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Extreme Great Lakes ice extent could portend cool summer for eastern U.S.

Time series of Great Lakes ice cover (NOAA)

Yesterday, I reported one-third of the Great Lakes was still covered in ice, an amount unheard of in over 30 years of records this late in April. An analysis reveals summers following icy Great Lakes winters tend to be cooler than normal in the central and eastern U.S.

CWG’s Matt Rogers computed U.S. summer temperatures for the 5 years with the highest and lowest peak ice extent over the Great Lakes, since 1973. He found summer temperatures average below normal in the high ice years and above normal in the low ice years. The maps below are pretty telling.

In Washington, D.C., I calculated the average summer temperatures during the five iciest and five least icy Great Lakes winters and found D.C.’s summer temperature was near normal during the icy winters and about 1 degree above normal during the ice-less winters.

Rogers says the cool summers following icy winters over the Great Lakes are not a direct result of the lake ice, but rather the large-scale weather pattern that helped the lake ice form in the first place.

“[T]he frozen lakes are a symptom of a much larger pattern dynamic,” Rogers says.

Specifically, Rogers points to the giant ridge of high pressure over Alaska and low pressure over the Hudson Bay that have forced cold air from the Arctic to spill southward into the eastern U.S. for months. In the past icy years over the Great Lakes, this pattern has tended to last into summer.

“The Hudson Bay low and Alaskan ridge have both been very persistent features this winter and the top ice years suggest they may mostly continue into the summer,” Rogers says.

Of course, the past does not always predict the future and the set of data we’ve used to establish the relationship between Great Lakes ice and summer weather is likely of inadequate length to draw firm conclusions.  A major wildcard in this summer’s weather forecast is the likely development of El Niño.

Related: Mid-Atlantic may enjoy cool summer if El Niño develops

The National Weather Service’s Climate Prediction Center is actually predicting normal to above normal summer temperatures for much of the central and eastern U.S.

The long-range Coupled Forecasting System (CFS)  model  (below) favors mostly near normal temperatures for the June through August period..

You may notice in the CFS forecast above the pocket of cooler than normal temperatures over the northern Great Lakes. The chilly Great Lakes water – a hangover from the icy winter and spring – will no doubt exert a cooling influence on the surrounding shores.  That’s what the model is simulating.  But as this region represents but a tiny fraction of North America, its direct effect on temperatures won’t extend far.  Yet the overarching weather pattern responsible for those icy waters to begin with may show considerable staying power. For those of us dreading a stifling summer, let’s hope that’s the case.