AO Forecast to Plunge

[donsutherland1]

To date, some of the major cities of the Middle Atlantic and New England regions have witnessed almost no snowfall. A few flurries have fallen, but that is about it. Select data seasonal data through 12/10:

Boston: Trace

New York City: Trace

Philadelphia: Trace

Washington, DC: Trace

However, the next 2-3 weeks should see accumulating snow in most or all of those cities. Almost in a replay of December 2009, the AO is forecast to fall to extremely negative levels just after midmonth and then remain generally negative for some time to come. If past extreme blocks are representative, the forecast block should last into at least the first week in January.

The GFS ensembles show the AO bottoming out in the -6 to -4 range.

If the AO falls to -4 or below, that will be the first time on record that it has fallen to such an extreme level when the ENSO Region 3.4 anomaly was -1.00°C or below. However, it has fallen to such levels during three La Niñas with December ENSO Region 3.4 values of -0.80°C or colder: 1950, 1995, and 2000. During Winter 1950-51, about a third of the Northeast’s and Mid-Atlantic’s seasonal snowfall occurred around the time of the extreme blocking. The extreme 1995 and 2000 blocks each saw a KU snowstorm.

Since 1950 when monthly ENSO data has been available on a regular basis, there has been no KU snowstorm when the ENSO Region 3.4 anomaly was -1.00°C or below. Hence, odds are probably against such a snowstorm during the possible extreme December 2010 block, but if the small sample has meaning (and the larger sample of extreme blocks supports the tiny La Niña sample), there should be some accumulating snow and perhaps a moderate snowstorm over the next 2-3 weeks.

[Kaner587]

To date, some of the major cities of the Middle Atlantic and New England regions have witnessed almost no snowfall. A few flurries have fallen, but that is about it. Select data seasonal data through 12/10:

Boston: Trace

New York City: Trace

Philadelphia: Trace

Washington, DC: Trace

However, the next 2-3 weeks should see accumulating snow in most or all of those cities. Almost in a replay of December 2009, the AO is forecast to fall to extremely negative levels just after midmonth and then remain generally negative for some time to come. If past extreme blocks are representative, the forecast block should last into at least the first week in January.

The GFS ensembles show the AO bottoming out in the -6 to -4 range.

If the AO falls to -4 or below, that will be the first time on record that it has fallen to such an extreme level when the ENSO Region 3.4 anomaly was -1.00°C or below. However, it has fallen to such levels during three La Niñas with December ENSO Region 3.4 values of -0.80°C or colder: 1950, 1995, and 2000. During Winter 1950-51, about a third of the Northeast’s and Mid-Atlantic’s seasonal snowfall occurred around the time of the extreme blocking. The extreme 1995 and 2000 blocks each saw a KU snowstorm.

Since 1950 when monthly ENSO data has been available on a regular basis, there has been no KU snowstorm when the ENSO Region 3.4 anomaly was -1.00°C or below. Hence, odds are probably against such a snowstorm during the possible extreme December 2010 block, but if the small sample has meaning (and the larger sample of extreme blocks supports the tiny La Niña sample), there should be some accumulating snow and perhaps a moderate snowstorm over the next 2-3 weeks.

Nothing like a good ole Donsutherand post to raise your spirits...you were epic last yr with your calls ( you're always spot on but last yr in particular)

[uncle W]

To date, some of the major cities of the Middle Atlantic and New England regions have witnessed almost no snowfall. A few flurries have fallen, but that is about it. Select data seasonal data through 12/10:

Boston: Trace

New York City: Trace

Philadelphia: Trace

Washington, DC: Trace

However, the next 2-3 weeks should see accumulating snow in most or all of those cities. Almost in a replay of December 2009, the AO is forecast to fall to extremely negative levels just after midmonth and then remain generally negative for some time to come. If past extreme blocks are representative, the forecast block should last into at least the first week in January.

The GFS ensembles show the AO bottoming out in the -6 to -4 range.

If the AO falls to -4 or below, that will be the first time on record that it has fallen to such an extreme level when the ENSO Region 3.4 anomaly was -1.00°C or below. However, it has fallen to such levels during three La Niñas with December ENSO Region 3.4 values of -0.80°C or colder: 1950, 1995, and 2000. During Winter 1950-51, about a third of the Northeast’s and Mid-Atlantic’s seasonal snowfall occurred around the time of the extreme blocking. The extreme 1995 and 2000 blocks each saw a KU snowstorm.

