Unusually well-organized 92L disturbance may become a tropical depression

By: Dr. Jeff Masters , 2:14 PM GMT on June 14, 2010

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Invest 92L, a remarkably well-developed African tropical wave for so early in the season, is midway between Africa and the Lesser Antilles Islands. Infrared satellite loops show a modest area of heavy thunderstorms along the north side of 92L's center of circulation, and the storm's heavy thunderstorms activity appears to be slowly increasing in intensity and areal coverage. Upper-level outflow is apparent to the west and north of 92L, and the outflow has been gradually improving this morning. Visible satellite loops do not show much in the way of low-level spiral bands, and my current take from the satellite imagery is that 92L is slowly organizing, and will not become a tropical depression any earlier than 11pm EDT tonight (Monday.) A 4:27 am EDT pass from the WINDSAT satellite saw a partially closed circulation at the surface (open on the south side), with top surface winds of 25 - 30 mph north of the center.


Figure 1. Morning satellite image of Invest 92L (left side of image) and a vigorous new tropical wave that has moved off the coast of Africa (right side.) None of models develop the new tropical wave, but it bears watching.

Sea surface temperatures
Climatology argues against development of 92L, since only one named storm has ever formed between Africa and the Lesser Antilles Islands in the month of June--Tropical Storm Ana of 1979 (Figure 2). However, sea surface temperatures (SSTs) underneath 92L are an extremely high 28°C, and will increase to 29°C by Thursday. In fact, with summer not even here, and three more months of heating remaining until we reach peak SSTs in the Atlantic, ocean temperatures across the entire Caribbean and waters between Africa and the Lesser Antilles are about the same as they were during the peak week for water temperatures in 2009 (mid-September.)

Dry air not a problem for 92L until Wednesday
The disturbance doesn't have to worry about dry air today or Tuesday--Total Precipitable Water (TPW) loops show a very moist plume of air accompanies 92L, and water vapor satellite loops show that the center of 92L is at least 200 - 300 miles from any substantial areas of dry air. As 92L continues to push northwest, though, the SHIPS model is predicting that relative humidity at middle levels of the atmosphere will fall from the current value of about 70%, to 60% on Wednesday. This dry air may begin to cause problems for 92L on Wednesday, especially since wind shear will be increasing at the same time. Tropical cyclones are more vulnerable to dry air when there is substantial wind shear, since the strong winds causing the shear are able to inject the dry air deep into the core of the storm.

Madden-Julian Oscillation
The 60-day cycle of enhanced thunderstorm activity called the Madden-Julian Oscillation is currently favoring upward motion over eastern tropical Atlantic, and this enhanced upward motion helps create stronger updrafts and higher chances of tropical cyclone development.


Figure 2. Tropical Storm Ana of 1979 was the only June named storm on record to form between Africa and the Lesser Antilles Islands.

Wind shear
A major issue for 92L, like it is for most June disturbances, is wind shear. The subtropical jet stream has a branch flowing through the Caribbean and tropical Atlantic north of 10° N that is bringing 20 - 40 knots of wind shear to the region. Our disturbance was located near 10°N, 40°W at 8am EDT this morning, a few hundred miles south of this band of high shear, and is currently only experiencing 5 - 10 knots of shear. This low amount of shear should allow for some steady development of 92L over the next two days as it tracks west-northwest or northwest at 15 mph. The latest run of the SHIPS model is predicting the shear will rise to 20 knots on Wednesday, which may start to cause problems for 92L.

The forecast for 92L
The National Hurricane Center is giving 92L a high (60% chance) of developing into a tropical depression by Wednesday morning, which is a reasonable forecast. The odds of development have increased since yesterday, as the storm has moved considerably to the northwest, away from the Equator. Now it can leverage the Earth's spin to a much greater degree to help get it get spinning. It is quite unusual for a tropical depression to form south of 8°N latitude.

I expect that 92L's best chance to become a tropical depression will come on Tuesday, and the storm could strengthen enough by Wednesday to be named Tropical Storm Alex. The farther south 92L stays, the better chance it has at survival. With the system's steady west-northwest movement this week, 92L will probably begin encountering hostile wind shear in excess of 20 knots by Wednesday, which should interfere with continued development. Several of our reliable models do develop 92L into a tropical storm with 40 - 55 mph winds, but all of the models foresee weakening by Thursday or Friday as 92L approaches the Lesser Antilles Islands and encounters high shear and dry air. I doubt 92L will be anything stronger than a 45 mph tropical storm when it moves through the northern Lesser Antilles Islands on Friday and Saturday, and it would be no surprise if wind shear has destroyed the storm by then. However, as usual, surprises can happen, and the GFS and the SHIPS model (which is based upon the GFS) do indicate that more modest levels of wind shear in the 15 - 20 mph range late this week may allow 92L to stay stronger than I'm expecting. Residents of the islands--particularly the northern Lesser Antilles--should follow the progress of 92L closely, and anticipate heavy rains and high winds moving through the islands as early as Thursday night.

Oil spill wind forecast
There is little change to the oil spill wind forecast for the coming two weeks. Light winds of 5 - 10 knots mostly out of the south or southeast will blow in the northern Gulf of Mexico all week, according to the latest marine forecast from NOAA. These winds will keep oil near the coast of Louisiana, Alabama, Mississippi, and the extreme western Florida Panhandle, according to the latest trajectory forecasts from NOAA and the State of Louisiana. The long range 8 - 16 day forecast from the GFS model indicates a typical summertime light wind regime, with winds mostly blowing out of the south or southeast. This wind regime will likely keep oil close to the coastal areas that have already seen oil impacts over the past two weeks.

