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Invest 93S Could Become Powerful Cyclone

By: MAweatherboy1 , 9:51 PM GMT on April 05, 2013

The tropics have been very quiet worldwide in recent weeks. We remain in the offseason of the Atlantic and East Pacific basins (though we grow closer every day!), and the West Pacific, South Pacific, and South Indian basins have been very quiet. This is likely to change in the coming days thanks to organizing invest 93S in the Southern Indian Ocean. This invest is currently located at approximately 11S and 74E, with the closest land area being the island of Diego Garcia a couple hundred miles NNW of 93S. The system is currently moving W/WSW at about 12kts, and this general motion is likely to continue in the coming days. The Joint Typhoon Warning Center (JTWC) has issued a Tropical Cyclone Formation Alert for the system as of 9AM EDT this morning, meaning there is a high probability of this system becoming a designated tropical cyclone in the next 24 hours. Global models strongly agree that 93S will indeed develop over the next day or two as it moves west, towards Madagascar and southeast Africa.

Figure 1: Invest 93S. Convection and banding on the west side is fairly impressive, but the center remains partially exposed east of the convection.

Forecast for 93S
93S has been prevented from developing in the past few days by wind shear, which is why the circulation has remained exposed or partially exposed east of the convection in the past couple days. This wind shear is relaxing, however, which will allow for invest 93S to develop in coming days, as the global models suggest. Environmental conditions should be favorable along the expected path of 93S, with the diminishing wind shear remaining quite low, and favorable sea surface temperatures of up to 29 degrees Celsius. The big question becomes how strong can 93S get? The models are in full agreement concerning development, and are coming into better agreement that significant strengthening is likely for this system. Regarding track, 93S should be steered on a general W/WSW motion for the next 4-5 days at least, and possibly up to a week depending on its speed. This is fairly well agreed upon by the models. The big question becomes how far west does it get, and what happens in the day 6-8 period. Right now, none of the major global models (GFS, Euro, CMC, UKMET) show a direct hit on Madagascar, which would be the first potential large land target, and likely the only one. The Euro and GFS are in fairly good agreement on a rather close pass, as it has been trending that way in recent days. There is likely to be a significant slowing or stalling of the system in the day 6-8 period as steering currents change up and briefly leave the cyclone caught in the middle. After this pause, models once again converge on a track featuring a recurve SE, away from Madagascar, in the 8-10 day period, though as one would expect timing and placement differences are very much a problem at this range. Once it does make the turn south and SE, weakening is likely, and the storm will eventually become extratropical. Overall, I would say that Madagascar, especially the northern part of the island, should keep an eye on future developments of this potentially powerful system, though right now a direct impact is unlikely. The islands off the coast of Madagascar, however, need to already be very careful with this situation as they could be dealing with major hurricane equivalent impacts. The following map presents my thinking. Due to timing differences in the models I haven't given a timeline but simply track and intensity. A word of warning, of course, is that this is certainly not an official forecast.

Figure 2: My forecast for 93S.

Figure 3: A graphic showing 93S's path thus far, and its likely short term motion of WSW.

It should be noted that rapid intensification is possible, if not likely, when the storm begins its pause off the northern Madagascar coast, likely in around 6 days. I will provide further blogs on this system as necessary. There is also the potential for another cyclone to develop on the other side of the Southern Indian basin, which could head SE towards western Australia, though a major impact seems unlikely at this time. If necessary, I will provide updates on that system in future blogs, though 93S will be the main priority.

I would greatly appreciate any feedback on my forecast graphic as I plan to make more graphical forecasts this year in the Atlantic than last year.

Thank you for reading, and I hope you have a great weekend!


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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8. WunderAlertBot (Admin)
10:50 PM GMT on April 08, 2013
MAweatherboy1 has created a new entry.
7. MAweatherboy1
9:46 PM GMT on April 07, 2013
Here's my updated track on 21S. As I suspected yesterday, the track needed an eastward shift, and I am now very confident in saying Madagascar will not see any direct impacts, and the Reunion Islands may even escape without a direct hit.

Shear has been putting a beating on 21S, keeping strengthening very slow. I still predict strengthening, however, and have only slightly lowered the intensity forecast as that forecast philosophy remains the same. Here's JTWC's track, I think they've come too far east, I'm not ready to make that big a shift. They forecast 110 mph for a peak intensity.

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6. MAweatherboy1
11:18 PM GMT on April 06, 2013
Here's the JTWC forecast track of 21S. They forecast a peak intensity of 100kts, or about 115mph.

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5. MAweatherboy1
11:12 PM GMT on April 06, 2013
Updated track forecast on former 93S, now Cyclone 21S. No need for a full new blog as I don't see much reason to change anything I said yesterday. New track is almost identical to yesterday's except a bit of a sharper SE curve near the end, and intensity forecast is also almost unchanged, lowered just barely on the peak.

If anything, the track may need to shift a little bit east with time as I'm noticing a bit of a model shift in that direction; need to wait for confirmation from the 0z suite to make any changes though. In addition, it should be noted that the intensity forecast could end up being too aggressive should the system not rapidly intensify near northern Madagascar, but right now I still think a heightened pace of intensification is likely enough to keep the system reaching Cat 3 equivalent status.
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4. trHUrrIXC5MMX
10:57 PM GMT on April 05, 2013
Thanks for the forecast MA
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3. PedleyCA
10:30 PM GMT on April 05, 2013
Thanks for posting about this, very well done.
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2. MAweatherboy1
10:03 PM GMT on April 05, 2013
Quoting wxchaser97:
I agree with your forecast. Thanks MA.

GFS turns 93S into a powerful cyclone.

So does the GEM.

Yep, it's possible that it ends up being even a little stronger than I forecast if it really blows up by Madagascar, but since this is an initial forecast I'm being a little more conservative.
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1. wxchaser97
10:02 PM GMT on April 05, 2013
I agree with your forecast. Thanks MA.

GFS turns 93S into a powerful cyclone. GFS also developed that other cyclone, but that was gone in this image.

So does the GEM. That other cyclone is seen bottom right.

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Average 20 year old weather nerd. Plymouth State University Meteorology, Class of 2018. NOAA Hollings scholar. Summer 2016 intern at NWS Boston.

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