Summer 2013 Outlook

   

Is it going to be a hot summer? 

   

Climate Prediction Center (CPC) outlooks strongly favor above normal temperatures. As with all things weather and climate, there are no guarantees. This is a probabilistic forecast. Historical temperatures are ranked from coolest to warmest, then divided into thirds, called terciles. These terciles are often described as below normal, near normal, and above normal. The outlooks then assign probabilities to each of these bins. For North and Central Texas, there is around a 45% chance that the mean temperature for June-August will fall within the warmest tercile, the warmest third of historical temperatures. (In the image to the right, our region falls between the 40 and 50 contours.) The middle (near normal) tercile is assigned a 33% chance. This means that the warmest tercile is twice as likely as the coolest tercile, which has around a 22% chance.

This outlook is largely based on a recent trend toward warmer summers. For Dallas/Fort Worth, the last 8 summers (2005-2012) have all been within the warmest tercile. In fact, four of the last five summers have been among the 12 warmest on record. If a roulette ball falls on a red number on successive spins, a gambler might suggest that a black number then becomes more likely. In climatology, the opposite is the case: a persistent condition typically continues to occur. In other words, after several warmer than normal summers, another warmer than normal summer is more likely than a random selection (which would be depicted on the outlook map as EC, meaning "equal chances" of falling in any tercile). This implies a trend toward warmer summers, which is reflected in the CPC outlook.

Climate Prediction Center Temperature Outlook
for June-July-August

CPC Temperature Outlook June-July-August

Soil moisture deficits prevail throughout much of the drought-stricken Lone Star State.  Dry ground increases the likelihood of above normal temperatures.  While soil moisture cannot account for the widespread above normal outlook throughout the country, it does improve confidence in the outlook for the Southern Plains where many areas continue to have significant soil moisture deficits.  

In addition, above normal sea surface temperatures prevail in the Atlantic basin within the latitudes of the subtropical ridges.  This includes the Gulf of Mexico.  These anomalies tend to enhance the strength and persistence of the subtropical ridges aloft, increasing the likelihood of above normal temperatures in Texas.

 

   

But we had a cool spring.  Could that mean a cool summer too?

Historically, a cooler than normal spring has favored a cooler than normal summer.  However, two of the warmest summers on record, 1952 and 1980, followed a cooler than normal spring.  The stronger correlation is with precipitation; after a drier than normal spring, the summer is twice as likely to be warmer than normal.  For much of North and Central Texas, including DFW and Waco, this spring was within the driest tercile, the driest third of historical data.

 

   

Will it be hotter than last summer?  Will it be hotter than the summer of 2011?

The outlooks are designed to assess the likelihood of above (or below) normal temperatures, not describe how much above (or below) normal the temperatures will be. There is nearly a 50% chance that the mean temperature for the summer will fall within the warmest tercile, the warmest third of historical data. This would be similar to the last several summers (2005-2012), all of which were in the warmest tercile. But it would take an extraordinary set of conditions to top the summer of 2011, which is the hottest on record for the region. Record events are rare for a reason; they are difficult to achieve.

 

Is El Niño or La Niña to blame?

The current ENSO phase is neutral (neither El Niño nor La Niña), and neutral conditions are likely to prevail into 2014.  However, the phase of ENSO (the El Niño-Southern Oscillation) often changes during the Northern Hemisphere summer.  As a result, describing a correlation between ENSO phase and summertime temperatures or precipitation has little value.

 

What about rainfall this summer?  Will it be wetter or drier than normal?

For North and Central Texas, summer is a dry season between the wetter spring and fall.  July and August are among the driest months of the year climatologically.  The upper ridge of high pressure that often prevails over Texas during the summer suppresses precipitation and when particularly persistent is associated with drier than normal (and warmer than normal) summers.  The ridge typically weakens or moves away from the region at various times during the summer, allowing for sporadic bouts of precipitation.  If the precipitation is sufficiently prolonged or widespread, it can further weaken the ridge, helping to continue the wet regime.

Positive summertime precipitation anomalies (wetter than normal periods) are often associated with tropical systems.  This need not be a named storm.  A tropical plume of moisture may persist for several days of even weeks, resulting in prolonged rainfall.  Unfortunately, in most cases, these events cannot be forecast more than a couple of weeks in advance.  As such, long term summer outlooks usually do not favor above normal precipitation for Texas.

We discussed how the above normal sea surface temperatures in the subtropical portions of the Atlantic basin can increase the likelihood of above normal temperatures in Texas.  The warm waters’ ability to enhance the strength and persistence of the subtropical ridges aloft may also limit the potential for precipitation this summer.  This is part of a multi-decadal warm phase in the North Atlantic basin that may inhibit summertime precipitation in subsequent years as well.  The warm phase of the Atlantic Multidecadal Oscillation (AMO) is expected to prevail the remainder of the decade if not longer.

Atlantic Multidecadal Oscillation (AMO)

Atlantic Multidecadal Oscillation (AMO)

 

Are we still in drought?

For much of the region, drought conditions have persisted since last summer.  (Portions of Texas have remained in drought since early 2011.)  For more information on the current state of the drought, see our Drought Information Statement.

U.S. Drought Monitor - June 11, 2013

Departure from Normal Precipitation Since October 1, 2012

Departure from Normal Precipitation
October 1, 2012 to June 12, 2013

 

How are the lake levels?

More than a year of drought has taken its toll on reservoir storage. Every river basin has seen significant declines since this time last year. In general, area reservoirs have lost 15-20% of conservation during the last 12 months.

    • Only half full, Lake Bridgeport is 18 feet low. Tarrant Regional Water District has said that runoff in the Lake Bridgeport watershed the past three years is less than the driest three years of the 1950s drought.

    • Lake Lewisville has lost 37 billion gallons of water in the past 12 months.

    • Hubbard Creek Reservoir (Stephens County) is more than 20 feet low, only 1/4 of its conservation value.

 

Statewide Reservoir Storage (Percent of Conservation)

Percent full graphic

The current level is a record low for this time of year.


 

Hydrologic issues will become a greater concern this summer with increasing evaporation and water usage.  Even where formal restrictions are not in place, residents are urged to be responsible about water usage.  Avoid watering between 10 a.m. and 6 p.m. when evaporation limits its effectiveness.  Grass watered deeply but infrequently has a stronger root system that is more drought tolerant.  Aim to water only one inch per week, preferably during the early morning (4 to 7 a.m.) when relative humidity is at a maximum and wind speeds are at a minimum.  Once water begins running off onto pavement, the topsoil has exceeded its capacity.  Assess such runoff and limit intervals of watering accordingly.

 

Municipal Water Storage (Percent of Conservation) - June 2012 vs. June 2013

Regional Water Districts Graphic

 


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