AO Winter Analogs When December Reaches +3.5

[bluewave]

I wanted to look at what type of blocking followed December +3.5 or greater daily AO readings like we just experienced.

I counted 9 Decembers when the AO rose this high and included the dates on which it occurred.

ftp://ftp.cpc.ncep.n...1.current.ascii

12/25/51......+3.5

Blocking developed during February and March of that season.

12/20/71....+3.8

JAN-FEB east based

12/21/75....+3.5

1/15-1/30 76

12/2/79 was the strongest December daily +AO reading that I could find coming in at a +5.0.

It was also the strongest JAN-MAR blocking on the list.

12/27/83...+4.4

MAR 84

12/26/90....+3.5

2/15-3/31, 91

12/17/91.....+4.7

1/15-1/31, 92 east based

12/24/99...+3.5

1/15-1/30, 00

12/11/06....+3.8

FEB 07

[usedtobe]

I'm not surprised at the ao going negative but wonder about the timeline of when it first went strongly negative and then ho wlong it lasted before the break.

[Feb]

Nice work. Gives some hope for at least some winter weather after December. Of course in the Philly regional thread i made a post about how the past few Decembers have spoiled us. At 47 years old i have seen many a winter not start until after the New Year in these parts. Of course in New England and other areas of the north December is a winter month but for Southern Pa on southward, any winter weather in December is usually a bonus.

[Huffwx]

That Dec 2499 is odd-- it was colder in the east that day. There was a clipper that kicked up 3 inches of snow in Lynchburg-- flurries was the forecast.

[uncle W]

_{12/27/83 was surprising to see after the coldest Christmas max ever...I like those analogs and I'm not expecting any big storms until later in the season if then...}

_{1971 1975 and 1999 are the three analogs I'd bet on now...I like 71-72 the best but all three were lacking the big snow in the city...There was a KU storm on 2/19/72 and 1/25/00...Both mixed with or changed to rain in the city...}

[bluewave]

I'm not surprised at the ao going negative but wonder about the timeline of when it first went strongly negative and then ho wlong it lasted before the break.

From what I could see, it was really variable from year to year.

Nice work. Gives some hope for at least some winter weather after December. Of course in the Philly regional thread i made a post about how the past few Decembers have spoiled us. At 47 years old i have seen many a winter not start until after the New Year in these parts. Of course in New England and other areas of the north December is a winter month but for Southern Pa on southward, any winter weather in December is usually a bonus.

Thanks, it will be interesting to see how things turn out this winter.

That Dec 2499 is odd-- it was colder in the east that day. There was a clipper that kicked up 3 inches of snow in Lynchburg-- flurries was the forecast.

Yeah, we had a big rise in the PNA at the time.

_{12/27/83 was surprising to see after the coldest Christmas max ever...I like those analogs and I'm not expecting any big storms until later in the season if then...}

_{1971 1975 and 1999 are the three analogs I'd bet on now...I like 71-72 the best but all three were lacking the big snow in the city...There was a KU storm on 2/19/72 and 1/25/00...Both mixed with or changed to rain in the city...}

Yeah, The -EPO and massive arctic high were responsible for the cold around Christmas.

[Dsnowx53]

The AO being this strongly positive during the darkest days in the Arctic allows for a very strong cold air source to be built. So when the AO finally goes negative, watch out! Though obviously we don't know when it actually will go negative. This concept was being discussed in one of the NYC subforum threads.

[BethesdaWX]

Perhaps include analogs in which the AO rose to +2.5 or +3 in December without the high threshold? Or maybe the mean December AO rather than a single spike? Might give us more years to mull over.

[bluewave]

Perhaps include analogs in which the AO rose to +2.5 or +3 in December without the high threshold? Or maybe the mean December AO rather than a single spike? Might give us more years to mull over.

The other +3's that I was able to find:

12/12/72....+3.1

12/05/94....+3.1

12/27/04....+3.0

1/15-3/31, 05 was the best blocking out of the three.

