Monday, June 24, 2013

Why is this early summer pattern different than 2012? More 90s in store?

In March at The Ohio State Weather Symposium, I  presented a paper on the conditions that preceded the Drought of 2012 and how those conditions set the stage for the summer of 2012. You can scroll through my POWERPOINT here. Its loaded with graphics that give a chronology leading up to last summer.

Last summer and the summers before it (2011 and 2010) are still fresh in everyone's mind. All three were above normal. Each one featured different degrees of heat. 2010 & 2011 had consistent heat. Last summer featured about 3 solid weeks of oppressive heat in late June and early July with 28 days in the 90s by mid September.  So its nature to think that this summer will be just as hot right? Not necessarily.  Why is this summer setting up to be different than last summer?  The answer lies partially in the amount and location of the spring rains.

Take a look at the Long Term Drought Indicator. This map blends a bunch of variables to include soil moisture, short term and long term precipitation, ground water and reservoir water levels. Last year's indicator for late June is on the left; this year is on the right.

The corn belt was in serious trouble last year with dry conditions that carried over from the winter before (SEE MY POWER POINT) while this year, above normal spring rains across Illinois, Indiana, Iowa and parts of Missouri are in great shape. Spring heat was absent so evaporation of soil moisture didn't occur as rapidly as it did in 2012. Look at the temperatures from March through May in 2012 and this year: POLAR OPPOSITES!
Evaporation was lower, rainfall in spring was above normal. The above normal rainfall for much of June this year has stayed above normal versus last June.
So what does ABOVE NORMAL SPRING RAIN, COOLER SPRING TEMPS AND HIGHER SOIL MOISTURE MEAN for our summer pattern? Last summer, the central US drought feed the heat which strengthened the ridge of high pressure which worsened the drought....etc. It was a huge drought/heat feedback loop with no end in sight.
This year, with WETTER AND COOLER CONDITIONS IN SPRING AND EARLY JUNE along with other pattern changes globally, the chances for a huge heat dome to develop in the center of the US and across Ohio are considerably smaller than last summer. The wet ground will reduce the propensity for the heat dome to develop. So far, the core of the heat has stayed out west with only small bursts of 90 degree heat across Ohio so far.

Remember that almost 50% of our 90 degree days occur in July so we have a lot of summer left.
More than half of the summers since 1980 have had less than 10 days above 90.
What's the bottom line?

1. This summer is starting out much different than the summer of 2012. 

2. The soil conditions and accumulative rainfall in the central US and across Ohio are far better.

3. They favor more frequent breaks from the 90 degree heat into the first 10 days of July. As long as we continue with frequent rainfall, this pattern could stick around longer. We'll watch carefully as we head into 4th of July week and beyond!

No comments: