December 16-31, 2005: Pattern Thoughts

[donsutherland1]

The cold pattern is likely to reload. Afterward, the second half of the month could see several periods of cold and storminess. Snowfall totals in the big cities of the East Coast should increase before the month is finished and prospects look promising for a snowier than average winter in many parts of the East.

NAO:
The NAO is likely to average negative through much of the above period. It could rise in the closing days of the month. At this time, the Artic Oscillation (AO) is forecast to go severely negative by around December 19 +/- 2 days. In December, when the Arctic Oscillation averages negative, the NAO has averaged negative 81% of the time. In January, the same relationship held 77% of the time. Hence, at this time, given how severely the Arctic Oscillation is likely to tank, I have good confidence that the NAO will likely be negative during much of the upcoming period before it rises toward the end of the month.

PNA:
The PNA is forecast to rise sharply in coming days and remain positive through at least the next two weeks.

NCEP ensembles:
Per the ensemble means, there are two important details that stand out, at least for me. First, is the return of the trough to the Eastern U.S. in the December 18-20 period after a brief cold shot in the Pacific Northwest. There, it appears that the best chance for some snow might be in the December 15-17 period, but such snowfall, especially in Portland and Seattle is not assured. Second is the idea that it seems to happen with a short wave dropping down from Manitoba that then blows up into a full-fledged negatively-tilted trough.

This hint of a “Manitoba Mauler” would be consistent with the overall stormy period I expect and the possible signal from the PNA regime change per Heather Archambault’s research for a storm sometime in the December 16-25 period. Needless to say, the December correlation is low but the January one is 4th highest. Hence, the correlation could be higher in late December than early December.

Also, troughs could be transient, with one deep trough passing and then another moving in shortly thereafter. This, too, could be a hint of a stormy pattern. With a fairly active southern branch, I believe odds favor storminess during the December 16-31 timeframe.

For more information on such systems, one can visit DT’s web page on such systems.

ENSO Analogs:
ENSO analogs--just those seasons with similar ENSO anomalies to those of this year--provide some support for what appears likely in terms of the overall pattern noted above. In the 15 most similar seasons, Boston received 0.1" or more snow during the 12/22-28 period (12/25 +/- 3 days) in almost all of those seasons. 13/15 (87%) saw 0.1" or more.

Median snowfall during the 12/22-28 period: 0.9"
Mean snowfall during the 12/22-28 period: 2.7"
Years with 4" or more during the 12/22-28 period: 3/15 (20%)

Both the snowfall probability and amounts increased when the sample was reduced to include only those years in which Boston saw 8" or more snowfall in December. Then 7/7 (100%) saw 0.1" or more snowfall during the 12/22-28 period.

Median snowfall during the 12/22-28 period: 2.0"
Mean snowfall during the 12/22-28 period: 4.4"
Years with 4" or more during the 12/22-28 period: 3/7 (43%)

In many of the above events, snowfall was more widespread e.g., it also impacted the Mid-Atlantic states. These analogs also offer a little weight to the hint on the NCEP ensemble, though the timing is somewhat later.

Summary:
Overall, I expect at least the following for the December 16-31 period:

∙ December 16-31 will see temperatures from Raleigh to Boston average below normal.
∙ The first half of the period could see an episode of severe cold with the lowest temperatures possibly reaching the single digits in Providence and Boston, and possibly in such cities as Washington., DC, Philadelphia, and New York. Teens are likely.
∙ Washington, DC, Baltimore, Philadelphia, New York, Providence, and Boston will likely add to their seasonal snowfall totals and possibly by 4” or more some of these cities.
∙ Some moderation in the closing 3-5 days of December but probably no much above normal temperatures.

Above Average Seasonal Snowfall Likely from at Least Washington, DC to New York City
The abundant December snowfall across the Mid-Atlantic and southern New England regions strongly suggests that most of the affected region is likely to experience above normal seasonal snowfall for Winter 2005-06.

∙ New York City: 30” or More Snowfall Highly Likely:
Through December 11, New York City had received 9.3” snowfall in December. With the upcoming pattern evolution, that figure is highly likely to exceed 10” before the month is finished and 12” is also very possible.