[bluewave]

The AO being this strongly positive during the darkest days in the Arctic allows for a very strong cold air source to be built. So when the AO finally goes negative, watch out! Though obviously we don't know when it actually will go negative. This concept was being discussed in one of the NYC subforum threads.

During the last Nina year on the list, we had an impressive arctic outbreak during the second half of January 2000.

NYC JAN 2000 HI/LOW Temps

[uncle W]

the lowest reading at the end of October was just before the snowstorm...Hopefully we will see some days with a lower index than that...We had one of the most negative Augusts on record and most of the really negative Augusts had a lower index reading for a month during the winter...I hope that continues this year...

[Bob Chill]

Going beyong just the December index brings up a couple of interesting connections. It's looking pretty safe to say that the Dec AO index will come out positive to potentially strongly positive. Also, the AO has been positive on a monthly basis since the end of August. The AO has had a positive monthly reading for Sept - Dec only twice since 1950 (71 & 75).

1971 was a second year Nina and 1975 was a third year Nina.

The NAO is also looking to be positive for the same Sept - Dec timframe. That has only happened 4 times since 1950 (56, 59, 99 & 07). There hasn't been a time since 1950 that both indices have had positive monthly reading at the same time. 99 is the closest statistical match to the current Sept - Dec timeframe. The AO was slightly negative during Oct of 1999. Other than that it is a good match. 1999 was also a second year Nina.

One thing that has me concerned is that we're coming out of one of the longest streaks of winters having an overall -ao/nao in the last 60 years. You gotta wonder if our time is up on the run we've recently had.

[donsutherland1]

Some quick thoughts on the AO+ regime...

First, today's reading of +4.830 is the 2nd highest December reading on record. The only higher figure was +5.040, which was reached on December 2, 1979.

Second, a key question concerns how long the current AO+ regime will last. The current AO+ regime began on November 20. The two longest such regimes that began in late November were 58 days (1951-52) and 56 days (2006-07).

Since 1950, there were 10 prior years on which the AO reached +3.500 or above in December: 1951, 1971, 1975,1979, 1983, 1984, 1990, 1991, 1999, and 2006. However, only four of those years saw the AO average +0.500 or above in November: 1975, 1990, 1999, and 2006.

The AO+ regimes for all 10 cases had a mean duration of 29.7 days and median duration of 26.5 days. For the four years in which the November AO averaged +0.500 or above, the durations of the AO+ regimes were as follows:

1975-76: 15 days

1990-91: 30 days

1999-00: 31 days

2006-07: 56 days

Mean: 33.0 days

Median: 30.5 days

75% of the cases had AO+ regimes that lasted 30 days or longer.

The most recent GFS ensemble forecast for the AO is below:

Very few ensemble members bring the AO < 0 through December 16. Hence, both the historic climatology (despite a small sample size) and the latest guidance suggest that the current AO+ regime is likely to persist for some time to come.

Third, another key question concerns the AO beyond December. The following table shows the AO for the four cases cited above:

For January, other factors generally trumped the AO except when the AO averaged well above +1 for January. Hence, temperature anomalies were as follows:

1976: Western half of CONUS: Areas of below normal and above normal readings; Eastern half of CONUS: Generally cooler than normal

1991: Western half of CONUS: Generally cooler than normal; Eastern half of CONUS: Generally warmer than normal

2000: Western half of CONUS: Generally warmer than normal; Eastern half of CONUS: Generally cooler than normal

2007: Western half of CONUS: Generally cooler than normal; Eastern half of CONUS: Generally warmer than normal

1975-76 and 1999-00 saw La Niñas, so those might be the most relevant cases.

