Hurricane Bertha: 6th strongest early season hurricane on record
Hurricane Bertha put on a remarkable burst of rapid intensification Monday afternoon, reaching major hurricane status--Category 3--with 120 mph winds and an estimated central pressure of 948 mb. Among early season hurricanes that have formed before August 1, Bertha is the sixth strongest early-season Atlantic hurricane on record. Only 12 early season major hurricanes have formed since record keeping began in 1851, though several were no doubt undocumented before the advent of the aircraft reconnaissance in 1944. Bertha holds the record for the farthest east a major hurricane has formed so early in the season (52°W longitude), easily beating the mark set in 1996 (67°W) by a previous incarnation of Hurricane Bertha. This year's Bertha now holds the record for farthest east formation as a tropical storm, hurricane, and major hurricane, so early in the season.
Figure 1. Bertha at peak intensity: 21:15 GMT Monday July 7, 2008. At this time, satellite estimates of Bertha's strength were 115 kt (135 mph), making the storm a weak Category 4 hurricane.
Bertha may even have intensified to Category 4 status between 1900 and 2300 GMT yesterday. Satellite estimates of the storm's intensity (Figure 2) from both NHC and the University of Wisconsin during that period were 115 knots (135 mph), which would have made Bertha a low-end Category 4 hurricane. However, it is unclear what Bertha's final official intensity maximum will be, since it reached maximum strength in between the official 6-hourly times used by NHC to document a storm's strength.
Figure 2. Satellite estimates of Bertha's strength using the Automated Dvorak Technique (ADT) showed the storm peaking at 115 knots (135 mph) for a four hour period on July 7, 2008. The "Best Track" line is the official NHC advisory strength, which peaked at 105 knots (120 mph); the AMSU dots are estimates from a polar orbiting instrument, the Advanced Microwave Sounding Unit; SATCON is a SATellite CONsensus estimate of Bertha's intensity combining several satellite strength estimates. Image credit: University of Wisconsin CIMSS.
Bertha's intensification was somewhat surprising, because environmental conditions for intensification were good, but not great. Wind shear was a modest 10-15 knots, Sea Surface Temperatures were 27°C--only 1° above the minimum needed to support a hurricane--and the oceanic heat content was only 20 KJ/cm**2, which is well below the value of 80 typically associated with rapid intensification.
Bertha is probably not a major hurricane any more. Wind shear has increased to 15-20 knots, and the eye has become less distinct and the cloud pattern degraded, as seen on visible satellite loops. The Hurricane Hunter mission scheduled for today has been canceled, and no future missions are planned.
The intensity forecast
SSTs will continue to warm to 28°C Wednesday morning underneath Bertha, but wind shear is expected to remain high and possibly increase some later this week. Bertha should gradually decline in strength to Category 1 status by the end of the week.
The track forecast
Bertha is slowing down and turning northward as it "feels" the approach of a trough of low pressure to the north. All of the computer models turn Bertha northwards east of Bermuda, and it currently appears that the island will feel only peripheral effects of Bertha. However, the trough of low pressure turning Bertha to the north may not be strong enough to fully drag the storm into the far North Atlantic, so Bertha may wander close to Bermuda early next week while it waits for another trough of low pressure to finish the job. It is very unlikely Bertha will threaten the U.S., but it could eventually affect Newfoundland, Canada.
Elsewhere in the tropics
There are no threat areas to discuss in the tropical Atlantic. The UKMET and GFS models are showing some possible development in the region between Africa and the Lesser Antilles Islands about a week from now.
I'll post an update Wednesday morning.