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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gwadaman, I'm in Barbados as well. I'm located in St. Joseph....
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Quoting yamil20:


it means that the convection is waning, the system is weakening


As expected during DMIN.
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Quoting STORMMASTERG:
Guys don't worry about 97l wind shear issue/its not an issue.Worry about it forimg a closed low level circulation.


wind shear is a huge issue becuase it can prevent 97L from forming that closed LLC

the loop confirms the increase in shear in the immediate vicinity of 97L
1262. K8eCane
Quoting STORMMASTERG:
Guys don't worry about 97l wind shear issue/its not an issue.Worry about it forimg a closed low level circulation.



agree
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Guys don't worry about 97l wind shear issue/its not an issue.Worry about it forimg a closed low level circulation.
I'm located in St. Phillip, not far from Bushy Park Race Track.
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Quoting WeatherStudent:
Oohh Lord, I give up, may soemoen plz expalint o him taht there isn't any wind shear there.


but there is wind shear there... they even have it color coded D:
Member Since: July 7, 2008 Posts: 2 Comments: 3237
Does anybody else notice that images shrink when you post them, regardless of the option to entire the size?

Also, is it true the CMC has been "toned down"?

LI (Lifted Index, another measure of convective currents and total column instability) is favorable for convection just ahead of 97L (~ -5), however, with the possibility of higher shear, this system may still swap good and impressive organization for more, disorganized and off centered storm activity.



Member Since: July 30, 2007 Posts: 76 Comments: 4063
1256. KEEPEROFTHEGATE (Mod)
your welcome 03
Member Since: July 15, 2006 Posts: 175 Comments: 54858
Quoting WeatherStudent:
Oohh Lord, I give up, may soemoen plz expalint o him taht there isn't any wind shear there.


look at the map I showed, stop being ignorant, the shear is there and increasing very close to the system

97L has a very small window to develop now, I dont think shear will ultimately kill it completely, but development in the short term is not likely at this point.
Quoting Funkadelic:
Well D-min is almost over, and it really wiped 97L out.... But it looked worse last night at this time si I believe tommrow morning we will be surprised

how much longer it it
Member Since: June 13, 2009 Posts: 2 Comments: 12504
Quoting BurnedAfterPosting:


Here is the regular shear map as well, all the shear circled is 20 knots or higher


That area seems to be moving slowly south
Member Since: March 19, 2009 Posts: 0 Comments: 1421
I believe they will run the NHC models (GFDL and HWRF) tonight, there's really no reason not to, the invest isn't falling apart.. yet.
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Quoting FloridaTigers:
What do warming cloud tops mean?
Its dmin,it happens this storm still looks good.I think it will respond to dmax and finally develop.Divergence reamins good/also lack of low level lift.I think it might form to ts by mid morning tomorrow.
Quoting KEEPEROFTHEGATE:
in another hr or so the bashing will begin


Thank you for the upcoming bashing possibility I will be lurking then
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1248. KEEPEROFTHEGATE (Mod)
Quoting FloridaTigers:
What do warming cloud tops mean?
clouid top temps are warming due to decrease convection cloud tops cooling is increasing convection blues and yellows warm dark red and black cold
Member Since: July 15, 2006 Posts: 175 Comments: 54858
Quoting FloridaTigers:
What do warming cloud tops mean?


Warm clouds mean weaker storms, Cold clouds mean stronger storms
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1244. yamil20
Quoting FloridaTigers:
What do warming cloud tops mean?


it means that the convection is waning, the system is weakening
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Well things can change, but looking at current forecast, if it doesn't form by the morning, it won't form at all.... Things can change though. The Anticyclone forming down near Costa Rica could be a big player in the fate of 97L.
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What do warming cloud tops mean?
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Here is what i think/it wil refire more deep storms once night approaches and morning in dmax.Shear will fluctuate at times during the night and remain favorable.It should begin to lift and based on wrf/model close to hwrf.I am sticking to wrf and some others show weakening shear/lifting north.


Here is the regular shear map as well, all the shear circled is 20 knots or higher
Can someone please explain the mechanism of the TUTT lifting and what it does to the development?

TIA
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Quoting HurricaneSwirl:
thanks 456


wow weatherwatcher, that shear map.. yikes. i got bashed earlier also for saying that shear would continue to increase in the east caribbean throughout the day.. i doubt the tutt can move fast enough for shear to fall 40 knots before 97L gets there.. im not saying 97L wont survive but im not saying it will develop.

