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 Orcasystems:


Unless I am doing my math wrong.... its 0218Z
So the OO would have been over 2 hours ago


Correct Orca...But most of the global models do not finish processing data and producing output until 6-9 hrs later.
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Quoting DestinJeff:
it's shear insanity in here tonight ...


ROFLMAO
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Quoting Orcasystems:


Unless I am doing my math wrong.... its 0218Z
So the OO would have been over 2 hours ago


Models take a while to run that's why there's lag. The early cycle models like the bams are probably already out.
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Westerly shear is more harmful than easterly shear.
Member Since: August 2, 2007 Posts: 19 Comments: 19234
Looks like rain chances every day for next 7 days here in Eastern NC. Is anything still showing a chance of something forming off the coast from one of these areas of low pressure?
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Quoting HurricaneKing:


Doesn't westerly shear come from the west? Easterly shear comes from the east? Like a west wind is from the west and an east wind is from the east.

So a storm going west would run straight into westerly shear.


Yeah, that was my point. 97L is running into westerly shear.
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Quoting HurricaneKing:


That's from early this morning. The latest model runs were the 18z. The new 00z is about to come out in a couple hours.

00z=late night/early morning
06z=morning
12z=afternoon
18z=late evening


Unless I am doing my math wrong.... its 0218Z
So the OO would have been over 2 hours ago
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1757. beell
Probably required reading for some. Some simple math included so you can calculate your own 200mb-850mb shear!

20 knots from the west at 200mb and 10 knots from the east at 850mb equals 30 knots of shear.

20 knots from the east at 200mb and 10 knots also from the east equals 10 knots of shear.

Wind Shear Tutorial-J. Masters
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Good night gents and ladies....how do you see the development of 97L? Enlight me...
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Quoting KoritheMan:


That is not what I heard, but I could have misinterpreted. Doubtful, though.

After all, why does westerly shear often destroy westward-moving tropical waves, and yet they can survive easterly shear?


Doesn't westerly shear come from the west? Easterly shear comes from the east? Like a west wind is from the west and an east wind is from the east.

So a storm going west would run straight into westerly shear.
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1754. KEEPEROFTHEGATE (Mod)
welcome back taz you missed all the fun at one point around 1 pm today we got pretty close to a T.C.F.A ony 1 away but as normal it waned away but the last hr is showing signs of some convective rebuilding whats to happen remains to be seen
Member Since: July 15, 2006 Posts: 169 Comments: 53296
Quoting LPStormspotter:


Good point. Thanks. So what is your thoughts for 97L?


It won't develop until 75W, when shear becomes more favorable.
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Quoting weatherwatcher12:

No it's 0z valid Monday 20 July
Link


That Valid Time is a little confusing...That is the time that the forecast is for...matter of fact that image doesn't even show what time the model was run.
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Quoting StormJunkie:
KM, I always thought that if the shear and the system were moving in the same direction it was more beneficial to the system? Storm moving 15 shear blowing 30 = 15knts of shear?


That is not what I heard, but I could have misinterpreted. Doubtful, though.

After all, why does westerly shear often destroy westward-moving tropical waves, and yet they can survive easterly shear?
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Approximately, when will the DMIN end for 97L?
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Quoting weatherwatcher12:

No it's 0z valid Monday 20 July
Link


no that is the 12Z from earlier today, it is even in the link
Quoting tropicfreak:


I never knew that could happen. Thanks for the educational info.


And in case it sounds like I'm making this up... From the NHC's report on Omar:

One can speculate that the impact of the vertical shear during the second rapid intensification period was mitigated by Omar's motion being in the same direction as the shear vector.
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Look like 97L is getting weak, nearly discipated. Strong winds from South America...
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Quoting weatherwatcher12:

No it's 0z valid Monday 20 July
Link


That's from early this morning. The latest model runs were the 18z. The new 00z is about to come out in a couple hours.

