Fig. 1-4: Yesterday’s synoptic map at 3PM CST when severe weather struck Mississippi/Alabama. Yesterday a likely record 581 storm damage reports were issued. The catalyst to the event was a deep West U.S. upper trough as identified by a robust negative Pacific North America (PNA) index. The southwest wind into the East U.S. brought abundant available moisture from the warmer ocean surface to the south.
Recipe for an unusually intense January severe weather outbreak: Yet another historic extreme event…581 severe weather reports on January 11th! Possibly the strongest severe weather episode on record for January occurred yesterday as a deep upper trough in the jet stream pattern across the U.S. turned northeastward across the East-central U.S. inviting very warm and humid air northward from the warmer-than-normal subtropical East Pacific and western North Atlantic basin causing a buoyant and unstable atmosphere. The result was a wide array of damaging weather events including 581 wind damage reports most concentrated in Alabama but covering a large portion of the South and East U.S. plus tornadoes in Mississippi, Alabama and Georgia. Across the entire Southeast, Interior East and Northeast high winds caused widespread power outages to the east of the primary cold front. On the back side of the frontal system snow and ice stretched from the northern Missouri Valley to Great Lakes region to Quebec and northern New England. High wind continues to plague the Northeast U.S. today.
The cause of this immense and unusual January severe weather event was the combination of a deep upper trough across the West U.S. as measured by an exceptionally negative phase of the Pacific North America (PNA) index. The –PNA pattern is notorious for producing cold and snowy upper troughs over western Canada/northwest U.S. during the winter months while downstream the Southeast U.S. can be record warm. The –PNA pattern inspires a southerly flow from the subtropics across the East U.S. and in this case that southerly flow was made warmer and more buoyant by the somewhat warmer than normal ocean surface in the Gulf of Mexico and eastern subtropical North Pacific. The largescale cause of the pattern described was an intense polar vortex located very close to the North Pole inspiring a mild and energetic flow into the U.S. storm track. Remember the cold and snowy “polar vortex patterns” of winter 2013-14/2014-15? The pattern described is just the opposite…polar vortex at home base very close to the North Pole.