The importance of a cross polar flow and Aleutian ridge in December
Multiple ensemble guidance indicate a major pattern change over the Pacific region. A ridge will builds near Aleutians then poleward into the Bering Strait and the Artic Ocean next week. This will cause cross-polar flow to finally setup over the North America. All the anomalous cold air that is has been over much of Eurasia for the last several weeks with finally see an open gateway into North America. Temperatures departures are forecast to between 10 to 20 below average over parts of the Rockies and Northern and Central Plains over next weeks.
The poleward Aleutian ridge was one the features we anticipated will be occur from winter forecast, especially during December. Given some research on +QBO and weak La Nina events. Currently 30-day Moving SOI data has been rising higher over next few days. So might be seeing more weak La Nina influence return during over next couple weeks.
Interestingly enough the winter of 2010-11, which moderate La Nina and +QBO, also started Aleutian ridge and -PNA signal in November. Eventually that pattern evolved to with major high-latitude blocking retrograde during from late November through mid-December. We don’t currently believe this pattern to evolve to quite this extreme for December. But despite some warmer than normal temperatures initially to start next month, the pattern is still appears to be evolving into one that favors wintry weather in mid-December over the more of the CONUS.
The GEFS model showing temperatures below normal over much CONUS with cross-polar in mid- December
The question is how much of this cold will reach the East Coast. This will depend on the amount of high-latitude blocking over Atlantic side to support more sustained cold airmass over the East. If the NAO remains more positive, then this ridge placement will support deep trough with more below normal temperatures over parts of the Western and Central US. This will cause stronger ridge build over the East Coast at times with temperatures after few days below, to moderating again to near or above normal.
At this time, ensembles data shows the AO/NAO rising again, as blocking ridge over Hudson Bay/Davis Strait region begins to break down next week as well. But there some hints of on the model ensembles ridging building over from the back Greenland and Scandinavia . However, the stratospheric polar vortex is forecast to recover somewhat over next few weeks. Stronger high-latitude blocking shown on some guidance should be used with more caution. Meanwhile the MJO/tropical forcing forecast by ensemble data to gradually weaken have less influence going into December.
Today’s GEFS pattern going into the middle December suggest a gradient may set uper over Northern Mid-Atlantic and Southern New England between confluence underneath possible -NAO and 50/50 blocking.
But some guidance also suggest there will be a gradient between a weak Southeast ridge and lower heights accompanied behind a deeper trough over Southeast Canada. If this happens, there will be s battleground between cold and warm airmasses setup near over Northern Mid-Atlantic and Southern New England. Pacific energy would eject out of the Southwest US and possibly run into a colder airmass before the departing the region. There will be more potential for storm events with wintry precipitation, especially over the Interior.
We will continue monitor these developments throughout the next few weeks. There will new long range outlookk update with a weekly breakdown of pattern changes through December and into January on Tuesday.