97L growing more organized, will bring heavy rain to the Lesser Antilles

By: Dr. Jeff Masters , 3:55 PM GMT on July 19, 2009

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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Dr.Masters,
Thank you for the lunch-time update enjoy your Sunday.
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drak from your loop i can see that the main blob of convection is expanding. interesting..
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Well dr. Masters cleared up everything. Now seems we may actually have orange code by 2pm. However, i still foresee favorable shear in the carib.
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Quoting hurricane23:


Hey Drak,

Lol little sucker just noticed 97 is back up just looking at obs out there and for now i cant fine a surface circulation but convection has somewhat consolidated overnight i'll give it that.Now as far as its future track a deepening trof off low pressure along the southeast coast should induce shear and a northwesterly turn eventually.

PS...Just ran an infrared loop and yes some pretty decent outflow is starting to take shape.


Hey Adrian,

Yea I saw that. There are signs that a LLC may be forming. System continues to get better organized. I'm using nowcoast to monitor the surface obs.
Member Since: October 28, 2006 Posts: 57 Comments: 32729
Dr M says shear will not be favorable though
Lotta folks weren't expecting this comeback..
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I guess TampaSpin may be right after all...shear might be a problem.
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Shiny new blog.....and now back to our regularly scheduled program.... Morn SJ!
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Jeff Masters goes with B LOL!
Member Since: October 28, 2006 Posts: 57 Comments: 32729
Quoting Drakoen:
That upper level trough is either going to make or break 97L.


Hey Drak,

Lol little sucker just noticed 97 is back up just looking at obs out there and for now i cant fine a surface circulation but convection has somewhat consolidated overnight i'll give it that.Now as far as its future track a deepening trof off low pressure along the southeast coast should induce shear and a northwesterly turn eventually.

PS...Just ran an infrared loop and yes some pretty decent outflow is starting to take shape.
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Here we go folks!
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he didn't wait till the 2pm, cool.

Thanks Dr. Masters!
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Thanks Doc!
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This is the only 1 I could find


Wx Update, E Caribbean, Sat18, 9a

Morning graphical QuickScat: ABCs & Venezuela W of 66W ENE-E@13-20 with a few squalls to 35k, mainly N of 12N / E of 66W to Trinidad & Grenada E@0-10 with isolated squalls to 30k / just W of Windwards ENE-E@10-17 / W of Leewards ENE@16-22 with scattered squalls to 30k / Virgins & PuertoRico ENE@15-20 / DomRep ENE-E@12-17.



TropicalWAVE near 60W:
--Strong, high-amplitude WAVE near E Caribbean along 17N/59W...12N/63W, moving W-WNW@15. To gauge risk for Tropical LO formation, I look for the following 3 criteria: squalls that are abundant, focused, and persistent at the focal point. This morning, squalls have become more-abundant & focused at 16N/59W. If these squalls persist over the next 12-24 hrs, a Tropical LO could form.

--Typically, such squall activity surges & wanes several times before becoming persistent-enough to generate a self-sustaining Tropical LO. Thus, it is unlikely (but not impossible - I'd estimate a 10% chance) a LO will form before this area moves W of VirginIslands & PuertoRico tomorrow. Wind-shear of 40-50k is decreasing, but should help prevent formation of a Tropical LO.

--NOAA this morning began mentioning a low (less-than-30%) chance of Tropical LOW formation over next 48hrs with this feature.

--Beyond tomorrow, Apex of WAVE should lie somewhere near the GreaterAntilles (Hispanola & Cuba), which, combined with likelihood of persistent wind-shear, should prevent formation of a Tropical LOW.

More here
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Thanks for the update.
Member Since: October 28, 2006 Posts: 57 Comments: 32729

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Dr. Masters co-founded wunderground in 1995. He flew with the NOAA Hurricane Hunters 1986-1990. Co-blogging with him: Bob Henson, @bhensonweather

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