Tropical Cyclone David

[Somerset Squall]

Invest 93S has become sufficiently organised to be classified as the fourth TC of the south indian ocean season. Quite an active start really, as it is only approaching mid-december.

05S' intensity is currently 35kts. Deep convection is persisting around a well defined LLC, and banding features are beginning to form. Shear is low, and waters are warm. Additionally, poleward outflow is being increased into a trough positioned east of 05S, which is serving to tighten the LLC and enhance convection. These factors suggest that 05S will strengthen over the coming days. Until I see what 05S does, I won't have a punt on just how strong 05S gets yet, but with the strong LLC and good environment, prospects are looking good for this cyclone.

05S is currently moving south-southwestward, but a ridge that is indicated by most of the models should stop much more southward progress and instead force 05S westwards throughout the next few days. This track doesn't put 05S near land, but the cyclone may need to watched thereafter- depending on how far west it gets.

[Cookie]

its becoming difficult to keep up

[Somerset Squall]

its becoming difficult to keep up

You can say that again! It's difficult to track them all, lol

[Somerset Squall]

05S has unexpectedly weakened to 30kts this morning, and now looks on it's last legs. Convection slowly decreased through the evening and night, and hasn't come back. This in turn has caused the LLC to become increasingly ill defined. The trough to the east of 05S got rather closer than anticipated and increased shear over 05S. Additionally, it cut off the outflow channel 05S was using to intensify, meaning that instead of unstable air rising through the column fuelling more convection, dry subsidient air choked convection. The final advisory has been isuued by JTWC, and MeteoFrance are no longer anticipating development. Unless the remnant low distances itself from the trough, re-devlopment will not occur.

[Somerset Squall]

The remnant low of 05S has moved generally westwards with little development over the last 4 days since dissipation. Dry air and shear made the lifespan of 05S very short. However, the LLC has now developed a good amount of convection over the LLC, and the remains have moved into a more favourable environment with a moister air profile and lower shear. However, outflow is still not great and this could hinder development. Don't be surprised, based on recent trends, that 05S redevelops into a tropical cyclone over the next day or so.

[Somerset Squall]

Like Cleo and Laurence did, 05S has regenerated this morning. Convection is cycling between deep and shallow over an increasingly well defined LLC. 05S has struggled to reform due to moderate shear and poor outflow, and these two factors suggest that strengthening will be slow, at least intially. Eventually, outflow should improve, and as 05S is over very warm water, some more significant intensification is likely beyond 24hrs. JTWC are currently predicting a peak of 60kts, though they have also said 05S could become stronger than that.

JTWC and MeteoFrance seem to disagree on future track. MeteoFrance are keen on breaking down the ridge and sending 05S to the southeast over the next day or so, eventually into colder waters and stronger shear. JTWC agree in as much as the ridge breaking down allowing 05S to turn south, but then are thinking the ridge will build back in sending 05S back west. MeteoFrance's track would support a maximum intensity of around 55-65kts, but if JTWC's track eventuates, than 05S could become even stronger in the long term as the cyclone would remain over warm waters and low shear.

[Somerset Squall]

05S has been slow to strengthen, with poor outflow and moderate shear keeping the LLC weak. At last, after forming 8 days ago, 05S has become a named storm, called David, with intensity at 35kts. David is in a slightly more favourable environment of low shear, fair outflow (though still not excellent) and hot sea temps. This should promote some intensification, though with the lack of a distinct outflow channel, intensification won't be rapid.

David is stationary at present, trapped between ridges to the southwest and northeast. The northeastern ridge is expected to become dominant, forcing David southeastwards along the southwestern periphery. This will eventually, in a few days time, take David over cooler waters which will initiate weakening. However, the ridge to the southwest will not entirely dissapear, and some models hint at it becoming stronger again in about 96hrs which could initiate a westward bend in the track around this time period. This may well be too late to keep David from slipping into hostile waters. It is really worth mentioning that this steering environment present around David is complex, so expect large shifts in track forecasts over the coming days.

A tenacious storm, it has really battled a lot just to get this far (I'm quite surprised the remnant low survived after David's first life).

[Cookie]

thanks for the updates mate.

this system well and truly came back from the brink.

[Somerset Squall]

Definitely agree there Cookie.

David is strengthening. The cyclone has developed a central dense overcast feature, along with increasingly impressive banding features. Intensity has increased to 55kts. David has the oppurtunity to further strengthen as the outflow is now good, and sea temperatures are warm. In about 48hrs, cooler waters and higher shear will begin to weaken David.

David is moving southeastwards along the southwestern periphery of a subtropical steering ridge to the northeast. Steering influence is still forecast to transition to the subtropical ridge to the south which will eventually enforce a westward turn, but not before David reaches cooler waters.

[Cookie]

strong burst of convection tonight?

[Somerset Squall]

Indeed, but it was rather shapeless. In that image, David was actually struggling, and didn't look as well organised as in my last posting. What was happening here was an increase in shear, but some good outflow managed to provide a very large area of deep convection before the shear removed it again later. Intensity was reduced to 50kts at this time.

However, the intensity has been increased to 55kts again this morning. Shear has eased a touch, though it is still in the moderate range of 10-20kts. Combatting this is some continued favourable outflow and warm sea temps. This should allow some slow intensification of David over the next couple days before shear increases and waters cool on the forecast track.

David is currently moving very slowly eastwards. This is because the ridge to the north is still influencing the storm but the ridge to the south is beginning to exert more influence again. Eventually, the ridge to the south will win, and bend David's track slowly to the west. But, whilst the steering environment is competitive, motion will be slow or quasi-stationary.

Edited December 24, 2009 by Somerset Squall

[Cookie]

Thanks for explaing that SS

[Somerset Squall]

No Probs Cookie mate.

David is being torn apart by shear, a little earlier than forecast. Intensity has been reduced to 40kts. The weakening LLC has drifted north, whilst the convection continues southeastwards, detached by shear. David will eventually push towards the west as forecast, but will shortly dissipate thereafter in continued shear. This system has certainly been a fighter, and never say never on a comeback, especially as David will degenerate into a remant low over waters that are still warm.

Edited December 25, 2009 by Somerset Squall

[Somerset Squall]

David has degenerated into a remnant low. The LLC became entirely exposed earlier today due to strong shear, but convection has recently developed to the southwest of the LLC. However, the flaring nature of the convection is not enough for David to be still classed as a tropical cyclone. The remnant low of David will be dragged westwards as the ridge to the south builds stronger than the one to the north. David may regenerate for a third life as some models are indicating, due to a relaxation in shear progged for beyond 48hrs, and the fact that the westerly track will keep the system over warm water. MeteoFrance and JTWC are keen on this idea. Will be interesting if this does occur! David is certainly an unusual system with it's highly erratic track and survival of several bouts of shear. Will David manage to come back again?