African tropical wave 99L slowly organizing

By: Dr. Jeff Masters , 1:26 PM GMT on July 31, 2012

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A tropical wave (Invest 99L) near 9°N 41°W, halfway between the Lesser Antilles Islands and the coast of Africa, has the potential to develop into a tropical depression later this week as it moves westward at 10 - 15 mph. Visible satellite loops show that the disturbance now has a moderate amount of poorly-organized heavy thunderstorms that continue to slowly increase in intensity and areal coverage. There is no surface circulation, but some counter-clockwise rotation of the large-scale cloud pattern is evident. Water vapor satellite loops show that 99L has a reasonably moist environment. The latest Saharan air layer analysis shows that the dry air from the Sahara lies to the north of 99L and is currently not affecting the storm. WInd shear over the disturbance is a light 5 - 10 knots, and ocean temperatures are 28°C, (82°F) which is well above the 26.5°C (80°F) threshold typically needed to allow formation of a tropical depression.


Figure 1. Morning satellite image of Invest 99L.

Forecast for 99L
Wind shear is expected to remain light through Friday, and ocean temperatures will remain near 28°C, according to the 8 am EDT run of the SHIPS model. However, a band of high wind shear of 20 - 40 knots associated with the subtropical jet stream lies just to the north of 99L, and it would not be a surprise to see 99L experience some higher shear conditions than are currently forecast. The farther north 99L gets, the higher the shear it will experience, and the SHIPS model is predicting shear in the moderate range, 10 - 20 knots, for Saturday - Sunday, as the storm works its way to 15°N. The disturbance is at 9°N, which is close enough to the Equator that the storm will have some difficultly getting spinning. Most of the models are showing some slow development of 99L. There are some major differences in the predicted forward speed of 99L, with the ECMWF and UKMET models predicting the storm will reach the Lesser Antilles on Friday, and the GFS predicting a later arrival, on Saturday. At 8 am Tuesday, NHC gave 99L a 20% chance of developing into a tropical depression by Thursday morning. I expect the storm will begin having trouble with tendrils of dry air reaching down from the north at times this week, but give 99L a 50% chance of eventually developing into Tropical Storm Ernesto sometime in the next ten days. Residents and visitors to the Lesser Antilles Islands should anticipate heavy rains and strong winds from 99L beginning to affect the islands as early as Friday morning. The long-range fate of 99L next week is uncertain. A track west to west-northwest through the Caribbean, or to the northwest towards the U.S. East Coast are both possible. The storm is less likely to survive if it heads northwest towards the U.S.

Extreme heat in the Central U.S.
The withering heat in America's heartland continued on Monday, with high temperatures of 112° recorded in Winfield, Kansas and Searcy, Arkansas. Little Rock, Arkansas hit 111°, their 3rd hottest temperature ever record, behind the all time record of 114° set just last year on August 3, and the 112° reading of 7/31/1986. Wichita and Coffeyville in Kansas both hit 111° Monday, and in Oklahoma, Enid, Tulsa Jones Airport, and Chandler all topped out at 111°. Carr Creek, Missouri hit 110°, the hottest temperature measured in the state so far this year. Highs temperatures Tuesday and Wednesday in this region could reach 110° again, as the most extreme heat this week will stay focused over Oklahoma and surrounding states.

Jeff Masters

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Quoting TropicalAnalystwx13:
Typhoon Saola and Tropical Storm Damrey:

Saola gathering a lot of moisture. Bad news for China.
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Quoting kmanislander:
If 99L has a solid CDO in this area it will likely survive ok and then really have a chance to ramp up near 75W and beyond where the TCHP is very high.

This system is potentially a dangerous one to be watched carefully, depending on where the steering takes it.


I am going to keep a very close watch on 99L as, like you, I see potential for intensification - time will tell.
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Wonder whats taking the SPC soo long for the next outlook. Was scheduled to be out at 12:30.
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Quoting kmanislander:
Good morning

I have not posted on 99L so far due to a busy schedule but thought I would throw out a few comments now. This system is a slow over which gives it time to build a solid CDO before reaching the Caribbean. The area between 50 and 60 W is also historically conducive to aiding rapid improvement in both the structure of a system as well as the extent of deep convection it can build.

