Airmass Changes Could Bring Widespread Relief from Bitter Cold
Monday, January 12, 2015, 2:37 PM - The atmospheric pattern that has dominated North America since late December has plunged much of the continent into an arctic deep freeze, but a short period of relief may be on the way as changes to the atmospheric landscape will allow milder conditions to temporarily take hold.
What’s behind this winter warmup? It’s all about source regions.
The atmosphere is a dynamic system – it is always in motion – which means that the characteristics of the air around you were likely determined somewhere else on the globe. One of the most important factors that determines the personality of an airmass is where it comes from: its source region.
The atmospheric pattern determines how airmasses move, and so establishes which source regions our weather is drawn from. This dictates whether we experience Pacific warmth, or Siberian cold, and is the reason that meteorologists follow pattern changes so closely.
This image shows the forecast atmospheric landscape on Wednesday, January 14th, and is very typical of what we’ve seen so far this month. The key driver of this pattern can been seen along the western margin of the continent. A strong ridge is in place from Oregon all the way into northern Alaska, while a trough can be seen extending southwestward from the California coast.
The combination of these two features form what we call an atmospheric “block”, which acts as a barrier to the normal west-to-east flow in the mid-latitudes. Air can’t flow easily through the block, so it is forced to travel around, both to the north and to the south.
This means that the warm airmass from the Pacific source region is bottled up behind the block (❶), and can’t strongly influence the weather across North America. Instead northerly flow sets up on the other side of the block (❷) which can allow airmasses to cross the pole from the Siberian source region, setting up a condition we call “cross polar flow”. This is how we get our most brutal cold outbreaks, such as the one we just experienced.
But fortunately these blocking patterns can’t hold forever, and eventually the atmosphere will return to its base state of west-to-east flow. Current forecast model guidance suggests that this will happen as we move later into January, as this forecast image from January 18th shows.
This image shows that the blocking pattern has relaxed, and the ridge and trough from the previous image are no longer in place. This opens the path for the airmass from the Pacific source region to overspread North America ❸ bringing widespread milder conditions, while the coldest air remains confined to the high polar latitudes.
The next image shows a forecast of the average temperature anomaly (departure from normal) for the period of January 17-22. If this forecast were to hold, it would mean warmer than seasonal temperatures from coast to coast across Canada.
Of course we are still in the heart of winter, and forecast guidance suggests this January thaw won’t last as long as we might hope. This next image indicates that late January could see the return of the blocking pattern on the west coast. If you compare this image with the first, you’ll notice that they are remarkably similar. This would mean the end of January may look much like the beginning, with widespread outbreaks of arctic air east of the Rockies.