Tropical Atlantic wakes up on peak day of hurricane season

Today is the climatological peak of the hurricane season and someone must have just sent the atmosphere the memo. Not only did Gabrielle regenerate, Humberto is expected to become the season’s first hurricane today, and Ingrid appears to be in the works.

Average cycle of tropical storm (red) and hurricane (yellow) activity in the Atlantic.  Today's date is marked by the green line.  (NOAA)

Average cycle of tropical storm (red) and hurricane (yellow) activity in the Atlantic. Today’s date is marked by the green line. (NOAA)

(How exactly is the peak defined? It is simply the date that historically –on average– has the highest number of tropical storms and hurricanes.  Of course, there is year-to-year variability, and individual years have their peaks scattered throughout August and September.)

Gabrielle

Since dissipating four days ago near Hispaniola, the remnants of Gabrielle were embedded in some rather strong vertical wind shear.  The shear is down to around 20-25 mph now, which is apparently just low enough to allow reorganization of the cyclone.

At 5 a.m. EDT today, it was re-classified as Tropical Storm Gabrielle.  It’s to pass over Bermuda later today and tonight (a tropical storm warning is in effect there), but the maximum sustained winds are 40 mph, so the effects there should be minimal.

Gabrielle heading for Bermuda. (NOAA)

Gabrielle heading for Bermuda. (NOAA)

As it interacts with a mid-latitude trough to its west, it is forecast to remain a tropical storm as it heads toward Newfoundland and Nova Scotia by Friday evening.

Humberto

Tropical Storm Humberto is very close to becoming the season’s first hurricane.  Since 1944, the latest date of first hurricane formation is September 11(set in 2002), and there has been plenty of talk lately about whether or not 2013 would set a new record.  But unless something very unexpected happens really soon, we will have Hurricane Humberto by this afternoon.

However, in the race for second place, Hurricane Diana formed on this date back in 1984 at 1200 UTC (8 a.m. EDT).  For now, Humberto will take second place, but it’s possible that in the post-season reanalysis, there could even be a tie.

Tropical Storm Humberto at 7:45am EDT today.  (NOAA)

Tropical Storm Humberto at 7:45am EDT today. (NOAA)

As of the 11 a.m. EDT advisory, Humberto is centered about 220 miles west of the Cape Verde islands and tracking WNW at 9mph.  Maximum sustained winds are 65 mph with a forecast for intensification to a Category 2 hurricane tomorrow.

The track forecast is essentially unchanged from yesterday… a sharp turn to the north should begin shortly, then a shift back to the west by the weekend.  As it heads north, it will encounter cooler ocean temperatures, drier air, and stronger shear, so its duration as a hurricane should be short.

Humberto track forecast (National Hurricane Center)

Humberto track forecast (National Hurricane Center)

In addition to the traditional visible and infrared satellite images, some satellites are equipped with microwave sensors too.  The longer wavelength of microwaves allows us to “see” through the cloud particles and observe the otherwise-hidden precipitation structure.  Earlier this morning, a Department of Defense weather satellite caught this view of Humberto… showing a very healthy primary spiral band and a nascent eyewall.

Enhanced microwave image of Humberto from 6am EDT today.  The Cape Verde islands are on the right side.  The red shades (indicative of ice/snow which would be found in strong thunderstorms) trace out key features in the developing storm.  Greens in this image represent liquid water particles.  (NRLMRY)

Enhanced microwave image of Humberto from 6am EDT today. The Cape Verde islands are on the right side. The red shades (indicative of ice/snow which would be found in strong thunderstorms) trace out key features in the developing storm. Greens in this image represent liquid water particles. (NRLMRY)

Ingrid?

A third area of interest is nearing the Yucatan peninsula.  For days, global models have been showing development of a disturbance near the peninsula, then intensification in the southern Gulf of Mexico.  That remains the case today.

Visible satellite image of the southern Gulf of Mexico and western Caribbean.  The disturbance that could become Ingrid is east of the Yucatan peninsula.  (CIRA/RAMM)

Visible satellite image of the southern Gulf of Mexico and western Caribbean. The disturbance that could become Ingrid is east of the Yucatan peninsula. (CIRA/RAMM)

Among the models, there is good agreement on formation once it enters the Bay of Campeche, then a W-WNW track into Mexico or even southern Texas.  We will keep a very close eye on any developments with this system.

Five-day GFS forecast (from the 0600 UTC run) of surface winds, valid Sunday morning.  (wunderground.com)

Five-day GFS forecast (from the 0600 UTC run) of surface winds, valid Sunday morning. (wunderground.com)

Season status

Today’s flare-up of activity notwithstanding, the Atlantic hurricane season has certainly gotten off to a slow start.  In terms of ACE (Accumulated Cyclone Energy), the 2013 season is at about 20% of an average season for this date.

As of the end of August, the ACE was 8.35 (the units are in 10,000 knots squared), the lowest since 2002 (and then 1988, 1984, and several other years prior).

Since the aircraft reconnaissance era began in 1944, eight seasons had a weaker start (through the end of August) than this year.

Among them, the most active ended up being 1967 with an ACE of 122, then 1988 with an ACE of 103, then the other six all fell between 22 and 84.  The ACE as of this morning is 10.

So, if history is any guide, this season will have a real hard time reaching much above average.  Sure, the remainder could still be quite active, but there’s a lot of a ground to make up just to reach “normal”.

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

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