SCENARIOS FOR EXTREME ARCTIC OUTBREAKS; Thursday, October 13, 2022

[Larry Cosgrove]

Preparation For Prominent Cold Spells

"DAM BREAK" EVENTS (also known as "BLUE NORTHER", or "SIBERIAN EXPRESS")

Slow but steady push of bitter cold from near the North Pole to about 25 N Latitude.

Formation Mechanism

A requirement for any massive, non-stop Arctic Outbreak is drainage from a cold air mass source region. If you review the 500MB height and wind charts, the flow from locations above the Arctic Circle to one at a subtropical position is unmistakable.

500MB observation chart

Surface Observation Map

Classic Case Study: February 12 - 18, 2021

Having a very cold-core cAk vortex at a near or below the U.S./Canada border, with a full-latitude trough from the Arctic Ocean and islands into Mexico to the right of a -EPO or +PNA styled ridge complex, with possible accompaniment by ridging in the -AO or -NAO positions, is the most probable way to see a far-south "ooze" reach of bitter cold temperatures. An intensifying surface storm to the right of affected areas will accelerate the advection of the bitter cold air mass.

This type of harsh cold advection can be compared to a person entering a water slide at an amusement park. Acceleration tends to happen in the last quarter of the event.

"BOMB DROP" (Associated With Bombogenesis Events)

Formation Mechanism

A particularly massive synoptic scale Arctic intrusion sometimes follows a bombogenesis event. The storm takes cold air aloft as well as draws down a cAk air mass from higher latitudes, and does so over a relatively short span of time. If the cyclone matures into a longwave feature that does not exit quickly, the bitter cold regime may stick around for as long as a week to ten days. Most "superstorms" form when there is a three way phasing of jet streams (Arctic, Polar, and Subtropical), creating extreme levels of vorticity, forcing, and baroclinicity that generates a wide variety, and large amounts, of precipitation, wind, and surface temperature differential. In these cases, extremely heavy falls of snow and ice act as a refrigerant and help to maintain the duration of the cold spell.

Classic Case Studies: 

March 12 - 14, 1993; Technical Research About Extent Of Cold Wave With The Superstorm

January 25-26, 1978; Technical Analysis Of Storm And Cold Wave

*****

"MULTIPLE BLOCKING Pattern"

Formation Mechanism

Up to four strong blocking ridge formations (-EPO, +PNA, -AO, -NAO) line up around or just above the Arctic Circle. This compresses shortwaves into lower latitudes, and prevents quick escape of cAk regimes from locations along and below 50 N Latitude. If one core of the ridge complex washes out, there is still the matter of the other positive 500MB height anomalies helping to maintain a cold surface anticyclone, preventing moderation.

Classic Case Studies

Long-Lived December 1983 Cold Wave

Record Cold DJF 1976-77

SUMMARY

There are three credible scenarios where widespread record cold air can overtake much of the lower 48 states. In all cases, a general set of five parameters seems to show up: deep snow and ice cover that expands out of the polar regions to normally warm climes; strong northerly wind component from Canada to Mexico; high-latitude blocking signatures; at 500MB, presence of a cAk vortex below 500dcm between 45 and 55 N Latitude; and a developing surface storm to the south and east of the hardest hit cold anomalies. Historic cold waves in the U.S. almost always have a long duration, and may reach into Central America and the Greater Antilles.