Story Published:
Nov 22, 2009 at 7:56 PM CDT
Story Updated:
Nov 23, 2009 at 3:41 PM CDT
UNUSUAL WEATHER THUS FAR
This past summer has been an odd one. July, August, September and October all finished in the top ten wettest or coolest on record, or both.
Rolla was the wettest and coldest October since record keeping began. Many other cities in both Missouri and Arkansas saw unusual conditions but are too numerous to name here. This has ultimately lead people to wonder and worry that this pattern will last through the winter. In order to understand what is going to happen in the future, we must begin by reviewing this summer’s weather pattern.
SUMMER 2009 PATTERN
A look back at the summer’s weather charts shows that several different types of upper-level patterns provided the Ozarks with the cool wet conditions. And three distinct patterns stand out: a deep Western trough, several cut-off lows and a northwest flow in the upper atmosphere. Each pattern brought clouds, rain or cooler than normal air to the region. Recently, we have seen a shift to a ridge of High pressure over the central U.S. which has given us warm and dry conditions.
FROM WEATHER HISTORY TO WEATHER FORECAST
Weather folklore suggests that if it rains on Easter it will rain for the next seven Sunday’s. The saying has some validity because as once a long wave pattern in the upper atmosphere has been established it takes a while to change. The trick to this example playing out and our currest winter forecast puzzle is to determine whether the break in the long term cool, wet pattern is an anomaly or is a genuine long term pattern shift?
WHAT IS A LONG WAVE AND WHY DO I CARE?As the upper level winds blow around the earth, they generally move from west to east at our latitude. However, they do not normally go directly from due west to due east. Instead, the winds form wavy patterns much like what happens when you flip a jump rope quickly with your wrist. A wave forms on the rope and travels down the length of the rope. In math class, you may remember this as a sine wave.
Notice (above) the wave (or dip) across the western states. This wave eventually resulted in weather that brought more than four inches of rain to the Ozarks. We call this type of pattern a long wave because it is very deep, or stretched across a long distance North to South. Also notice the second short wave located along the Mississippi River between Missouri and Illinois and extending into western Tennessee. Short waves often rotate through long waves. Waves, whether they are short or long, bring unsettled weather. The longer the atmospheric wave, the slower the wave’s movement will be... therefore, the unsettled or nasty weather generally lasts longer.
FINDING A PATTERN TO THE WAVES OF WEATHERWhen trying to figure out what the weather is going to do in a long range forecast we need to look at the position of the long wave pattern. This is called the mean, or average, trough position. During the summer, the mean trough position was either out west or over the central United States. Even though there may be temporary changes to the long wave pattern, the atmosphere tends to return to the mean, or average position. Once a long wave pattern has been established it is difficult for this to change, especially during the summer and winter months. This is why the sudden change we noticed during the first part of November is being viewed with caution and skepticism as a long-term trend.
USING CLIMATE TO PREDICT THE WEATHER
Climate is a measure of our weather over a long period of time. And many things can affect the climate of the earth. Some of the more well known factors include El Nino or La Nina. Still other factors can be sunspots, volcanic dust and what the high pressure system over the North Atlantic does. Several factors are beginning to emerge which, when viewed together, indicate that we may be in store for a mild winter.
EL NINO INFLUENCES GLOBAL WEATHER
The first and most important factor is El Nino. El Nino is a warming of the waters of the Pacific Ocean off the coast of Peru in South America. Since June, the waters in the Pacific Ocean have been warming and recently have been the warmest since 2006.
According to the latest information from NOAA, a moderate to strong El Nino is predicted for this winter. A local study done about 12 years ago showed that during an El Nino year the Ozarks can expect slightly warmer and wetter conditions during the winter. In the above graphic, notice the orange shading. This is the above average warmth. Blue indicates the coolness of the ocean in relation to the average temperature. One can easily see the ocean is much warmer than normal which is a sign of a healthy El Nino period.
HOW DOES EL NINO AFFECT THE OZARKS?
The air is warmed or cooled by the surface it sits over. During an El Nino episode, air over the Pacific Ocean is warmed by the above-average temperatures of the water itself. Winds blow from high to low pressure, so wind is a result of differences in temperature and pressure.
Jet streams are found where the pressure and temperature differences are greatest. In the
Northern Hemisphere, there can three distinct jet streams in the winter, an Arctic jet, a Polar jet and a Subtropical jet. El Nino enhances the Subtropical jet, the fast-moving river of air closest to the equator. The Subtropical jet normally flows over the southern tier of the
United States and is responsible for bringing storm systems and moist air from the Pacific Ocean across the Deep South. The Subtropical jet also ushers in warmer than average air to the central United States including the Ozarks. This is why we typically find warmer and wetter conditions in the Ozarks during an El Nino period.
OTHER FACTORS INFLUENCE WEATHER
One of the other factors to consider is the output of the Sun's energy. Over the last several years the
sunspot activity has been very low. Sunspots are an important measure of solar activity. When there are no sunspots, the Sun is quiet and the energy reaching the Earth is not as great. 2009 saw a period of no visible sunspots for 243 days (or 76% of the year) which is very quiet.
Recently there has been an up tick in solar activity. The sun has an
11 year sunspot cycle and this recent new cycle may be an indication the sun is waking up and beginning an upswing in solar output. The combination of increasing solar output plus El Nino may eventually help to verify a warmer than normal winter.
THE RESULTING WINTER OUTLOOK
Changes will not occur immediately. It takes time for the atmosphere to ramp up to above-normal conditions. It would be unusual to suddenly go from wet and cool settings to a warm and dry pattern.
My feeling about the upcoming winter is that the first part of winter may in fact be cooler than average with slightly heavier precipitation. However, by the middle part of January conditions will begin to warm and we might experience an early spring. This is not to say we will not have individual snow or ice storms, rather that overall conditions should average out to be slightly above what is considered 'normal'.
More Winter Information
