Wednesday, September 30, 2015

"Hurricane Warnings for the Bahamas From Joaquin; Threat to U.S. East Coast Grows"

Hurricane Joaquin
From Wunderblog:
Joaquin is now a hurricane, and Hurricane Warnings are up for the Central Bahama Islands as the slowly intensifying storm moves southwest at 6 mph. An Air Force hurricane hunter aircraft made two penetrations of Joaquin's center on Wednesday morning, and found top surface winds of 80 mph, a central pressure of 971 mb, and a huge 54-mile diameter eye with a fully closed eyewall. Joaquin continues to battle high wind shear of 20 knots due to strong upper-level winds out of the north-northwest, but this wind shear had fallen by about 5 knots since Tuesday morning. Water vapor satellite loops show that a large area of dry air lay to the northwest of Joaquin, and the strong wind shear was driving this dry air into Joaquin's core, keeping intensification slow. Visible and infrared satellite loops show that Joaquin has developed a large Central Dense Overcast (CDO) of high cirrus clouds over the center, characteristic of intensifying storms, and the hurricane's large eye was beginning to be apparent. Upper level winds analyses from the University of Wisconsin show that the hurricane has developed an impressive upper-level outflow channel to the southeast, which is supporting the intensification process. Ocean temperatures in the region are near 30°C (86°F)--the warmest seen there since record keeping began in 1880.

The U.S. outlook for Joaquin
A hurricane watch could be required for portions of the U.S. East Coast as early as Thursday night. The forecast for Joaquin is very complex, and the confidence in both the intensity and track forecast for the storm is low. Joaquin is trapped to the south of a high pressure system whose clockwise flow will push the cyclone very slowly to the southwest or west-southwest at about 3 - 6 mph. As the storm progresses to the southwest, the strong upper-level winds out of the north currently bringing high wind shear of 20 knots will gradually decrease, continuing to allow Joaquin to strengthen. The 8 am EDT Wednesday run of the SHIPS model predicted that wind shear over Joaquin would fall to the moderate range, 10 - 20 knots, on Thursday and Friday. These conditions should allow Joaquin to intensify to a Category 2 hurricane by Thursday. As Joaquin progresses to the west, the storm will also increasingly "feel" the steering influence of a strong upper-level trough of low pressure situated over the Eastern United States on Friday, and begin to turn north. These winds may also open up another upper-level outflow channel to the northwest of Joaquin on Friday, potentially allowing the storm to intensify to Category 3 strength. However, as Joaquin gets closer to this trough, its winds will bring high wind shear of 20+ knots, likely halting the intensification process and causing weakening by Sunday....MORE

Figure 2. Our two top models for forecasting hurricane tracks, both run at 8 pm EDT Tuesday September 29, 2015 (00Z Wednesday) , came up with two very different solutions for the path of Joaquin. The GFS model showed Joaquin making landfall in Virginia, while the European model took the storm to the northeast out to sea without hitting the U.S. Image credit: wundermap with the "Model Data" layer turned on.