Since 1869, the following has held true:

November: Trace or more/December: 9” or more: 19/22 (86%) seasons received 30” or more snowfall; Median snowfall: 42.0”; Mean snowfall: 42.3”

November: Trace or more/December: 10” or more: 17/19 (89%) seasons received 30” or more snowfall; Median snowfall: 43.6”; Mean snowfall: 43.2”

November: Trace or more/December: 12” or more: 11/11 (100%) seasons received 30” or more snowfall; Median snowfall: 46.6”; Mean snowfall: 46.4”

∙ Washington, D.C.: 20” or More Snowfall Likely:
In 7/8 (88%) years in which the November temperature came to 50.2° +/- 1° and December saw 3.0" or greater snowfall, seasonal snowfall reached or exceeded 20" and 5/8 (63%) reached or exceeded 30". Median seasonal snowfall came to 31.2” and mean seasonal snowfall amounted to 28.5”

∙ How much for Philly?
In seasons in which Washington, D.C. received 20” or more snowfall and New York City received 30” or more snowfall, the following has occurred in Philadelphia:

- 18/22 (82%) seasons with 25” or more seasonal snowfall
- 15/22 (68%) seasons with 30” or more seasonal snowfall
- Median seasonal snowfall: 35.7”
- Mean seasonal snowfall: 37.4”

Based on what has already occurred, I believe it is likely that Winter 2005-06 will see 30” or more snowfall in New York City for the 4th consecutive winter, 25” or more seasonal snowfall in Philadelphia and 20” or more seasonal snowfall in Washington, DC.

If Washington, D.C. receives 20” or more snowfall in 2005-06, it would be the first time since Winter 2002-03 that Washington, D.C. has received that much snowfall. Then, Washington, D.C. was blanked by 40.4” snowfall.

If Philadelphia picks up 25” or more snowfall, Winter 2005-06 would be the second consecutive winter for such snowfall. The last consecutive winters were 1981-82 (25.4”) and 1982-83 (35.9”). The longest such stretch is 5 consecutive winters with 25” or more snowfall (1913-14 through 1917-18). If Philadelphia picks up 30” or more snowfall this winter, it would be the second consecutive winter with such snowfall. The last consecutive winters with 30” or more snowfall were 1977-78 (54.9”) and 1978-79 (40.2”). The longest such stretch is 5 consecutive winters with 30” or more snowfall (1913-14 through 1917-18).

If New York City receives 30” or more snowfall, Winter 2005-06 would be the 4th consecutive winter with such snowfall. The last similar streak occurred during Winters 1945-46 through 1948-49 with the following amounts of seasonal snowfall: 1945-46: 31.4”; 1946-47: 30.6”; 1947-48: 63.2”; 1948-49: 46.6”. The longest such stretch is 5 consecutive winters with 30” or more snowfall (1880-81 through 1884-85). If New York City receives 40” or more snowfall, it would also be the 4th consecutive winter with such snowfall. The current stretch of 3 consecutive winters is the record.

Historic Tidbit: December 25-26, 1872 Snowstorm:
Amidst a severe cold period that sent temperatures to as low as 3° in New York City, -8° in Boston, -26° at Chicago, -50° to -45° in New Hampshire, brought ice 2”-3” thick for a time to the Patapsco River and Baltimore Harbor, and froze the Arkansas River at Little Rock, the biggest snowstorm since the Blizzard of 1857 buried the Midwestern, Mid-Atlantic and Northeastern United States.

[ETXHAMXYL]

Don,
I know most of your forecasting is for the Eastern Seaboard cities, but wondering what are your thoughts on Texas for the same time period. We have heard rumors about blizzards and brutal cold outbreaks here in Texas.
I know its harder to predict the Texas Winter Weather since one day we are in shorts and 80's and next we have 30's and ice.
Sometimes our local mets and NWS don't want to hype up something but then the bad part is they wait until the day of or day after to notify us of potential ice or frozen. We don't have enough time to prepare like the folks up north.
This next weekend and Christmas weekend are very important in my business and I'd like to have some kind of information about the weather so I can make preparations in advance. Especially if we could get frozen stuff. We normally get ice and not snow here in NTX.
So if you have any info on a possible blizzard, ice storm, or frozen anything heading down to NTX/ETX for those weekends or period, I'd appreciate hearing from you. I enjoy reading your post and appreciated your weather knowledge.
Thanks Loads!!

[cctxhurricanewatcher]

Right on ETX. Don your thoughts? We appreciated your Huricanne insights during that season.

[AnthonyC]

Don,

The models have basically thrown out the idea of retrogression for the PNW...have your thoughts changed from your original post? I appreciate your insite.

Anthony

[donsutherland1]

Everyone,

I'll comment with my ideas outside the East later tonight or tomorrow morning.

[southerngale]

Great! Thanks Don.

[Yankeegirl]

Thanks Don, Inquiring minds really want to know...!

[wx247]

I will be waiting patiently as well. Thanks for taking the time to do this Don.

[donsutherland1]

Additional Thoughts:

Pacific Northwest:

I believe the December 16-17 period could start cool but moderation will likely quickly commence as the PNA builds to strongly positive levels. Per ensemble guidance and the PNA forecast, the 12/20-25 period could be especially warm. Readings could run 4°-8° above normal, especially in interior sections and parts of British Columbia. Later, the PNA+ could decay--the ensemble guidance is a little more enthusiastic on this--but with December correlations concerning a strongly negative Arctic Oscillation (strongly supportive of a big ridge in the West/deep trough in the East in approximately 70% cases), the process might be gradual. Thus readings will probably trend toward normal in the closing days of the month.

Texas:

The cold is coming. It appears that based on the pattern with good support on the computer guidance and ensembles, the period after mid-month should turn colder to possibly much colder than normal. That cold could persist toward the Christmas/Hanukkah holidays before it fades.

With an active southern jet, some parts of Texas will probably see some snow and, if the timing is right, even eastern Texas might have a shot at some frozen precipitation but that is not the most likely scenario. Probably the best chance for accumulating snow would be in such cities as Dallas in the December 17-19 timeframe.

[donsutherland1]

Wx247,

It appears that Missouri will likely be colder than normal through the Holidays. A storm coming out of the Gulf of Mexico might bring some snow, especially to eastern Missouri, in the 12/18-19 period. In the wake of the storm, it could turn bitterly cold.

Moderation appears likely after the Holidays.

[AnthonyC]

Thanks Don!

Anthony

[wxwatcher91]

Don, what are your ideas about the upcoming storm (Thursday through Friday)

looks like a big ice event for the Mid Atlantic and a mix for a lot of southern New England...
what are your ideas about inland accumulation?I know precip type will be a big factor...

[donsutherland1]

Wxwatcher91,

Right now, I believe Keene will likely see 2"-4" of snow before a change to a prolonged period of freezing rain. The 18z NAM suggests that some rain is also possible for a short time.

[david30]

Don can you please check your email? Thanks!

[donsutherland1]

David,

I didn't get any messages from you in my e-mail account. Please, e-mail me again or PM me.

Thanks.

[wx247]

Wx247,

It appears that Missouri will likely be colder than normal through the Holidays. A storm coming out of the Gulf of Mexico might bring some snow, especially to eastern Missouri, in the 12/18-19 period. In the wake of the storm, it could turn bitterly cold.

Moderation appears likely after the Holidays.

Your insight is always appreciated Don. The weekend pattern according to the EURO looks very interesting.

[invisible]

Don,

Its interesting to know what will the things that happen to most areas in the two countries. I agree with your incidies such as PNA, NAO, and ENSO for the month of Decmeber because all the things you posted are making sense. Do u think the PNA will be negative in January ?? I think it will be negative because the PNA is going to be positve overall in December. Its time to change the position of the PNA.

[donsutherland1]

Invisible,

So far, the teleconnection indices you note have performed quite well in terms of offering guidance for the neutral (biased on the cool side) ENSO. With regard to the PNA, I don't have sufficient skill to indicate whether it will average negative in January. I do believe that it will go negative at least for a time in January and probably before mid-month.

