A weakening Bill generating a major wave event for Bermuda and the U.S. coast

Wind shear and dry air are weakening Hurricane Bill, which may no longer be a major hurricane. However, the hurricane is still large enough and strong enough that it will generate a major wave event for much of the Western Atlantic. Visible and infrared satellite imagery show that the hurricane is no longer as symmetric as it once was, with a squashed oval shape to its cloud pattern. Upper-level cirrus clouds are restricted on the storm's southwest side, indicating that upper-level winds from the southwest are shearing the storm. The University of Wisconsin CIMSS wind shear analysis shows about 10 - 15 knots of southwesterly wind shear impacting Bill. Satellite intensity estimates of Bill's strength show essentially no change since 2am EDT this morning, and Bill is at the borderline of becoming a Category 2 hurricane. There are currently no Hurricane Hunter aircraft in Bill, and the next mission is scheduled for 2pm EDT this afternoon.

Wind shear is forecast to remain low to moderate, 5 - 15 knots, for the next two days, and it is possible Bill may see a relaxation of the wind shear affecting it, allowing some re-intensification today. Sea Surface Temperatures (SSTs) will be plenty warm today, as Bill traverses a region of ocean with SSTs near 29°C. Total ocean heat content is steadily declining, though, as the warm waters Bill is currently traversing do not extend to great depth. By Saturday, SSTs decline to 27.5°C, and intensification becomes less likely.


Figure 1. Visible satellite image of Bill at from Friday morning 8/21/09. Bill had an oval shape, and was missing upper-level cirrus clouds on the southwest side, indicating that wind shear from strong upper-level southwesterly winds was affecting it.

The computer model track forecasts for Bill have been very consistent the past two days, giving confidence that the trough of low pressure developing along the U.S. East Coast this weekend will turn Bill to the north then northeast as expected. Both Nova Scotia and Newfoundland, Canada can expect tropical storm conditions from Bill, which should be rapidly weakening on Sunday afternoon when it makes its closest pass to Nova Scotia. Category 1 hurricane conditions are possible on Nova Scotia if Bill makes a direct hit there. Cape Cod and eastern Maine may receive sustained winds of 35 mph from Bill, just below tropical storm force, but the chance of a direct hit by Bill are slim.

Bill's waves
Hurricane Bill is generating huge waves, thanks to its enormous size and major hurricane intensity. Output from NOAA's Wavewatch III model suggests that significant wave heights near Bill's center will peak at 50 feet by Saturday. Large swells from Bill have reached Bermuda, and are generating waves of 10 - 20 feet in the offshore waters. The Bermuda Weather Service predicts seas will increase to 20 - 30 feet on Saturday as Bill makes its closest approach to the island.

In the U.S., Bill's swells will reach New York's Long Island on this afternoon, and seas will build to 7 - 10' on Saturday and 12 - 16' on Sunday in the near shore waters. By tonight, Bill's swells will be affecting the entire U.S. East Coast from Florida to Maine. Maximum sea heights in near shore waters over the weekend will be about 6 - 8' from Florida to South Carolina, 11 - 14' along the North Carolina coast, 8 - 11' along the mid-Atlantic coast, and 11 - 14' along the coast of Maine. The highest waves along the U.S. coast will occur at Cape Cod, Massachusetts, where waves of 18 - 23' are being forecast by NOAA for Sunday. Bill's high waves will likely cause millions of dollars in coastal erosion damage and create very dangerous rip currents and swimming conditions along the coast.

CloudSat slices through Bill
The CloudSat satellite, launched in 2006, carries the first satellite-based millimeter wavelength cloud radar. It is the world's most sensitive cloud-profiling radar, more than 1000 times more sensitive than current weather radars. It collects data about the vertical structure of clouds, including the quantities of liquid water and ice, and how clouds affect the amount of sunlight and terrestrial radiation that passes through the atmosphere. The satellite has a narrow field of view, so can image only a small portion of the planet each day. About once per year, CloudSat happens to slice through the eye of an Atlantic hurricane. This happened Wednesday, when CloudSat caught a remarkable view of Hurricane Bill (Figure 2).


Figure 2. Top: conventional 89 GHz microwave "radar in space" image of Hurricane Bill on Wednesday, 8/19/09, from the AMSR-E instrument on the Aqua spacecraft. Bottom: cross section through Bill's eye taken at the same time from the CloudSat cloud radar instrument. The CloudSat pass occurs along the red line in the top image. The CloudSat pass runs from south (left side of CloudSat image) to north (right side of CloudSat image). At the time of the image, Hurricane Bill was strengthening into a Category 4 hurricane (135 knot winds, 947 mb pressure), while completing an eyewall replacement cycle. In the 89 GHz Microwave image, half of the eyewall is already completed (half red circle on right side of the image). In the CloudSat image, the southern eyewall shows weak echoes at the low levels, and slopes outward with height. The northern eyewall is much more intense, and a core of high reflectivity echoes extends high into the troposphere, to 16 km altitude, forming a "hot tower". These "hot towers" a characteristic of intensifying hurricanes. Interestingly, the hot tower is tilted into the eye, so that the eyewall does not slope outward with height along the northern eyewall. This is not the typical behavior of a hurricane, and may be due to the unusual strength of the hot tower. Cirrus clouds with a base at 8.5 km can be seen to the south of the hurricane in the CloudSat image. The thin grey line at 5 km marks the 0°C temperature line. Ice particles falling inside the hurricane melt at an altitude just below the 0°C line, creating a "bright band" of orange echoes throughout most of the hurricane. Image credit: NASA/Colorado State University.

Elsewhere in the tropics
There are no threat areas in the Atlantic worth mentioning today. The GFS model calls for a tropical cyclone to develop just off the coast of Africa about seven days from now.

I'll have an update Saturday.

Jeff Masters