western pacific Super Typhoon Dolphin

[Somerset Squall]

Yet another tropical depression has formed in this already very active Western Pacific season. 07W is located southeast of Pohnpei in the east of the basin. The depression is currently looping southeast near the equator, but should turn northwestwards soon as ridging to the north becomes better established. Shear is currently moderate, so strengthening initially should be quite slow, but when 07W moves away from the equator should strengthen quite decently, with the potential to become a fairly intense typhoon.

Edited May 8, 2015 by Somerset Squall

[Somerset Squall]

07W drifted erratically eastwarda over the couple of days, but has now made a definite turn towards the northwest. The depression has also strengthened into a tropical storm, named Dolphin, with winds of 40kts according to JTWC. Shear has kept the system in check over the last few days, but now appears to be easing, so Dolphin should strengthen at a faster rate from now on.

[Sainsbo]

A lot of the models are showing that Dolphin may become a monster in size and intensity in a couple of days.(Almost all of the models going sub-900mb, some down to ~880mb, though we know how unreliable these can be) Definitely one to watch.

 

JTWC forecasting 115kts about 4 days from now. Woudn't be surprised if it's much higher than that. Conditions look near perfect when the shear eases away.

[Somerset Squall]

Week old, erratically moving Dolphin has been struggling over the past few days as it has drifted northwards in moderate to high shear. Now Dolphin has made a more definite turn back to the west, shear has reduced, and Dolphin is already responding. Winds are up to 65kts according to JTWC, a typhoon. A central dense overcast has formed, from which an eye is forming. As Sainsbo has said, Dolphin has the potential to become a pretty intense typhoon over the coming days. Of first concern however, is just how close Dolphin gets to Guam...

[Somerset Squall]

Dolphin's eye is clearing out and is small and well defined. The inner core is also pretty small, looks like rapid intensification is likely to occur. Winds are at 75kts currently.

[Vorticity0123]

Typhoon Dolphin has not strengthened as much today as previously anticipated. It appears to be undergoing an eyewall replacement cycle. Moreover, the cyclone struggling with dry air entraining from the west and 15-20 kt wind shear from the south.

 

Latest satellite imagery shows that the eye of Dolphin has disappeared in the visible channel, though it is still apparent in microwave satellite imagery. Also, the central convection diminished significantly over the past couple of hours. Most recently, however, much more vigorous central convection has developed, which also covers a much larger area than previously seen (see image from Somerset Squall for example, the inner convection only covers a relatively small area). An eye is not yet present, though.

 

DVORAK satellite image loop of Dolphin (click to animate). Courtesy: NOAA.

 

Eyewall replacement cycle

 

If one takes a look at microwave imagery from CIMSS, it can be seen that Dolphin has been through quite some structural wobbles during the past few days.

 

24 hours loop MIMIC imagery of Dolphin (click to animate). Courtesy: CIMSS

 

What can be seen is that Dolphin initially had a very small eyewall (not very clear, but it is visible as a small circular feature). Thereafter, the eyewall weakened considerably. In the last few images, a new, nearly circular, and quite intense eyewall has become apparent again. Judging from this it appears that the cyclone has undergone an eyewall replacement cycle (EWRC), with a small eyewall collapsing and a new, larger one emerging.

 

Given that a new, well-defined eyewall ahs finally emerged in microwave imagery, it is possible that we see an eye also emerging shortly in visible imagery, with a possible subsequent episode of rapid intensification. However, upon closer inspection of the Dvorak satellite loop, it appears that the low level circulation center (LLCC) is located in the northeastern edge of the convection, possibly delaying the onset of the formation of an eye. Furthermore, dry air could still be impacting the cyclone, which is discussed in more detail below.

 

Dry air

 

Along with the eyewall replacement cycle, dry air also seems to have been a limiting factor for Dolphin so far. Water vapor imagery illustrates this quite nicely:

 

Water vapor satellite loop of Dolphin (click to animate). Courtesy: NOAA

 

Although there seems little in the way of dry air in the upper circulation judging from the image itself, a large area of dry air is evident to the west of the cyclone. If one pays close attention to this area, it seems that this air is actually moving entraining into the circulation from the west, probably in the middle part of the atmosphere.

