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Earl a threat to the Lesser Antilles; Danielle misses Bermuda; 97L organizing

By: Dr. Jeff Masters, 3:40 PM GMT on August 28, 2010

The Atlantic's first major hurricane of 2010, Hurricane Danielle, is undergoing an eyewall replacement cycle, and has weakened to a Category 2 storm. Danielle peaked in intensity yesterday as a low-end Category 4 storm with 135 mph winds. Danielle is expected to pass well east of Bermuda tonight, bringing winds of 25 - 35 mph to the island. So far this morning, winds have remained below 20 mph at the Bermuda airport. Bermuda radar shows the outermost spiral bands of Danielle are beginning to approach the island. Infrared satellite loops show that Danielle's cloud tops have warmed, indicating weakening. This is due to the collapse of the storm's eyewall. A new eyewall will form today, but by the time the new eyewall forms on Sunday morning, wind shear will rise to a high 20 - 40 knots, ending any chance Danielle has of further intensification. Danielle is destined to recurve out to sea without hitting any land areas. The largest impacts Danielle will have are from its waves. The Bermuda Weather Service is predicting 12 - 18 foot waves today for Bermuda's offshore waters, and large waves of 6 - 8 feet will affect much of the U.S. East Coast today through Sunday. The latest near shore water forecast for Cape Hatteras calls for 6 - 8 foot waves this weekend.

Figure 1. True color image of Danielle taken at 10:25am EDT Friday, August 27, 2010, by NASA's Aqua satellite. At the time, Danielle was a Category 4 hurricane with 135 mph winds.

Tropical Storm Earl
The bigger concern in the Atlantic today is Tropical Storm Earl. Earl continues to follow a track very similar to Danielle's, but is now farther south than Danielle was. Recent satellite imagery shows the storm has changed little in organization this morning. Water vapor satellite images show a large region of dry air from the Sahara lies to the northwest of Earl, and this dry air is keeping Earl's heavy thunderstorm activity relatively meager. The wind shear analysis from the University of Wisconsin CIMSS group shows that northerly upper-level winds due to the outflow from Hurricane Danielle are creating a moderate 10 - 15 knots of shear over Earl. This shear is keeping any heavy thunderstorms from developing on the north side of Earl's center of circulation. The center of circulation is now exposed to view, which is always a sign of a tropical storm struggling with high wind shear. The first flight into Earl is scheduled for 4pm EDT this afternoon, and will be a research mission by NOAA's P-3s. The NOAA jet will also fly tonight to sample the large-scale steering currents. The first regular hurricane hunting mission by the Air Force is scheduled for Sunday morning.

Figure 2. Morning satellite image of Earl.

Intensity forecast for Earl
The latest SHIPS model forecast for Earl predicts that wind shear will remain moderate this afternoon, 10 - 15 knots, then fall a bit to the low to moderate range, 5 - 15 knots through Monday, as Danielle pulls away and its upper-level outflow stops bringing northerly wind shear to Earl. This reduction in shear should allow the storm to build heavy thunderstorms around the entire center of circulation on Sunday, and close off Earl's core from the dry air to the northwest. SSTs will warm from 29°C today to nearly 30°C on Sunday. These very high SSTs, combined with the low shear environment expected Sunday and Monday, should allow to Earl to intensify into a hurricane by Monday. Earl will likely be a major hurricane by Tuesday and Wednesday, as it moves northwestward between Bermuda and the U.S. East Coast.

Track forecast for Earl
Earl is being steered to the west by the same ridge of high pressure that steered Danielle. As Earl approaches the Lesser Antilles on Sunday, a weakness in the ridge left behind by the passage of Danielle will allow Earl the opportunity to move more to the west-northwest, likely bringing the core of the storm just to the northeast of the northernmost Lesser Antilles Islands Sunday night and Monday morning. However, it is possible that Earl could move directly over some of these northernmost islands. NHC is giving Saint Maarten in the northern Lesser Antilles a 57% chance of receiving tropical storm force winds from Earl, and an 18% chance of getting hurricane force winds. Heavy rain squalls and tropical storm force winds should begin affecting the islands Saint Maarten, Barbuda, and Antigua late Sunday night. The odds of Earl bringing tropical storm force winds to Puerto Rico are lower--25%, according to NHC.

