1:00 PM | *No April Fool's joke...major cold air outbreak arrives in the US around April 1st*

12Z GFS 500 millibar height anomaly 216-hour forecast map for April 1st; map courtesy Weather Bell Analytics, NOAA

Overview
There is a blizzard raging today across the Rockies and it’ll greatly impact the Northern Plains and Upper Midwest over the next 12 to 24 hours or so with significant snow and wind. By no means, however, is this winter’s last gasp for that part of the country and indeed, there are numerous signs for a major cold air outbreak for much of the central and eastern US as the calendar turns from March to April.  Multiple “teleconnection” indices support the idea of a pattern change to colder-than-normal around here by the time April begins and it could last right into the second week of the month.  Looking even farther ahead, once this upcoming cold spell disintegrates likely by the middle of next month, we could very well go from “winter-like” to “summer-like” in a hurry.  
  

AO and NAO indices are forecasted (red) to drop into negative territory by early April; courtesy NOAA

Teleconnections
“Teleconnection” in atmospheric science refers to climate anomalies being related to each other at large distances (typically thousands of miles). All teleconnection patterns are a naturally occurring aspect of our chaotic atmospheric system and can arise primarily as a reflection of internal atmospheric dynamics.  Meteorologists can track changes in teleconnection patterns through index values and when important phase changes take place between positive and negative values, significant pattern changes are often the result.  In fact, three such teleconnection indices are pointing to a pattern change to colder-than-normal for much of the central and eastern half of the nation in the coming days as they are forecasted to drop from positive-to-negative.  In fact, one of these indices known as the Eastern Pacific Oscillation (EPO) is forecasted to go to nearly “off the charts” negative territory before it rebounds dramatically.

Composite temperatures anomaly maps for April when AO (left) and NAO (right) indices are negative where blue=colder-than-normal; maps courtesy MadUSWeather, NOAA

Arctic Oscillation (AO)
The Arctic Oscillation (AO) is a large scale mode of climate variability, also referred to as the Northern Hemisphere annular mode. The AO is a climate pattern characterized by winds circulating counterclockwise around the Arctic at around 55°N latitude. When the AO is in its positive phase, a ring of strong winds circulating around the North Pole acts to confine colder air across polar regions. This belt of winds becomes weaker and more distorted in the negative phase of the AO, which allows an easier southward penetration of colder, arctic air masses and increased storminess into the mid-latitudes.  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.  During the month of April, negative AO index values are typically correlated with high-latitude blocking and colder-than-normal conditions in much of the nation (above, left). 

North Atlantic Oscillation (NAO)
A closely-related cousin to the Arctic Oscillation is known as the North Atlantic Oscillation (NAO) and it too will be undergoing a drop into negative phase territory by early April.  Should the NAO index actually fall into negative territory - much like the AO index is expected to do - then the analog temperature composite map for April suggests colder-than-normal weather for much of the nation (above, right). 

EPO index forecasted to drop deep into negative territory (left); Composite temperature anomaly map for April with negative EPO; courtesy MadUSWeather, NOAA

Eastern Pacific Oscillation (EPO)
While the AO and NAO are index values related to patterns in the North Atlantic Ocean, the EPO index involves wind and pressure patterns over the eastern Pacific Ocean.  When in a positive phase, the EPO generally is reflected by dominant stronger zonal flow and/or low pressure troughing along the west coast of the U.S. This combination, in turn, tends to funnel milder Pacific Ocean air well inland into the country and thus, limits Arctic outbreaks by holding them at bay up in Canada. When the EPO is dominated by a negative phase (as with the AO, NAO), more high pressure ridging develops along the west coast as higher pressure extends from the Gulf of Alaska south along the west coast of Canada (opposite of the positive phase). The latest GFS model forecast of 500 millibar height anomalies for April 1st (top) clearly shows strong (warm) ridging along the west coast of Canada and a deep (cold) upper-level trough centered over southern Canada.  This, in turn, encourages a northwesterly flow from Canada into the middle and eastern sections of the US and thus, the delivery of polar or Arctic air. As with the negative AO and NAO in the month of April, a negative EPO is historically correlated well with colder-than-normal temperatures in this part of the country (above, right).

Bottom line – still one more hurdle to jump before we get sustained warmth in the eastern US.  

Meteorologist Paul Dorian
Vencore, Inc.