Two Disturbances to Watch in the Atlantic; Megh and 96B Active in the North Indian

Cyclonic Storm Megh continues to slowly organize over the Arabian Sea as the storm heads west towards Yemen and Somalia. As of 7 am EST Friday, the Joint Typhoon Warning Center put Megh's winds at 50 mph, and predicted the storm would take advantage of low wind shear and warm ocean waters near 28°C to intensify into a Category 1 storm by Sunday--although Megh could encounter some cooler patches of water upwelled last week by the passage of Tropical Cyclone Chapala. Cyclone Chapala left at least 8 fatalities and more than 200 injuries and destroyed dozens of structures in Yemen, including Socotra Island, which may receive a direct hit from Megh. As explained in more detail in Thursday's post, Megh may be a threat to Somalia, which has had more experience with tropical cyclones than Yemen in recent years.


Figure 1. MODIS image of Tropical Cyclone Megh over the Arabian Sea on November 6, 2015 at 09:15 UTC. Image credit: NASA.

A second tropical cyclone in the North Indian Ocean this weekend?
The North Indian Ocean may witness a very rare event this weekend: the existence of two simultaneous tropical cyclones in November--one in the Bay of Bengal and one in the Arabian Sea. Recent satellite loops of Invest 96B in the Bay of Bengal show that this area of heavy thunderstorms moving westwards towards Sri Lanka and the east coast of India has acquired some spin, and the European model shows this disturbance developing into a tropical depression by early next week. A 2011 study by Evan and Camargo, A Climatology of Arabian Sea Cyclonic Storms, found that between 1979 - 2008, only 1986 saw November tropical cyclones form in both the Bay of Bengal and the Arabian Sea. The authors theorized that favorable conditions for development in the Arabian Sea tend to cause unfavorable conditions over the Bay of Bengal, and vice-versa.

Two Atlantic areas of disturbed weather to watch
An area of heavy thunderstorms near the northern Lesser Antilles Islands extending eastwards several hundred miles is associated with a tropical wave interacting with an upper-level trough of low pressure. Wind shear is a high 20 - 30 knots over the region, so development is not expected for the next two days while the disturbance moves west-northwest at about 10 mph. However, on Sunday, when the disturbance will be near the Dominican Republic and Southeast Bahamas, wind shear is expected to drop to the moderate range, 10 - 20 knots, and some development could occur. In their 8 am EDT Friday Tropical Weather Outlook, NHC gave the wave 2-day and 5-day odds of development of 0% and 30%, respectively. The disturbance should head north by Tuesday and then northeast by Wednesday, and could bring heavy rains to Bermuda by the middle of the week. A number of members of the GFS and European ensemble models develop the system into a tropical depression.

A broad area of low pressure over the Yucatan Peninsula and the adjacent waters of the Gulf of Mexico is producing disorganized shower activity, and will move west-northwestward into the southwestern Gulf of Mexico by Saturday. Wind shear is moderate at 10 - 20 knots, which may allow some slow development until the disturbance gets entangled with a cold front early next week. In their 8 am EDT Friday Tropical Weather Outlook, NHC gave the wave 2-day and 5-day odds of development of 20% and 20%, respectively. Very few members of the GFS and European ensemble models develop the system into a tropical depression, and the ones that do show the disturbance staying weak and stuck in the extreme southern Bay of Campache.


The science of deciphering how much long-term climate change influences shorter-term weather and climate events (attribution studies) continues to blossom. On Thursday, the American Meteorological Society (AMS) released its fourth annual special issue of the Bulletin of the AMS devoted to these attribution studies. Bob Henson will offer his take on the studies in a post later today.

Jeff Masters