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Winter 2015-2016 Medium-Term Discussion


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No - I saw DT's post about 2 hours ago and he pulled part of it (I think) to correct "trough" with "ridge" on a graphic and now it's all gone; Isotherm isn't DTWXRISK (I don't think - and there is no substantive post from Isotherm that I can find).

 

????????

Isotherm's substantive post was #111 in this thread. 

 

Also, I believe DT's post to which you are referring can be found here: http://www.americanwx.com/bb/index.php/topic/47483-the-coming-pattern-change-dec-26/?p=3817379

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This morning, the Arctic Oscillation (AO) was +0.805 vs. +4.485 a week ago. The PNA was +1.351 vs. -0.261 a week ago. The latest guidance is in strong agreement that an AO-/PNA+ (and EPO-) pattern is likely to develop early in January. Should that pattern be sustained, it would favor a more expansive area of cold anomalies in North America than has been the case so far this winter. For illustrative purposes, below are composite 500 mb and temperature anomaly maps for January cases when the monthly AO average is -1.000 or below, monthly PNA average is +1.00 or above, and when the AO monthly average is -1.000 or below and the PNA monthly average is +1.00 or above.

 

Right now, it's too soon to be sure that the January averages will reach such magnitudes, but the maps are useful in illustrating the growing probability that cold anomalies will be more expansive than they were in December. It remains plausible given the super El Niño that much of Canada would wind up warmer than normal and the northern tier of the U.S. from Montana or North Dakota eastward into northern New England (possibly somewhat south or north of that line) would see monthly temperatures average on the warm side of normal. Some blend of the composite for PNA anomalies of +1.00 or above, January 1878 (message #118), and January 1983 (message #118) might be reasonable barring a January AO average of perhaps -2.000 or below.

 

AO12292015_1.jpg

 

AO12292015_2.jpg

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Long-Awaited Pattern Evolution Now Underway…

 

The long-awaited pattern regime change from a persistent super ridge centered over eastern North America is now underway. The SOI has now fallen to -23.25 after peaking at +30.72 on December 24. The rate of decline in the ongoing super El Niño has shown some indications of picking up. The most recent weekly changes were as follows:

 

Region 1+2: -0.3°C

Region 3: -0.2°C

Region 3.4: -0.2°C

Region 4: -0.1°C

 

In the larger scale, the kind of historic warmth that covered the eastern half of December during super El Niño events is typically unsustainable, not just in magnitude (otherwise that kind of warmth wouldn’t be such a rare event), but also in coverage. Even as the sample size is limited to two super El Niño cases (1877-78 and 1982-83), super warm winters for the entire December-February period are rare. Hence, with a degree of caution, one can argue that it is more likely than not that the warm anomalies should be expected to retreat to the northern tier (perhaps running from Montana or North Dakota to northern New England and northward) if the current ENSO event is reasonably similar to the two cases and the larger historical experience has relevance. The latest CFSv2 guidance shows just such an outcome.

 

ENSO12282015.jpg

 

The ensembles have persistently been forecasting a return of the PNA+, which is a favored state when the Pacific Decadal Oscillation is positive. More recently, ensemble support for AO- and EPO- blocks has increased. The 12/28/2015 GFS ensembles now show strong agreement that the AO will be falling to negative values with a cluster of members supporting severely negative values during and after the first week in January. Even a few ensemble members support a negative NAO during the extended range.

 

AO12282015.jpg

 

At this point, the guidance strongly supports the idea that a pattern evolution is now underway. There will likely be a short lag before the impact of the blocking takes hold on the hemispheric pattern. Before then, there will likely be a transitional pattern that lasts 1-2 weeks. During that transitional pattern, eastern North America will likely be much cooler than during December, even as readings average somewhat above normal in some of the areas that saw extraordinary warmth during December. There may yet be opportunities for systems to cut to the Great Lakes, producing short periods of much warmer than normal readings in the East. The West will generally remain colder than normal, but toward the end of the transitional period, a warming trend may begin to appear in the Pacific Northwest and northwestern Canada on account of the PNA+.

 

What happens afterward will depend on whether the forecast blocking develops. The turnaround that occurred during winter 1965-66 when sustained blocking developed in late January could provide some insight.

