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.
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.
This shift is consistent with the expected eastward progression of the Madden Julian Oscillation (MJO) toward the tropical Indian Ocean. As shown in the map to the right, on average, there is a tendency for upper-level troughing near western North America (outlined in red) when the MJO moves like it is expected to next week.
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.
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.
So, what does this mean for our part of the world? In general terms, a trough near the West Coast is not something you want if you’re a winter lover in the East. There are, of course, lots of variations on this theme. But the pattern that the models are beginning to zero in on for the latter stages of February is one that would give the eastern half of the country atypically mild and wet conditions. It primarily consists of, as shown below, an upper-level trough in the west and a mild, westerly flow aloft in the East.
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.
The image below on the left shows temperature maps from recent La Nina events (yellows are warm and blues are cold). Though the 2011-12 La Nina has not always presented itself with a jet stream like the one shown above, it may well have set the table for the kind of temperature anomalies we’ve observed thus far this winter (map on the right).