS AND SHOULD
NOT BE CONSIDERED AS A FINAL PRODUCT. PLEASE SEE THE TPC/NHC
OFFICIAL FORECAST.


FORECAST STORM POSITION

HOUR LATITUDE LONGITUDE HEADING/SPEED(KT)

0 11.9 52.2 270./21.0

STORM DISSIPATED AT 0 HRS AT THE ABOVE PSN.

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I don't see that 97L has lost any of its structure, just the thunderstorm activity to a degree. That's what is expected during this time of the evening.

Am I missing something?
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the 18Z GFDL dissipates the system after 0 hours (immediately)....
Quoting HurricaneSwirl:


Hmmm, that's pretty sweet deal for the cyclone lol. when is it predicted to form?


Look on the shear map near Costa Rica.. Notice the arrows circulating in a clockwise fashion, that is an anticyclonic flow, and I think the reason why models forecasted low shear was maybe they noticed the Anticyclone.
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nope StormW, don't like those runs at all...throw those out and lets start over...
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1331. Drakoen
NHC is going to keep development chances at <30%
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that system by the CV islands is really far south, I estimate it is around 7N
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Quoting WaterWitch11:


where is this map from?

The Satellite Services decision's. Tropical Formation branch
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It appears according to what is going on to the west of the main burst of convection, that it might be trying to wrap.
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Quoting weatherwatcher12:
A new AOI by the Cape Verde's?


where is this map from?
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1313. reedzone 11:19 PM GMT on July 19, 2009
Quoting Nolehead:
everyone wrote 97l off last night and WALLA today..so lets see what it does tonight and wait to see what the new models are saying..before you bash the daylights out of it...


Ahhem... not everyone ;)


oh i was with ya on it too..let's just sit back and wait...
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Quoting reedzone:


Anticyclones are basically shields for tropical systems.


Hmmm, that's pretty sweet deal for the cyclone lol. when is it predicted to form?
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Quoting BurnedAfterPosting:


LMAO you caught that huh? No ignorance means not admitting something that is right in front of you

shear maps and the visible clearly shows that there is shear in the immediate vicinity of 97L, yet he claims over and over there is no shear

that is being ignorant, it is taken out of context as being an insult


I have a question...while I see plenty of shear from avil blow off, how do you know the shear maps are accurate? Do they launch ballons over the water? Do they cover every 10 square miles? What is the process? Seems like a lot of estimation to me.
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97L really needs a good DMAX to get going again....
Isn't the shear map everyone is looking at the 200-850 mb one? I am not sure that this system even has cloud tops through 200 mb. Whatever wind speed difference occurs at that height won't tear up the circulation unless the clouds actually reach that height. DMIN may well have saved it from this small pocket of shear that has sprung up just to its west, and odds are that the shear pocket is a random temporary thing that will lift out with the rest of the trough. The atmosphere is chaotic like that.

The mid-level shear map might -- for now -- be the one to actually look at in terms of it affecting 97L. It goes from 850 to 500 mb.

I think that if 97L DOES bomb out tonight and then it hits whatever high shear may be there, it would have a greater impact on it than if it had not bombed out. Of course, if the TUTT lifts out quickly as some models suggest, that's a moot point.
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Quoting KoritheMan:


For days, I've been saying that 97L isn't going to develop until at least 75W. The GFS and NOGAPS continue to forecast a dramatic decrease in shear in that general area over the next few days.

I'm obviously no expert, and the NHC is definitely smarter, this is just my opinion from my experience and observations.

I'm fully prepared to be wrong, however, if the system unexpectedly develops prior to approaching the Windwards.


I am in the camp you are, development around 75W, SSE of Jamaica
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Quoting victoria780:
It will be like hitting a brick wall,like what happened to the other disturbance as it entered the lesser antiles..


Yes, exactly.
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Storm: if you are still around. Any ideas why the models didn't pick this up?

It also shows that the models are not perfect, because there is obviously a tropical event occurring, regardless of whether it ever becomes a full blown tropical storm.
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Quoting BurnedAfterPosting:
I still think we will eventually get Ana out of 97L, but I think it will be in the long term and not the short term

this situation with the shear is very fickle and it could go either way, right now though it looks like shear will prevent 97L from organizing further in the short term

of course I could be wrong, we will see


For days, I've been saying that 97L isn't going to develop until at least 75W. The GFS and NOGAPS continue to forecast a dramatic decrease in shear in that general area over the next few days.

I'm obviously no expert, and the NHC is definitely smarter, this is just my opinion from my experience and observations.

I'm fully prepared to be wrong, however, if the system unexpectedly develops prior to approaching the Windwards.
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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.