Bertha slowly intensifies, continues west-northwest

Tropical Storm Bertha has slowly intensified overnight, with new heavy thunderstorm activity building up around the center. Sea Surface Temperatures (SSTs) are now slightly below the threshold of 26°C considered beneficial for tropical storms, and this is limiting Bertha's intensification. This morning's 4:01am EDT QuikSCAT pass revealed top winds of about 30 mph, but the satellite was not able to reliably detect Bertha's strongest winds, since QuikSCAT does poorly in heavy rain. Bertha's current intensity is based mostly on satellite imagery of the cloud patterns. The storm is under about 10 knots of wind shear. Bertha should continue to slowly intensify today.


Figure 1. Track chart of all Atlantic tropical storms that have formed east of 40°W longitude since 1851.

The forecast
Not much has changed in the forecast, with all of the computer models foreseeing a west-northwest track into the mid-Atlantic over the next five days, with a possible recurvature to the north by the end of the period. Whether this recurvature takes place depends on how strong Bertha gets. A larger, stronger storm will be more likely to "feel" the approach of the trough of low pressure expected to recurve Bertha, which a shallower, weaker storm might be able to avoid recurvature and continue west-northwest. Wind shear is expected to remain below 10 knots the next three days, then increase to 30-40 knots by day five, according to the GFS model, when Bertha hits a branch of the Subtropical jet stream. The GFDL model does not go along with this high shear forecast, and makes Bertha a Category 2 hurricane that begins recurving to the east of Bermuda. In contrast, the HWRF model keeps Bertha a weak tropical storm for the next six days. Suffice to say, there is a lot uncertainty in the long range intensity forecast for Bertha!

The hurricane season of 2008 sets a new record
Bertha's at 25° West longitude is the farthest east a tropical storm has ever formed in the Atlantic so early in the season. Reliable records of Eastern Atlantic storms go back to 1967, the beginning of the geostationary satellite era. It's remarkable that no other early July storm even comes close to matching how far east Bertha formed (Figure 1).

Is the formation of Bertha a harbinger of an active hurricane season?
Probably. According the the Hurricane FAQ, "as shown in (Goldenberg 2000), if one looks only at the June-July Atlantic tropical storms and hurricanes occurring south of 22°N and east of 77°W (the eastern portion of the Main Development Region [MDR] for Atlantic hurricanes), there is a strong association with activity for the remainder of the year. According to the data from 1944-1999, total overall Atlantic activity for years that had a tropical storm or hurricane form in this region during JJ have been at least average and often times above average. So it could be said that a JJ storm in this region is pretty much a "sufficient" condition for a year to produce at least average activity."

Elsewhere in the tropics
A tropical wave (93L) that passed through the Lesser Antilles Islands Wednesday night is now in the central Caribbean and is very disorganized, thanks to high wind shear. This wave is not expected to develop, and no computer models are predicting development anywhere else in the Atlantic over the next seven days.

I'll post an update Saturday morning. Happy 4th of July weekend!
Jeff Masters