Tropical Storm Gabrielle, the seventh named storm of the season, is bringing stormy conditions to the northern Caribbean today before heading out into the open Atlantic.  We still await the first hurricane of the 2013 Atlantic season.

Enhanced water vapor image of Tropical Storm Gabrielle from 7am EDT this morning. While the heaviest thunderstorms are east of the center, eastern Puerto Rico is also experiencing heavy rain.

Gabrielle formed from an easterly wave that exited the African coast around August 25.  It tracked through the deep tropics and was just shy of the Lesser Antilles this past Sunday (see “Disturbance heading into eastern Caribbean“).  Since then, it battled with some mid-level dry air, but began to really get its act together on Wednesday afternoon.

An Air Force reconnaissance plane reached it by mid-afternoon and was quick to find a closed surface circulation — at which point it was upgraded to tropical depression 7.  By the next advisory at 11 p.m. EDT, satellite data indicated it had intensified and it was upgraded to Tropical Storm Gabrielle.

As of 8 a.m. EDT this morning, Gabrielle is centered over eastern Puerto Rico and has sustained winds of 40mph.  Tropical storm warnings are in effect for all of Puerto Rico and the northeast coast of the Dominican Republic.  I have a very long radar loop (starts Wednesday morning and is still growing) from San Juan available that shows the storm organizing and condensing as it approaches Puerto Rico.

Early morning radar image of Gabrielle from San Juan. (NOAA)
Early morning radar image of Gabrielle from San Juan. (NOAA)

Although Gabrielle is lashing Puerto Rico with some gusty winds and heavy rains, it has become quite disorganized this morning due to interaction with land.   It is becoming increasingly difficult to find a circulation center.

Despite its current troubles, the forecast from the National Hurricane Center is for gradual intensification over the next 5 days as it heads north then northeast… out into the open Atlantic, perhaps brushing close to Bermuda by early next week.  The GFS deterministic and ensemble models have fairly high confidence in the storm tracking north for a couple more days before recurving to the northeast.


Still no hurricanes?

We have reached September 5 and still have not had a hurricane in the Atlantic.  While not a record, it’s getting close.

Before diving into the records, let’s think about what we’re really measuring.  The boundary between a tropical storm and a hurricane is hard to accurately measure.  It’s a matter of distinguishing between sustained winds (found anywhere in the storm) of 60kts and 65kts.  All other features can be virtually identical, but that single number determines its status.

Even with aircraft reconnaissance, that measurement can be uncertain.  The aircraft will sample the storm as thoroughly as possible, but there are limitations to coverage and to diagnosing winds at the surface.  While aircraft have been used to reconnoiter tropical cyclones in the Atlantic for about 70 years, they can only reach storms in the western part of the basin, and are not flying into storms continuously.

Then along comes the satellite era in the early 1960s.  The satellite era is much more reliable and accurate when it comes to tropical cyclone climatology.  While technology has improved the instruments aboard the satellites over the years, having around-the-clock images of the entire basin has made the record much more robust during this entire period.

So, using the best available data we have and going back 50 years (1963), the latest date of first hurricane formation is September 11, and that occurred in 2002 with Gustav.  The latest five dates during this period are:

September 11: Gustav (2002)

September 10: Diana (1984)

September 9: Erin (2001)

September 3: Arlene (1967)

September 2: Debby (1988)

You can see that even if Gabrielle became a hurricane today, it would still be in fourth place.

All fifty dates of first hurricane formation are shown in the charts below.  Some dates have had multiple instances of the season’s first hurricane forming. August 22 has had four.  To see how variable these dates are in time, the second chart plots the same data but by year rather than by date.  The median date of first hurricane formation is August 6 and is marked with a green line in both charts.

In those five years with the latest first hurricanes, it’s of course interesting to look at how active they turned out to be.  The table below summarizes the ACE (Accumulated Cyclone Energy) as well as any major hurricanes that occurred.  And by the way, 2013’s ACE is now down to 21 percent of normal for this date.

YEAR ACE                            MAJOR HURRICANES

2002    67 (below normal)       Isidore, Lili

1984    84 (near normal)          Diana

2001    110 (near normal)        Erin, Felix, Iris, Michelle

1967    122 (above normal)     Beulah

1988    103 (near normal)        Gilbert, Helene, Joan

It’s quite possible that Gabrielle will never reach hurricane status, and the clock will keep ticking.  There are three other areas of interest scattered across the basin this morning, but none of them pose the imminent threat of developing into a named storm.

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