Just about the same upper air pattern producing excessive cold in April also caused the record warm May in the U.S.
Discussion: May 2018 is the warmest last month of meteorological spring on record (1895-2018). In May 42 of the 48 contiguous states were much warmer than normal while 8 states observed their warmest May on record (Fig. 1). The super warm month of May is in contrast to one of the coldest months of April on record when 20 Central/East-Central U.S. states were much below normal and 2 states were the coldest on record for the month (Fig. 2). The evolution of the weather pattern causing these extremes began in March and lead to not dissimilar upper air patterns in April and May that caused very different types of thermal regimes across the U.S.
Fig. 1: NOAA state rankings of average temperature departures from normal in May 2018 given the 1895-2018 climatology. May 2018 was the warmest month of May on record.
Fig. 2: NOAA state rankings of average temperature departures from normal in April 2018 given the 1895-2018 climatology. April 2018 was the 13th coldest month of April on record.
You may recall the bitter cold that reached Ireland and U.K. in March associated with unusual early spring snow cover. That set of extreme circumstances was caused by a late February stratospheric warming event over north-northwest Russia causing the troposphere below to cool. A strong polar vortex emerged (Fig. 3). The coldest arctic air of winter occurred very late in calendar winter across Russia and extended westward to Europe with widespread unprecedented snows. As a result of the snow cover the polar vortex expanded westward to Europe.
The seasonality of the warming subtropics to mid-latitudes of the northern hemisphere in April caused by the rapidly increasing sun angle and coupled with the snow covered northern latitudes caused a pattern sustaining thermal contrast in the upper atmosphere. The polar vortex maintained intensity continuing its westward shift to Canada (Fig. 4). The attendant unusual widespread snow cover enabled cold air masses to form in Canada and release into the U.S. with steady frequency causing the excessive U.S. cold pattern. The warm SSTA pattern in the southeast North Pacific enabled a strong warm ridge in the Southwest U.S. in April further enhancing the thermal regime between the warm ridge and the polar vortex.
Fig. 3: Evolution of the APR/MAY 2018 upper air pattern began in March when a stratospheric warming event caused a cold and snowy polar vortex from Russia to Europe.
Fig. 4: The snowy/cold polar vortex shifted to eastern Canada in April and the far south reach of snow cover into mid-spring caused unusually cold weather in the eastern U.S.
In May as the subtropical/middle latitude sun angle strengthened further the Southwest U.S. warm ridge gained the upper hand on the U.S. thermal pattern (Fig. 5). The polar vortex was still present and very intense and still causing lingering late season snow cover in Canada. The tropics became activated in May with abundant convection bringing rains particularly to the Southeast U.S. The forcing in-between the convective heat release from the subtropical rains in the Southeast U.S. and the polar vortex to the north caused the May warmth to become extreme. The same pattern, northeast Canada trough and Southwest U.S. ridge as observed in April only this time the ridge won the battle of influence between the two outstanding features.
Fig. 5: The polar vortex lingers in May shifting slightly northward and keeping northeast Canada snow covered. Just south of the jet stream axis separating the Southwest U.S. ridge and northeast Canada polar vortex atmospheric forcing caused the record warm May to occur.
As usual the dominant upper air features causing our climate are reflected in the global SSTA regime (Fig. 6). The warm pattern across the U.S. in May was flanked by anomalous warm SSTA west and east of the U.S. Meanwhile the cold polar vortex lay across cool SSTA of the North Atlantic south of Greenland. The dramatic change in amplitude of the SSTA pattern over a short distance represent increased risk of dramatic climate across nearby land masses.
Fig. 6: Global SSTA from the past month or so yields a reflection in ocean temperature of the prevailing upper level influences. The record warmth across the U.S. was associated with very warm SSTA west and east of the U.S. and forced warmer by the cold trough across northeast Canada.
Summary: Climate Impact Company identifies the April and May upper air patterns causing opposing thermal regimes across the U.S. as similar in character although the same features switched off during each month as the dominating force. The pattern is an extension of dramatic events taking place in the stratosphere over northern Russia in March. Helping to cause the placement of these dramatic climate influencing features is the constricted polar ice cap and the influence open water northwest and north of Europe has on the cold season climate.