As arctic air drained out of northern Canada last weekend, a very different pattern began to emerge. The northern territories, no longer dealing with -30°F to -50°F temperatures like they were just days ago, are now basking in above-zero weather.

The upper-level ridge over western North America so prominently displayed in last week’s weather maps (shown to the left below) is breaking down.


The jet stream (outlined by the black line) last Thursday (left) and in about 5 days (right). Black text denotes upper-level low (L) and high (H) pressure systems. (Penn State)

According to the weather model consensus, mild air has flooded western Canada in a way that will soon allow a broad cyclonic (counterclockwise) flow to develop west of the Continental Divide (as shown above on the right). This will provide relief from the recent frigid temperatures in the Plains, and more likely than not, set the stage for …and winter lovers will hate this… a mild and wet finish to February.


High-altitude temperatures and winds near North America, averaged over many MJO events in the Indian Ocean region. (N. Sakaeda, University of Albany.)

Interestingly, the current MJO behavior may also be related to the recent uptick in tropical cyclone activity in the southern Indian Ocean, as it tends to enhance thunderstorm activity in the areas shaded in green in the map below.


Anomalous precipitation rates, averaged over many MJO events in the Indian Ocean region. Green (brown) shading denotes relatively wet (dry) areas. (NOAA)

Indeed, just days ago, a powerful tropical cyclone moved right through the area circled in red. As noted by Jason Samenow, this storm (Giovanna) made landfall on the island of Madagascar last night.


A forecast of high-altitude weather from the GFS ensemble mean for next week (NOAA)

In the larger context, this high-altitude pattern fits nicely within the La Nina framework, where a southwesterly jet stream (black arrow in the image above) separates mild conditions in the East from colder conditions in the West.


Temperature anomalies averaged over many La Nina winter months (left), and over Dec-Jan this season (right). Yellows and blues indicate warm and cold regions, respectively (NOAA/ESRL)