Since 1950 when monthly ENSO data has been available on a regular basis, there has been no KU snowstorm when the ENSO Region 3.4 anomaly was -1.00°C or below. Hence, odds are probably against such a snowstorm during the possible extreme December 2010 block, but if the small sample has meaning (and the larger sample of extreme blocks supports the tiny La Niña sample), there should be some accumulating snow and perhaps a moderate snowstorm over the next 2-3 weeks.

thanks Don...Maybe we can get a White Christmas again...The last two years it snowed on the 19th...1961 and 1966 had a real White Christmas and were a weak neg enso...1970 had some snow and sleet on the 22-23rd and some of it was left on the ground Christmas morning...1955, 1964 and 1967 had some snow before Christmas but it melted on the 24th...1962 and 1975 had some snow before Christmas with a trace on the ground Christmas morning and light snow in the afternoon changing to rain at night...1995 had snow on the ground from the major storm on the 20th...1998 had a snowfall on the 23-24th...None of the la nina years except 2000 had a KU event in December...But a 1961, 1962, 1966 or 1970 event just before Christmas is possible...6" storm not out of the question...

[BullCityWx]

pin plz

[Typhoon Tip]

Don,

agreed, I wrote and extensive discussion about this subject matter that has scrolled (apparently without much notice)

if anyone is interested....

[skierinvermont]

Don,

agreed, I wrote and extensive discussion about this subject matter that has scrolled (apparently without much notice)

http://www.americanw...-lead-forecast/ if anyone is interested....

So is this AO event linked to the stratospheric warming event you were talking about? Did that event propagate?

[Typhoon Tip]

So is this AO event linked to the stratospheric warming event you were talking about? Did that event propagate?

It is too early to tell unfortunately, but it was in the modeled guidance and with all other leading factors in support, the event would appear more plausible than not.

[WVclimo]

Don, I agree that is unlikely that all these cities get through this pattern without some snowfall. Looking specifically at DCA; since Reagan airport became the observation site in 1948 there have been 25 Decembers that finished below the 1971-2000 normal monthly temp for December of 39.5. 23 of them had measurable snowfall. The AO should contribute to solidifying the negative departure there, which favors some snow before the end of the month.

Glad you made it to Americanwx.

[ptb127]

Hopefully it won't be so strong that things get suppressed... a cold/dry pattern is depressing.

[jbcmh81]

I seem to remember from discussion last year that episodes of extreme -AO tend to repeat later in winter... or am I remembering that wrong? It is interesting to see extreme -AO two years in a row.

[cpickett79]

If the AO falls to -4 or below, that will be the first time on record that it has fallen to such an extreme level when the ENSO Region 3.4 anomaly was -1.00°C or below. However, it has fallen to such levels during three La Niñas with December ENSO Region 3.4 values of -0.80°C or colder: 1950, 1995, and 2000. During Winter 1950-51, about a third of the Northeast’s and Mid-Atlantic’s seasonal snowfall occurred around the time of the extreme blocking. The extreme 1995 and 2000 blocks each saw a KU snowstorm.

Since 1950 when monthly ENSO data has been available on a regular basis, there has been no KU snowstorm when the ENSO Region 3.4 anomaly was -1.00°C or below. Hence, odds are probably against such a snowstorm during the possible extreme December 2010 block, but if the small sample has meaning (and the larger sample of extreme blocks supports the tiny La Niña sample), there should be some accumulating snow and perhaps a moderate snowstorm over the next 2-3 weeks.

Don if you have it....what does the data say for times when enso region 3.4 is -1 or below and we have a tanking AO REGARDING 12" snow storms for SNE or even 8" for that matter. I understand there are no KU's (Yet) but trying to get an idea if a large snow storm (regardless of hitting the MA) could be in the cards for SNE. Thank you for you continued excellent work. (i used to read you way back on storm2k)

[donsutherland1]

I seem to remember from discussion last year that episodes of extreme -AO tend to repeat later in winter... or am I remembering that wrong? It is interesting to see extreme -AO two years in a row.

They can repeat. 1976-77 and 1977-78 is one case where extreme AO- episodes occurred during consecutive winters.

[usedtobe]

They can repeat. 1976-77 and 1977-78 is one case where extreme AO- episodes occurred during consecutive winters.

Don nice post, the pattern does look much better than it's been in terms of where the blocking is forecast to settle. Like you, I doubt there will be a KU even unless we can get some southern stream help.