00z=late night/early morning
06z=morning
12z=afternoon
18z=late evening
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Quoting futuremet:

Mid level shear

you need 200-850mb

Oops. Guess I was wrong. I will search for the correct one.
Member Since: May 16, 2009 Posts: 1 Comments: 1231
Haha, Welcome back Taz...
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Quoting weatherwatcher12:

No it's 0z valid Monday 20 July
Link

Mid level shear

you need 200-850mb
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hell all my 24hr ban time is up



am now out the the watering hole



how is old 97L doing
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Quoting tropicfreak:


If the hurricane (or storm) is going against shear, then it will get ripped apart. If it travels in the same direction as the shear then it won't have as much as an impact on the system.


What I said in different words. LOL
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I'm reading all the back and forth on whether their is going to be shear ahead of 97L. This is from the TPD -

UPPER-LEVEL WINDS ARE EXPECTED TO
BECOME LESS FAVORABLE FOR THE DEVELOPMENT OF THIS SYSTEM OVER THE
NEXT 24 TO 48 HOURS.


This is Dr. Masters comment -

Monday night, shear is expected to rise to 20 - 30 knots

Based on their comments I have to believe shear will be out in front of 97L.
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Sullivan weather posted this(same info thats in post #1721, hours ago), but in much greater detail. Could be interesting Tues/ Wed.
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Quoting tropicfreak:


If the hurricane (or storm) is going against shear, then it will get ripped apart. If it travels in the same direction as the shear then it won't have as much as an impact on the system.


and 97L is going against the shear
Quoting StormJunkie:
ww12

That means it's really old...Over 24hrs...

No it's 0z valid Monday 20 July
Link
Member Since: May 16, 2009 Posts: 1 Comments: 1231
Quoting HurricaneKing:


If a storm is going with the shear it lessens the effect the shear has on the storm. If a storm is going against the shear you have additional directional shear on the storm so the shear is worse.


If the hurricane (or storm) is going against shear, then it will get ripped apart. If it travels in the same direction as the shear then it won't have as much as an impact on the system.
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Quoting StormJunkie:
KM, I always thought that if the shear and the system were moving in the same direction it was more beneficial to the system? Storm moving 15 shear blowing 30 = 15knts of shear?


That's correct.
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http://www.sailwx.info

No buoys, but one ship report of SSW breeze.
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Quoting DestinJeff:
it's shear insanity in here tonight ...


That was bad...
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Quoting KoritheMan:


I'll reiterate a very important meteorological fact that almost no one seems to ever mention. If vertical shear is coming from the direction in which a tropical cyclone (or in this case, a tropical disturbance) is moving, then it will be more detrimental than normal, because it is blowing directly across the circulation center.

This is why Omar was able to intensify into a hurricane despite 20 knots of westerly shear; because it was moving in the direction opposite the shear.


Good point. Thanks. So what is your thoughts for 97L?
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ww12

That means it's really old...Over 24hrs...
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Quoting HurricaneKing:


If a storm is going with the shear it lessens the effect the shear has on the storm. If a storm is going against the shear you have additional directional shear on the storm so the shear is worse.


and based on what I am seeing it is going head on into the shear
KM, I always thought that if the shear and the system were moving in the same direction it was more beneficial to the system? Storm moving 15 shear blowing 30 = 15knts of shear?
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Quoting Stormchaser2007:


Is that the 18z?

No, 0z
Member Since: May 16, 2009 Posts: 1 Comments: 1231
Quoting tropicfreak:


I never knew that could happen. Thanks for the educational info.


If a storm is going with the shear it lessens the effect the shear has on the storm. If a storm is going against the shear you have additional directional shear on the storm so the shear is worse.
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Quoting KoritheMan:


I'll reiterate a very important meteorological fact that almost no one seems to ever mention. If vertical shear is coming from the direction in which a tropical cyclone (or in this case, a tropical disturbance) is moving, then it will be more detrimental than normal, because it is blowing directly across the circulation center.

This is why Omar was able to intensify into a hurricane despite 20 knots of westerly shear; because it was moving in the direction opposite the shear.


