southwest indian ocean Severe Tropical Storm Fobane

[Somerset Squall]

MeteoFrance have declared the next tropical depression, to the northeast of Severe Tropical Storm Edilson, and a few hundred miles south of Diego Garcia. Convection is increasing over the LLCC, with formative banding features becoming evident. JTWC have issued a tropical cyclone formation alert. Shear is low and waters are warm, so tropical depression 11 should strengthen decently as it heads southeastwards.

[Somerset Squall]

11 has struggled under unexpected shear over the last 12hrs. JTWC have upgraded the system to a 35kt tropical cyclone, but MeteoFrance have kept the system as a 30kt tropical depression. Convection is flaring near the LLCC, but there isn't really any strong banding features as yet. 11 is drfting south but should turn east-southeastwards soon as ridging builds to the east. This motion is not expected to last long as a bend in track back towards the southwest (an S shape track) should occur as a new ridge builds in south of the system. Shear is expected to ease, which should allow the system to finally strengthen.

 

[Somerset Squall]

Tropical Depression 11 has been upgraded to Moderate Tropical Storm Fobane by MeteoFrance. Fobane is still struggling under shear and supressed outflow, although it has managed to sustain some deep convection over the LLCC. Winds remain at 35kts according to JTWC. Fobane seems to be finally heading east-southeastwards as ridging to the east gains steering influence. The S shaped track is still expected as ridging builds to the south and guides Fobane back towards the southwest. Fobane is expected to strengthen, but only modestly. It seems that a nearby trough is not expected to pull away from Fobane, and whilst it is there, some at least moderate shear will continue, but also outflow will remain poor.

 

 

 

[Somerset Squall]

Fobane has strengthened to 55kts. The trough impinging on the outflow and sustaining shear over Fobane has actually moved sufficiently far enough way to decrease shear, though it is still supressing poleward outflow. Equatorward outflow however, is good. The strong equatorward outflow and lower shear should allow Fobane to intensify further, and JTWC are expecting a peak of 70kts, cat 1 on the SS scale. Fobane is still heading southeastwards, but should turn south then southwest soon.

[Vorticity0123]

Fobane has strengthened to 55kts. The trough impinging on the outflow and sustaining shear over Fobane has actually moved sufficiently far enough way to decrease shear, though it is still supressing poleward outflow. Equatorward outflow however, is good. The strong equatorward outflow and lower shear should allow Fobane to intensify further, and JTWC are expecting a peak of 70kts, cat 1 on the SS scale. Fobane is still heading southeastwards, but should turn south then southwest soon.

 

The JTWC has made a rigorous change in their intensity forecast with significant strengthening now being forecast. This has occured multiple times with this storm, indicative of a difficult track to forecast. It has been a rather interesting cyclone so far.

 

For now, it seems that Fobane is strengthening on par, or slightly quicker, than forecast by the JTWC. CIMSS satellite estimates are already at hurricane strength (65, 67 and 72 kt, respectively). However, IR satellite imagery shows a rather disorganized cyclone, as can be seen in the link below:

 

http://oiswww.eumetsat.org/IPPS/html/MTP/IMAGERY/IR115/COLOR/EINDIANOCEAN/IMAGESDisplay/u6tMNMTB64vgK

 

The cyclone consists of some thin central convection with some banding to the north and a large area of clouds not associated with the cyclone to the south. Moreover, CIMSS satellite position indicates the center is located to the east of the forecast track.

 

The forecast track indicates a track parallel to the 26*C isotherm in the long term, indicative that any deviation toward the north/south could have big implications on the forecasted intensity of the system. The track forecast (by the GFS) can be seen below:

 

The forecast track of Fobane by the GFS (12Z run) and the SST isotherms.

 

For now, it seems that the track of Fobane will govern at least partially its intensity. A track to the south of the current forecast would mean the system would traverse cooler SSTS, and therefore weakening. A track to the north of the current forecast would mean that Fobane would be over SSTS sufficiently warm to sustain the tropical cyclone. 

