Post by Miles Schumacher – Senior Forecaster
The winter of 2014-15 temperatures averaged very close to normal. With December and January both averaging warmer than normal, February was much colder than normal. Temperatures in the Pacific Ocean were in large part responsible for the late season cold.
The state of temperatures of the equatorial Pacific Ocean exhibited a weak El Niño temperature distribution. During the early winter, the warmest water, relative to normal, was located more over the east Pacific. The warmest water, relative to normal, has shifted into the central Pacific during the past few months. The typical expectation for an El Niño winter is for warmer than normal winters over the Rockies and Plains, cooler than normal over the southeast U.S. When the warmest water shifts west into the central Pacific, as it has this winter, the pattern shifts west with the cooler than normal weather extending over the eastern U.S. into the Plains. That occurred as the transition into the central Pacific took place during the last half of January and February. Another factor affecting the winter pattern was the pool of warm water from the Gulf of Alaska, extending south along the west coast. The warm waters favored the development of an upper level ridge of high pressure. That ridge also aided in the delivery of cold air into the U.S.
The circled area in figure 1 shows the overall warmer than normal temperatures along the equatorial Pacific, especially in the central Pacific. Little additional warming is expected, though above normal water temperatures are likely to remain this summer. The warm pool of water from the Gulf of Alaska south along the west coast is expected to cool slowly over the next several months.
Figure 1: Global sea surface temperature departure from normal.
The atmosphere typically follows a three to seven year cycle between El Niño and La Niña. Depending on the phase of the Pacific Decadal Oscillation (PDO), El Niño/La Niña is favored during warm/cold phase of the PDO. The Pacific is currently in the cold phase of PDO. La Niña conditions are favored by a two to one margin during the cold phase. The reason for that change is that during the warm phase of PDO, El Niño typically lasts 10 to 12 months. In contrast, during the warm phase of PDO, it will persist for 20 to 22 months. The development of El Niño was favored by many of the world models for this winter going into the summer. During the past six months, the PDO has exhibited a warm PDO signal. Indications are that the weak El Niño pattern may persist into next winter. Below is a set of forecasts of equatorial Pacific temperature departures from the Japan Agency for Marine-Earth Science and Equatorial Technology (JAMSTEC), see figure 2. Note that most model runs indicate above normal sea surface temperature (SST) is expected to remain through the next two years with significant cooling not taking place until later next winter. For SST departures to be considered either an El Niño or La Niño, the average temperature departure must be at least 0.5°C above or below normal, respectively, or more for three consecutive 90 day seasons.
Figure 2: Sea surface temperature departure for the past year plotted in blue. The forecasts for the next two years follow. The red line indicates the mean of the nine forecasts made through January 2017. The gray lines are the individual model runs. Departure in degrees C is shown on the ordinate, with time on the abscissa.
Although in meteorology no two years are the same strictly speaking, one can look at weather patterns of the recent past to give some indications of near term weather trends in the future. This forecast is based in large part on the best fit from several of the years that were the most similar to late last fall and the winter season just past. Considerations were also made for the state of the Pacific and the expected change to an El Niño state as well as the warm pool off the west coast and other factors that influence the weather pattern.
The Pacific SSTs are yielding a signal for the upcoming spring and summer months. The cooling effect brought on by the warm pool off the west coast, as well as the position of the El Niño warm pool, has become quite evident during the last winter. This is likely to persist into the spring. March is likely to be the coldest month relative to normal with temperature returning to normal or a little above for April and May. Other factors to consider are the development of drought areas. The southern U.S. drought and developing dry conditions over the north central states can affect temperatures, especially during the late spring and summer months. The near record extend of ice on the Great Lakes can be a factor, contributing to shunting Gulf moisture to the south as dry high pressure will be favored over the cold water.
For this spring, it is likely that the effects that have been seen this winter will persist for at least the first month to month and a half. Overall, temperatures will likely be a little cooler than normal. The trend for less than normal precipitation is expected to continue, though extremely dry conditions are not expected. See figure 3.
Figure 3a: Mean temperature departure for March of 2015 through May of 2015.
Figure 3b: Mean precipitation departure for March of 2015 through May 2015.
Looking ahead toward the summer for what is most typical of the El Niño patterns observed is that temperatures are not extreme. The summer temperatures over Iowa are likely to be warmer than normal. Though warmer than normal, it appears very unlikely that Iowa will see a return to the heat experienced in 2012 and 2013. Rainfall is expected to be a little below normal, but once again not by a great deal. It is unlikely that there will be a return to the extremely dry conditions of the summers of 2012 and 2013. Development of dry soil conditions, such as are currently present north and south of Iowa, need to be monitored for potential expansion. That change could result in less rainfall than is currently expected. See figure 4 for details.
It will be important to monitor the oceanic and atmospheric patterns over the next several months. Although the signs point more toward a warmer and drier summer than normal, at this time extreme conditions are quite unlikely.
These outlooks are based more heavily on statistics than many of the methods used by the Climate Prediction Center. The complete set of official forecasts from the Climate Prediction Center can be found on our website.
Figure 4a: Mean temperature departure forecast for June of 2015 through August of 2015.
Figure 4a: Mean precipitation departure forecast for June of 2015 through August of 2015.