For February, unless the AO averaged below -1, the East basked in warmth. The February cases were as follows:

1976: Western half of CONUS: Western 1/4 cooler than normal but rest warmer than normal; Eastern half of CONUS: Generally much warmer than normal

1991: Western half of CONUS: Generally much warmer than normal; Eastern half of CONUS: Generally much warmer than normal

2000: Western half of CONUS: Generally much warmer than normal; Eastern half of CONUS: Generally warmer than normal

2007: Western half of CONUS: Generally warmer than normal; Eastern half of CONUS: Generally cooler than normal

The 1975-76 and 1999-00 cases might again be most relevant as they are the La Niña cases.

[usedtobe]

Don, Thanks, that's just the type of information I was interested in. Thanks again, good stuff.

[Rainman]

Going beyong just the December index brings up a couple of interesting connections. It's looking pretty safe to say that the Dec AO index will come out positive to potentially strongly positive. Also, the AO has been positive on a monthly basis since the end of August. The AO has had a positive monthly reading for Sept - Dec only twice since 1950 (71 & 75).

1971 was a second year Nina and 1975 was a third year Nina.

The NAO is also looking to be positive for the same Sept - Dec timframe. That has only happened 4 times since 1950 (56, 59, 99 & 07). There hasn't been a time since 1950 that both indices have had positive monthly reading at the same time. 99 is the closest statistical match to the current Sept - Dec timeframe. The AO was slightly negative during Oct of 1999. Other than that it is a good match. 1999 was also a second year Nina.

One thing that has me concerned is that we're coming out of one of the longest streaks of winters having an overall -ao/nao in the last 60 years. You gotta wonder if our time is up on the run we've recently had.

It would certainly seem so. If the cause of the strong -AO was, in fact, the 100 year solar minimum as many believe, then it makes sense that we are now seeing its demise. The only thing that I have to add to that is that I don't think we are really seeing the end of the -AO trend now. It ended in February. Last year's la Nina episode was quite strong, so it's not surprising to see that its strength was eventually manifested in the form of a strengthening polar vortex and +AO period that helped create a pattern of spring flooding and an active severe weather season. It is not fair to characterize the last cold season as -AO, seeing as how it was only moderately to strongly negative for Dec-Jan.

And that was the end of that. The -AO hype and excitement lives (understandably so). The stretch of the super negative -AO is dead.

February through April were definitively positive before NH summer set in. Now that we are firmly under the influence of this 2nd year la Nina, we can reasonably expect a return of the +AO (already underway) and probably a pattern similar to what we saw last spring for the eastern two thirds.

My thoughts: This long term +AO period will probably stick around through the better part of spring 2012, which makes sense given recent trends (discussed in this thread), long term trends (last cold season, minus the early part), and the physical implications of a la Nina...especially a multi-year episode. I like the idea of a record or near record cold outbreak later this winter, hinted at by another poster. If the +AO does turn out to be a fairly robust one (once again, it looks like it could be heading in that direction already), then we will spend a decent fraction of the winter chilling the Arctic air to below normal levels. Then a switch to a -AO for any period of time would really mean business! Until then, if you live anywhere near the Ohio valley, get out your umbrellas and snow shoes!

P.S. That last comment and the one before it are my official winter outlook. Sorry, no big writeups from me. Take it for what it's worth...I'm no HM )

:santa: May everyone win in the winter of 2011-2012! Except for the southeast. They're probably screwed

[showmethesnow]

With the NAO, when you have a switch from + to - or - to +, many times you see a major East Coast event. I was just curious if the AO was known for an increased chance of a weather event when it's values switched or even if there was a sharp increase or decrease in it's value.

[donsutherland1]

With the NAO, when you have a switch from + to - or - to +, many times you see a major East Coast event. I was just curious if the AO was known for an increased chance of a weather event when it's values switched or even if there was a sharp increase or decrease in it's value.

