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Saturday, June 7, 2014

New England has Atlantic Ocean to thank for cool, cloudy May

Doug Webster

We have now endured the three months of spring that meteorologists and climatologists have long defined as March, April and May.

I say “endured” because in our little northeast corner of the U.S. known as New England, spring doesn’t bloom with the warmth and sunshine as it does through many other areas of the nation. ...

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We have now endured the three months of spring that meteorologists and climatologists have long defined as March, April and May.

I say “endured” because in our little northeast corner of the U.S. known as New England, spring doesn’t bloom with the warmth and sunshine as it does through many other areas of the nation.

Our expectations of sunny, warm weather are frequently dashed by that pool of water to our east known as the Atlantic Ocean. Winds from the east and south can be our friends during the summer, bringing refreshing breezes, and these same onshore winds during the winter can turn a snowy scene into puddles of water.

During the spring, when the land heats much more quickly than the ocean, the rising currents of relatively light, mild air are replaced by heavier cool air from the ocean. The air over the ocean usually has a decent amount of moisture and tends to bring clouds with it, especially when one adds in the effects of the air moving uphill and cooling further, producing more clouds.

Such is the climate of eastern and southern New England during the spring. May can be a disappointing month, because we see all that warm weather across the Midwest and South, but it has trouble making it to our doorstep except for a few brief interludes.

May 2014 was characterized by much of what I’ve described. May is normally one of our most cloudy months of the year, and the frequency of onshore winds is at the top. May is one month when places such as Lebanon and North Conway see more sun and milder days than we do in Nashua because the ocean’s effects diminish farther west and north.

The most recent version of May brought only four days that we would define as sunny, while 27 days were partly cloudy or cloudy. Blue Hill Observatory in Milton, Mass., the last location to record sunshine data in New England, saw 47 percent of the possible sun, a deficit of 5 percent.

Another interesting tidbit was that while measurable precipitation was noted on 16 days – 24 days if you include trace amounts – rainfall was below normal. That means we had a lot of days with just a little light rain or drizzle and no big rain producers, just plain dank, damp and dreary.

The good news was that for the first time in half a year, temperatures averaged a little milder than normal. The trend to milder readings was more because of lots of cloudy nights, which helps hold in the warmth, than it was from warm days. In fact, the average high temperature for May was right on target.

The average temperature for Nashua during May was 57.9 degrees, 1.2 degrees above normal and the 38th mildest May since records started in 1885. No record temperatures occurred, and the high of 87 on the 13th was but a brief spike of warmth. The very next day saw temperatures barely rise above 50.

A couple of frosty mornings were noted during the month, most notably the 7th, when readings fell a little below freezing in many areas just outside the city. Some patchy frost was also noted in a few spots on the morning of May 29. We don’t see frost after Memorial Day too often.

While the frequency of rain was high during May, amounts were low. Readings taken at Pennichuck Water Works show a total of 3.52 inches for the month, a deficit of 0.55 inches. Last month was the 75th driest May since records began in 1884.

Most of our precipitation total fell in the form of drizzle or light showers as a result of persistent onshore winds bringing moist marine air inland. Those were the cool days, while the handful of days with a west wind brought us sunshine and dry conditions.

Wind direction is the ballgame during the spring and into early summer. Winds from the east and south generally bring dull, damp weather, while west or northwest winds are dry and warm.

Spring’s final numbers show few surprises. The Gate City saw a cold spring with precipitation a little lacking. The average temperature of 43.5 degrees was 2.3 degrees below normal, making spring 2014 the 20th coolest since records began. The very cold March helped bring down the mean temperature for the season.

The spring precipitation total of 11.82 inches was 0.89 inches short of normal, giving spring 2014 a rank of 83rd from driest to wettest out of 130 years.

Our weather during the summer and even into next winter could be influenced by the ocean that just brought you the dull May. About 20 years ago, a couple of scientists noticed that North Atlantic Ocean temperature trends had a strong correlation to weather across North America and even other areas of the globe.

The Atlantic Multidecadal Oscillation, or AMO, was coined, and data was developed all the way back to 1856, since our oceans have been well sailed for hundreds of years with some pretty good data at hand.

Like its brother in the Pacific, the PDO, the warm and cold cycles tend to last for a few decades. The AMO’s warm cycle produces southern Plains droughts and warm U.S. temperatures, while the cold cycles bring more rain to the heartland and a colder temperature regime across North America, much like the last 12 months have brought.

It so happens that the AMO has been trending into negative territory during the last several months and might have been a player in the harsh winter across North America.

If the AMO values remain negative during the next several months, we may want to buckle up for another harsh winter in 2014-15. This could be the start of a new negative AMO cycle, but only time will tell.

The graph accompanying this column online shows plotted data since 1856. It’s interesting to note that Concord’s warmest decade was during the 1870s, when the AMO was very warm, and our state capital’s coldest decade, the 1970s, came during the coldest of the AMO records. Concord’s warmest year on record, 1878, matches the warmest spike on the graph.

Summer is here with the longest days of the year. By month’s end the days will already be getting shorter. Typical for June around southern New Hampshire are a few hot, humid days, but a few cool, damp days can still occur if that old east wind sets in.

June can also bring brilliant blue skies dotted with a few clouds along with refreshing temperatures and humidity as summer polar air slides south from Canada.

Weather & Climate appears the first or second Saturday of the month, depending on when final weather data are available. Doug Webster, of Hudson, is senior meteorologist at DTN in Woburn, Mass.