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Spring and summer is on the way and while human beings make subjective forecasts based on a multitude of factors, there is a model that relies on one basic component....soil moisture.

Soil moisture is the amount of water contained in the soil pores above the saturated groundwater zone that's available for plants to use or for evaporation into the atmosphere. Soil moisture affects runoff because snowmelt fills soil pores first, then it either flows through the soil (interflow) or over the soil (surface runoff) and eventually into streams. Once the soil pores are saturated with moisture, the water percolates down into the groundwater. Soil moisture is measured directly by sensors in the ground at specific locations, or indirectly by using radio waves to observe the amount of moisture in the ground.

The Climate Prediction Center (CPC) calculates soil moisture because it is an important factor in monthly and seasonal temperature and precipitation outlooks. Forecasters at CPC have noticed that precipitation one month has a large impact on the temperature the next month in the summer due to its contribution to soil moisture. Soil moisture is more predictive in temperature forecasts than precipitation alone, and the correlation between soil moisture and temperature is highest in warm months when evaporation is the highest. It works like this: solar radiation is used either to evaporate soil moisture or to warm the air. If it is evaporating the soil moisture, then the solar radiation cannot raise the air temperature. Therefore, higher soil moisture will effectively lower air temperature below what it would normally be. Researchers also found that soil moisture is a good predictor for future precipitation because increased evaporation from the soil and resulting humidity increases the likelihood of future precipitation.

Forecasters have developed two methods for creating soil moisture outlooks from forecasts of temperature and precipitation. While both methods use forecasts of temperature and precipitation in the soil moisture calculation explained above, they differ in forecast methodology and how far into the future they forecast soil moisture. One method uses an atmospheric model and the other uses a statistical model.

This is a different approach to long range forecasting and after a year of observation I've actually found the results have generally verified or at the very least are good trend indicators. With that in mind here's what the model indicates for spring and summer.




Summer on the CAS looks this way (June-August). Interestingly enough its shown as cooler and drier than normal.

Here's how this compares with the CFSv2...the U.S. climate model. Some similarities in the precipitation forecast but the CFSv2 is much cooler this summer across much of North America. I have a hard time buying the CFSv2 temperature least to that degree.



For good measure I'll throw in the official forecast from the Climate Prediction Center.



I would say it's fair to say that around the Midwest there's a fairly strong trend towards below normal precipitation this summer. I would also note there's a nod towards below normal temperatures. We'll find out soon enough. Roll weather...TS