Atlantic hurricane season still lacking hurricanes: any sign of a change?

Infrared satellite image of the quiet tropical Atlantic and Africa this morning. (NRLMRY)

Infrared satellite image of the quiet tropical Atlantic and Africa this morning. (NRLMRY)

With the first day of September coming this weekend, will we see the expected increase of activity in the tropical Atlantic?  There are signs storminess is about to pick-up some.

The last time we made it this far into the season without a hurricane was 2002 (Gustav on September 11) and before that was 2001 (Erin on September 9).  It’s worth noting that 2001 was a transitional year for ENSO — from La Niña to El Niño – then 2002 was a moderately strong El Niño.  The El Niño phase of ENSO is correlated with less active hurricane seasons in the Atlantic.  This year, ENSO is near neutral.

In terms of Accumulated Cyclone Energy (ACE), the 2013 Atlantic season is really slipping behind climatology or “normal” in a hurry now.  The last week of August is typically quite active, so every day without activity increases that gap.  As of today, this season’s ACE is about 28 percent of normal. To put the actual values into perspective, if Hurricane Katrina (2005) were transplanted to this year, we would still fall just short of climatology.

However, as anticipated, several environmental-scale factors appear to be coming together to kick the season into gear.  The stable air across the deep tropics that I referenced in last Friday’s post is finally coming back into the normal range (with the exception of the extreme eastern Atlantic near the African coast).  The dry stable air over the tropical Atlantic has been a major factor in suppressing storm formation and halting the growth of any storms that did form.

Mid-level (700mb) relative humidity anomaly averaged over August.

Mid-level (700mb) relative humidity difference from average in August. The large area of dry air in the tropical Atlantic has been very unfavorable for tropical storm development. (NOAA)

Surface pressures are low all over the eastern Atlantic and Caribbean, and sea surface temperatures are slightly warmer than average in those same areas.  Vertical wind shear is near normal in the typical formation areas this time of year.  The climatological peak of the season is during the second week of September, so – on average – that is when all of the ingredients typically do come together.

This is the time of year when we look to Africa for the “hurricane seedlings”- those westward-moving waves or disturbances generated over eastern Africa.  They are easy to track as they make their way across the continent, a trip that lasts about 5-6 days.  On average, only about 20 percent of these waves become tropical storms, but the most intense hurricanes can commonly be traced back to easterly waves.

Schematic and example of easterly waves moving across Africa and developing into tropical cyclones over the Atlantic.  Note that this is not a current satellite image.  (UCAR)

Schematic and example of easterly waves moving across Africa and developing into tropical cyclones over the Atlantic. Note that this is not a current satellite image. (UCAR)

A noteworthy wave is actually just exiting the African coast today, and can be followed back to the Ethiopian Highlands on August 25.  The National Hurricane Center finds it has a 40 percent chance of becoming a tropical storm in the next 48 hours, and a 60 percent chance within the next 5 days.

The disturbance is centered near 13N 16W, and the majority of forecast models predict it to intensify, though probably not reaching hurricane strength.  The models also are in good agreement that it will recurve to the north by the time it reaches 50W… so likely won’t be a concern for land.  If it does strengthen into a tropical storm, the next name on the list is Gabrielle.

Visible satellite image of an easterly wave on the African coast today.  This image was taken by GOES East at 1145Z (7:45 a.m. ET) on August 30, 2013. (NOAA)

Visible satellite image of an easterly wave on the African coast today. This image was taken by GOES East at 1145Z (7:45 a.m. ET) on August 30, 2013. (NOAA)

* Brian McNoldy is a senior researcher at the University of Miami’s Rosenstiel School of Marine and Atmospheric Science.

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