Discussion: Likely the coldest April on record for the U.S. finds the coldest major city in Minneapolis, MN where a departure from normal for April (so far) of -13.3 has occurred. 840 heating degree days (HDD) are projected for April, 530 is the 30-year normal. Wow! The most recent (past 30 years) cold April was in 2013 when 716 HDD (temperature anomaly -6.5) occurred in Minneapolis. What caused this unprecedented cold?
After a record warm February in parts of the eastern U.S. the March pattern reversed colder and more snowy. According to the Rutgers Snow Laboratory U.S. snow cover ranked 6th highest in the 52-year climatology. The late season snow cover continued into April touching Arkansas April 7th (Fig. 1) and Midwest to Northeast U.S. April 17th (Fig. 2). The snow cover enhanced an already cold air mass produced by the polar vortex (Fig. 3) colder and enabled a southern sprawl of the big chill.
Fig. 1-2: NOAA snow cover analysis across the U.S. for April 7 and April 17, 2018.
The key to the weather pattern generating the late season polar vortex was the Madden Julian oscillation. The MJO strengthened during early April and shifted east toward tropical Africa (Fig. 4). The correlation to North America climate when MJO performs this transition during April is evolution of a mid-continental upper trough shifting east to the U.S. Atlantic Coast. The upper trough generated cold air made much colder by the late season snow cover.
Fig. 3: 500 MB anomaly pattern for April indicates a polar vortex centered on Ontario.
Fig. 4: Madden Julian oscillation observations for April indicate moderate-to-strong intensity (plot beyond inner circle) shifting from the Western Hemisphere toward Africa. The correlation to climate of this MJO signature is a cold upper trough moving toward/into the East U.S.