Once Earl passes the Lesser Antilles, steering currents favor a northwesterly course towards North Carolina. History suggests that a storm in Earl's current location has a 30% chance of making landfall on the U.S. East Coast. Earl's chances of making a U.S. landfall are probably more like 20%, due to the steering influence of a strong trough of low pressure predicted to move off the U.S. East Coast next Friday. This trough, if it develops as predicted by the long-range GFS, NOGAPS, and ECMWF models, should be strong enough to recurve Earl out to sea late in the week, with the storm missing landfall. However, six-day forecasts can be off considerably on the timing and intensity of such features, and it is quite possible that the trough could be delayed or weaker than expected, resulting in Earl's landfall along the U.S. East Coast Thursday or Friday. The most likely landfall locations would be North Carolina on Thursday, or Massachusetts on Friday. The GFS model predicts that Earl will come close enough to North Carolina on Thursday to bring the storm's outer rain bands over the Cape Hatteras region. The other models put Earl farther offshore, and it is possible that Earl could pass close enough to Bermuda to bring tropical storm force winds to that island. It is possible that if 97L develops into Hurricane Fiona and moves quickly across the Atlantic, as predicted by the GFS model, the two storms could interact and rotate counterclockwise around a common center. Predicting these sorts of interactions is difficult, and the long-term track forecast for Earl has higher than usual uncertainty because of the possibility of a storm-storm interaction with Fiona.

Links to track Earl
Martinique radar
Wundermap of the northern Lesser Antilles Islands

The tropical wave (Invest 97L) about 300 miles southwest of the Cape Verdes Islands remains well-organized, and appears destined to develop into a tropical storm and follow the path of Danielle and Earl. 97L already has a broad, elongated surface circulation, as seen on satellite loops, but only limited heavy thunderstorm activity. The storm is experiencing a moderate 10 - 15 knots of winds shear, is over warm 28°C waters, and is battling a region of dry air associated with the Saharan Air Layer (SAL) to its northwest. The latest SHIPS model forecast calls for shear to drop to the low range, 5 - 10 knots, tonight through Monday, and this should allow 97L to organize into a tropical depression. The storm will follow a track very similar to Danielle and Earl westward towards the Lesser Antilles Islands, and the storm should arrive near the northern Lesser Antilles Wednesday or Thursday. A more northwesterly path is likely for 97L as it approaches the Lesser Antilles, as the storm follows a break in the high pressure ridge steering it, created by Danielle and Earl. It currently appears that the Northern Lesser Antilles Islands may be at risk of at close brush or direct hit by 97L. If 97L moves relatively quickly, arriving at the Lesser Antilles on Wednesday, it is likely to be a weaker system, since it will have less time over water, and will be closer to big brother Earl. Earl is likely to be a large and powerful hurricane at that time, and the clockwise upper level outflow from Earl will bring strong upper-level northerly winds to the Lesser Antilles, creating high wind shear for 97L. However, if 97L moves relatively slowly, and arrives in the Lesser Antilles on Thursday, Earl will be farther away, the wind shear will be lessened, and 97L will have had enough time over water to potentially be a hurricane. Depending upon how fast they have 97L moving, the computer models have a wide variety of solutions for 97L, ranging from a making it a major hurricane five days from now (GFDL model) to a weak tropical storm five days from now (several models.) History suggests that a storm in 97L's current location has a 20% chance of making landfall on the U.S. East Coast. NHC is giving 97L a 80% chance of developing into a tropical depression by Monday.

Figure 3. Morning satellite image of 97L.

Elsewhere in the Tropics
In the Eastern Pacific, Tropical Depression Frank is dissipating off the coast of Baja California in Mexico.

There is at least one more tropical wave over Africa that will be a candidate to develop into a tropical depression late next week once it emerges from the coast.

Jeff Masters


The views of the author are his/her own and do not necessarily represent the position of The Weather Company or its parent, IBM.