Winter is — still — coming. As we move into the New Year, forecast models and ensemble guidance continues to show changes in the mid level atmospheric pattern that will support much colder temperatures for January than what we saw in December. As explained in our previous post post, the very anomalous Kara Sea Ridge and ridge over Western Canada will cause splitting of the polar vortex at 500mb. This will support polar jetstream shortwave energy diving on the downstream side the Western Canadian ridge (like a slide), carving out a trough over the Northeastern United States.
This pattern will bring a seasonably cold airmass into our region this weekend and early next week. Temperatures will run near or several degrees below normal. An Arctic front on Monday will make Monday afternoon through Tuesday feel bitterly cold, especially when compared to the lack of cold we’ve seen so far. Afterward, temperatures will moderate to several degress above average as the Western Canadian ridge starts to roll over and the trough shifts east later in the week. But this warm up is temporary, as the pattern starts to reshuffle in the higher latitudes.
Next weekend, the ridge will rebuild over Western Canada. Then mid level heights from the ridge will build into Alaska, North Pole, and Siberia. This will cause the EPO and WPO to go negative. A polar vortex at the 500mb level is expected to begin to regroup over Hudson Bay and Southeast Canada, which also helps build heights into Greenland. This pattern will cause a much larger trough to form over the Central and Eastern United States with colder than average temperatures for the second full week of January.
Meanwhile, the subtropical jet will become active as deep trough forms over Northeast Pacific. At this time, the ensemble means have the trough splitting. This will a support split flow, with a number of systems tracking underneath the Western US ridge into the Southwest United States. These systems will bring much needed to rainfall to California over the next few weeks.
For our region, we will have to keep an eye on all of these systems — and watch to see if that can track far enough north to impact us over the next three weeks. We are watching, in particular, the Jan 10-13th period. Ensembles indicate the mid level pattern will be more conducive for the southern and northern streams to phase, and cause significant storm to track further north into Mid-Atlantic region and Northeast.
The high-amplitude ridge over the West Coast supports a deepening and amplification of the trough over the Eastern US. The polar vortex looks likely to elongate west to east over the Hudson Bay & Southeast Canada. More elongation or a splitting polar vortex supports polar jet disturbances digging and phasing with subtropical jet disturbances.
A somewhat more uncertain aspect of this threat is the strength of high-latitude blocking over the NAO region. More blocking in those areas typically supports a colder airmass remaining in place — and helps the storms to track south and east of the region. Positive height anomalies support a negative NAO, and a bending back west in the height field is more indicative of a true blocking ridge developing. Some of the long-range ensemble mean suggest that both will occur to some extent.
These changes in the atmospheric pattern over the Northern Hemisphere are largely due to major polar vortex disruptions in the stratosphere and troposphere. A wave 2 event will be causing some elongation or partial splitting of the stratospheric polar vortex at various levels over the next several days. A stronger wave 1 event will cause a bigger displacement and stretching of the polar vortex in about 8-10 days. This will start to downwell into the troposphere to some extent during the second full week of January. This could support more high-latitude blocking during that period.
At this time model guidance is not showing an official Sudden Stratospheric Warming event. 10mb temperatures are not warming quite enough, nor are there a full reversal of zonal winds to the east. But pattern changes in the troposphere will be occurring, regardless, over next three weeks.
The Madden Julian Oscillation will be propagating into phase 7 next week. This supports tropical forcing focused near the dateline, enhancing a trough southwest of the Gulf of Alaska and ridge over the Western North America. High-latitude blocking, an active subtropical jet, and below normal temperatures for the Eastern US is also typically seen in this phase, during El Nino for January.
The MJO should continue to propagate into phases 8/1 in a weakening state. This will support more cold temperatures for the Eastern United States. One to thing to note, is that research published from American Geophysical Union shows the MJO decaying in phases 8/1 then growing again in phases 2/3, occurring with Sudden Stratospheric Warming events.
So, the pattern change is still on its way. While we are not necessarily going from record warmth to extreme cold, in more of step-down process with more typical winter cold outbreaks, the change will be notable. Stay tuned for more medium and long-range premium updates, as the magnitude of the cold and any winter storm threats becomes clearer as we get closer.