March and April 2013 have brought us several near-record cold spells, and this is far from typical for a Texoma spring.
It’s quite common to get one chilly spell during the transition from winter to spring, and our average “last freeze” dates run into late March.
However, it’s rare to see freezes the third week of April in the Red River Valley. We can blame the Arctic Oscillation (AO) and a persistent northwesterly wind flow.
The AO is an index which measures pressure differences around the Arctic Ocean; when it is negative like it has been much of March and April, pressures are higher than average over Siberia and Alaska and lower than average over the northern and eastern United States:
The northwesterly flow is related to a nearly stationary low-pressure area over Hudson Bay in eastern Canada. This low has kept the eastern U.S. rather stormy during the past month, while northwesterly winds on its western side have helped to “dislodge” the high-pressure areas building over the Arctic and send them southward:
The cold snaps have kept the U.S. tornado count much below average; March of 2013 had the lowest national tornado count for the month in 35 years, and April is also running much below average for twister activity through the date of this post.