It should be noted that the PNA remained positive from September 9 through October 14 this year, so a prolonged stretch of PNA+ is very possible. In fact, the ensemble means and all of the ensemble members keep the PNA > 0 through December 29 0z. This is quite remarkable given that the PDO has been so negative lately. However, it is also another indication that the Arctic Oscillation has been the proverbial 800-pound gorilla this month. King Kong could not even begin to compare with the Arctic Oscillation that has ruled with an icy fist in the East so far all month.

The MJO phases would suggest that sometime late in the first week in January/early in the second week, it could switch to negative. But, the MJO passages have been very weak this winter, so the correlation needs to be tempered.

In the end, I believe that B.C. will see more wintry weather return next month. However, before then, it could become truly warm (with some days 3°C-6°C above normal). Both the Canadian ensembles and GFS ensembles are very bullish on the magnitude of the warmth.

Best wishes.

[JETSTREAM BOB]

Comment To Don:

I love your analysis of the weather and please keep up the good work, my only comment is per the 12Z, I don't see any major warmups like everybody is talking about, I do see a relaxed jet stream in the east after Christmas but digs later near New Years...it appears that the 0 line may get back to northern Va., but after that it drops back into S.C. toward New Years...As far as the AO it stays strongly negative and I believe this has been the governing force behind this winters brutal cold spells...JSB

[donsutherland1]

Several quick points before I leave for vacation...

1) NAO: The ensembles indicate a generally neutral NAO through December (alternative between somewhat positive and somewhat negative). I believe the NAO will be predominantly negative.

At the beginning of this thread, I noted "In December, when the Arctic Oscillation averages negative, the NAO has averaged negative 81% of the time. In January, the same relationship held 77% of the time. Hence, at this time, given how severely the Arctic Oscillation is likely to tank, I have good confidence that the NAO will likely be negative during much of the upcoming period before it rises toward the end of the month." On November 16, I noted, "Moreover, the ensemble forecasts for both the Arctic Oscillation and NAO suggest that the closing days of November should see negative to strongly negative readings for both indices. In general, in 70% of the cases where the Arctic Oscillation has averaged negative in November or December, the NAO was negative."

If one examines the NAO data, the Arctic Oscillation has proved to be the 800-pound gorilla that historic climatology has suggested:

Since November 22:

∙ The Arctic Oscillation has been negative 22/23 days
∙ The NAO has been negative 20/23 days (and 20 of the last 21 days)

With the AO still progged to tank and generally stay much below 0 for the remainder of the month (except by a few ensemble members), I have good confidence that the NAO will also be predominantly negative.

2) The pattern has continued to evolve very much in line with what I've been thinking. On November 30, I observed, "The December 8-15 period could be especially cold, with minimum temperatures possibly falling into the teens on one or more days in the Mid-Atlantic and Northeast (from at least RIC to BOS)."

Yesterday, Boston, New York City, Newark, and Philadelphia saw lows fall to the teens. Today, the following lows were reported:

Baltimore: 15°
Boston: 10°
New York City:12°
Newark: 11°
Philadelphia: 15°
Providence: 11°
Richmond: 19°
Washington, DC: DCA: 19°/ IAD: 13°

As things have worked quite well according to expectation and the fundamental factors that led me to expect such conditions have not changed much--with the Arctic Oscillation continuing to be the dominant factor--I have little reason to abandon the idea that much of the second half of December will be colder than normal in the East. Some moderation is possible late, but it does not appear to me that the warmup will be extreme.

Of course, past performance is no guarantee of future results. But it does give me some confidence that I've had a decent handle on the pattern so far.

3) The idea that cities from Philadelphia to Boston and probably Baltimore and Washington, DC would see additional accumulations of snow in the December 16-31 period remains very viable. Given a fairly active southern jet and a predominantly cold pattern, such opportunities are realistic.

4) The December 15-16 event will be mostly rain from DCA to BOS after perhaps a minor accumulation of snow/sleet (1" or less in most places). Interior New England i.e., Hartford and Concord should see 2"-4" snow before a prolonged period of freezing rain. Northern New England should see greater accumulations of snow e.g., 3"-7" with local amounts of 8" in and around Burlington. Some sleet is possible even there.