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Vorticity0123

Severe Tropical Cyclone Kate

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After a week of (global) inactivity in tropical cyclones, a new one appears to be on the verge of forming. 04U has a large area of deep convection over and to the south of the center, as well as some banding features (especially in the south).

 

The low is located to the southwest of Indonesia, just to the north of Cocos Island. The Bureau of Meteorology (BOM) expects the cyclone to intensify into a category 3 cyclone (Australian scale, about equal to a category 1-2 hurricane on the Saffir-Simpson hurricane scale). Regarding track, 04U will move southeastward intially, before curving back to the southwest possibly impacting Cocos Island in about 3 days.

 

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Track forecast of 04U (as of 23-12-2014, from the BOM)

 

Sources:

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

http://www.bom.gov.au/cyclone/index.shtml

http://en.wikipedia.org/wiki/Tropical_cyclone_scales

Edited by Vorticity0123

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04U has now been upgraded to Tropical Cyclone Kate by BOM. Winds are at 35kts currently. Kate currently has some fairly deep centralised convection and some fairly good banding features. Shear is moderate, but waters are warm and outflow is good, meaning Kate will likely intensify at least steadily over the next few days before reaching cooler waters. The track forecast keeps swinging back and forth from a track to the west of Cocos Island to a track to the east of Cocos Island. It's fair to say that Cocos Island will receive some adverse weather from Kate, but how severe will depend on the final track.

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It's expected to pass close to the northern side of the islands early afternoon today. Not a nice visitor to have on Christmas Day. The highest topographical point on the islands is about 10 metres asl.

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Kate has now passed the Cocos Islands and has been upgraded to category 2. It's expected to get to category 3 but then move into higher wind shear and cooler sea surface temperatures.

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Kate has significantly intensified overnight, with winds now at 65kts (which is a cat 3 severe classification on the Australian scale). An eye has emerged from the central dense overcast, indicative that the moderate levels of shear have lessened. Kate has the potential for a little more strengthening today before shear rises and waters cool as the track takes a more southerly component.

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post-1820-0-42855100-1419599652_thumb.pn

Edited by Somerset Squall

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IDW27600
TROPICAL CYCLONE TECHNICAL BULLETIN: AUSTRALIA - WESTERN REGION
Issued by PERTH TROPICAL CYCLONE WARNING CENTRE
at: 1902 UTC 26/12/2014
Name: Severe Tropical Cyclone Kate
Identifier: 04U
Data At: 1800 UTC
Latitude: 12.6S
Longitude: 93.9E
Location Accuracy: within 30 nm [55 km]
Movement Towards: west southwest [238 deg]
Speed of Movement: 3 knots [5 km/h]
Maximum 10-Minute Wind: 80 knots [150 km/h]
Maximum 3-Second Wind Gust: 110 knots [205 km/h]
Central Pressure: 963 hPa

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Kate has rapidly strengthened and winds are now at 105kts (1 min sustained JTWC), which is cat 3 major hurricane strength on the SS scale. JTWC expect a peak of 125kts, which is considerably stronger than all intensity forecasts so far.

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Kate has upped the ante, now a category 4. However, wind shear is beginning to increase.

IDW27600
TROPICAL CYCLONE TECHNICAL BULLETIN: AUSTRALIA - WESTERN REGION
Issued by PERTH TROPICAL CYCLONE WARNING CENTRE
at: 0136 UTC 27/12/2014
Name: Severe Tropical Cyclone Kate
Identifier: 04U
Data At: 0000 UTC
Latitude: 12.8S
Longitude: 93.7E
Location Accuracy: within 30 nm [55 km]
Movement Towards: southwest [230 deg]
Speed of Movement: 4 knots [7 km/h]
Maximum 10-Minute Wind: 90 knots [165 km/h]
Maximum 3-Second Wind Gust: 125 knots [230 km/h]
Central Pressure: 953 hPa

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Kate has upped the ante, now a category 4. However, wind shear is beginning to increase.

 

Yes, it has been the first potent tropical cyclone of the Southern Hemisphere of this season, and it has been a real overachiever so far!

