9:25 AM | *Now a two-to-three week stretch of warmer weather for the eastern US which is likely to then be followed by an extended colder period*
Paul Dorian
Temperature anomalies for the month of January so far with well below-normal conditions in the central and eastern US; map courtesy Weather Bell Analytics
Overview
The month of January has been much colder-than-normal so far in the central and eastern US, but there are strong indications that we are about to enter a warmer pattern that may last for the next two-to-three weeks. This does not mean that each and every day going forward in this stretch will be warmer-than-normal and there will continue to be cold air outbreaks, but the cold air outbreaks should be relatively short-lived compared to recent weeks and it'll likely average above-normal for each 5-day period going forward into the first week of February. The latest forecast of the Madden Julian Oscillation (MJO) index is supportive of the idea for an extended period of warmer weather in the central and eastern US. Looking beyond this extended warmer stretch; however, the MJO longer-range outlook and a comparison to some analog years, suggest that a colder pattern is likely to return in February and it could very well stick around for awhile.
Euro model forecast of the MJO index between now and February 1st, 2018; map courtesy NOAA
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 over 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.
Different phases of the MJO index this time of year typically result in different temperature anomalies; map courtesy NOAA
Research has found that the location or “phase” of the MJO is linked with certain temperature and precipitation patterns around the world. The MJO phase diagram illustrates the 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 will usually move in a counter-clockwise direction as the MJO moves from west to east. The very latest European model MJO index forecast propagates the MJO through phase 4 and then into and through phases 5 and 6 as we progress through the remainder of January and into the first part of February (follow green line between arrows). This time of year, phases 4, 5 and 6 of the MJO index typically signal warmer-than-normal temperatures in the central and eastern U.S. Even the next phase (7) that the MJO is likely to enter is typically a warmer-than-normal result for the central and eastern US and this could be the case during the first week of February.
12Z GEFS temperature anomalies averaged over 5-day periods (days 1-5 on left, days 6-10 on right) with above-normal temperatures over the central and eastern US; maps courtesy NOAA/tropicaltidbits.com
Looking beyond the warmer stretch
It is likely that the MJO will continue propagating in a “counter-clockwise” pattern during February and head into typically colder phases for the central and eastern US (8, 1 and 2). This notion of a return to an extended colder period in February following a warm stretch is also supported by a look at analog years featuring similar oceanic temperature anomalies around the world that we have today. Specifically, in those years with La Nina conditions in the tropical Pacific Ocean and were preceded by active tropical seasons in the Atlantic Basin, cold weather patterns tended to dominate in the month of February. In fact, in many of those analog years, the cold weather pattern in the central and eastern US usually stuck around and continued through much of the month of March – not something many of us want to hear at this point.
Meteorologist Paul Dorian
Vencore, Inc.
vencoreweather.com
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