It's been an unusually quiet August for hurricane activity in the Atlantic, and if we finish the month without a hurricane, it will mark the first year since 2002 without an August hurricane. However, the quiet weather pattern we've been blessed with is about to come to an end, as conditions favorable for hurricane formation move into place for the last few days of August and the first week of September. The big guns of the African Monsoon are firing off a salvo of African tropical waves over the next two weeks that will find the most favorable conditions for development that we've seen this year. While there is currently a new outbreak of dry air and dust from the Saharan Air Layer (SAL) over the Eastern Atlantic, the latest European model forecast calls for a reduction in dry air and dust over the Tropical Atlantic during the 7 - 14 day period, accompanied by low wind shear. The Madden Julian Oscillation (MJO),
a pattern of increased thunderstorm activity near the Equator that moves around the globe in 30 - 60 days, has begun a new active phase. The most active part of the MJO has not yet crossed into the Atlantic, but is expected to do so during the period 7 - 14 days from now. The MJO will bring rising air that will aid strong thunderstorm updrafts and thus tropical storms--and their subsequent intensification into hurricanes. According to Dr. Michael Ventrice,
an MJO expert at WSI, Inc., the latest run of the GFS model predicts that this MJO event will be the 3rd strongest in the Western Hemisphere since 1989. During the last four comparable strong MJO events, 68% of all the tropical depressions that formed during these events (21 out of 31 storms) intensified into hurricanes. The MJO will likely continue to support Atlantic hurricane activity through September 15. The MJO is then expected to progress into the Western Pacific for the last half of September, which would likely bring sinking air over the Atlantic and a quieter portion of hurricane season.Figure 2.
Saharan Air Layer analysis at 8 am EDT on August 26, 2013. A burst of dust and dry air had emerged over the Eastern Atlantic, along with a new tropical wave to watch just south of the driest air. Image credit: University of Wisconsin CIMMS and NOAA/HRD.
The first tropical wave to watch is one that came off the coast of Africa on Sunday. This disturbance is moving westward at 10 - 15 mph, has a modest amount of spin, but is relatively thin on heavy thunderstorm activity. It has not yet earned status as an area of interest ("Invest") by NHC, but they are giving the wave a 30% chance of developing by Saturday. The wave will encounter an eastward-moving Convectively-Coupled Kelvin Wave (CCKW)
that moved off the coast of South America on Monday.
This atmospheric disturbance, moving eastwards across the tropical Atlantic at about 25 - 40 mph, has a great deal of upward-moving air, which may help the tropical wave develop when the two interact beginning on Wednesday. The UKMET model is predicting that the wave will develop into a tropical storm by Saturday, about 800 miles east of the Lesser Antilles Islands. The other models show limited or no development. There will be a trough of low pressure off the U.S. East Coast at the end of the week that will be capable of causing the wave to recurve and miss the Lesser Antilles, but it is too early to say how likely this is to occur.
There is much greater model consensus on developing a tropical wave expected to emerge from the coast of Africa on Friday. This wave would appear to have a high chance of recurvature, according to the latest run of the GFS model.