Watching 96L off South Carolina Coast; Tropical Storm Ida Forms in Central Atlantic

By: Dr. Jeff Masters , 3:47PM,GMT on September 19,2015

An area of disturbed weather off the coast of South Carolina (Invest 96L) is bringing heavy rains to the waters from the northern Bahamas to the Outer Banks of North Carolina, but is not a major wind or heavy rain threat to the coast. The disturbance is under high wind shear of 20 - 30 knots, and there is plenty of dry air around it, which is inhibiting development. Satellite loops on Saturday morning showed that 96L had developed a weak surface circulation, but the storm's heavy thunderstorms were removed over 200 miles to the north and east of the center, characteristic of the structure of a subtropical storm. Our three reliable models for predicting tropical cyclone genesis show little or no development, and the 12Z Saturday run of the SHIPS model showed wind shear remaining a high 20 - 30 knots though Monday, which should keep any development slow. In their 8 am EDT Saturday Tropical Weather Outlook, NHC gave the disturbance 2-day and 5-day odds of development of 20% and 40%, respectively. 96L's slow movement to the northeast through Monday should keep the heaviest rains offshore, but strong winds from the system will bring high surf to the Outer Banks of North Carolina. By Tuesday, 96L may move more to the north or north-northwest, bringing the storm close to the U.S. mid-Atlantic coast near Virginia.


Figure 1. Latest satellite image Invest 96L.

Tropical Storm Ida forms
Tropical Storm Ida formed Friday night in the waters of the Central Atlantic midway between the Lesser Antilles Islands and the coast of Africa, and was headed west-northwest at 12 mph on Saturday morning, well away from any land areas. Satellite images on Saturday morning showed that Ida was slowly organizing, but lacked a large area of heavy thunderstorms, and the center of circulation had been exposed to view by high wind shear. Conditions over the next few days favor slow strengthening, with the 12Z Saturday run of the SHIPS model predicting moderate wind shear of 10 - 20 knots, warm ocean temperatures near 28°C (83°F), and a moist atmosphere. The long-range fate of Ida is unclear, since the storm will experience a collapse in its steering currents from Monday through Wednesday that will cause a very slow, erratic motion. The European model predicted with its 00Z Saturday run that Ida would lift out to the north by the middle of the week, caught up in a strong trough of low pressure passing to its north. The GFS model showed that the trough would bypass Ida and leave it stranded in the Central Atlantic to wander for many days. Unless Ida can pull off some non-Euclidian movements like her namesake character in my favorite computer game, Monument Valley, Ida is unlikely to threaten any land areas in North America or the Caribbean.

Ida is the ninth named storm of the 2015 Atlantic hurricane season, which already surpasses the total number of named storms that occurred during the last two strong El Niño events, 1997 (eight named storms) and 1982 (six named storms.) This year's strong El Niño event is different than the previous two, in that we are seeing much lower levels of wind shear over the far Eastern Atlantic, which is allowing storms to form in that region.


Figure 2. Latest satellite image of Tropical Storm Ida.

Tropical Depression Nine near dissipation
Tropical Depression Nine in the Central Atlantic is barely a tropical depression, and is likely to dissipate by Sunday.

Tropical Depression 5-C forms in Central Pacific
Yet another tropical cyclone has formed in the Central Pacific, where Tropical Depression 5-C is moving northwards on a path that will take it several hundred miles west of the Hawaiian Islands. A Tropical Storm Warning has been issued for central portions of the Papahanaumokuakea Marine National Monument, from French Frigate Shoals to Maro Reef to Lisianski Island; a Tropical Storm Watch has been issued for the area from Lisianski Island to Pearl and Hermes.

Invest 91E a heavy rain threat to Baja Mexico
An area of disturbed weather off the coast of Mexico's Baja Peninsula (Invest 91E) is headed north-northwestwards at 10 mph, and will likely make landfall on the central Baja coast on Monday. Moisture from this system will produce heavy rains across portions of the Baja California Peninsula and northwestern mainland Mexico through Tuesday, potentially causing dangerous flash flooding and mudslides. 91E will bring isolated heavy rains to Southern California and Arizona beginning on Monday night.

Jeff Masters