91L could develop over the central Atlantic; Bopha in the Western Pacific
Even though hurricane season officially ended yesterday in the Atlantic, there is a low pressure system that could briefly become a subtropical or tropical cyclone. It is situated over the central Atlantic roughly 1150 miles southwest of the Azores Island. The low pressure system was designated as Invest 91L by the National Hurricane Center (NHC). Satellite imagery depicts that the system is producing some shower and thunderstorm activity and has an asymmetric structure, which suggests that it is more subtropical than tropical in nature. The ASCAT pass revealed that 91L has a relatively broad circulation center, which is partially exposed as the relatively deep convective activity remains to the north of it.
Figure 1. Evening infrared satellite image of Invest 91L. Image credit: NOAA/RAMMB.
Forecast for 91L
91L is forecast to move northward between 10 and 15 mph as it is steered by a high pressure ridge over the Azores. Thereafter, it is anticipated to turn generally northeastward and then eastward over the next several days, which model guidance agree on. 91L is not anticipated to impact any landmasses.
91L has a small window of opportunity to become a subtropical or tropical cyclone in the next day or so before being absorbed by a frontal system in 48 hours. If it becomes a subtropical or tropical cyclone, it will be named Valerie as it is the next name used in this year’s list. Although sea surface temperatures are cool and vertical wind shear is high, 91L could still further organize and strengthen as it is more subtropical than tropical in nature. Subtropical cyclones are more accustomed to withstand high wind shear, cool sea surface temperatures and stable environment than tropical cyclones. All in all, I give 91L a 50% chance of becoming a subtropical or tropical cyclone in the next 48 hours.
Bopha in the Western Pacific
In the Western Pacific, there is a very powerful typhoon named Bopha heading west-northwestward to Palau and the Philippines. Based on satellite images, Bopha is probably a super typhoon at Category 5 intensity on the Saffir-Simpson hurricane scale. It will likely remain a powerful typhoon when it hits the Philippines around middle of next week. An interesting fact to note on Bopha is that it developed below 10° N latitude, which is one of the rarest tropical cyclones to form below 10° N. The southernmost tropical cyclone on record is Tropical Storm Vamei of 2001, which became a tropical storm at 1.5° N. Generally, most tropical cyclones do not form close to the equator, due to the lack of Coriolis effect near the equator; without the atmospheric spin, tropical cyclones cannot develop. Typhoon season in the western Pacific happens nearly all year long but more active in the summer months.
Figure 2. Morning infrared satellite image of Typhoon Bopha. Image credit: NOAA/RAMMB.
The GFS and the ECMWF models are forecasting a potential subtropical or tropical cyclone to develop by five days in nearly the same spot as 91L developed.