 

500 hPa relative humidity fields from COAMPS-TC nicely illustrate this feature:

 

COAMPS-TC 500 hPa relative humidity values, as of 00 UTC 14-05. The arrow delineates the dry air intrusion from the west.

 

From the fields you can clearly see a band of relatively dry air wrapping in from the (south)west into the circulation. This has been an issue for the cyclone over the past few days, and prevents deep convection from forming in the core of the circulation. If this dry air manages to mix in further into the circulation, development will be severely hampered.

 

Another indication for the dryness of the air is a sounding taken at Guam (which is visible in the imagery to the westnorthwest of the cyclone) yesterday afternoon:

 

13-05 12 UTC Skew-T sounding of Guam. Courtesy: University of Wyoming.

 

As can be seen from the image, the air is very dry (large difference between temperature, rightmost line, and dewpoint, leftmost line) from about 600 hPa upward. This is also the type of air that has been entraining into the circulation of Dolphin so far.

 

Forecast

 

The JTWC forecasts the system to intensify steadily into a 130 kt typhoon. hitting Guam as a 110 kt typhoon before that. After peak intensity, Dolphin is forecast to recurve out to sea with the island of Iwo To probably on its path.

 

Forecast track and intensity from the JTWC.

 

Summary

There have been mixed signals concerning the near future of Dolphin. On one hand, there is dry air and some wind shear affecting the system, arguing against significant strengthening in the near future. However, recently a rounded-off eyewall has appeared in MIMIC imagery, which would argue for rapid strengthening. It is hard to say which of these signals will appear to be the dominant one. Regardless of the exact intensity of the system, it is certain that Guam will have to brace itself for a rather dangerous tropical cyclone. Let's hope the people will stay safe.

 

Finally, the formation of Dolphin on May 9 by JMA has been the earliest 7th tropical cyclone to form ever in the Western Pacific. This could have to do with the El Nino event.

 

Sources:

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

http://en.wikipedia.org/wiki/2015_Pacific_typhoon_season (naming record)

http://weather.uwyo.edu/upperair/sounding.html

http://www.nrlmry.navy.mil/TC.html

http://www.ssd.noaa.gov/PS/TROP/floaters/07W/07W_floater.html

Edited May 14, 2015 by Vorticity0123

[Somerset Squall]

Winds are at 95kts currently. It seems like Dolphin is under some moderate shear, and this, coupled with the factors Vorticity has discussed, has prevented Dolphin strengthening quickly. JTWC still insist on Dolphin becoming a cat 4 eventually however. Guam is still at threat, especially as Dolphin has taken a southward wobble this morning, increasing the threat here.

[Somerset Squall]

Dolphin has passed just north of Guam, and brought typhoon conditions to the island as it did so. Dolphin is now strengthening again, as the eye has cleared out once more, and the central dense overcast has become more symmetrical. Winds are up to 110kts, cat 3 on the SS scale. JTWC are now forecasting a 145kt, cat 5 peak, as poleward outflow greatly improves as Dolphin links up with the mid-lattitude westerlies to the north. These westerlies will eventually cause Dolphin to weaken as it recurves northeastwards.

[Sainsbo]

Dolphin's still blowing up now.

 

Eye temperature is up to 15C, and cloud tops are still at -75C. The system looks like it's becoming much more symmetrical too, and with raw T numbers of T7.0, I don't see anything to suggest Dolphin won't be a Category 5 on the next advisory. They are still expecting a peak of 145kts, but only time will tell.

 

 

It looks even more formidable when you see the size of it

 

Edited May 16, 2015 by Sainsbo

[Somerset Squall]

Indeed Sainsbo, Dolphin has certainly become a lot larger over the last couple of days!

Winds are up to 130kts, making it the season's third Super Typhoon! Interestingly, this is also the first time on record that two Super Typhoons have formed in May (JTWC data). What a start to the season!!

[Somerset Squall]

Dolphin became the third cat 5 of the season, with winds of 140kts. The super typhoon has started to weaken now as shear is increasing, and winds are down to 130kts. Dolphin is currently moving northwards, but should accelerate northeastwards soon as it gets fully caught up in the mid-lattitude westerlies.

[Somerset Squall]

Dolphin's winds have fallen to 65kts according to JTWC. Convection is being stripped away from the LLCC due to strong shear. Extratropical transition will begin soon as Dolphin races northeast south of Japan.