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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2215. SLU
000
ABNT20 KNHC 201140
TWOAT
TROPICAL WEATHER OUTLOOK
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
800 AM EDT MON JUL 20 2009

FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO...

CLOUDINESS AND SHOWERS CONTINUE IN ASSOCIATION WITH A TROPICAL WAVE
LOCATED A COUPLE HUNDRED MILES EAST OF THE WINDWARD ISLANDS.
UPPER-LEVEL WINDS ARE EXPECTED TO REMAIN UNFAVORABLE FOR
DEVELOPMENT OF THIS SYSTEM FOR THE NEXT COUPLE OF DAYS. REGARDLESS
OF DEVELOPMENT...LOCALLY HEAVY RAINFALL AND GUSTY WINDS WILL AFFECT
THE WINDWARD ISLANDS DURING THE NEXT DAY OR SO AS THE WAVE MOVES
WESTWARD AT 15 TO 20 MPH. THERE IS A LOW CHANCE...LESS THAN 30
PERCENT...OF THIS SYSTEM BECOMING A TROPICAL CYCLONE DURING THE
NEXT 48 HOURS.

ELSEWHERE...TROPICAL CYCLONE FORMATION IS NOT EXPECTED DURING THE
NEXT 48 HOURS.

$$
FORECASTER BRENNAN


oh well ..
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2214. WxLogic
Quoting willdunc79:
so in layman terms wxlogic what are you saying?


Shear has decouple the once coupled Vorticity engines that where helping the tropical system get organized. If they're not stacked together then you won't see organization needed for a TD to even develop.

Of course there are other ingredients... but this one should over one of them.
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Quoting SLU:


Anyone living in Barbados, St. Lucia, St, Vincent & Martinique may want to treat this system with respect. Interesting to hear the NHC's comments in a few minutes.

Wow!! We must have posted quikscat at same time basically.
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2212. IKE
TROPICAL WEATHER OUTLOOK
NWS TPC/NATIONAL HURRICANE CENTER MIAMI FL
800 AM EDT MON JUL 20 2009

FOR THE NORTH ATLANTIC...CARIBBEAN SEA AND THE GULF OF MEXICO...

CLOUDINESS AND SHOWERS CONTINUE IN ASSOCIATION WITH A TROPICAL WAVE
LOCATED A COUPLE HUNDRED MILES EAST OF THE WINDWARD ISLANDS.
UPPER-LEVEL WINDS ARE EXPECTED TO REMAIN UNFAVORABLE FOR
DEVELOPMENT OF THIS SYSTEM FOR THE NEXT COUPLE OF DAYS. REGARDLESS
OF DEVELOPMENT...LOCALLY HEAVY RAINFALL AND GUSTY WINDS WILL AFFECT
THE WINDWARD ISLANDS DURING THE NEXT DAY OR SO AS THE WAVE MOVES
WESTWARD AT 15 TO 20 MPH. THERE IS A LOW CHANCE...LESS THAN 30
PERCENT...OF THIS SYSTEM BECOMING A TROPICAL CYCLONE DURING THE
NEXT 48 HOURS.

ELSEWHERE...TROPICAL CYCLONE FORMATION IS NOT EXPECTED DURING THE
NEXT 48 HOURS.
Member Since: June 9, 2005 Posts: 23 Comments: 37858
2211. Mikla
97L with latest SSTs, Water Vapor image, and some 00Z models...
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2210. crownwx
Crown Weather Tropical Weather Discussion: Link
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13N/57W, quite a few 35-40mph winds .
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2208. SLU


Anyone living in Barbados, St. Lucia, St, Vincent & Martinique may want to treat this system with respect. Interesting to hear the NHC's comments in a few minutes.
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So whats the guess on the NHC at 8. Its currently pouring with rain here in Barbados. I am in the south of the island
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so in layman terms wxlogic what are you saying?
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2205. WxLogic
Quoting sporteguy03:
Hi Wx,
After more rambling about shear in a day the end result is that 97L is still there effected by shear but there.


Hehe... Hi.

Yeah... but it is good that shear conflicts rise since that's the only way to clear up misconceptions about it, so people can be in the same page in regards certain topics. :)
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2204. MahFL
"The storm is currently under about 20 to 25k..."

What storm ? It's a tropical wave right now. Not even a depression.....
Save the word "Storm" for the correct meteologial entity.
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Quoting willdunc79:
group question. How can you tell when a tropical wave/system has convection that is from shear versus building it's own convection?


Upper diffluence enhanced convection has little surface convergence and plenty upper divergence

Self-induce convection is surrface based hence surface convergence is often high or exceeds upper divergence.

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2201. WxLogic
Seems 97L has finally hit the trough.

Now one interesting thing to note is that the time and location of this meeting point should prove interesting... since in my point of view 97L is in the eastern side of the trough which at this time is the worst part and later today it should a not so head on position with the shear... so should be interesting to see if the DMAX/Diffluence induced convection can stick around while it enters the E/C Carib. Of course not saying conditions are favorable in the CARIB as there's still shear left in there...

Also just in case... I've combined the latest 850MB through 500MB level VORT MAX charts and you can see the mid level shear has been displaced from the low shear VORT MAX due to shear.

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2200. crownwx
Crown Weather Tropical Weather Discussion: Link
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group question. How can you tell when a tropical wave/system has convection that is from shear versus building it's own convection?
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Quoting Weather456:
Good Morning;

Tropical Invest 97L heads for the Windward Islands



Good update W456, Have to say I follow your post closely.
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2197. SLU


No closed surface low but unflagged tropical storm force winds! Plus it appears to be getting better organised on satellite pictures.

