Tropical Storm Bertha forms--and sets a record

Tropical Storm Bertha is here. Overnight, a significant amount of heavy thunderstorm activity built up around the center, aided by Sea Surface Temperatures (SSTs) about 26°C (1°C above average for this date). This temperature is right at the threshold of where tropical storms can form. Also aiding the storm is 5-10 knots of wind shear. Bertha has good organization and a favorable environment for intensification, and should continue to slowly intensify today. There is not much African dust or dry air near the storm, and the main impediment to future intensification will be a region of colder SSTs the storm will track over on Friday and Saturday. These cooler SSTs have created some stable air to the northwest of Bertha. Evidence of this stable air in satellite imagery (Figure 1) can be seen in the form of a large area of stratocumulus clouds to the northwest of Bertha.


Figure 1. Visible satellite image of Bertha shortly before becoming a tropical storm. Note the large field of stratocumulus clouds to the storm's northwest, indicating stable air lying over cooler SSTs.

The forecast
Most of the computer models foresee 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 approaching trough of low pressure and recurve. A shallower, weaker storm might be able to avoid recurvature and continue west-northwest towards Bermuda.

The hurricane season of 2008 sets a new record
Today's formation of Bertha at 25° West longitude is the farthest east a tropical storm has ever formed in the Atlantic so early in the season. It is also the farthest east a tropical storm has formed in the month of July. Reliable records of Eastern Atlantic storms go back to 1967, the beginning of the geostationary satellite era.

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 last night has grow less organized this morning, due to an increase in wind shear to 15-20 knots. The disturbance is a rather small one, and thus vulnerable to wind shear. QuikSCAT satellite imagery from this morning shows no sign of a surface circulation. The SHIPS intensity model predicts wind shear will increase above 30 knots by Saturday morning over 93L, and it is unlikely this disturbance will develop into a tropical depression.

Both the GFS and NOGAPS models are forecasting the development of a tropical disturbance in the southern Gulf of Mexico's Bay of Campeche on Monday. The tropics are getting active!

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