December 20-31 Pattern Discussion

[donsutherland1]

Last week’s discussion leading into this period highlighted a number of developments:

• A look at a number of the indices and other data now points to the establishment of a more wintry regime in the East, particularly from after next weekend through perhaps the end of the month.

Last week saw the start of this transition and the promise of a snowfall in some parts of the East and a shot of bitter cold highlight the idea that a more wintry regime is establishing itself. During the December 13-18 period, one saw the following anomalies for select cities:

Boston: 3.3° below normal
New York City: 1.3° below normal
Washington, DC: 1.7° below normal

• Right now, the air in Canada is not sufficiently cold for there to be such a severe Arctic outbreak. However, if cross-polar flow can be established, things could begin to fall in place after the 20th.

The timing on this might be off just a little. December 20 will see the arrival of by far the coldest air mass of the season. While it probably won’t reach extreme levels (single digits in all the major cities from Philadelphia to Boston), it will be quite impressive in sending temperatures into the teens in all these cities for lows.

• I believe that temperatures across a large part of the East should be running 3°-5° or more below normal on a regular basis this week onward, albeit with some modest warmups. I don't expect to see the temperature at Central Park reach 59° again this month.

Boston has already fallen into that average for the December 13-31 period. I expect to see this downward trend in mean temperatures continue this week and maybe somewhat beyond. At this point in time, there is no strong data that would suggest that the high temperature would reach the 59° mark in Central Park noted last week.

• Afterward, given the MJO things could begin to get quite interesting, especially as abundant cold air remains available. Should blocking develop, the risk of snowfall across the Ohio Valley, Mid-Atlantic, and Northeast should increase markedly after next weekend. Whether or not any more significant storms will develop remains to be seen. Even as the risk of accumulating snow grows, should blocking occur in tandem with cross-polar flow, the possibility of severe cold could also be increasing. The best chance at that is probably 12/20 or afterward.

All said, I believe things still look good for December to end with all the major cities from Washington, DC to Boston having picked up at least some accumulation of snow and likely having seen the lowest minimum temperature in at least the upper teens.

That point is really the bridge to this week’s discussion. First, as evidence that the opportunities for snowfall are increasing markedly is the development of the storm that should affect parts of the East tonight into tomorrow.

This storm should deliver some accumulating snow to portions of the northern Mid-Atlantic and Northeastern United States. With the exception of perhaps Boston and Providence, none of the major cities should see anything more than a light snow out of this system. The bigger story will likely be the number-busting cold that blasts into the region, sending temperatures into the teens from Philadelphia to Boston. Afternoon highs in New York City might not exceed 22° and the mercury could remain in the teens for most or all of the afternoon in Boston. This morning’s discussion on the event pretty much offers my thinking on this situation.

Snow geese from Washington, DC through Boston should be cheering for at least some of the snowfall opportunities to materialize for a reason other than their love for snow. When it comes to seasonal snowfall, winter really does seem to remember December.

In Boston, 21/33 (64%) winters that saw 50” or more in seasonal snowfall experienced 8” or more snow in December. Of the 39 winters that received 8” or more snow in December, 21 (54%) had 50” or more snowfall and 32 (82%) had 40” or more snowfall. The average seasonal snowfall in such winters came to 54.2”.

In New York City, 32/52 (62%) winters that saw 30” or more in seasonal snowfall experienced 6” or more snow in December. Of the 49 winters that received 6” or more snow in December, 32 (65%) had 30” or more snowfall and 45 (92%) had 20” or more snowfall. The average seasonal snowfall in such winters came to 37.8”.

In Washington, DC, 27/44 (61%) winters that saw 20” or more in seasonal snowfall experienced 2” or more snow in December. Of the 48 winters that received 2” or more snow in December, 27 (56%) had 20” or more snowfall and 42 (88%) had 10” or more snowfall. The average seasonal snowfall in such winters came to 22.6”.

Impressive lake effect snows in parts of Ohio, Pennsylvania, and New York also bear witness to the changed situation. Indeed, these snows developed a little ahead of what I had been thinking last week.

Based on an examination of the global indices, ensemble forecasts, SSTAs and the evolution of 500 mb height anomalies, it appears that although the NAO will probably deep below 0 around December 22 or so, it will not lock in at the negative phase. Instead, several days later, it will likely head back into positive territory. Consequently, I believe the period for blocking will probably be fairly brief.

At the same time, there is increasing data that suggests that the PNA could turn negative beginning later this week and the Arctic Oscillation should remain generally positive for the remainder of this month.

The 5-day moving average of the 500 mb height anomalies shows an area of much above normal heights remaining well south of Greenland and slowly retrograding toward the west.