For KU snowstorms, the state of the AO is important during El Niño and La Niña conditions, but apparently less important during neutral ENSO conditions:

El Niño: 18/19 (94.7%) KU storms commenced during AO-

Neutral: 7/14 (50.0%) KU storms commenced during AO-

La Niña: 6/7 (85.7%) KU storms commenced during AO-

El Niño conditions were defined as having a +0.5 or above ENSO Region 3.4 anomaly during the KU storm; La Niña conditions were defined as having a -0.5 or below ENSO Region 3.4 anomaly during the KU storm.

[thunderbolt]

its a start . http://ds.data.jma.go.jp/tcc/tcc/products/clisys/STRAT/html_e/pole30_n.html

[donsutherland1]

Today, the AO reached +5.647. That demolishes the December record of +5.040 set on December 2, 1979 and is the second highest AO reading on record. Recordkeeping began in 1950.

The 10 highest AO readings are as follows:

1. +5.911, February 26, 1990

2. +5.647, December 4, 2011

3. +5.582, January 14, 1989

4. +5.359, February 27, 1990

5. +5.337, January 15, 1989

6. +5.293, January 11, 1989

7. +5.245, January 14, 1993

8. +5.165, January 12, 1989

9. +5.147, February 9, 1990

10. +5.142, February 25, 1990

All of the events that saw top 10 AO readings were super AO+ regimes.

1989 event: Lasted 78 days

1990 event: Lasted 120 days

1993 event: Lasted 58 days

If one includes the other prior events that saw an AO reading of +5.000 or above, those events had the following durations:

1957: 41 days

1979: 16 days

[showmethesnow]

For KU snowstorms, the state of the AO is important during El Niño and La Niña conditions, but apparently less important during neutral ENSO conditions:

El Niño: 18/19 (94.7%) KU storms commenced during AO-

Neutral: 7/14 (50.0%) KU storms commenced during AO-

La Niña: 6/7 (85.7%) KU storms commenced during AO-

El Niño conditions were defined as having a +0.5 or above ENSO Region 3.4 anomaly during the KU storm; La Niña conditions were defined as having a -0.5 or below ENSO Region 3.4 anomaly during the KU storm.

Very strong correlation between needing neg AO values for KU events showing with those numbers.

The question I was curious more so about was if the rapid increase or decrease in the AO values or the switch from pos to neg was a harbinger of a chance of any sort of weather event in the US such as the many occurrences of East Coast storms with the switch of states with the NAO. For example, a massive invasion of arctic air, a strong great Lakes low, etc....

[donsutherland1]

Very strong correlation between needing neg AO values for KU events showing with those numbers.

The question I was curious more so about was if the rapid increase or decrease in the AO values or the switch from pos to neg was a harbinger of a chance of any sort of weather event in the US such as the many occurrences of East Coast storms with the switch of states with the NAO. For example, a massive invasion of arctic air, a strong great Lakes low, etc....

No. That's probably because the AO is a larger-scale phenomenon than the NAO.

[Organizing Low]

great post bluewave and great stats Don.

what was the enso during the winter of 92-93 (and January 93 onwards)?

[donsutherland1]

great post bluewave and great stats Don.

what was the enso during the winter of 92-93 (and January 93 onwards)?

Thanks Organizing Low.

ENSO R3.4 Anomaly:

January 1993: +0.12

February 1993: +0.25

March 1993: +0.44

April 1993: +0.81

[donsutherland1]

Today's AO reading is +4.850. The GFS ensembles again initialized it at a much lower figure (~+3.500). Hence, the brief period of a negative AO shown by some members of the GFS ensembles is suspect. In contrast, the ECMWF ensembles initialized the AO around +4.000 and they show no turn to negative through the next 10 days. I believe the ECMWF ensembles have a better grasp on the persistence of the current AO+ regime.

[weathafella]

Today's AO reading is +4.850. The GFS ensembles again initialized it at a much lower figure (~+3.500). Hence, the brief period of a negative AO shown by some members of the GFS ensembles is suspect. In contrast, the ECMWF ensembles initialized the AO around +4.000 and they show no turn to negative through the next 10 days. I believe the ECMWF ensembles have a better grasp on the persistence of the current AO+ regime.

