11:30 AM | Winter to take a bit of a break, but stratospheric warming ensures additional cold air outbreaks during February

Temperatures (red) in the stratosphere (10 mb) in the region between 90N and 65N with spike during last few days (arrow); courtesy NOAA

Overview
One of the ways to monitor the potential for Arctic air outbreaks in the northern U.S. is to follow what is happening in the stratosphere over the polar region of the northern hemisphere. Sudden stratospheric warming (SSW) events are large, rapid temperature rises in the winter polar stratosphere occurring primarily in the Northern Hemisphere. They have been found to often set off a chain of events in the atmosphere that ultimately lead to Arctic air outbreaks from northern Canada into the central and eastern U.S. Indeed, there is stratospheric warming taking place right now over portions of the polar region and another significant warm up in the upper part of the atmospheric is predicted ten days from now. This unfolding stratospheric warming event provides reason to believe that after a bit of a break in the winter weather pattern in the eastern U.S., cold air outbreaks will return in full force for much of February beginning as early as late next week.  
 
Winter’s break
Now that it appears the threat for snow at week’s end is off the table, the weather pattern is shaping up rather nicely for the next week or so.  In fact, by the weekend, high temperatures in the Mid-Atlantic region should reach the 40’s and the milder conditions are likely to continue into the early part of next week.  Later next week, a major storm will likely cut across the nation’s mid-section producing blizzard conditions out there, and it is likely to usher in a return to some serious widespread cold around here by the latter part of next week.  The notion of a return to some serious cold in the eastern U.S. after a break is supported by the stratospheric warming event now underway.  

Todays' temperature analysis at 10 mb showing stratospheric warming (arrow); courtesy NOAA

Stratospheric warming
During the winter months in the lower polar stratosphere, temperatures are typically lower than minus 70° Celsius (purple area above). The cold temperatures are combined with strong westerly winds that form the southern boundary of the stratospheric polar vortex. The polar vortex plays a major role in determining how much Arctic air spills southward toward the mid-latitudes. This dominant structure is sometimes disrupted in some winters or even reversed. Under these circumstances, the temperatures in the lower stratosphere can rise by more than 50°K in just a few days (temperature plot shows recent spike in temperatures during the past few days at 10 millibars).  This sets off a reversal in the west-to-east winds and the collapse of the polar vortex or a shifting of it from its usual position over the North Pole to another part of the polar region.  In response to the stratospheric warming at the high latitudes, the troposphere in turn cools down dramatically and this cold air displacement is then transported from the tropospheric high latitudes to the tropospheric middle latitudes. The current temperature pattern at 10 millibars shows stratospheric warming (arrow) on one side of the North Pole and there is another substantial area of warming ten days from now (arrow) as forecasted by the GFS computer model.

Meteorologist Paul Dorian

Vencore, Inc.

Forecast of 10 mb temperatures ten days from now with additional substantial stratospheric warming (arrow); courtesy NOAA