June is turning out to be another warmer-than-normal month (so far about 4 degrees F above the 30-year mean). This would make 3 out of the last 4 on the warm side, with May being the outlier finishing right at normal. In the absence of any context, this statistic is hardly unusual. Above-average monthly-mean temperatures of this magnitude are common symptoms of the natural cycles our weather pattern goes through. But when you consider how wet it’s been this June -with many locations in eastern Kansas already measuring several times the normal amount of rain expected for the entire month- this data point needs an asterisk.

Why? Well, when analyzing monthly averaged temperatures at a particular location during Spring and Summer, wetter than normal conditions are correlated with cooler than normal temperatures. This is partly because more of the sun’s incoming radiation is used to evaporate the relatively wet soils rather than heating the ground. The bottom line is that it’s just harder for the sun to heat a wet ground than it is a dry ground. Because of this relationship, climate models use soil moisture content as a predictor in their summer temperature forecasts. And we often see that wet soil conditions sometimes lead to cool summer forecasts.

This is exactly what we’re seeing in some of the long lead forecasts for this upcoming summer. With all the rain we’ve had in the last several months, the water content in the ground is unusually high across much of the Central Plains. The following image, courtesy of our Climate Prediction Center (CPC) division of NOAA, shows the soil moisture anomalies for the month of May (their most recent measurement does not include June)

http://worldonline.media.clients.ellingtoncms.com/img/blogs/entry_img/2010/Jun/18/soil_anom.JPG

The areas shaded in green represent abnormally wet ground.

Now, the temperature forecast for the entire month of July issued by CPC predicts that the best chance for below-normal temperatures anywhere in the country is basically right over us, where the ground is particularly moist.

http://worldonline.media.clients.ellingtoncms.com/img/blogs/entry_img/2010/Jun/18/CPC__temp.JPG

In their forecast, the areas shaded in blue indicate chances for below average temperatures are higher than for above average temperatures.

Here’s the twist. Just because the ground is wet doesn’t mean it’s going to be cool. Case in point; abnormally warm weather (nearly 3F above the long-term mean) has been in place over eastern Kansas since turn of meteorological Spring (March 1) during which time we’ve had way more rain than average. Go figure, right?

Not really. Hot weather patterns can thoroughly overwhelm the cooling effects associated with enhanced soil moisture. In my opinion, CPC’s cool summer forecast is relying too heavily on soil moisture anomalies and, in particular, their own climate model (called CFS) which has shown no skill in predicting our warm Spring. According to CFS, our temperatures should have been several degrees below normal since March.

http://worldonline.media.clients.ellingtoncms.com/img/blogs/entry_img/2010/Jun/18/CFS_temp.JPG

The chart at the top left shows the temperature forecast for North America for April, May, and June 2010 that was made back in March. The areas shaded in blue were forecasted to be colder than normal for the entire period. Comparing with what’s happened so far from April 1 through June 15, with the areas shaded in green/yellow/red being above normal,

http://worldonline.media.clients.ellingtoncms.com/img/blogs/entry_img/2010/Jun/18/temp_anom.JPG

CFS gave us a nearly useless forecast.

So where can we turn for a better summer forecast? There are climate models from various meteorological agencies that generally outperform CFS (e.g., UKMET, ECMWF) that are predicting a summer pattern that favors warmer than normal temperatures for us … a sure sign that these models favor a hot airflow regime that far outweighs wet soil effects. The question is, how warm would these forecasts be if we were in a drought?