Discussion: A large mass of cooler than normal water emerged south of Greenland during spring 2018 (Fig. 1). The reason for this vast area of much cooler than normal ocean water is a combination between the possible long-term phase change of the Atlantic multi-decadal (AMO) oscillation which has been in the warm phase since the mid-to-late 1990’s and vast ice melt from the Greenland ice sheet flowing into this region. There is also speculation that the global ocean thermohaline conveyor belt, a subsurface ocean current which originates in the North Atlantic is changing phase.
The vast area of cooling waters also cools the atmosphere above inviting presence of the polar vortex. During spring and into meteorological summer 2018 the polar vortex has anchored over or near this cool water surface south of Greenland and into the Canadian Maritimes (Fig. 2).
Meanwhile a warmer than normal ocean surface is present in the southeast North Pacific, Gulf of Mexico and east of the Mid-Atlantic U.S. contributing to warming the nearby atmosphere propelling a strong upper level high pressure ridge over the Great Plains. Occasional entrainment of moist air from the Gulf of Mexico and southeast North Pacific has enhanced a sharp boundary in the atmosphere between unusually cold climate in northeast Canada including late season snow cover caused by the polar vortex and record warmth in the U.S. related to the warmer than normal ocean surface either side of the U.S.
The described enhanced boundary in the atmosphere has caused excessive rainfall in the Midwest U.S. to Virginia (Fig. 3-6). Rainfall amount of 10-25 in. during the past 30 days is common in this stretch with isolated reports near 30 in. The indicated rainfall is generally near or higher than 5 times the normal amount.
Fig. 1: Clash of an upper level high pressure ridge over the Central U.S. (dark red) and lingering winter polar vortex (blue) during the past 30-40 days is driven by warm and cool sea surface temperature anomalies including the unusual vast cooler-than-normal waters south of Greenland.
Fig. 2: The past 40 days has observed a dramatic contrast in the North America upper air pattern featuring an intense upper ridge over the Upper Midwest of the U.S. and lingering winter polar vortex south of Greenland.
Fig. 3: Rainfall amount is in the 10-25 in. range with high spots of 30 in. across Virginia the past 30 days.
Fig. 4: The 30-day rainfall amount across Virginia ranks in the >500% of normal range.
Fig. 5: Rainfall amount is in the 10-20 in. range across the Midwest U.S. the past 30 days.
Fig. 6. The 30-day rainfall amount across the Midwest U.S. ranks in the >500% of normal range.
Summary: Rainfall amount observed during the past 30 days in the Midwest U.S. to Virginia is generally >500% of normal. The rainfall regime is estimated as a 1-in-50 to 1-in-100 year (or worse) event especially given the broad nature of this wet episode.
The cause of this heavy rainfall event is mostly related to the vast cooler-than-normal waters south of Greenland cooling the atmosphere aloft and allowing a lingering winter-time polar vortex to persist.
High pressure across the U.S. causing a record warm late spring clashed with the cold polar vortex to the northeast creating a boundary of instability leading to day after day of gully-washer thunderstorms drenching the Midwest U.S. to Virginia.