Morning guys
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I think we may get a little surprise from 97L. It's still holding on fairly well in my opinion.
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Hi Wx,
After more rambling about shear in a day the end result is that 97L is still there effected by shear but there.
Member Since: July 7, 2005 Posts: 0 Comments: 5120
2194. WxLogic
Good Morning...
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bright and sunny with winds of 15 mph this carnival monday. few are are inform about 97L and the impending weather. it will be a total surprise when the weather starts to go down hill
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Good Morning;

Tropical Invest 97L heads for the Windward Islands

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Whew! Looks like 97L is walking headlong into a gale in November. Look at her hair fly!
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Quoting leftovers:
remember old rule if tw does not develop before the islands it wont


rgr that.


eastern carrib is the dead zone...
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2188. beell
Tropical Update:
Quiet.
Member Since: September 11, 2007 Posts: 137 Comments: 15297
true but Leftovers not def. written in stone
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the wave @ 32w 7n could/should become 98L sometime today if it sticks to the trend of how it's doing now.
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raining here with strong wind gusts...
Good Morning Barbados, our system seems to have slowed a bit and convection is firing again as we hit DMAX. Rotation is still evident and she's in the sweet spot now, should be an interesting morning.

Current conditions are cloudy with light winds ENE with periods of heavy rain.

Info taken from Station 41101

Wind Direction (WDIR): E ( 80 deg true )
Wind Speed (WSPD): 22.9 kts
Wave Height (WVHT): 6.6 ft
Dominant Wave Period (DPD): 7 sec
Atmospheric Pressure (PRES): 29.90 in
Pressure Tendency (PTDY): +0.02 in ( Rising )
Air Temperature (ATMP): 81.3 °F
Water Temperature (WTMP): 81.5 °F
Dew Point (DEWP): 75.6 °F
Heat Index (HEAT): 87.8 °F
Combined plot of Wind Speed, Gust, and Air Pressure
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morning
convection is once again on the increase with 97L. this morning"s infra red pics are showing that 97L was able to fight off the high shear and retain it"s structure. water vapour loops show that the system is now at the base of the trough which is giving it some breathing space. last night 97L moved southwest for awhile but has resumed a west course shear is still 20 knots over the system and that will prevent much organisation. deveopment or not the system will bring heavy rains to barbados and the central windwards with gusts near storm force, most of today and a portion of tomorrow
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The storm is currently under about 20 to 25kt shear about 10kt more than i expected it to be tonight.Shear has reached its max for the enxt 3 to 4 days over this system.SShear will begin lifting today from the latest runs of wrf model.
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2178. HadesGodWyvern (Mod)


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Quoting AstroHurricane001:
The NHC says 97L is becoming less organised. What are they talking about? Convection seems to have flared up in just the last two hours.


That convection is induced by diurnal maximum and upper level diffluence. If it can make it through today, then we have something to watch.
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2176. HadesGodWyvern (Mod)
don't think it is going to become a deep depression with the way it looks now.
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2175. MahFL
Dr M says conditions will improve then get a lot worse, 30 kts of shear will kill 97.
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2174. HadesGodWyvern (Mod)
India Meteorological Department
Tropical Cyclone Bulletin Number TWO
DEPRESSION BOB03-2009
11:30 AM IST July 20 2009
===================================

Subject: Depression over northwest Bay of Bengal

At 6:00 AM UTC, Depression BOB03-2009 over northwest Bay of Bengal remained practically stationary and lay centered near 21.0N 88.5E, about 120 km southeast of Digha, 160 km east-southeast of Balasore and 200 km southwest of Khepupara (Bangladesh).

The system is likely to intensify further and move in a west-northwesterly direction and cross north Orissa-West Bengal coast between Balasore and Digha today

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The NHC says 97L is becoming less organised. What are they talking about? Convection seems to have flared up in just the last two hours.
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good morning from Barbados

It's raining very heavily here now along with occasional gusty winds. Will get worse as the day goes on....
Quoting weatherwatcher12:
I think I see what could be an anti-cyclone forming in the central Caribbean
Looks like this is true,after 97L goes by the upper air feature it might make it into a anticyclone,which may at 1st be detrimental then more positive for growth..I post at this time of the morning in case Im wrong ,not many viewers..LOL
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Quoting willdunc79:
Is shear suppose to be high in the carib.? If it's as high as they say it's suppose to be then this is the final hoorah for 97L and the wave out behind it could maybe become 98L.
Looks like the upper level feature to the north is slightly pulling away,in which 97l might enter a area of favorable conditions???
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Quoting weatherwatcher12:

When cloud tops cool and convection explodes. Strongest just before suunrise
DMAX is this Dirunal Maximum ?
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Is shear suppose to be high in the carib.? If it's as high as they say it's suppose to be then this is the final hoorah for 97L and the wave out behind it could maybe become 98L.
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Now I could be completely wrong about this. And I know its dmax. But it looks like 97L is pulling the clouds/convection from the NW trying to wrap them around itself. Anyway its interesting. :)
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Quoting natrwalkn:


What is DMAX?

When cloud tops cool and convection explodes. Strongest just before suunrise
Member Since: May 16, 2009 Posts: 1 Comments: 1231
Quoting weatherwatcher12:
Approaching DMAX


What is DMAX?
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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.