As a result, I believe the rest of the month will feature no prolonged sieges of bitter cold air, though there should be periodic pushes of cold-to-very cold air. As the month nears an end, assuming that the PNA goes negative, there is some danger that ridging might begin to retrograde toward the East. I believe last night’s run of the GGEM, which showed some ridiculously high height anomalies into the Northeastern U.S. in the medium range might have been sensing this danger. I’m not sold on this just yet and teleconnections argue against it, but it is something for which one might need to watch as the month nears an end. Another possibility would be for this area of anomalous heights to head more northward toward Greenland. If that were to happen, one might see the NAO swing negative in early January.

For now, there appears to be only a slim window of big opportunity for a blockbuster East Coast snowstorm (approximately in the 12/24-12/29 period). Given the changes taking place as discussed above, I’d have to argue that odds lean against any major or historic East Coast snowstorm this month. By major or historic snowstorm, I’m referring to a storm that would deliver a foot or more of snow to a wide area that would include at least four of the following major cities: Richmond, Washington, DC, Philadelphia, New York City, and Boston.

Having said that, the pattern does not rule out the prospect of a significant snowstorm. In fact, I am confident that there will be additional accumulations of snow after the upcoming event (12/19-20) and cities from Washington, DC through Boston should see additional accumulations. In fact, if everything comes together, Boston, New York City, and Washington, DC should at least have a chance at attaining the December snowfall benchmarks described earlier. The same should hold for the Ohio Valley.

In the Deep South, I believe opportunities remain for at least the threat of snow. Best prospects for that, in my view, lie across northern Alabama, northern Georgia, and northern Mississippi.

The SOI has now crashed to –21.90 and is still trending downward. As the more active southern jet runs into continuing intrusions of very cold air, at least the potential for snow does exist there. If the threat grows more serious, I’ll post in greater detail as any such possible event draws closer.

Overall, for the remainder of the month, expect additional intrusions of much below normal cold but there will also be times where temperatures do moderate to near or somewhat above normal. The warmth in the West should peak this week and then likely begin to fade.

[rainstorm]

thanks for the info don. personally, i think if the dec23 low is as strong as indicated for the midwest, then the only snow the east big cities will get this month will come tonight in the form of snow showers, and 1 inch or less

[donsutherland1]

Rainstorm,

I don't believe that low will be as strong as the 12z GFS suggested. I expect that it will be weaker and will likely track near or just west of the Appalachians. Afterward, there may yet be one or two opportunities for accumulating snow in the East before December ends, though I can't say I'm bullish on any major storms this month. It's not impossible but not the most likely scenario at this time.

[stormman]

at this point I would be happy to get just 2 inches here in ne philly. Got a little dusting tonight.

[sertorius]

Excellent discussion Don as usuall and thanks so much. The central plains, despite today and the cold Wed-Sat, will wind up a bit above normal this month and so far here in Lawrence, we are about half an inch below normal precip. All in all, a real bland month here. Hopefully January will give us a good week of winter weather!!

[donsutherland1]

Thanks, Sertorius.

If some of the guidance is right, Thursday and Friday could be bitterly cold in Lawrence. Unfortunately, as has been the case for much of this month, the pattern will remain progressive and moderation should set in as the weekend progresses.

[Chris the Weather Man]

I remember last year, it got warm the last week of Dec, warm 1st week of Jan, then BOOM..... Ole Man Winter puts Arctic Cold on Northeast, record cold everywhere.... But I been hearing that Feb 2005, could be a COLD, VERY COLD, and SNOWY!

[donsutherland1]

Chris the Weather Man,

Per analogs, February will probably be the coldest month relative to normal this winter in the Mid-Atlantic/Northeast. It should also be the snowiest. I don't expect January 2005 to rival the extreme cold of January 2004.

[sertorius]

Don:

Yea-Wed-Friday look to be down right frigid-I should have known to go with the monthly trend and not have gotten gung ho with the models-even the Euro has trended warmer for the weekend and next week looks just down right warm!! If I am correct, I think the PNA is going below negative 1 and if that is the case, winter weather here is hard to come by based on my research of the past 40 years-the PNA needs to be between 1 and negative 1 for us to get our winter weather-as it is falling from the plus 2 or 3 it has been, we get the cold-as it goes below negative 1, we switch back. I thought maybe it would stay around negative 1 or so and we could maybe have a week or two of winter weather and I got caught up with the models (of course 4 days ago, they had this zonal pattern for next week-lost it and now its back) I'm still thinking I could squeeze an inch or so out of this week with the low ejecting out of Texas and the arctic push-if that low tracks just a 100 miles North or can kick some more mositure North, we could get an inch-if that happened, I would be happy as seeing clouds have made me happy this month We did have a low in Louisana give us 8 inches in 2000 with this exact same set up. Again Don, thanks for responding to my post-you provide excellent analysis of the trends and I really enjoy reading your posts and try to learn from them-have a great night!!

[Chris the Weather Man]

Thanks, Don... I read DT maps for his winter Forecast, and according to him, he says Feb 2005, will be the coldest since 1989, when the polar vortex plunged.....