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Danielle steams towards Bermuda; Earl organizing over eastern Atlantic

By: Dr. Jeff Masters, 3:20 PM GMT on August 26, 2010

Hurricane Danielle continues on its steady northwesterly path towards Bermuda, as a respectable Category 2 hurricane with 105 mph winds. However, the hurricane has had its troubles this morning--Danielle wrapped a significant amount of dry air into its core between 4am - 8am EDT this morning, and Infrared satellite loops show that Danielle's cloud tops warmed late this morning, indicating that this dry air reduced the vigor of Danielle's thunderstorms. Danielle has managed to quickly mix out most of this dry air and reform its broken eyewall early this afternoon, as seen on recent visible satellite imagery (Figure 1.) We'll get a better idea of Danielle's intensity on Friday afternoon, when the first Hurricane Hunter mission is scheduled. The storm is still too far from land to reach with their airplanes today.

Figure 1. Morning satellite image of Danielle. A huge thunderstorm has blown up on the west side of the eyewall, giving the hurricane a rather lopsided appearance.

Intensity forecast for Danielle
Once Danielle recovers from its current troubles with dry air, warm 29°C SSTs combined with low wind shear of 5 - 10 knots should allow the storm to intensify. There is substantial dry air to the west and south of Danielle, though, and it is quite possible that some unexpected increase in shear could inject dry air into the hurricane at any time over the next few days. The latest SHIPS model forecast predicts that this shear will remain low, 5 - 10 knots, over the next three days, but then rapidly rise to a high 20 - 40 knots on Sunday. Danielle will probably reach peak intensity Saturday night or Sunday morning, near the time of its closest approach to Bermuda. NHC is giving Danielle a 41% chance of becoming a major Category 3 hurricane.

Track forecast for Danielle
On Saturday, a trough of low pressure that is expected to move off the East Coast of the U.S. and Canada should begin pulling Danielle due north, with the hurricane passing east of Bermuda Saturday night through Sunday. NHC is giving Bermuda a 25% chance of receiving tropical storm force winds of 39 mph or greater from Danielle, and a 1% chance of getting hurricane force winds. This is a substantial reduction from the odds given in the 5am forecast, which were 37% and 6%, respectively. The Bermuda Weather Service is predicting 15 - 20 foot waves this weekend for Bermuda's offshore waters. The most likely scenario once Danielle passes north of Bermuda is for the storm to recurve out to sea, missing the U.S. and Canadian coasts. All of the computer models currently call for this scenario. The possibility of Danielle stalling north of Bermuda and potentially tracking northwest towards the U.S. and Canadian coasts, as called for by the NOGAPS model in some of its runs yesterday, is looking less likely.

History suggests that a storm in Danielle's current location has only a 5 - 10% chance of making landfall on the U.S. East Coast. If Danielle passes east of Bermuda, as forecast, these percentages drop to less than 5%. However, Danielle will bring high surf conditions to the U.S. East Coast beginning this weekend. The latest wave forecast from NOAA's Wavewatch III model (which uses the GFS model as its prediction for the position and intensity of hurricanes), calls for waves from Danielle to begin hitting the coast of North Carolina on Saturday. These waves will build to 7 - 10 feet in the offshore waters from Northern Florida to North Carolina by Sunday. The latest near shore water forecast for Cape Hatteras calls for 5 - 8 foot waves Saturday, and 7 - 10 feet on Sunday.

Figure 1. Morning satellite image of Danielle, Earl, and a new tropical wave emerging from the coast of Africa (possibly to be designated 97L?)

Tropical Storm Earl
Tropical Storm Earl developed late yesterday afternoon, one week ahead of when climatology suggests the fifth named storm of the year should occur in the Atlantic. Earl is currently very weak and ragged looking, thanks to dry air and wind shear. The wind shear analysis from the University of Wisconsin CIMSS group shows an upper level trough of low pressure to the north of Earl is bringing a high 20 - 30 knots of wind shear to the northern portion of the storm, though the center of Earl is under low wind shear of 5 - 10 knots. The shear has injected dry air into the storm, disrupting the circulation. As a result, Earl has a rather disorganized clumpy appearance on satellite imagery, and the center jumped 30 miles to the south this morning to reposition itself farther from the shear, and closer to the main region of heavy thunderstorms. Water vapor satellite images show a large region of dry air from the Sahara lies to the west of Earl, and this dry air will likely be the primary inhibiting factor for development over the next three days. Sea surface temperatures are warm, around 28°C. Earl is too far from land for the Hurricane Hunters to reach, and the first flight into the storm is not scheduled until Sunday afternoon.

Forecast for Earl
Wind shear is predicted to remain low to moderate, 5 - 15 knots, for the next five days. SSTs will steadily warm--to 28°C on Friday, and 29°C by Sunday. Earl may cross Danielle's cold water wake at some point, which could interrupt development. Dry air will probably be the main inhibiting factor for Earl over the next three days. In combination, these factors should allow for a slow intensification of Earl into a hurricane 4 - 5 days from now.

History suggests that a storm in Earl's current location has a 15 - 20% chance of making landfall on the U.S. East Coast, but the long term fate of Earl remains unclear. The storm is being steered by the same ridge of high pressure steering Danielle, and Earl will initially follow a track similar to Danielle's. As Earl enters the central Atlantic 4 - 5 days from now, the storm will encounter the same mid-Atlantic trough that is steering Danielle to the north. This trough should be able to turn Earl far enough to the northwest so that the storm will miss the Lesser Antilles Islands. It is unclear at this point whether this trough will be strong enough to fully recurve Earl out to sea, east of Bermuda. The long range 6 - 10 day forecasts from the GFS and ECMWF models have consistently been calling for Earl to turn north and follow Danielle, bringing Earl very close to Bermuda next week. This is a reasonable forecast, since Danielle is large enough and strong enough to create a low pressure "path of least resistance" for Earl to follow northwestward towards Bermuda.

Elsewhere in the Tropics
A new tropical wave emerged from the coast of Africa yesterday, and has the potential to develop into a tropical depression early next week. Several of the computer models develop this wave 3 - 6 days from now, and I can't see any reason why this would not occur. Conditions for tropical cyclone development will remain favorable in the Eastern Atlantic for at least the next week, and several of these models successfully predicted the development of both Danielle and Earl well in advance. The new wave will follow a track similar to that of Danielle and Earl, with an unknown potential for eventually affecting any land areas.

In the Eastern Pacific, Hurricane Frank has moved away away from the Mexican coast, and does not appear to be a threat to bring heavy rains to Mexico.

Jeff Masters


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