SAT 7-5 AM UPDATE: The Tropical Atlantic is starting to produce noticeable activity, view the latest infrared satellite loop. As of 11AM Thursday morning, the robust tropical wave off the West African coast was upgraded to Tropical Storm Bertha, and may approach hurricane strength late in the weekend.
SYNOPSIS: Bertha is the first named tropical storm known to have developed this far East in the Atlantic. At present, upper level air flow patterns indicate that a landfall on the U.S. mainland cannot be ruled out. In the southeast Caribbean, a weak area of low pressure had been producing scattered showers and thunderstorms, but remains disorganized. However, activity continues to flare up west of the estimated center as the overall system drifts west-northwestward. A broad area of showers and thunderstorms is also present near the coastal Texas/Mexico border and will be monitored this weekend.
FIRST, THE CAPE VERDE SYSTEM: View this enhanced infrared loop. Ironically the last notable tropical cyclone to form in this region at this time of the year was...Bertha, in July 1996. With a classic summertime Bermuda High ridge forecast to start building next week, one might initially think the storm could travel under the ridge and in the general direction of the Caribbean. When considering the air flow at upper levels, a different trend is revealed, and you would see why some computer models are indicating recurvature east of Bermuda. A brief explanation for this is provided in the "upper level analysis" section. Below is the current NHC projected path from 5PM 7-4, and preceding it is the 11AM Advisory.
Notice the westward shift in the cone of uncertainty from one map to the next.
UPPER LEVEL ANALYSIS: This next map is among the many new features I've been learning about in my Weather Forecasting and Climate Change class here at the picturesque College of the Atlantic. As I experiment with a new feature or skill from the class, it will be added here for use during this year's hurricane season. The newest (to me) resource is the 500 mb 24-hour forecast for the Central and North Atlantic issued by the Ocean Prediction Center. The inference I am making from this map is that once Bertha nears the Low and High progged at the 500 mb (~18,000 foot) level, the air movement between these two centers will create convergent flow south to the south side, with air diverging on the north side. This will act to pull the system toward the vacuum created by this flow, and begin curving the storm northwest and eventually northward into the central Atlantic. The NHC refers to this as a weakness in the subtropical ridge. This is an example of the importance of knowing movement of air masses and pressure differences at upper levels, which ultimately dictate what is observed on the surface for us ground observers.
BACKGROUND: A tropical wave developing off the West African coast is referred to as a "Cape Verde" system due to the promixity of an island cluster of the same name. Were this occuring between mid August and late September, we would have great cause for concern. This early in July, waves way out there usually don't become the Katrinas, Ivans, Floyds or Glorias and come terrorizing the Gulf or East Coast. Each tropical season, one hundred or more disturbances drfit off the African coast and cross the Atlantic. In July 2005, two of these systems reorganized once near the Caribbean and developed into named storms. Dennis made landfall as a Category 3 Major Hurricane on the Florida panhandle July 10 of that year. Just eight days later, Emily has already crossed the same southern Caribbean waters, and struck the Yucatan on July 18 as a Category 4. Emily also had a second landfall south of Brownsville in Northeast Mexico as a Category 3. I realized in reviewing the archives on both storms...back on July 6, 2005 the Atlantic basin had nearly an identical setup: A tropical wave just off the African coast (which would become major Hurricane Emily), AND a small feature in the southern Caribbean which became Dennis.
While this system has decated, compare the earlier projected path to the NHC official tracks for Hurricane Dennis in July 2005, as recorded in this link to the archives for that period.