[Ji]

Don nice post, the pattern does look much better than it's been in terms of where the blocking is forecast to settle. Like you, I doubt there will be a KU even unless we can get some southern stream help.

we dont always need a KU to have a great snowstorm. An 6-10 inch storm would be great

[donsutherland1]

we dont always need a KU to have a great snowstorm. An 6-10 inch storm would be great

After having seen almost nothing to date, any snow would be fine for me. FWIW, looking farther ahead to January, when the ENSO 3.4 anomaly has been -1.00°C or colder, Washington, DC has more than twice the probability of seeing accumulating snow when the AO is negative than when it is positive.

[donsutherland1]

Don if you have it....what does the data say for times when enso region 3.4 is -1 or below and we have a tanking AO REGARDING 12" snow storms for SNE or even 8" for that matter. I understand there are no KU's (Yet) but trying to get an idea if a large snow storm (regardless of hitting the MA) could be in the cards for SNE. Thank you for you continued excellent work. (i used to read you way back on storm2k)

Cpickett79,

Since 1950, there have been no days on which the AO fell to -3 or below in December when the December ENSO Region 3.4 anomaly was -1.00 or below. 1950, 1995, and 2000 saw extreme blocks when the La Niña was a little weaker. 1950 saw no such snowstorms. For the 1995 blocking episode, there were 3 such storms highlighted by the January 1996 blizzard just before the block ended. The 2000 blocking episode produced none for the Boston area. So, my confidence in such a storm is low. I do believe there is a much better chance of one or two 3"-4" events before the blocking episode ends.

[okie333]

They can repeat. 1976-77 and 1977-78 is one case where extreme AO- episodes occurred during consecutive winters.

And 1978-1979...

So is this AO event linked to the stratospheric warming event you were talking about? Did that event propagate?

Likely yes, but it is likely NOT related to the other, slightly stronger warming event that will likely happen in 3 days or so. This second event looks to be the first major deviation of 2010 from 2009 in the entire year (remarkable similarity has existed between the two years in the stratospheric temperature pattern... but 2009 has no event in mid-December whereas 2010 will have another warming). Possible "Have you seen us?" event (on the new charts) near January 10?

[donsutherland1]

Don nice post, the pattern does look much better than it's been in terms of where the blocking is forecast to settle. Like you, I doubt there will be a KU even unless we can get some southern stream help.

Thanks, Wes. I strongly agree with you.

[usedtobe]

Thanks, Wes. I strongly agree with you.

Don, How long did the strong blocking episodes last during the nina years?

[donsutherland1]

Don, How long did the strong blocking episodes last during the nina years?

Wes,

The 1950, 1995, and 2000 all lasted 4-5 weeks, but the blocking was noticeably weaker in the last week or so:

1950: 27 days

1995: 35 days

2000: 29 days

FWIW, here are the mean high and low temperatures for DCA during AO- regimes in January when the ENSO 3.4 anomaly was -1.0 or below:

AO:

-3 or below: Mean high: 41.0; Mean low: 25.5

-2.99 to -1.00: Mean high: 43.5; Mean low: 28.9

-0.99 to 0: Mean high: 45.2; Mean low: 30.0

0 to +0.99: Mean high: 48.1; Mean low: 31.7

Also, FWIW, GWO Phase 2 seems to coincide with the coldest temperatures at DCA in January during those La Niñas. When the AO < 0 and the GWO was in Phase 2, DCA's mean high was 39.0 and its mean low was 23.8.

[usedtobe]

Wes,

The 1950, 1995, and 2000 all lasted 4-5 weeks, but the blocking was noticeably weaker in the last week or so:

1950: 27 days

1995: 35 days

2000: 29 days

FWIW, here are the mean high and low temperatures for DCA during AO- regimes in January when the ENSO 3.4 anomaly was -1.0 or below:

AO:

-3 or below: Mean high: 41.0; Mean low: 25.5

-2.99 to -1.00: Mean high: 43.5; Mean low: 28.9

-0.99 to 0: Mean high: 45.2; Mean low: 30.0

0 to +0.99: Mean high: 48.1; Mean low: 31.7

Also, FWIW, GWO Phase 2 seems to coincide with the coldest temperatures at DCA in January during those La Niñas. When the AO < 0 and the GWO was in Phase 2, DCA's mean high was 39.0 and its mean low was 23.8.

Don, thanks, good stuff.