99L may therefore enter the Caribbean in good form sufficient to fight off the fast trades near the N coast of SA near Aruba that tend to disrupt the circulation on the South side of a tropical cyclone in that area. In addition, another reason the eastern Caribbean is hostile is due to dry air ingestion from the SA continent being caught up in the inflow on the Southern side of the circulation.

If 99L has a solid CDO in this area it will likely survive ok and then really have a chance to ramp up near 75W and beyond where the TCHP is very high.

This system is potentially a dangerous one to be watched carefully, depending on where the steering takes it.

Hey Kman.

When's the last time you saw TCHP values in excess of 140 kJ/cm^2..in July?

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Goodnight all. Stay cool, stay safe.
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Good morning

I have not posted on 99L so far due to a busy schedule but thought I would throw out a few comments now. This system is a slow mover which gives it time to build a solid CDO before reaching the Caribbean. The area between 50 and 60 W is also historically conducive to aiding rapid improvement in both the structure of a system as well as the extent of deep convection it can build.

99L may therefore enter the Caribbean in good form sufficient to fight off the fast trades near the N coast of SA by Aruba that tend to disrupt the circulation on the South side of a tropical cyclone in that area. In addition, another reason the eastern Caribbean is hostile is due to dry air ingestion from the SA continent being caught up in the inflow on the Southern side of the circulation.

If 99L has a solid CDO in this area it will likely survive ok and then really have a chance to ramp up near 75W and beyond where the TCHP is very high.

This system is potentially a dangerous one to be watched carefully, depending on where the steering takes it.
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414. 7544
hmm you would think the mjo is here the way all these blobs are lighting up the three stooges return still watching that pr wave as it heads to the bahamas
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Quoting TropicalAnalystwx13:

I know the NHC really dislikes bumping percentages up on a system that is still connected to the ITCZ, but this deserves higher than 20%.


One in 5 99L type systems will be a TD in 48 hrs?

sounds good to me...but they may need to raise it to 30% this update.
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Quoting Stormchaser2007:


There is some definite circulation which means there is one dominant better defined LLC. I would say 40% at 2pm and 50% at 8pm with it developing Wednesday evening.
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Quoting MississippiWx:
I see that the NHC is still unimpressed with our ITCZ system, which is to be expected. However, the 2PM TWO will probably be bumped to 30% due to the marked improvement in circulation and convection since last night/early this morning.

99L is still moving due west along the ITCZ, just south of 10N (very far south). Unless 99L strengthens quickly, it's not moving north of the islands this late in the game. If I lived in the islands, I would be preparing for at least a minimal tropical storm within the next week.

Back later after lunch.

I want lunch. Can I come? Lol.
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Typhoon Saola and Tropical Storm Damrey:

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There is a lot of wind shear in the W Caribbean and GOM.
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As you can see by the BAM model suite a stronger storm will likely move on a more wnw-nw path.

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12z Nogaps..continues the NW pursuit
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I see that the NHC is still unimpressed with our ITCZ system, which is to be expected. However, the 2PM TWO will probably be bumped to 30% due to the marked improvement in circulation and convection since last night/early this morning.

99L is still moving due west along the ITCZ, just south of 10N (very far south). Unless 99L strengthens quickly, it's not moving north of the islands this late in the game. If I lived in the islands, I would be preparing for at least a minimal tropical storm within the next week.

Back later after lunch.
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Quoting Stormchaser2007:

I would say that has some spin to it.
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Quoting Pocamocca:

It may look okay now, but I think the NHC is gauging this off the likelihood of it to develop over the next 48 hours based off the conditions within the 48 time frame.

Which are favorable.
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Still has a noticable connection to the first wave ahead of it that ultimately spawned 99L.

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Good Afternoon, as what Stormchaser2007 said it looks like 99L is starting to organize. It has some banding features which so a developing LLC also note a weak looking anticyclone is developing over it keeping out shear to its north.
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12z Dynamical models.

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Quoting Articuno:

I like Carl Parker, he is the best Meteorologist on TWC JMO


Yeah I agree..and I almost would swear he reads this

blog by the way he comes up with graphics to
illustrate debated topics in here...
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Quoting Stormchaser2007:

I know the NHC really dislikes bumping percentages up on a system that is still connected to the ITCZ, but this deserves higher than 20%.
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Quoting RayT:
Hey guys. I dont often post, but I was just curious.

I've always noticed that there is a sizeable difference in strength between a 40 MPH tropical storm and a 70 MPH tropical storm.