 

However, it seems that Kate has started a steady weakening trend. The eye is no longer discernible, and it seems that the LLCC (low level circulation center) is located close to the eastern edge of the deep convection. This can be seen on the latest visible satellite image of Kate below:

 

post-20885-0-73385200-1419724038_thumb.g

Visible satellite image of Kate. The image does not auto-update itself. Courtesy: NOAA

 

CIMSS analysis of the cyclone also agrees with the observation, and the LLCC might become exposed on the eastern edge of the deep convection if current trends continue. The main cause is easterly shear impinging on the cyclone.

 

The Bureau of Meteorology (BOM) expects the cyclone will continue to weaken due to easterly wind shear and cooler SSTS (sea surface temperatures) as the cyclone will continue moving to the southwest.

 

Sources:

http://www.ssd.noaa.gov/PS/TROP/floaters/04S/04S_floater.html

http://www.bom.gov.au/cgi-bin/wrap_fwo.pl?IDW27600.txt

http://tropic.ssec.wisc.edu/#

http://www.usno.navy.mil/NOOC/nmfc-ph/RSS/jtwc/warnings/sh0415web.txt

Edited by Vorticity0123

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Kate is now category 2, although still 60 knots (10 minute interval). As it moves southwards it appears to be dragging the monsoon trough down. BOM is predicting the monsoon trough to be over the Top End on Friday 2nd January by which stage Kate should be well and truely dissipated.

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Kate is now category 2, although still 60 knots (10 minute interval). As it moves southwards it appears to be dragging the monsoon trough down. BOM is predicting the monsoon trough to be over the Top End on Friday 2nd January by which stage Kate should be well and truely dissipated.

 

Can this have an effect on the MJO?

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Kate weakened to 65kts (1 min sustained), but has since unexpectedly restrengthened to 75kts, as shear has lessened. Kate should resume weakening soon as it moves over cooler water and increased shear again. Interesting the way this cyclone keeps defying forecasts!

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Unexpectedly, Kate has put up a second round of intensification.The cyclone has become quite a bit better organized since yesterday, deceiving the forecasts from various agencies from yesterday.

 

Structural changes

 

A cloud-filled eye has become visible, which is surrounded by deep, though irregular, convection. The eye is not well visible on VIS imagery (probably this has to do with the resolution), but Dvorak imagery shows this well:

 

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Dvorak satellite image of Kate. This image does not update itself.

 

Note that the eyewall is not completely circular on the southern side. This indicates the system still has some room left for structural improvements.

 

Further analysis also shows the eyewall is not completely encompassing the cyclone; there seems to be a gap in its southeastern quadrant. This can be seen in MIMIC imagery from CIMSS:

 

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CIMSS MIMIC imagery loop of Kate. The loop can be activated by clicking on the link provided. The image does not auto-update itself.

 

 

Apart from the broken eyewall, it can also be seen that the structure of the cyclone has improved quite a bit since yesterday, as an eyewall was largely absent a day earlier.

 

Intensity assessment

 

Given the increase in organization, the JTWC has upped the intensity of Kate to 75 kt from 65 kt in the previous advisory. Furthermore, the BOM (Bureau of Meteorology) has re-upgraded the cyclone to a category 3 storm (Australian intensity scale, this equals about a category 1 hurricane on the Saffir Simpson hurricane scale).

 

CIMSS ADT satellite intensity estimates suggests that the cyclone may be even stronger, as can be seen in the satellite intensity estimate trend below:

 

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CIMSS ADT satellite intensity estimate trend over the lifetime of Kate. The image does not auto-update itself.

 

As can be seen on the image, a very sharp increase in intensity can be observed over the last few hours. Currently, the assessed intensity is about 96 kt, and this may increase even further in the short-term. Though these intensity estimates might have a high bias over the last few days (as the structure of the system, as well as the official intensity estimations from JTWC suggested the system was much weaker than the CIMSS ADT intensity estimate would suggest), the observed rapid increase in organization argue that the CIMSS intensity assessment may not be far off the mark.

 

Causes

 

The cause for this unexpected intensification might be that shear has been not as strong as expected. In fact, the cyclone is currently located in an area with wind shear between 10 to 20 knots, though this value might be lower judging from CIMSS shear analysis. Also, the equatorward outflow, which was expected to decrease from yesterday has not decreased yet (as assessed by the JTWC).

 

Future of the cyclone

 

Kate is expected to continue moving southwestward, which will bring it into a higher shear environment. Furthermore, the southwestward motion will direct Kate toward cooler sea surface temperatures, reaching the 26*C isotherm by tomorrow. This can be seen on the GFS forecast of the cyclone below:

 

2.track.png

GFS forecast track of Kate + sea surface temperatures (12Z 28-12-2014 run).

 

A major caveat to the forecast track is that any motion to the north of the forecast will result in Kate moving over warmer waters, which can delay its weakening. On the other hand, any south of the forecast given will cause Kate to encounter even cooler SSTS, hastening its demise. Note that even though the forecast from the GFS is from yesterday, the same rules apply regarding deviations in track.

 

So even though the system may gain a little intensity over the next few hours as shear remains low and SSTS sufficiently warm (this possibility is also mentioned by BOM), its southwestward track willl move it into much less favorable conditions, which will eventually lead to its demise. The rate of weakening will mainly depend on its track, as described above.

 

Regardless of any possible north-south deviations in the track of Kate, it will soon move west of 90W, where RSMC La Réunion will take over responsibility of the cyclone.

 

 

 

Can this have an effect on the MJO?

 

The question is a rather difficult one to answer from scratch, and requires thorough understanding of the processes involved. The connection between the MJO and tropical cyclone activity is that certain phases of the MJO favor tropical cyclone formation in certain parts of the world. A scientific article containing more in-depth information is given below:

 

Link

 

Basically, the same occurs for the ITCZ. Enhanced ITCZ activity at certain times can often be explained by the MJO being in a certain phase. However, what the influence is of a southward moving ITCZ on the MJO is unclear to me. Perhaps somebody more knowledgeable than me might be able to provide a better answer to this question.

 

EDIT: Somerset Squall, you just beat me on this one :wink: .

 

Sources:

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

http://tropic.ssec.wisc.edu/real-time/mimic-tc/2014_04S/webManager/mainpage.html

http://tropic.ssec.wisc.edu/real-time/windmain.php?&basin=austwest&sat=wgms∏=sht&zoom=&time=

http://www.ssd.noaa.gov/PS/TROP/floaters/04S/04S_floater.html

http://en.wikipedia.org/wiki/Tropical_cyclone_scales

http://moe.met.fsu.edu/cyclonephase/gfs/fcst/index.html

http://www.bom.gov.au/cyclone/index.shtml

http://www.meteofrance.re/cyclone/activite-cyclonique-en-cours

http://journals.ametsoc.org/doi/abs/10.1175/JCLI-D-13-00483.1

Edited by Vorticity0123
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Can this have an effect on the MJO?

I'm by no means an expert but my understanding was that it was more the other way around, that disturbances are heightened by the MJO.

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Low VWS and warmer waters have ramped Kate up again, now a category 4 with 10 minute winds at 90 knots, 3 second gusts to 125 knots. It's now moving out of the Australian western region and into that monitored by RSMC La Reunion.

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Poleward outflow into an area of high shear to the south has caused Kate to rapidly intensify overnight. Winds are up to 105kts, cat 3 on the SS scale (1 min sustained, JTWC). The eye has cleared out and is surrounded by deep convection. As the poleward outflow remains strong, Kate could strengthen a little more, and reach a new peak intensity.

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Edited by Somerset Squall

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Kate is, once again, weakening. Shear is rising again, giving Kate a lopsided look. Winds are down to 85kts (1 min sustained). As waters continue to cool along track, Kate should continue to weaken from hereonin.

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Amazing to see how quickly tropical cyclones can intensify and weaken. Two days ago, Kate put up an unexpected round of intensification, intensifiying 40 kt in less than a day. Now, the cyclone has lost 50 kt in just 24 hours! The LLCC of the cyclone became exposed a couple of hours ago, and according to JTWC, the system has weakened to a 35 kt tropical storm from a 85 kt hurricane. As a result, the JTWC has issued their final warning on the system.

 

Source:

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

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