I guess it's a wait and see
Member Since: May 16, 2009 Posts: 1 Comments: 1231
1236. KEEPEROFTHEGATE (Mod)
in another hr or so the bashing will begin
Member Since: July 15, 2006 Posts: 175 Comments: 54858
thanks 456


wow weatherwatcher, that shear map.. yikes. i got bashed earlier also for saying that shear would continue to increase in the east caribbean throughout the day.. i doubt the tutt can move fast enough for shear to fall 40 knots before 97L gets there.. im not saying 97L wont survive but im not saying it will develop.
Member Since: July 7, 2008 Posts: 2 Comments: 3237


seems to me you are ignoring this area of increased shear that I have circled. This is moving southward right into the path of 97L
Be careful how you look at those shear maps compared to what is ctually being observed on satellite imagery such as the orientation cloud clusters.
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The more southerly a track it takes, the better a chance it has at development. Whether it takes a more northerly track towards the Leeward Islands or a southerly track closer to due west through the Windward Islands is up in the air.
Member Since: July 30, 2007 Posts: 76 Comments: 4063
Quoting BenBIogger:


the shear tendency shows shear starting to increase to its west TUTT ain't moving much.

watch the shear loop Link
Member Since: May 16, 2009 Posts: 1 Comments: 1231
Hey all, dont forget to check the right hand side of your atlantic wide views on occassion :) This is how we see things sneak up on his...because we are all so focused on our persistant invest we forget about anything else that might need watching.
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In Reeds words it looks like it is firing another little "bubble" of stronger convection to the east again.
Member Since: October 9, 2008 Posts: 0 Comments: 8426
Quoting weatherwatcher12:

The TUTT continues to leave


the shear tendency shows shear starting to increase to its west TUTT ain't moving much.
Member Since: March 19, 2009 Posts: 0 Comments: 1421
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The shear currently is what wrf shows.Wrf is the most accurate right now.Plus if its right 97l will not deal with higher than 15kt shear/max as low as 5kt or lower.
Quoting Weather456:
HurricaneSwirl,

The 18Z ships show northely shear 17 knots around this time 2mr. This is contridicting the actual forecast shear maps which show 5-10 knots



weatherwatcher12, i cannot say since I'm unsure of development. But normal tropical wave conditions should be expected regardless of development.
Quoting Weather456:
HurricaneSwirl,

The 18Z ships show northely shear 17 knots around this time 2mr. This is contridicting the actual forecast shear maps which show 5-10 knots



weatherwatcher12, i cannot say since I'm unsure of development. But normal tropical wave conditions should be expected regardless of development.

thanks
Member Since: May 16, 2009 Posts: 1 Comments: 1231
Quoting BurnedAfterPosting:


shear isnt looking promising for this to develop at all


agree
Member Since: March 19, 2009 Posts: 0 Comments: 1421
HurricaneSwirl,

The 18Z ships show northely shear 17 knots around this time 2mr. This is contridicting the actual forecast shear maps which show 5-10 knots



weatherwatcher12, i cannot say since I'm unsure of development. But normal tropical wave conditions should be expected regardless of development.
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http://cimss.ssec.wisc.edu/tropic/real-time/atlantic/winds/wg8vor4.html

Vorticity has increased some on 97L
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Quoting BurnedAfterPosting:


shear isnt looking promising for this to develop at all

The TUTT continues to leave
Member Since: May 16, 2009 Posts: 1 Comments: 1231
Quoting weatherwatcher12:
New shear map


shear isnt looking promising for this to develop at all

are you completely ignoring the shear that is east of the Islands? that shear has been increasing and dipping south for the last 24 hours
gwadaman, where are you located in Barbados? Which Parish?
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New shear map

There goes the TUTT
Member Since: May 16, 2009 Posts: 1 Comments: 1231
Actually shear all depends on what that developing Anticyclone does in the Western Carribean. So the WRF could or could NOT be on to something. the WRF shows shear lifting to the north allowing 97L to travel under it with shear 5-10 knots. This is (again) a long shot. I have a feeling Weather456 is right, a red code might appear in the morning once it gets through DMAX. This is not soo much of a long shot, has a good chance of happening.
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Quoting hurricane23:
Cloud tops over the past 2hrs or so have warmed.

Hi 23. What do you think of the system?
Still looking pretty good on Shortwave.
Link
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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.