Jeff Masters

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2790. msphar
thanks, seems low or hardly significant given the small size of the satellite image.
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2789. xcool
15/0545 UTC 12.0N 42.6W T1.0/1.0 92L
Member Since: September 26, 2009 Posts: 2 Comments: 15620
Interesting, 92L was taken off the CIMSS map on their home page.
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2786. xcool
25wind speed
Member Since: September 26, 2009 Posts: 2 Comments: 15620
2785. EricSFL
Quoting btwntx08:

its on the blog directory scroll down a bit and its on the bottom right hand side it says blog community...also i figured it out cause i had 4,099 comment early sunday am then now im up to 4,315 which equal 216 and that is increasing now lol cause im posting this lol


thanks
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Quoting msphar:
Knots are fine with me, I prefer them actually but what is 25 knots ?? Is this an estimate of wind speed or track over ground ??


The former.
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2783. msphar
Knots are fine with me, I prefer them actually but what is 25 knots ?? Is this an estimate of wind speed or track over ground ??
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2781. EricSFL
Oh my God, it IS an eye after all!
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Quoting msphar:
xcool what does the 25 kts in your last post refer to ???


25 kt = 30 mph (rounded).
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2778. msphar
xcool what does the 25 kts in your last post refer to ???
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2777. xcool
92L POPOP NOW
Member Since: September 26, 2009 Posts: 2 Comments: 15620
2776. EricSFL
Quoting btwntx08:
om i didnt relialied i went over 100 comments again wow total for the last 42 hrs is now 216 comments


How can you see that?
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2775. xcool
ECMWF 0000%
Member Since: September 26, 2009 Posts: 2 Comments: 15620
2773. EricSFL
If the NHC does not give this thing at least TD status by 5AM, we should all sign a petition (somehow) for 92L to appear in the record books. lol
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2772. xcool


Member Since: September 26, 2009 Posts: 2 Comments: 15620
Quoting EricSFL:


But we cannot really say it will track over the islands for sure.


That's this forecasters opinion
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92L is very close to (if not already) a TD...reasoning? persistance of TS around the COC that it didnt have 12hrs ago
Member Since: January 22, 2006 Posts: 0 Comments: 1600
2768. EricSFL
Quoting louisianaboy444:


I think once its done its done...with the tideous track near the mountainous islands i think once the shear decouples the system that may be all she wrote


But we cannot really say it will track over the islands for sure.
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2767. xcool


Member Since: September 26, 2009 Posts: 2 Comments: 15620
Quoting KoritheMan:


If it develops today, I think it will pull an Ana, dying off very quickly, only to regenerate later.


I think once its done its done...with the tideous track near the mountainous islands i think once the shear decouples the system that may be all she wrote
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2764. xcool


Member Since: September 26, 2009 Posts: 2 Comments: 15620
2763. xcool
EricSFL :)
Member Since: September 26, 2009 Posts: 2 Comments: 15620
We need to remember the NHC has been around the block a couple times and if a conserative NHC is still giving it a 50/50 chance they are apparently seeing something
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2760. Hattie
hey guys , Id just like to say good night.see you all tomorrow..
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2759. EricSFL
Quoting xcool:
i see nww & west move now ?


xcool, I'm so tired that maybe I'm seeing things moving erratically...
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2757. xcool
i see nww & west move now ?
Member Since: September 26, 2009 Posts: 2 Comments: 15620
2756. xcool
lmao
Member Since: September 26, 2009 Posts: 2 Comments: 15620
2755. EricSFL
Based on the last few frames, I actually see 92L moving more NNW, as if it was trying to relocate under the convection...
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Quoting SouthDadeFish:
Korithe I agree with you that it doesn't look amazing, but I still feel it is a TD. And it's fired convection for over a full day away from the ITCZ. If Ana could be a TS last year than this could at least be a TD lol. Night all.


If it develops today, I think it will pull an Ana, dying off very quickly, only to regenerate later.
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2752. xcool
btwntx08 .i rember him from 2009 ???
Member Since: September 26, 2009 Posts: 2 Comments: 15620
Quoting KoritheMan:


Persistent convection. And it persisting through DMAX means absolutely nothing.
Korithe I agree with you that it doesn't look amazing, but I still feel it is a TD. And it's fired convection for over a full day away from the ITCZ. If Ana could be a TS last year than this could at least be a TD lol. Night all.
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Quoting SykKid:


Nothing too impressive. Looks like a disorganized TD to me....Nothing to get excited about...at best this thing makes it to a minimal TS.


Agreed. And even TS status is pushing it.

Though I should note that I do expect development, just not until it reaches the Caribbean in a few days, when models relax the shear substantially.
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Quoting GTcooliebai:

Is that an eye I see on that last frame...j/k it must be a pocket of dry air wrapping around it's center.

I saw it too lol. Definitely a partially formed eye.
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Quoting SouthDadeFish:
Nonetheless, there is still convection over the center and a closed low. I thought those were the requirements for a TD. Well they used to be at least.....


Persistent convection. And it persisting through DMAX means absolutely nothing.
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Well this thing isn't going anywhere overnight, so Imma head of to bed. Interesting start to the season. Glad it most likely wont amount to anything more than a weak TS.
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2744. xcool


Member Since: September 26, 2009 Posts: 2 Comments: 15620
Quoting KoritheMan:
Not really sure why everyone is impressed? The deepest convection isn't even over the center...
Nonetheless, there is still convection over the center and a closed low. I thought those were the requirements for a TD. Well they used to be at least.....
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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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