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3:15 PM | *All quiet in the Atlantic Basin, but that’s about to change*

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Weather forecasting and analysis, space and historic events, climate information

3:15 PM | *All quiet in the Atlantic Basin, but that’s about to change*

Paul Dorian

Warmer-than-normal sea surface temperatures have returned to the Gulf of Mexico; courtesy NOAA, tropicaltidbits.com

Warmer-than-normal sea surface temperatures have returned to the Gulf of Mexico; courtesy NOAA, tropicaltidbits.com

Overview

It has been quite awhile since Barry reached tropical storm status over the Gulf of Mexico and then made landfall (briefly) as a hurricane in southern Louisiana – the first hurricane of the 2019 Atlantic Basin tropical season.  In fact, there is currently no activity across the Atlantic Basin and none is likely over the next five days or so.  There are signs, however, for activity to ramp up later this month and a more active pattern is looking more and more likely for September as well.

Atlantic update storm names 2019.png

General Discussion

Frankly, it is not a real bold prediction to say that tropical activity will ramp up later this month in the Atlantic Basin as from a climatological viewpoint, the real tropical season runs from around August 15th to October 15th.  There are, however, a few solid reasons described below as to why tropical activity should ramp up in the near term.

Favorable signs

First, sea surface temperatures have turned around in the Gulf of Mexico and are now running warmer-than-normal in most spots.  In fact, much of the western Atlantic is now running at above normal levels and warmer-than-normal water this time of year is a favorable factor for the development and potential intensification of tropical systems.  The sea surface temperatures in the Gulf of Mexico were actually colder-than-normal for a few weeks following the passage of Barry which created upwelling as it churned overhead.  [Upwelling results in colder water from underneath to rise up to the surface level and this is generally an inhibiting factor for tropical storm formation as long as it remains in tact]. The western Gulf of Mexico or western Atlantic Ocean may very well turn out to be the first places where increased activity begins in about a week to ten days in an area where sea surface temperatures are especially warm (i.e., “home-grown” type tropical systems). So far, this is likely to be the quietest start to the Atlantic Basin tropical season since around 1982 in terms of Accumulated Cyclone Energy (ACE) which is a measure of both tropical storm strength and longevity.

On July 13, Barry came ashore in southern Louisiana as a category 1 hurricane and there has been no other system since then to reach named tropical storm status and it looks like there may be none for the next five days or so.  The last time there was no tropical storm in the Atlantic Basin between July 14 and August 19 was 1982 (credit Philip Klotzbach, CSU) – a season that was well below average with only six tropical storms.  This year we may not be quite so lucky.

High pressure ridging aloft may dominate the pattern later this month in the NE US and SE Canada; courtesy NOAA, tropicaltidbits.com

High pressure ridging aloft may dominate the pattern later this month in the NE US and SE Canada; courtesy NOAA, tropicaltidbits.com

A second favorable sign for an increase in tropical activity as we progress through the month of August has to do with the likely unfolding pattern at 500 millibars in coming weeks.  High pressure ridging in the upper part of the atmosphere is likely to dominate over the northeastern US and southeastern Canada as we progress through the latter part of August and this is often a favorable pattern for tropical storm formation in the western Atlantic this time of year.  When high pressure remains in place for an extended period of time over the northeastern US and/or southeastern part of Canada, there is often compensating low pressure over the western Atlantic and this can result in tropical systems sliding “underneath” the ridge into the southeastern US or Gulf of Mexico.  Longer range computer forecasts indicate there will be such a pattern in coming weeks.

Warmer-than-normal water dominated the equatorial Pacific Ocean three months ago; Courtesy NOAA

Warmer-than-normal water dominated the equatorial Pacific Ocean three months ago; Courtesy NOAA

Finally, changes have been occurring in the central Pacific Ocean that can actually result in a more active second half of the tropical season in the Atlantic Basin.  Specifically, there are signs that El Nino conditions (warmer-than-normal water) in the equatorial part of the Pacific may actually subside as we progress into the fall and winter seasons.  In fact, there have been noticeable changes in sea surface temperatures during the past three months off the west coast of South America. When El Nino is an on-going event, there tends to be an increase in wind shear (change in wind speed and direction with altitude) in the tropical part of the Atlantic Ocean and this is usually an inhibiting factor for tropical storm formation and/or intensification.  Conversely, when El Nino fades away this time of year, this tends to provide a boost to activity in the Atlantic Basin and that could very well be the case over the next couple of months.

Cooler-than-normal water now exists off the west coast of South America in the equatorial part of the Pacific Ocean; Courtesy NOAA

Cooler-than-normal water now exists off the west coast of South America in the equatorial part of the Pacific Ocean; Courtesy NOAA

Stay tuned…the current stretch of quiet weather is quite likely going to come to an end during the second half of the month of August. 

Meteorologist Paul Dorian
Perspecta, Inc.
perspectaweather.com