2:45 PM | *”January thaw” to give time for the atmosphere to reload…signs point to a return of cold weather after several warm days*

Tremendous high-pressure ridging forms across northern Canada in ten days (purple area) according to 00Z Euro 500 mb height anomaly forecast and cold air reloads across Alaska, western Canada and the western US; maps courtesy tropicaltidbits.com

Overview
It is not uncommon for a “break in the action” to occur in the northeast US during a given winter season.  In fact, this extended period of warmer-than-normal weather often takes place right in the heart of the winter season during the middle or latter part of January and has become known as the “January thaw”.  It looks like we are indeed in store this year for a “January thaw” with warmer-than-normal weather beginning around the middle of next week and lasting for perhaps a week or so.  Beyond that, however, there are numerous signs (high-latitude blocking, MJO, stratospheric warming) that point to a return of a cold weather pattern for the northeast US – one that could last well into February.

Very high probabilities of warmer-than-normal weather in the 6-10 day period; courtesy NOAA

  
The “January Thaw”
Arctic high pressure will keep us cold this weekend, but once that system shifts off the coast early next week, it’ll open the flood gates for milder air to pour into this part of the country. Temperatures should climb to well above normal levels during the middle and latter part of next week following the cold start to the week.  In fact, Inauguration Day (next Friday) could turn out to be one of the warmest ever with the “January thaw” well underway.  NOAA’s latest forecast of temperature anomalies in the 6-10 day time period (i.e., January 18-22) features very high probabilities of warmer-than-normal conditions in the eastern half of the nation. Temperatures here could reach the 60 degree mark for highs during this extended “thaw” period.

Cold signals for late January/February

High-latitude blocking 
The 00Z Euro forecast map of 500 millibar height anomalies for the northern hemisphere in the 120-240 hour time period from January 18 to January 23 shows an extreme anomalous high-pressure (purple) across northern Canada by the last day of this loop (loop in 24-hour increments).  In addition, there is the look of an active southern branch of the jet stream (blues) and cold air masses are likely traveling in this time period from Alaska to western Canada to the western US and then ready to pounce on the eastern US by day ten (1/23).  This predicted path of cold air from Alaska to western Canada/western US is actually quite similar to the movement of the initial cold air outbreaks this winter in December.  This extreme ridging of high pressure across the northern latitudes (e.g., northern Canada, Greenland) in the winter season is often referred to as high-latitude blocking - if it remains in place for awhile. Without this type of pattern in the upper atmosphere, it would be quite difficult to get sustained cold air masses in the Mid-Atlantic region during the winter season.  

High-latitude blocking is tracked by meteorologists through indices known as the Arctic Oscillation (AO).  The Arctic Oscillation refers to opposing atmospheric pressure patterns in middle and high latitudes.   When the AO is positive, for example, surface pressure is low in the polar region and this helps the mid-latitude jet stream to blow strongly and consistently from west-to-east keeping Arctic air locked up in the polar region. When the AO index is negative, there tends to be high pressure in the polar regions (i.e., high-latitude blocking), weaker zonal winds, and greater movement of polar air into the middle latitudes such as the Mid-Atlantic region.  The medium-term forecast for the AO is for a significant drop into negative territory. 

MJO index forecast next few weeks according to the 00Z Euro model with progression from warm phase into cold phases

Madden Julian Oscillation (MJO) 
The MJO is a tropical disturbance that propagates eastward around the global tropics with a cycle on the order of 30-60 days. It is a large-scale coupling between atmospheric circulation and tropical deep convection. The MJO has wide ranging impacts on the patterns of tropical and extratropical precipitation, atmospheric circulation, and surface temperature around the global tropics and subtropics. Furthermore, the MJO influences both precipitation and surface temperature patterns across the US. Specifically, one significant impact of the MJO in the U.S. during the northern hemisphere winter is an increase in the frequency and intensity of cold air outbreaks across the central and eastern US.

Temperature composites for different phases of the MJO index during this time of year; courtesy NOAA

MJO Phases 
Research has found that the location of the MJO, or phase, is linked with certain temperature and precipitation patterns around the world. The MJO phase diagram illustrates the recent and forecasted progression of the MJO index through different phases which generally coincide with locations along the equator around the globe. When the index is within the center circle, the MJO is considered weak, meaning it is difficult to discern. Outside of this circle, the index is stronger and usually moves in a counter-clockwise direction as the MJO moves from west to east. The very latest Euro model forecast of the MJO index propagates it from its current phase 6 location into phases 8, 1 and 2 later this month (follow green line in figure in a counter-clockwise fashion). Phases 8,1 and 2 for the MJO index this time of year typically signals colder-than-normal temperatures in the eastern U.S. (see temperature composite map).

Stratospheric warming event to take place across Canada and northern US during the next ten days; courtesy NOAA

Stratospheric Warming
Another way to monitor the potential for Arctic air outbreaks in the northeastern U.S. is to follow what is happening in the stratosphere over the polar region of the northern hemisphere. Sudden stratospheric warming (SSW) events in the region of the North Pole have been found to set off a chain of events in the atmosphere that ultimately lead to Arctic air outbreaks from central Canada into the northeastern U.S. Indeed, there appears to be a developing stratospheric warming event headed for much of Canada and the US over the next 10 days that could lead to a return for a cold weather pattern in late January after we experience the prolonged period of a “thaw”.  

Stratospheric Warming Consequences 
During the winter months in the lower polar stratosphere, temperatures on average are below minus 70 degrees Celsius. 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 degrees in just a few days. This sets off a reversal in the west-to-east winds and the collapse of the polar vortex. 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 (in this case, into Canada and the US).  Based on historical similarities, a stratospheric warming event like this one expected in late January for areas on this side of the north pole, could lead to colder-than-normal weather in the eastern US through much of February and perhaps even into March.

Meteorologist Paul Dorian
Vencore, Inc.
vencoreweather.com