Entering the second half of May, the weather continues to look quite nice with abundant sunshine and pleasant temperatures for many days.   This of course means no rain and a continuation of the developing dry pattern.  While we did have some rain over the past few days, we are still below seasonal normal for May and over 20 percent below since March 1^st.

While you might not think a dry pattern in April and May has anything to do with a summer heat wave, that’s actually not the case.  If you are someone who doesn’t enjoy summer heat waves and several days in a row of 90-plus temperatures, you should be wishing for rain.

Dry ground brings more heat

In order to get the air to heat up, the ground must be warmed by the sun.  The hotter the ground, the hotter the air will become.  Deserts heat up really fast every day and cool off quickly at night.  One of the reasons for this is the dry ground.  If the ground is moist then some of the sun’s energy must be used up to evaporate moisture.  After a very wet period, which we haven’t seen in a while, the ground is saturated and it’s more difficult for it to be heated.

The dry weather this spring may build on itself.  As the ground becomes drier, there is less moisture for evaporation and fewer showers.  As the saying goes, drought begets drought.

Predicting the weather is of course exponentially more complicated than saying a dry April and May will mean a hot summer, but there are other factors that point in that direction.  The El Nino of last fall and this winter continues to weaken and is likely going to become a La Nina this fall.  Summers following a strong El Nino year average hot.

This idea of forecasting seasonally is still far from perfect.  The government forecasts are based on odds, rather than a specific number.  Much of northern New England will typically see anywhere from no 90-degree days in the northern mountains to three or four along the southern coast and double that inland. The last really hot summer we had was 2013 when Portland reached 90 degrees a whopping 10 times. 2010 was also a hot one with a similar number of 90-degree days.  For those of us who don’t like heat, that was a miserable summer.  2010 was also a summer following an El Nino, although a different type of El Nino than the one currently waning.

Already, several summer forecasts have been issued that call for a warm summer.  The official forecast from NOAA below shows the northeast with the greatest odds of a warmer than average summer.

The Weather Network, in Canada, is also forecasting a hot and dry summer for our region.

We managed to get through much of last summer with about three 90-degree days in Portland, the average number.  You might remember one of those came in September, the others in May and July.  This year, signs are pointing to the fact we will not only reach, but likely exceed that number. Get out the air conditioners.

You can follow Dave Epstein on twitter @growingwisdom

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