Severe Tropical Cyclone Ilsa

[Somerset Squall]

Invest 92P has continued to get much better organised overnight and it is now Tropical Cyclone Ilsa. Intensity is currently estimated by BOM to be at 50kts, and further intensification is likely over the next couple days. Ilsa is displaying some good banding features and plenty of deep, central convection, a characteristic of a healthy system. Ilsa will move generally westwards along the northern periphery of a subtropical steering ridge to the south, strengthening in an environment of low shear, warm sea temperatures and good poleward outflow. The storm is not likely to efdect the Australian mainland and will probably pass south of Cocos Island and Christmas Island, which is just as well as Ilsa has the potential to become quite strong.

[kold weather]

Ilsa has got some very deep convection wrapping around the circulation with hints of an eyewall setting up in the eastern quadrant on IR, though no eye is yet showing. Looking at the way it has developed recently I see no reason why this won't get above 100kts.

Edited March 18, 2009 by kold weather

[Somerset Squall]

Ilsa has got some very deep convection wrapping around the circulation with hints of an eyewall setting up in the eastern quadrant on IR, though no eye is yet showing. Looking at the way it has developed recently I see no reason why this won't get above 100kts.

Indeed, shear was a problem for the system yesterday but it has continued to ease right down. Sea temps are really warm and will continue to be as long as Ilsa maintains it's westwards track and doesn't dip too far south. Outflow is really good at the moment as well, and this is allowing some really intense convection over the centre as you say. Rapid intensification is likely and I agree that Ilsa may become an intense system.

[Somerset Squall]

Ilsa is on the verge of cat 1 status on the SS scale, as intensity is now at 60kts. Ilsa has continued to rapidly strengthen in a favourable environment. No major changes are forecast to occur in the environmental conditions, so more intensification is expected.

[Cookie]

thanks for the updates mate

[Somerset Squall]

Ilsa continues to rapidly intensify, and is now an 80kt, cat 1 cyclone on the SS scale. An increasingly well defined eye is visable on satellite imagery, along with maintained excellent banding features wrapping neatly around the the very strong circulation. Shear is low, waters are very warm and Ilsa is also making use of some strong dual outflow channels to rapidly strengthen. Ilsa is expected by BOM to peak at 100kts, I think Ilsa could go a little higher, if I were to guess I reckon around 110-120kts based on the current rapid intensification.

Beyond 48hrs, Ilsa will move over cooler waters, and as shear is expected to increase around this time, weakening is forecast. Just how agressive depends on how strong the shear is, as waters could well still be around 25-26C (depending on just where Ilsa tracks), which is not low enough to cause rapid weakening on it's own.

[Cookie]

shes a bit of beast

[Cookie]

new tracker

Edited March 19, 2009 by cookie

[Somerset Squall]

Ilsa has continued to rapidly intensify. She is now a 100kt, cat 3 cyclone. Further intensification is likely over the next day or so before cooler waters and elevated shear may induce slow weakening. I reckon Ilsa may peak at 110-120kts before the weakening begins. The system certainly has a well defined eye and a good environment to reach this kind of intensity.

[Cookie]

still looking decent

[Somerset Squall]

Ilsa peaked at 110kts this morning, but has since weakened to 85kts, as shear has increased from the east and sea temps are gradually cooling. These two factors will cause continued weakening over the next couple days as the storm drifts westwards.

You can see on latest satellite imagery that the clear eye has gone but there are still some pretty powerful winds near the centre as Ilsa slowly winds down:

Edited March 20, 2009 by Somerset Squall

[Cookie]

thanks for the great updates mate

[Somerset Squall]

Ilsa continues to gradually weaken. Intensity is now at 65kts. The slow weakening trend is expected to continue as sea temps continue to fall, and shear remains moderate. However, Ilsa is expected to remain a tropical cyclone for the next 48hrs at least as she drifts slowly west-southwestwards.

[Somerset Squall]

Ilsa has maintained intensity overnight, an in latest satellite imagery, you can see an eye re-appearing. Outflow is improving, especially in a poleward direction, and shear is low. Although waters are fairly cool beneath the system, the low shear and good outflow has allowed Ilsa to maintain the 65kt intensity. Ilsa may remain a cat 1 on the SS scale for the next 12 hours as she moves generally westwards and the upper level environment remains favourable. However, after this, shear is expected to increase again, cutting off outflow, and coupled with the marginal sea temps, weakening should resume. Interesting to note that Ilsa is expected to persist west of 90E, which will transfer the storm from BOM's area of responsibility to MeteoFrance. If this occurs, Ilsa will gain a second name (much like TC Lee-Ariel in 2007 for example).

Edited March 22, 2009 by Somerset Squall

[Somerset Squall]

Ilsa has weakened considerably this evening, and you can clearly see by the satellite image below that Ilsa doesn't look anywhere near as healthy as it did this morning. Convection has decreased markedly over the last few hours in particular, as dry air wraps around the system. The LLC remains well defined and is clear to see, due to there being very little convection covering it. Shear is still low, but sea temps are marginal and the environment is quite dry. Intensity has been lowered to 45kts, and further weakening is expected. Ilsa may dissipate within the next 48hrs.

[Cookie]

its amazing time and time again how fast they can intensify and weaken!

[Cookie]

All but gone now

[Somerset Squall]

All but gone now

Agreed. Dry air is really surpressing convection, and waters are quite cool aswell, which is cancelling out the low shear. Intensity is now at 35kts and Ilsa looks to be on the verge of dissipation unless convection makes a comeback soon.