Earl hits Category 4; Fiona forms

Powerful Hurricane Earl, now a Category 4 storm with 135 mph winds, continues to lash Puerto Rico and the northern Lesser Antilles Islands with heavy rain and high winds this afternoon. Hardest hit was Anegada in the British Virgin Islands, population of 200. The eye of Earl passed just north of Anegada at noon EDT, and the island probably saw sustained winds of 100 mph in the south eyewall of Earl. Second hardest hit was probably the island of Anguilla. Amateur weather observer Steve Donahue at anguilla-weather.com estimated gusts of 100 mph on Anguilla this morning; his anemometer broke at 88 mph. Juliana airport on neighboring St. Martin Island recorded sustained winds of 47 mph, gusting to 68 mph at 8am EDT. Winds were below tropical storm force on Antigua, but heavy rains of 5.71" have deluged the island. Heavy rains have hit Puerto Rico, where radar-estimated rainfall amounts of up to 5" have occurred southwest of San Juan. A heavy rain band moved across the island late this morning, with a tornadic thunderstorm that prompted issuance of a tornado warning.


Figure 1. Afternoon satellite image of Earl.


Figure 2. Radar image of Earl taken this afternoon from the San Juan, Puerto Rico radar.

Outlook for the Caribbean islands today
Latest radar animations out of Puerto Rico show that the eye of Earl has now moved past the Virgin Islands, and winds will begin to subside on most of the islands this evening. Heavy rains will continue through Tuesday, however, bringing the risk of flash flooding and mudslides.

Intensity forecast for Earl
Wind shear as diagnosed by the latest SHIPS model forecast is nearly non-existent over Earl--just 4 knots--but is expected to increase to the moderate range, 10 - 15 knots, tonight through Thursday afternoon, due to upper level winds out of the southwest from a trough of low pressure to Earl's west. This shear should not appreciably affect Earl between now and Thursday, since the hurricane is so large and strong. Ocean temperatures are a near-record 30°C, and very warm waters extend to great depth, resulting in a total ocean heat content highly favorable for continued intensification. Earl should continue to intensify until reaching Category 4 or 5 strength on Tuesday, and will probably maintain major hurricane status through Thursday, when it will make its closest approach to North Carolina. The hurricane will probably undergo at least one eyewall replacement cycle during that period, which will diminish its winds by 20 - 30 mph for a day or so. By Friday, when Earl will be making its closest approach to New England, wind shear will rise to a high 20 - 30 knots and ocean temperatures will plunge to 20°C, resulting in considerable weakening. Earl will still probably be a Category 2 hurricane on Friday, when it could potentially make landfall in Massachusetts. Earl is more likely to be a Category 1 hurricane on Saturday morning, when it could potentially make landfall in Maine or Nova Scotia, Canada.


Figure 3. Swath of surface winds from Earl predicted by the 8am EDT Monday August 30, 2010 run of NOAA's HWRF model. Hurricane force winds (yellow colors, above 64 kt) are predicted to stay off the coast, except over Nova Scotia. Tropical storm force winds (light green colors, above 34 knots) are predicted to affect virtually the entire U.S. coast from North Carolina to Maine. Winds between 58 mph - 73 mph (dark green colors) are predicted to affect North Carolina's Outer Banks, Southeast Massachusetts, and Eastern Maine. Image credit: Morris Bender, NOAA/GFDL.

Track forecast for Earl
The latest set of computer models runs from 8am EDT (12Z) this morning push Earl's projected track a little closer to the U.S. East Coast, but still keep hurricane force winds offshore. History suggests that a storm in Earl's current location has a 25% chance of making landfall on the U.S. East Coast, and Earl's chances of making a U.S. landfall are probably close to that. We now have one model predicting a U.S. landfall--the latest HWRF model predicts Earl will hit the Maine/Nova Scotia border region on Saturday morning as a Category 1 hurricane. None of the other computer models show Earl hitting the U.S., but the storm will likely come uncomfortably close to North Carolina's Outer Banks and to Massachusetts. Several models now predict Earl will being tropical storm force winds of 39+ mph to North Carolina's Outer Banks, beginning on Thursday evening. The Outer Banks of North Carolina and Cape Cod, Massachusetts are both at the edge of the cone of uncertainty, and could potentially receive a direct hit. NHC is giving Cape Hatteras a 11% chance of receiving hurricane force winds. These odds are 10% for Nantucket, 5% for Boston, and 3% for New York City. The main determinant of whether Earl hits the U.S. or not is a strong trough of low pressure predicted to move off the U.S. East Coast Friday. This trough, if it develops as predicted, should be strong enough to recurve Earl out to sea, but it is not unusual for the models to miss the timing and intensity of these troughs significantly in 4 - 5 day forecasts.

Regardless of Earl's exact track, the U.S. East Coast can expect a long period of high waves beginning on Thursday. Significant beach erosion and dangerous rip current will be the rule, due to waves that will reach 10 - 15 feet in offshore waters.

Hurricane History for the northern Lesser Antilles
The last Cape Verdes-type hurricane to affect the Barbuda and the surrounding northern Lesser Antilles Islands was Hurricane Debby of 2000, which passed over the islands on August 28 as a Category 1 hurricane. Damage was less than $1 million, and no fatalities were reported. The last hurricane of any kind to pass through the northern Lesser Antilles Islands was Category 4 Hurricane Omar, on October 16, 2008. Omar took an unusual track, moving towards the northeast, and the storm's eyewall missed all of the islands. Omar did $80 million in damage to the Caribbean, mainly on the islands of Antigua, Barbuda, Dominica, the SSS Islands (Saba, St. Eustatius, and St. Maarten), and the U.S. Virgin Islands. No direct deaths were attributed to Omar, and the name Omar was not retired from the 6-year rotating list of hurricane names.

Links to track Earl
Long range radar out of San Juan, Puerto Rico
Visible rapid scan satellite loop

Fiona forms
Tropical Storm Fiona finally gained enough heavy thunderstorm activity to be given a name, but continues to struggle with dry air. Satellite loops show that heavy thunderstorm activity comes and goes, and there are not many intense thunderstorms near the storm's center. The storm is experiencing low wind shear of less than 5 knots, and is over warm 29°C waters. The main impediment to development continues to be dry air associated with the Saharan Air Layer (SAL) surrounding the storm. The latest SHIPS model forecast calls for shear to increase to moderate, 10 - 15 knots, by Tuesday. Fiona is moving quickly to the west, at about 24 mph. This means it is catching up to Earl, which is moving at 15 mph. By Tuesday night, Earl is expected to be a large and powerful major hurricane with a well-developed upper-level outflow channel heading clockwise out from Earl's center at high altitudes. These strong upper-level winds will bring high levels of wind shear, 20 - 30 knots, to Fiona, and probably arrest the storm's development. A scenario predicted by the GFS, GFDL, and HWRF models is for Fiona to be drawn into the low pressure wake of Earl and pass to the northeast of the Lesser Antilles. Earl would then eventually destroy Fiona through high wind shear, and by robbing the storm of its moisture. An alternative scenario, championed by the ECMWF and NOGAPS models, is for Fiona to stay far enough away from Earl that it will be able to potentially threaten the Bahamas and U.S. East Coast early next week. At this point, it is difficult to choose between these two scenarios. History suggests that a storm in Fiona's current location has a 25% chance of making landfall on the U.S. East Coast.


Figure 4. Afternoon satellite image of Fiona.

Danielle
Danielle is now a tropical storm, and is on its way to oblivion over the cold North Atlantic waters.

Jeff Masters