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Somerset Squall

Very Intense Tropical Cyclone Bansi

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The first named tropical storm of 2015 has formed east of Madagascar, and north of La Reunion. Moderate Tropical Storm Bansi has winds of 35kts. The storm has well defined banding features wrapping into some moderately deep convection over the LLCC. Shear is moderate, but equatorward outflow is good. Poleward outflow is fairly poor however due to a nearby upper level low supressing the outflow mechanism in this direction. Due to the warm waters beneath Bansi, and the good equatorward outflow, Bansi should at least intensify slowly over the coming days as it moves slowly south eastwards along the southwestern flank of a ridge to the northeast.

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Edited by Somerset Squall

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Well scrap slow intensification, Bansi is definitely undergoing rapid intensification! A solid central dense overcast has formed, and an eye is already apparent on satellite imagery!

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Edited by Somerset Squall

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RSMC La Reunion has elevated the system to Tropical Cyclone. It's situated 395 kms to the north of La Reunion and moving south at 11 kph. However later Monday it's expected to turn more to the east avoiding a direct hit. The island has been placed under cyclone pre-alert (or early warning).
 

(Interesting perspective, "The first named tropical storm of 2015...." looks rather odd. We never see it as based on calendar year, this is the 2014/15 cyclone season and we look at it seasonally.)
 

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Bansi's winds are up to 65kts according to JTWC and MeteoFrance. JTWC now expect a peak of 120kts (cat 4 on SS scale, 1 min-sustained). Outflow has vastly improved in all quadrants, and shear has dropped out, so further robust strengthening appears likely.

Edited by Somerset Squall

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Bansi's winds are up to 65kts according to JTWC and MeteoFrance. JTWC now expect a peak of 120kts (cat 4 on SS scale, 1 min-sustained). Outflow has vastly improved in all quadrants, and shear has dropped out, so further robust strengthening appears likely.

 

The cyclone has definitely put up a siginficant round of intensification. In fact, the cyclone looks more like a category 2 cyclone (Saffir-Simpson Hurrncane Scale), judging from Dvorak satellite imagery:

 

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Dvorak satellite image of Bansi. Courtesy: NOAA.

 

As can be seen from the image, a distinct eye feature is present, accompanied by a circular, though broken, eyewall. If Bansi is able to close off its eyewall, further intensification is likely. Luckily, there do not appear to be any landmasses on the path of the cyclone (though any deviation to the south may bring La Reunion in the danger zone of the cyclone.

 

EDIT: Added MIMIC TC-imagery to show the broken eyewall more evidently.

 

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MIMIC TC-imagery of Bansi. The image does not auto-update itself. Courtesy: CIMSS

 

Sources:

http://www.usno.navy.mil/JTWC/

http://www.ssd.noaa.gov/PS/TROP/floaters/05S/05S_floater.html

http://tropic.ssec.wisc.edu/real-time/mimic-tc/2015_05S/webManager/mainpage.html

Edited by Vorticity0123

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Bansi has continued to rapidly intensify, with winds now at 100kts according to JTWC, cat 3 on the SS scale. The eye has continued to become better defined and has cleared out also. Very favourable multi-directional outflow, low shear and warm waters are responsible for this rapid intensification, and similar conditions will allow for further intensification for the next 36hrs or so. Bansi is slowly moving eastwards at present, well north of Mauritius. The current JTWC track spares Mauritius the worst of Bansi's weather, though any deviation south will bring more adverse weather here.

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Edited by Somerset Squall

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Bansi has continued to intensity rapidly. Winds are up to 130kts, according to JTWC, which is amazing considering Bansi was only named early yesterday! JTWC expect a peak of 150kts (cat 5 on SS scale), if this was to occur it would be one of the strongest storms recorded in the South Indian Ocean!

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Bansi has continued to intensity rapidly. Winds are up to 130kts, according to JTWC, which is amazing considering Bansi was only named early yesterday! JTWC expect a peak of 150kts (cat 5 on SS scale), if this was to occur it would be one of the strongest storms recorded in the South Indian Ocean!

 

Incredible... Such explosive intensification events are rarely seen, I am not aware of any of such events happening in the Southwest Indian Ocean (though I might be mistaken).

 

Bansi is looking stellar onm satellite imagery, The cyclone has an almost symmetric ring of deep convection surrounding a clear eye. Fortunately, no landmasses are impacted directly by the cyclone.

 

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Visible Satellite image of Bansi.

