May is cooling in parts of Canada and the USA. Looking at the NOAA monthly data for May, this is the recent trend (over 10 years or more).

Any ideas why?

Highlighted are more than -1F/decade cooling.

Alabama May is cooling at -0.21F/decade from 1933 to 2012
Arizona May is cooling at -4.2F/decade from 2000 to 2012
Arkansas May is cooling at -0.31F/decade from 1998 to 2012
California May is cooling at -3.41F/decade from 2001 to 2012
Colorado May is cooling at -2.58F/decade from 2000 to 2012
Connecticut May is cooling at -0.11F/decade from 1979 to 2012
Florida May is cooling at -0.04F/decade from 1995 to 2012
Georgia May is cooling at -0.19F/decade from 1933 to 2012
Idaho May is cooling at -3.53F/decade from 2001 to 2012
Illinois May is cooling at -0.26F/decade from 1998 to 2012
Indiana May is cooling at -0.06F/decade from 1998 to 2012
Iowa May is cooling at -0.62F/decade from 1985 to 2012
Kansas May is cooling at -0.71F/decade from 1998 to 2012
Kentucky May is cooling at -0.2F/decade from 1998 to 2012
Louisiana May is cooling at -0.31F/decade from 1998 to 2012
Maine May is cooling at -0.36F/decade from 1998 to 2012
Massachusetts May is cooling at -0.1F/decade from 1975 to 2012
Michigan May is cooling at -0.71F/decade from 1998 to 2012
Minnesota May is cooling at -1.26F/decade from 1985 to 2012
Mississippi May is cooling at -0.1F/decade from 1933 to 2012
Missouri May is cooling at -0.43F/decade from 1998 to 2012
Montana May is cooling at -2.46F/decade from 2000 to 2012
Nebraska May is cooling at -1.09F/decade from 1998 to 2012
Nevada May is cooling at -4.51F/decade from 2001 to 2012
New Hampshire May is cooling at -0.4F/decade from 1975 to 2012
New Mexico May is cooling at -3.07F/decade from 2000 to 2012
North Carolina May is cooling at -0.14F/decade from 1933 to 2012
North Dakota May is cooling at -2.12F/decade from 1985 to 2012
Oklahoma May is cooling at -0.56F/decade from 1996 to 2012
Oregon May is cooling at -3.57F/decade from 2001 to 2012
Pennsylvania May is cooling at -0.12F/decade from 1930 to 2012
South Carolina May is cooling at -0.17F/decade from 1933 to 2012
South Dakota May is cooling at -1.44F/decade from 1985 to 2012
Tennessee May is cooling at -0.14F/decade from 1932 to 2012
Texas May is cooling at -1.16F/decade from 1998 to 2012
Utah May is cooling at -4.18F/decade from 2000 to 2012
Vermont May is cooling at -0.47F/decade from 1975 to 2012
Virginia May is cooling at -0.17F/decade from 1930 to 2012
Washington May is cooling at -2.38F/decade from 2001 to 2012
WestVirginia May is cooling at -0.13F/decade from 1932 to 2012
Wisconsin May is cooling at -0.82F/decade from 1998 to 2012
Wyoming May is cooling at -3.07F/decade from 2000 to 2012

Regions:
Northeast Region May is cooling at -0.02F/decade from 1930 to 2012
East North CentralRegion May is cooling at -0.87F/decade from 1998 to 2012
Central Region May is cooling at -0.04F/decade from 1932 to 2012
Southeast Region May is cooling at -0.13F/decade from 1933 to 2012
WestNorth Central Region May is cooling at -1.78F/decade from 1998 to 2012
South Region May is cooling at -0.75F/decade from 1998 to 2012
Southwest Region May is cooling at -3.47F/decade from 2000 to 2012
Northwest Region May is cooling at -3.19F/decade from 2001 to 2012
West Region May is cooling at -3.82F/decade from 2001 to 2012

National

National( contiguous 48 States) May is cooling at -0.78F/decade from 2000 to 2012