Josh Eachus is a meteorologist at WBRZ News 2 in Baton Rouge, Louisiana.

Ridge, Retreat, Repeat: Understanding One of Summer’s Most Important Weather Drivers

  • May 31, 2016

Weather fanatics caught in the minutia of day to day forecasting would be thrilled with the subtle forecasting challenges along the Gulf Coast recently. Most of the ground truth is actually playing out via small shifts 19,000–35,000 feet over our heads.

Of course, the jet stream is ALWAYS important to the forecast. But particularly during southern summers, understanding the position of ridges and troughs can make all the difference in a bam or bust forecast.

polar and subtropical jet streams

An illustration of polar and subtropical jet streams.

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To preface, we recall that the jet stream is a river of wind aloft that divides regions of differing air density. Knowing that cold air is more dense than warm air, forecasters look at select atmospheric heights based on the average temperature from the surface to a given pressure level. Kinks, or north to south variations in this wind flow, are called ridges and troughs.   

ridge and trough pattern

The 500 millibar eastern U.S. ridge and western U.S. trough pattern.

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An upper level ridge is a region of less dense air and warmer temperatures. Stability and thus warmer, drier weather, is often the result. On the other hand, an upper level trough represents an area of more dense air and cooler temperatures. Instability is easier to come by, resulting in cooler and unsettled weather.  

ridge over southeastern U.S.

A 500 millbar ridge over the southeastern U.S.

Of late, a slight ridge bulging north from the Gulf of Mexico has put a cap on what would otherwise be a highly unstable environment along the coast. With summer heat and humidity already engaging cities like Houston, Baton Rouge, New Orleans and Mobile, plenty of surface energy exists every afternoon. However, because hot and humid air only rises effectively to develop precipitation when much cooler air is present aloft, the upper level ridge has squashed widespread convection. (The western periphery of this same ridge is adjacent to the eastern flank of the trough causing severe weather in the Midwest).   

During the summer months, a ridge promoted by the continental warm and dry air over Mexico will shift across the southern U.S. as the jet stream goes through natural undulations. The position of this ridge over the southeastern U.S. is a key factor in forecasting whether or not afternoon thunderstorms will erupt or surface energy will stay put. For coastal areas of Texas, Louisiana, Mississippi, Alabama and Florida, slight retreat in the upper ridge can allow a simple sea breeze to become a microscale, pseudo-cold front with widespread showers and intense thunderstorms.

While it can be confidently inferred that the Gulf of Mexico ridge was responsible for preventing many pesky afternoon pop-ups this week, can anything be said about its presence over the summer? You bet.

surface temperature anomalies march 2016

Global surface temperature anomalies for March 2016. 

Don’t jump to blame El Niño. The transition from the positive to negative phase of El Niño-Southern Oscillation is underway, but this has been found to have a weak impact on summer climate in the U.S. (Of course, the tropics are a different story) Much more important are Atlantic and Pacific ocean temperatures along with short-term trends in the atmosphere. Of late, sea surface temperatures in the Eastern Pacific, Western Atlantic and the Gulf of Mexico, have been running above average — essentially encapsulating the U.S. in a bubble of warmth. And because heat energy is transferred through water much more slowly than through air, these trends shouldn’t change any time soon.

Summer precipitation and temperature outlook

Three month temperature and precipitation outlooks for the summer.

Warmer sea surface temperatures will lead to warmer land surface temperatures over the U.S. Particularly in the southern U.S. where summer jet stream interactions are infrequent to begin with, warmer temperatures will result in a more prevalent ridge. Therefore, a warmer summer is the likely result. Should the ridge grow increasingly stubborn, (barring any tropical impacts) the southeastern U.S. may face a drier summer as well.