 

To put the intensification in some perspective: just 36 hours ago, Bansi was still a 35 kt tropical storm, and now it is a 130 kt huricane! This means an increase of 95 knots in just 36 hours, or 2,67 kt/hour (compare the forecast that Somerset Squall posted in his first post with the current intensity estimate).

 

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Current forecast track and intensity of Bansi from the JTWC.

 

Sources

http://www.meteo.fr/temps/domtom/La_Reunion/webcmrs9.0/anglais/:

http://www.ssd.noaa.gov/PS/TROP/floaters/05S/05S_floater.html

http://www.usno.navy.mil/JTWC/

Edited by Vorticity0123

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Bansi is going through a weakening trend due to eye wall replacement and is moving eastwards at 4 knots. Central pressure 938 hPa, wind speed (10 minute) 105 knots, radius of maximum winds 13 kms. Due to a weak steering environment Bansi is expected to move slowly over the next 24 hours and then track southeastwards accelerating over the following few days. Although today and tomorrow conditions are expected to be favourable with good poleward and equatorward outflow from Thursday it's expected to lose the equatorward outflow and then begin to come under increasing vertical wind shear and lower sea surface temperatures.
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Indeed, so difficult to predict when these will occur, and yes, the process has weakened Bansi to 115kts using 1 min-sustained winds (JTWC). JTWC are expecting Bansi to recover and still go on to become a 140kt cat 5 on the SS scale after the cycle finishes. Whether this occurs is a little more touch and go than previously thought, but the potential is still there.

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The eye wall replacement certainly shook up Bansi. Central pressure now is 960 hPa and wind speed (10 minute) is 80 knots. However, radius of maximum winds has spread out to 65 kms and gales may reach 300 kms. It is expected to regain a peak around 90 knots from 24 to 36 hours. Thereafter weakening should occur as conditions become less favourable.

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Bansi now has a huge eye, and is a much larger cyclone than before the eyewall replacement cycle. As Tropicbreeze has stated, winds have reduced significantly, and are at 90kts (1 min sustained). Bansi still has the chance to regain some strength over the next day, as it nears Rodrigues. The track for Bansi has been shifting east, putting the island in the firing line from dangerous Bansi.

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Bansi has started to contract, radius of maximum winds is now 48 kms. Wind speed (10 minutes) at 75 knots, central pressure 963 hPa. It's expected to track further east taking it further from Rodrigues. Wind speed is expected to increase to 90 knots in 12 hours, at which stage it will be near to its closest point to Rodrigues, and then weaken from 36 hours on.
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Bansi is rapidly intensifying again. Winds are up to 120kts, cat 4 on the SS scale (1 min sustained). JTWC are forecasting Bansi to reach 145kt winds whilst the cyclone is near Rodrigues, even if the track shifts further east it looks like the island will still be in for some pretty rough weather.

Edited by Somerset Squall

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Bansi is currently moving east south east at 11 knots and could pass within 100 kms of Rodrigues. Central pressure 940 hPa, wind speed (10 minute) 100 knots. radius of maximum winds 39 kms. It should remain in a favourable environment overnight and then come into increasing vertical wind shear with decreasing sea surface temperatures.

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The second peak of Bansi appears to be reached according to the JTWC. Nevertheless, the cyclone has intensified into a 130 kt tropical cyclone (1 min. average), and now contains a large eye surrounded by a nearly circular eyewall. This can be seen in Dvorak imagery below:

 

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Dvorak satellite imagery of Bansi.

 

This is a good example of the volatility that the intensity of a very intense tropical cyclone caused by inner core dynamics.

 

Sources:

http://www.usno.navy.mil/JTWC/

http://www.ssd.noaa.gov/PS/TROP/floaters/05S/05S_floater.html

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0600UTC 16/01/2015 Bansi had a central pressure 923 hPa, wind speed 115 knots and radius of maximum winds 37 kms.

The latest, 1900UTC 16/01/2015, central pressure 935 hPa, wind speed 100 knots and radius of maximum winds 46 kms. Movement to the south east at 11 knots.

The eye wall is showing erosion in the northwest.

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Bansi is moving east south east at 15 knots. Central Pressure 956 hPa, wind speed (10 minute) 70 knots, radius of maximum winds 59 kms. It's expected to commence extratropical transition Tuesday.

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It is an amazing shot, it's shown up on a few threads on Netweather and other weather sites. There's also one taken further along so it's at much more of an angle. And also one with a (that?) storm well away from the eye and showing up as a bright luminescent spot. But it's a long way to go to get a good photo. They were taken by Astronaut Sam Cristoforetti.

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