Look at April temperatures versus the 30 year average:
Remember that across the mid-latitudes, troughs and ridges (valleys and peaks in the upper levels of the atmosphere) travel like a fluid through the atmosphere. When one location is lucky enough to be blanketed by a ridge, fair weather typically is the result. Once the succession of trough and ridges is changed by an interruption in the wave pattern, the weather will change at the surface. These changes (too much to go into here. See Southern Oscillation Index) often originate over the Pacific Ocean and propagate east. Currently, these changes are reflected over the northern Atlantic and Greenland.
|Notice the red (high pressure) at the top inching west blocking/stalling the blue (low pressure systems) over the NE|
|NAO drops to -300 soon|
But be warned, historically, this pattern over Greenland in May doesn't always produce a cold and rainy May. Look at the high temperatures in Cleveland during these years of record low May NAO:
Look at the resultant upper level pattern across the US in each of these years. All but 1948 and 2008 show dominant warmth.
|May 1948 - DOMINANT COLD OVER GREAT LAKES/NEW ENGLAND|
|MAY 1951 - WIDESPREAD WARMTH ACROSS US. COLDER SOUTHWEST|
|MAY 1962 - WIDESPREAD WARMTH ACROSS US|
|MAY 1980 - WIDESPREAD WARMTH ACROSS US|
|MAY 1993 - MORE US WARMTH. COLDER DEEP SOUTH|