 

Another aspect that could affect the intensity of Fobane is vertical wind shear, which is forecast to increase by the end of the forecast period (from JTWC).

 

One final question, Somerset Squall (or anybody else), is such a track like the forecasted westward track at such high latitude common in the Southern Indian Ocean? Many thanks!

 

Sources:

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

http://tropic.ssec.wisc.edu/real-time/storm.php?&basin=indian&sname=14S&invest=NO&zoom=4&img=1&vars=11111000000000000000&loop=0

http://oiswww.eumetsat.org/IPPS/html/MTP/IMAGERY/IR115/COLOR/EINDIANOCEAN/IMAGESDisplay/u6tMNMTB64vgK

http://moe.met.fsu.edu/cyclonephase/gfs/fcst/archive/14020912/1.html

Edited February 9, 2014 by Vorticity0123

[Somerset Squall]

Storms at the the latitude showed on that track forecast often head southeast, there must be a strong blocking high stopping it from taking the usual path out of the tropics. TC Favio from 2007 is the most recent example of a cyclone I can think of at high latitude refusing to sink south and instead stubbornly moving west.

Edited February 9, 2014 by Somerset Squall

[Vorticity0123]

Storms at the the latitude showed on that track forecast often head southeast, there must be a strong blocking high stopping it from taking the usual path out of the tropics. TC Favio from 2007 is the most recent example of a cyclone I can think of at high latitude refusing to sink south and instead stubbornly moving west.

 

 

Indeed, TC Favio also didn't follow the usual southeastward track, curving from the southwest to the northwest instead!

 

What does make me wonder, though, is the high pressure that should be blocking the southeastward track of Fabio. The JTWC states the following:

 

 

 

AFTER TAU 12, A MID-LAYEREDSUBTROPICAL RIDGE TO THE SOUTHEAST WILL ASSUME STEERING AND DEFLECT THE SYSTEM SOUTHWESTWARD.

 

However, upon looking at the forecasted 500 hPa heights around Fabio, I am unable to find a well-defined ridge to the southeast of Fabio. For example, when looking at the GFS forecast from T54:

 

 

A weak area of high heights (red color) can be seen to the east of the cyclone. Is such a ridge strong enough to assume steering of the cyclone? What I would think is that the ridge to the west of Fabio would assume steering (as it seems to be more intense). However, this doesn't seem to be the case.

 

The difficult part in interpreting those 500 hPa heights is that they have a direct relationship with latitude, as the 500 hPa level is usually located at lower heights at higher latitudes (because the air is generally colder at higher latitudes). This implies that it is a significant challenge to define the ridges/troughs at such altitude.

 

Many thanks for the reply!

 

Sources:

http://www.usno.navy.mil/NOOC/nmfc-ph/RSS/jtwc/warnings/sh1414web.txt

http://www.wetterzentrale.de/topkarten/fsavnet.html

Edited February 9, 2014 by Vorticity0123

[Somerset Squall]

Maybe it's to do with the fact that Fobane is now a vertically shallow system, thus the relatively weak ridge is enough to steer it?

Fobane peaked at 60kts but is now weakening on it's southwesterly leg of the forecast track. Winds are down to 45kts. The cyclone retains well defined circulation but only shallow convection, which is probably due to the fact Fobane is surrounded by dry air and is now over cooler waters. Further slow weakening is forecast as Fobane drifts towards the southwest, and JTWC indicate dissipation within 36 hours.

[Somerset Squall]

Fobane is hanging on as a tropical cyclone, just. The cyclone continues to drift slowly southwestwards over marginal sea temps, and through dry air. Some shallow convection remains near the tightly wrapped LLC. Winds are down to 35kts. As dry air continues to surround Fobane, and sea temps continue to fall as Fobane drifts poleward, the convection should completely decay, which will be followed by a spin down of the LLC. JTWC expect Fobane to degnerate into a remnant low in about 12 hours.