I never knew that could happen. Thanks for the educational info.
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Quoting weatherwatcher12:
Gfs 12 hour shear forecast:


Gfs 24 Hours:



Gfs 36 Hours:



Gfs 48 Hours:


Is that the 18z?
Member Since: June 9, 2007 Posts: 4 Comments: 15776
1722. surfmom
Quoting melwerle:
I realize I shouldn't complain but it's HOT here...go figure. I think I brought the GA heat with us. It's 90, 67% humidity. The heat is supposed to break at some point this week. Not good though when you have no a/c.


arrrrrvvvvvvy - where in the world are you now??
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A friend of mine sent me this. Thought it was kinda interesting.

THIS HAZARDOUS WEATHER OUTLOOK IS FOR SOUTHERN
CONNECTICUT...NORTHEAST NEW JERSEY AND SOUTHEAST NEW YORK.

.DAY ONE...TONIGHT.

HAZARDOUS WEATHER NOT EXPECTED AT THIS TIME.

.DAYS TWO THROUGH SEVEN...MONDAY THROUGH SATURDAY

AN APPROACHING WARM FRONT STALLING SOUTH OF LONG ISLAND COULD
SERVE AS THE FOCUS FOR HEAVY SHOWERS AND THUNDERSTORMS TUESDAY
INTO WEDNESDAY...AS TROPICAL MOISTURE SURGES NORTHWARD AND
OVERRIDES THE FRONT
. LOCALIZED URBAN AND FLASH FLOODING
ARE POSSIBLE.

.SPOTTER INFORMATION STATEMENT...

SPOTTER ACTIVATION MAY BE NEEDED TUESDAY INTO WEDNESDAY.

&&
Member Since: June 9, 2007 Posts: 4 Comments: 15776
Quoting BurnedAfterPosting:


First off none of the three of us said the shear would kill it

Second, anyting over 20 knots of shear can definitely kill a tropical system

Third, dont assume where this system is going and where it will find favorable conditions when we dont really know if it will survive the next 24 hours or how shear will be like the next few days.


I never said you said shear would kill it i said a 50/50 chance. Maybe i'm getting a little too ahead of myself. we'll just wait and see what happens...
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Quoting tropicfreak:


we'll see what happens. If convection flares up tomorrow mornig and it continues to look organized like it has been then it will survive the shear. Again you are talking 15-30 kt shear so it could have a 50/50 chance of surviving the shear. Whan it does survive, it will head into very favorable area (GOM) with hot SST's and low wind shear and a very moist environment then this baby will fire up into a TS, perhaps a cat 1 at strongest at landfall.


I'll reiterate a very important meteorological fact that almost no one seems to ever mention. If vertical shear is coming from the direction in which a tropical cyclone (or in this case, a tropical disturbance) is moving, then it will be more detrimental than normal, because it is blowing directly across the circulation center.

This is why Omar was able to intensify into a hurricane despite 20 knots of westerly shear; because it was moving in the direction opposite the shear.
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I honestly think it may be time to watch the Bahamas. We have the old front moving out of the gulf. We have the surge of moisture coming from the wave in the caribbean and maybe eventually from part of 97L's wave. We also have a ULL in the area that could hold something there for a while. I'm thinking subtroipcal development is a possibility in a couple of days.
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Good to see you KEH

And yes the point Ike left out is that while we are just over a quarter of the way through The Season™; we still have about 85 percent of the worst portion of The Season™ to go.

That 85% is a guestimate...not sure exactly what it is, but we have a large portion of the worst of it to come was the point.

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Quoting weatherwatcher12:
Gfs 12 hour shear forecast:


Gfs 24 Hours:



Gfs 36 Hours:



Gfs 48 Hours:


Slowly but surely it shows that shear is becoming more and more favorable, right?
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Quoting tropicfreak:


we'll see what happens. If convection flares up tomorrow mornig and it continues to look organized like it has been then it will survive the shear. Again you are talking 15-30 kt shear so it could have a 50/50 chance of surviving the shear. Whan it does survive, it will head into very favorable area (GOM) with hot SST's and low wind shear and a very moist environment then this baby will fire up into a TS, perhaps a cat 1 at strongest at landfall.


First off none of the three of us said the shear would kill it

Second, anyting over 20 knots of shear can definitely kill a tropical system

Third, dont assume where this system is going and where it will find favorable conditions when we dont really know if it will survive the next 24 hours or how shear will be like the next few days.

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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.