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Tropical weather analysis - December 2, 2012

By: KoritheMan , 8:55 AM GMT on December 02, 2012

Invest 91L

A non-tropical area of low pressure in the central Atlantic located about 1000 miles southwest of the Azores ("91L") continues to produce widespread shower activity, mainly to the north of the center in a large band.

Figure 1. Latest infrared satellite image of Invest 91L. Image credit: NOAA

Evening scatterometer data suggests that the low possesses a well-defined circulation at the surface, and is generating winds to near tropical storm force in the primary convection to the north. While upper-level winds are currently light as the system sits beneath the axis of a large cyclonic circulation, the fast forward motion to the north suggests that the storm will move closer to the high-level westerlies in about 24-36 hours. The GFS continues to show only a narrow area of light shear over the system, small and fragile enough that any sudden increase in the shear will likely decapitate our little invest. For now, however, environmental conditions are not unfavorable, and some development of this low is still possible over the next day or so before the low begins to interact and eventually merge with a developing extratropical cyclone to the west. Given the cold waters, any development will be subtropical. I should note that, given the well-defined nature of the low-level center, it would probably only take a modest increase in organization to result in the formation of a subtropical storm.

This low is expected to continue moving northward over the next 24 hours, followed by a turn to the north-northeast with acceleration.

Probability of development in 48 hours: 50%


Dangerous Super Typhoon Bopha is headed toward the central Philippines, and poses a very dangerous threat to that nation. As of the latest JTWC advisory, the following information was posted on the storm:

Wind: 155 mph, with higher gusts
Location: 6.3N 136.0E
Movement: W at 18 mph
Category: 4 (Saffir-Simpson Hurricane Scale)

Bopha carries with it an unusual legacy: initial formation was all the way down at 4N. This is remarkably close to the equator, and storms generally have a lot of difficulty generating adequate spin so far south. For comparison, the lowest formation of an Atlantic tropical cyclone was Hurricane Isidore of 1990's 7.2N. Bearing that in mind, this is fairly impressive. However, Tropical Storm Vamei became a tropical storm at only 1.5N in December of 2001, so Bopha does have some competition.

Satellite estimates still support a 150 mph super typhoon, and the system does not appear to be weakening at this time. In fact, temperatures within the eye have warmed again, while the central dense overcast has expanded and once again become more symmetrical.

Figure 2. Latest infrared satellite image of Super Typhoon Bopha. Image credit: NOAA

With little increase in shear anticipated over the system, there is little reason, barring inner core dynamics (i.e., eyewall replacement cycles) to assume the system will weaken, and Bopha is expected to be of considerable strength, probably 130 to 140 mph, when it makes landfall in the central portion of the Philippines. Unlike when storms approach the United States coast in the Atlantic, there is much less influence from mid-latitude weather in the western Pacific, which makes intensity forecasts a little easier. In short, Bopha probably isn't going to just abruptly fall apart as it approaches land like so many Atlantic storms do. First and foremost though, we need to be concerned about the island of Palau, which lies directly in the path of the system. Earlier satellite pictures showed a turn to the south of the forecast track, and indeed the 0900Z forecast package just released from the JTWC shows that the cyclone has lost about a degree of latitude since the 0300Z advisory. This will probably place Palau closer to the more dangerous quadrants of the storm. Once the system emerges into the South China Sea in about 72 hours, some reintensification is possible, but it will depend on how disrupted the inner core structure is from passage over the Philippines.

The forecast track is quite straightforward. Water vapor imagery indicates an enormous zonal flow anchored to the north of the typhoon. This is generally indicative of strong ridging aloft. UW-CIMSS steering data suggests that this ridge is abnormally strong, probably comparable to the one that steered Andrew into south Florida twenty years ago. With this and tightly clustered model guidance, there is little reason to deviate from the expected west-northwest course of the system, although it will be interesting to see how much longer the system continues moving west. Since it is entirely possible that the magnitude and scope of the ridge could be underestimated in the global model forecast fields, I would not be surprised to see some additional southward adjustments to the forecast track, especially in the short-term.

Subsequent to emerging into the South China Sea, models suggest a slowing of the forward speed and a gradual turn to the north as a longwave trough develops between China and Japan. The GFS actually shows the storm looping back around and striking only a little to the north of the area of expected landfall.

Unfortunately, due to having lost the map I drew forecasts for Jelawat with, I'll have to delay doing a forecast track graph with Bopha this morning. I will try to scour it from the confines of the Internet in my next forecast.

People in the path of this storm should be taking it very seriously. It is exceptionally powerful.

5-day intensity forecast

INITIAL 12/02 0900Z 135 KT 155 MPH
12 hour 12/02 1800Z 135 KT 155 MPH
24 hour 12/03 0600Z 130 KT 150 MPH
36 hour 12/03 1800Z 125 KT 145 MPH...APPROACHING THE PHILIPPINES
48 hour 12/04 0600Z 105 KT 125 MPH...INLAND
72 hour 12/05 0600Z 75 KT 85 MPH...EMERGING INTO SOUTH CHINA SEA
96 hour 12/06 0600Z 75 KT 85 MPH
120 hour 12/07 0600Z 80 KT 90 MPH

The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.

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12:35 AM GMT on December 13, 2012
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Basement-dwelling pseudo-otaku with a thrill for forecasting on the side.

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