We missed the record Don. Nice job on the play by play here. It will be interesting to see where this trends over the next 10 days.

[tamarack]

_{12/27/83 was surprising to see after the coldest Christmas max ever...I like those analogs and I'm not expecting any big storms until later in the season if then...}

_{1971 1975 and 1999 are the three analogs I'd bet on now...I like 71-72 the best but all three were lacking the big snow in the city...There was a KU storm on 2/19/72 and 1/25/00...Both mixed with or changed to rain in the city...}

Somewhat OT: 12/25/83 max was 13. Three years earlier the official max was 20, at 12:01 AM as the temp plunged toward its low of -1. Since Boston's low that day was -7 and I once read (probably in New England Climo Data from NOAA) that their afternoon high was zero, I'd guess that NYC never reached 10 on the afternoon of 12/25/80. Would you have any info on that? Lots more wind with that earlier cold snap as well.

[uncle W]

Somewhat OT: 12/25/83 max was 13. Three years earlier the official max was 20, at 12:01 AM as the temp plunged toward its low of -1. Since Boston's low that day was -7 and I once read (probably in New England Climo Data from NOAA) that their afternoon high was zero, I'd guess that NYC never reached 10 on the afternoon of 12/25/80. Would you have any info on that? Lots more wind with that earlier cold snap as well.

I found the local climate page I have for 12/25/1980...These are the three hour temps...

1am...18

4am...11

7am.....2

10am. -1

1pm.....4

4pm.....7

7pm.....7

10pm ..7

[tamarack]

I found the local climate page I have for 12/25/1980...These are the three hour temps...

1am...18

4am...11

7am.....2

10am. -1

1pm.....4

4pm.....7

7pm.....7

10pm ..7

Thanks!

Their low for 12/26 was 8, so that was probably the PM high for the 25th.

[donsutherland1]

After spending four consecutive days at or above +4, the AO has now fallen below that level. The decline should continue for a few more days. However, consistent with historic experience for extreme AO+ regimes during La Niña conditions, the AO is not likely to dip below 0 anytime soon. The 12/7 0z ECMWF ensembles keep the AO positive through its 10-day forecast period, as does the operational ECMWF.

Beyond that timeframe, both the 12/7 0z and 12/7 6z runs of the operational GFS hint that the AO could push toward extreme positive figures beyond 12/20. Although a continuation of the AO+ regime is probably quite likely beyond 12/20, it's perhaps a little soon to have significant confidence in another spike. Nonetheless, past super AO+ regimes have seen multiple spikes to extreme levels, so that's a scenario that is at least somewhat plausible. Should such a spike in the AO occur, it could coincide with a positive EPO. If so, the stage could be set for a warm holiday season.

[Kevin Druff]

After spending four consecutive days at or above +4, the AO has now fallen below that level. The decline should continue for a few more days. However, consistent with historic experience for extreme AO+ regimes during La Niña conditions, the AO is not likely to dip below 0 anytime soon. The 12/7 0z ECMWF ensembles keep the AO positive through its 10-day forecast period, as does the operational ECMWF.

Beyond that timeframe, both the 12/7 0z and 12/7 6z runs of the operational GFS hint that the AO could push toward extreme positive figures beyond 12/20. Although a continuation of the AO+ regime is probably quite likely beyond 12/20, it's perhaps a little soon to have significant confidence in another spike. Nonetheless, past super AO+ regimes have seen multiple spikes to extreme levels, so that's a scenario that is at least somewhat plausible. Should such a spike in the AO occur, it could coincide with a positive EPO. If so, the stage could be set for a warm holiday season.

I really don't know what the policy on profanities are on the board, but I don't know of any other way to say this:

You're ****ing depressing, man. Educational. But, depressing.

If it's a no-no, I'm sure a mod will be along shortly to edit...