Bertha becomes the season's first hurricane
The 2008 hurricane season's first hurricane is here--Hurricane Bertha. Bertha is the earliest forming July hurricane since Hurricane Cindy (July 6, 2005) and Hurricane Dennis (July 7, 2005). Bertha took advantage of warmer Sea Surface Temperatures (SSTs) of 27°C and modest wind shear of 10-15 knots to put on a burst of intensification to hurricane strength overnight. The storm has continued to intensify since the 5 am EDT NHC advisory, with some satellite estimates giving Bertha 90 mph winds--just below the threshold of Category 2 status. Visible satellite loops show a well-formed eye with excellent upper-level outflow to the north. The Hurricane Hunters are scheduled to make their first visit to Bertha on Tuesday afternoon to get a better idea of the storm's true strength.
Figure 1. Bertha at 7:45am EDT Monday July 7, 2008.
The intensity forecast
SSTs will continue to warm to 28°C Tuesday morning underneath Bertha, but wind shear is expected to increase some, to 15-20 knots. Bertha should intensify into a Category 2 hurricane today. Higher wind shear should halt intensification on Tuesday. Wind shear is expected to stay in the 15-25 knot range for the remainder of the week, and it is unlikely that Bertha can intensify beyond Category 3 status. The GFDL predicts Bertha will peak at Category 2 status, and the HWRF takes it to Category 3 strength (941 mb) five days from now.
The track forecast
Bertha will start to slow down over the next few days as it "feels" the approach of a trough of low pressure scheduled to move off the U.S. East Coast on Thursday. This trough will force Bertha on a more northwesterly track towards Bermuda, and most of the computer models foresee that Bertha will pass close to Bermuda 5-7 days from now. The exception is the GFS model, which predicts that Thursday's trough of low pressure will not be strong enough to recurve Bertha so far to the north. The GFS keeps Bertha farther south, bringing the storm on a track to pass close to the Carolinas early next week. So far, the GFS has been the most reliable model (Figure 2) tracking Bertha. However, the current run of the GFS depicts a considerably weaker storm than Bertha has become, and its track for Bertha will likely be too far south. A stronger Bertha will "feel" the upper-level westerly winds of the approaching trough more strongly than the GFS is indicating. In short, the best forecast is to assume Bertha will recurve to the north and pass close to Bermuda by the end of the week. Whether or not this trough will be strong enough to fully recurve Bertha northeastward into the hurricane graveyard of the North Atlantic is uncertain at this time.
Figure 2. Animated .gif showing the official NHC forecast every 12 hours (gray line) along with the actual track Bertha took (black line) and the forecasts from several of the computer models. The GFS model has had the best overall performance of the models. The NOGAPS and HWRF model tracks are not shown here, but did not do as well as the GFS.
Elsewhere in the tropics
There are no threat areas to discuss in the tropical Atlantic, and none of the models are forecasting tropical storm formation in the next seven days.
I'll post an update Tuesday morning.