Why do we not have two classifications of tropical storm?

Perhaps cateogory 1 tropical storms for up to 55 MPH and category 2 for anything between 55 and 75 MPH?

Maybe there is some reason for it.

Mostly just curious.

Actually, the WPac and the SHem have two categories for tropical storms
It's called a severe tropical storm (category 2). Just in case you didnt know.
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99L enters the Western Caribbean as a 1008 Low@ 168 hours.

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The storm is less likely to survive if it heads northwest towards the U.S.


This one sentence seems to contradict what most bloggers were saying yesterday...i.e Tradewinds
tearing it apart....
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The 12Z GFS keeps a well defined area of low pressure all the way through the Caribbean, and shows it making landfall in Nicaragua a little over a week from now. The model still lies on the southern edge of the model guidance, and the main reason it weakens the storm in the east Caribbean is because it interacts with the northern coast of South America. Should see it shift north some eventually.

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Once 99l organizes and has a discernable low pressure center, the model tracks will be more reliable, right?
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I am starting to think that 99L will run into South america. :(
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If it doesn't develop by the time it reaches the Caribbean...it may be lights out for this invest.
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Quoting GeorgiaStormz:


its just a bunch of rain


Well, no it's not just rain. There is alot of lightening and some hail as well. It is a dangerous storm
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Im done talking about long range for now.

99L appears to be organizing somewhat today as some low-level banding begins to setup just to the north of the LLC. I'm still not sure why Masters thought that this does not have a LLC. Granted it's weak, but it's definitely there.

Not sure about a % increase from the NHC since they hate monsoon trof systems, but I'd go with a 40% over the next 72 hours.



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So far, the GFS keeps 99L alive, which is much different from past runs..
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Quoting Tropicsweatherpr:


What is RH?

Relative Humidity.

And yeah, it may look ragged for a while after it detaches. Should still make tropical storm status by 50-55W.
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Quoting Bluestorm5:
99L is not gonna be like Ivan... chill.


I do not expect it to and am chilled - was rather passing comment on, IMHO, a somewhat unguarded or ill-considered statement
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99L, still a Low Pressure system entering the Western Caribbean at 138 hours on the 12Z GFS (still running)
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GFS seems to think 99L has a hard time detaching from the ITCZ and keeps it on a more southerly course...which makes sense.
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Quoting GTcooliebai:
We need at least a Tropical Depression then these models will have a better idea as to where this is going.


Bingo!
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Quoting Stormchaser2007:


The rule with monsoon trof systems is that there is a 24-36 hours period where they have to sustain themselves.

Some of the storm-specific models show a sharp decrease in RH associated with 99L after it detaches.


What is RH?
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We need at least a Tropical Depression then these models will have a better idea as to where this is going.
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Quoting TropicalAnalystwx13:
Carl Parker said this is getting close to tropical depression status. And it would be if it weren't connected to the Intertropical Convergence Zone still. Once we see it detach, we should see classification.


The rule with monsoon trof systems is that there is a 24-36 hour period where they have to sustain themselves.

Some of the storm-specific models show a sharp decrease in RH associated with 99L after it detaches.
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....its embedded..its going straight like all the others
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Quoting TropicalAnalystwx13:
Carl Parker said this is getting close to tropical depression status. And it would be if it weren't connected to the Intertropical Convergence Zone still. Once we see it detach, we should see classification.

I like Carl Parker, he is the best Meteorologist on TWC JMO
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Quoting RayT:
Hey guys. I dont often post, but I was just curious.

I've always noticed that there is a sizeable difference in strength between a 40 MPH tropical storm and a 70 MPH tropical storm.

Why do we not have two classifications of tropical storm?

Perhaps cateogory 1 tropical storms for up to 55 MPH and category 2 for anything between 55 and 75 MPH?

Maybe there is some reason for it.

Mostly just curious.


From a damage perspective, structural damage starts in the range from 80-100 mph depending on the strength/type of the structure.

Most tropical storms don't cause much structural wind damage, with mainly power lines/trees experiencing some moderate damage.

As you get closer to hurricane force (60-70mph sustained tropical storm with higher gusts) you could have winds in the low end of the structural damage category but overall there needs to be a break somewhere between TS and HU and it seems to be in a fairly reasonable place.

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99L is not gonna be like Ivan... chill.
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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.

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