As we move forward into the final days of December and into January, the theme of this winter will continue, that is, a step-down to colder/snowier weather in the Eastern half of the United States. Notice the following two maps; first, we have the departure from normal temperatures over the last 30 days, followed by the departure from normal temperatures in the last week.
This further illustrates the step-down nature of December -- previous posts mentioned how the core of arctic chill would largely target the Rockies and Mid-west for Dec 1-15 due to unfavorable tropical forcing (MJO phases 4-6). However, the transition period would occur around the 18th of the month, and it certainly did - in a big way. Major winter storm with some locations recording historic snow totals, especially interior Virginia, northeast through Maryland, Delaware, southeastern PA, central/southern NJ, Long Island (where Upton recorded just over 26") and southeastern New England.
Looking ahead at the tropical forcing progression in the Pacific, MJO will be entering phases 1-2 over the next 7-10 days, which are historically very cold ones for the Eastern part of the US. Thus we have support from all global indices once again for a potentially very cold next two weeks -- off the charts negative AO, strongly negative NAO, favorable north pacific and tropical forcing (at least for another 10-15 days before the MJO moves into phase 3).
Phase 2 MJO composite 500mb pattern for December:
In terms of previous extreme arctic blocking events throughout history, the severe -AO period tend to persist at least 30-40 days before breaking down. Considering the AO went into the tank around December 10th, I don't believe we'll see the AO recover until January 10th at the earliest. And even then, the moderating affects won't be felt on the sensible weather regime until several days later. Therefore, I can't find any reason to forecast sustained above normal temperatures in the Eastern/Northeastern US through January 15th. Most great winters feature at least a 7-10 day relaxation period while the pattern reloads, and I do think this year will have that as well. However, I do not see that relaxation occurring any time before January 10th-15th. My guess if we'll have one right after, with a reload to more colder than normal weather into February.
The bottom lines:
This further illustrates the step-down nature of December -- previous posts mentioned how the core of arctic chill would largely target the Rockies and Mid-west for Dec 1-15 due to unfavorable tropical forcing (MJO phases 4-6). However, the transition period would occur around the 18th of the month, and it certainly did - in a big way. Major winter storm with some locations recording historic snow totals, especially interior Virginia, northeast through Maryland, Delaware, southeastern PA, central/southern NJ, Long Island (where Upton recorded just over 26") and southeastern New England.While the next couple days will feature fairly large positive departures (generally + 5 to +10), it'll be very transient in nature, particularly considering quantity of below normal temperature days in our future, beginning early next week. The trigger that got the ball rolling in my opinion was the -QBO induced stratospheric warming event back in mid November, which propagated downward below the tropopause, thus causing the polar vortex to become greatly perturbed and displaced well south of its normal position. Essentially what a -QBO (easterly phase) does is enhance the irregularity of stratospheric waves, which can lead to these major winter warmings of that push into the troposphere. The end result is a significant height rise across the northern polar region and a tanking A0 -- in this case it went off the charts about 30 days after the big stratospheric warming event. Since the NAO and AO have a correlation of about .60, considered statistically significant, the severely negative AO aided in producing the enormous north atlantic blocking as well. The west based moderate-strong el nino event is also a contributing factor to the favorable polar regime. A third possible influence is the SSTA profile in the Atlantic, which has actually tranformed into tripole appearance, with warmer than normal SST's in the ITCZ south of 30N, north of 60N near Greenland, and cooler than normal waters in the central atlantic. This is the type of water profile one would like to see for negative NAO tendency, although it's not perfect, it's much better looking than a month ago (*see image below).
Looking ahead at the tropical forcing progression in the Pacific, MJO will be entering phases 1-2 over the next 7-10 days, which are historically very cold ones for the Eastern part of the US. Thus we have support from all global indices once again for a potentially very cold next two weeks -- off the charts negative AO, strongly negative NAO, favorable north pacific and tropical forcing (at least for another 10-15 days before the MJO moves into phase 3).
Phase 2 MJO composite 500mb pattern for December:
In terms of previous extreme arctic blocking events throughout history, the severe -AO period tend to persist at least 30-40 days before breaking down. Considering the AO went into the tank around December 10th, I don't believe we'll see the AO recover until January 10th at the earliest. And even then, the moderating affects won't be felt on the sensible weather regime until several days later. Therefore, I can't find any reason to forecast sustained above normal temperatures in the Eastern/Northeastern US through January 15th. Most great winters feature at least a 7-10 day relaxation period while the pattern reloads, and I do think this year will have that as well. However, I do not see that relaxation occurring any time before January 10th-15th. My guess if we'll have one right after, with a reload to more colder than normal weather into February.The bottom lines:
- Transient warm spell this weekend with a total departure of at least +20 by the time early next week rolls around.
- MJO progression into phases 1-2 in conjunction with a persistant severe -AO/NAO couplet will yield a much colder than normal early January for much of the nation, Plains eastward.
- AO may not come out of the tank until the 10th or later of January, meaning any thaw waits until after.
- Snow potential increases significantly around the New Years with model data hinting at a short wave emerging from the sub tropical jet for a few days now. Did not mention it much in the post due to high uncertainty, but the pattern depicted with a massive west based (Greenland) block and an undercutting jet could yield another major to historic storm somewhere in the Eastern US.
- Merry Christmas and Happy New Year, could be a snowy start to 2010.
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