[NJwinter23]

Don,

I was hoping you'd come out of the woodwork (well at least I hadn't yet come across a post of yours since the switch to amwx) regarding the tanking AO. It really was your big subject matter last year and you gave great statistics & forecasts throughout the winter. I recall most that when the severe(record?) Dec/early Jan -AO episode of last winter was relaxing, you were all over it's return to severity for February based on events of the past (I think the likelyhood of a SSW/MMW event played in?). I wish we could bring back the old thread from Eastern to read through because I do not fully remember the numbers/thresholds as far as peak strength and episode length that could signal for later in the winter. That may be getting a little too ahead of things as nothing has happened yet with this episode, but I am intrigued if you would care to comment. It would be VERY interesting if we repeated an episode like early last winter, and had the prospects of a return later in winter vs. strong nina climo in Feb. for our area.

[jbcmh81]

Two of those mentioned winters had severe arctic outbreaks the first week of February: 1951 and 1996.

[VAwxman]

And 1978-1979...

Likely yes, but it is likely NOT related to the other, slightly stronger warming event that will likely happen in 3 days or so. This second event looks to be the first major deviation of 2010 from 2009 in the entire year (remarkable similarity has existed between the two years in the stratospheric temperature pattern... but 2009 has no event in mid-December whereas 2010 will have another warming). Possible "Have you seen us?" event (on the new charts) near January 10?

We already deviated from last year, as there was a good warming up above in early Dec last year, which this year has not seen ( a little bump up, but really more or less normal at best). And we're not seeing a notable warming on the horizon yet, so the intensity of this block can definitely not be completely linked to the stratosphere. In fact, the warming events this year can actually be traced back to tropospheric events (MJO, mountain torque, and subsequent wave breaking into the stratosphere, thus the warming). Now we should get another MT event, but it's unclear what impact this one will have up above.

[Snow Wizard]

They can repeat. 1976-77 and 1977-78 is one case where extreme AO- episodes occurred during consecutive winters.

Those were both El Nino winters though. Do you think this happening with a La Nina will result in a different outcome than what was seen those winters? Living in the NW I sure hope so!

[usedtobe]

We already deviated from last year, as there was a good warming up above in early Dec last year, which this year has not seen ( a little bump up, but really more or less normal at best). And we're not seeing a notable warming on the horizon yet, so the intensity of this block can definitely not be completely linked to the stratosphere. In fact, the warming events this year can actually be traced back to tropospheric events (MJO, mountain torque, and subsequent wave breaking into the stratosphere, thus the warming). Now we should get another MT event, but it's unclear what impact this one will have up above.

Here's a temperature profile of last winter and the nao. There was a warming event that was for the time of year was quite significant in November and then another in December which was related to wave breaking and then the big daddy in January.

I think a nuber of us thought that December would be colder than normal and probably would have a negative ao especially the first half of the month. However, I don't think anyone saw and event of this magnitude except those that think the low solar drives everything. I know I didn't see this strong of a negative ao coming.

[Snow Wizard]

So is this AO event linked to the stratospheric warming event you were talking about? Did that event propagate?

Nope. Makes it even more mysterious.

[PAblizzardlover]

Since K&U found that KU storms were common when a a strong neg. NAO moderated, is there a similar correlation/causation when coming out of a strongly neg. AO?

[Kaner587]

like clockwork the 12z euro shows an MECS for DC-PHL and HECS for NYC

[donsutherland1]

Don,

I was hoping you'd come out of the woodwork (well at least I hadn't yet come across a post of yours since the switch to amwx) regarding the tanking AO. It really was your big subject matter last year and you gave great statistics & forecasts throughout the winter. I recall most that when the severe(record?) Dec/early Jan -AO episode of last winter was relaxing, you were all over it's return to severity for February based on events of the past (I think the likelyhood of a SSW/MMW event played in?). I wish we could bring back the old thread from Eastern to read through because I do not fully remember the numbers/thresholds as far as peak strength and episode length that could signal for later in the winter. That may be getting a little too ahead of things as nothing has happened yet with this episode, but I am intrigued if you would care to comment. It would be VERY interesting if we repeated an episode like early last winter, and had the prospects of a return later in winter vs. strong nina climo in Feb. for our area.

Thanks NJwinter23.

It's unfortunate that the EUSWX archive wasn't preserved, but it is what it is.

Unfortunately, extreme blocking situations haven't occurred in December with a La Niña as strong as the present one. Nevertheless, if 1950, 1995, and 2000 are representative (even as ENSO Region 3.4 was warmer), one would likely witness a return of strong blocking. 1951 saw a brief episode of strong blocking and severe cold in February and then a sustained period of strong blocking in March. In 1996, there was another period of strong blocking from late January into mid-February. 2001 saw extreme blocking from late February well into March.