 

Great stuff Don. I've always been intrigued by the 1877-78 El Nino event which resulted in the warmest winter on record in the northern Plains and eastern Prairies of Canada. Would you have an idea how strong the EL Nino was that year compared to 1982, 1997 or this year? ENSO values only go back to 1950, so I'm trying to put the 1877 event into some historical perspective. Also, is there any info on what other oscillations may have been like that year to produce such an extremely anomalous winter over the central continent?  

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Great stuff Don. I've always been intrigued by the 1877-78 El Nino event which resulted in the warmest winter on record in the northern Plains and eastern Prairies of Canada. Would you have an idea how strong the EL Nino was that year compared to 1982, 1997 or this year? ENSO values only go back to 1950, so I'm trying to put the 1877 event into some historical perspective. Also, is there any info on what other oscillations may have been like that year to produce such an extremely anomalous winter over the central continent?  

The hadISST dataset shows both the 1877-78 and 1997-98 El Niño events had an equal peak in Region 3.4. The 1997-98 El Niño's greatest anomalies were in Region 1+2 and that region was much warmer than in 1877-78.

 

Region 1+2: http://www.esrl.noaa.gov/psd/gcos_wgsp/Timeseries/Data/nino12.long.anom.data

 

Region 3.4: http://www.esrl.noaa.gov/psd/gcos_wgsp/Timeseries/Data/nino34.long.anom.data

 

So far, no official ERSSTv4 data has been published to compare the two El Niños.

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This morning, the SOI had fallen to -27.30. That is its lowest figure since December 4. At the same time, the PNA remained positive and the ensembles remained in agreement that the AO will very likely go negative over the next week. In short, a pattern evolution remains underway.

 

However, if one looked at the latest CFSv2 forecast, one finds warm anomalies forecast across much of North America with the exception of the southern portion of the United States and Mexico. One also finds an expansive area of much below normal precipitation forecast for the Ohio and Tennessee Valley regions.

 

Such an outcome would not be unprecedented. In fact, it is remarkably similar to the outcome for January 1983, though much warmer across northern Canada and Alaska and with more expansive warmth in the CONUS.

 

CFSv212302015.jpg

 

With that model now in its skillful range, is there any hope for colder outcome? A comparison of the 1983 teleconnection indices and ensemble forecast may provide some insight.

 

In January 1983, the PNA averaged +1.18. Moreover, the PNA was positive on 28 out of 31 days. The latest ensemble guidance forecasts a PNA+ and the current PDO+ also favors such an outcome.

 

When it comes to the AO, January 1983 had an AO average of +1.359. 22 out of 31 days saw the AO positive. A short period of blocking occurred during the January 16-23 timeframe when the AO was negative for 6 out of 8 days. A long-duration period of blocking developed on January 29.

 

In contrast, the current ensemble forecast shows the AO going negative early in January and remaining there through at least mid-month. That's a significant difference from what happened in 1983.

 

All said, assuming the blocking develops and is sustained, my thinking remains that the warmth will be less expansive than what is shown on the latest run of the CFSv2. For a rough idea of my thinking, one can expand the areas of cool anomalies, anomalies <0.5°C above normal, and anomalies less than 1°C above normal northward by about 5° latitude.

 

If, however, the blocking proves transient and the polar vortex rebuilds for the second half of January, then the warmer CFSv2 idea might well have merit. Right now, I think that's not a high probability outcome, but there is sufficient uncertainty in ensemble forecasting skill related to the AO to make it impossible to completely rule it out.

 

In sum, a pattern evolution away from the predominant December pattern is underway. A 1-2 week transitional period lies ahead. Afterward, especially if blocking is sustained, the potential for increasing cold shots with greater geographic coverage exists. The second half of January would wind up colder relative to normal than the first half of January in much of the CONUS and parts of Canada. That's my baseline thinking as December nears an end.

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Don,

 

I am curious.  Do you have any idea what the atmospheric mechanism is that the CFSv2 is latching onto to predict that virtually all of Canada will have a much warmer than normal January?  Is this an El Ninio effect?

It favored a predominantly EPO+ pattern. Moreover, toward the end of its run, it tried to rebuild ridging near, but somewhat west, of where it predominated in December. Canada would see far more Pacific air masses than Arctic ones according to such a pattern.

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It favored a predominantly EPO+ pattern. Moreover, toward the end of its run, it tried to rebuild ridging near, but somewhat west, of where it predominated in December. Canada would see far more Pacific air masses than Arctic ones according to such a pattern.

Thatnks for the response Don.  My understanding is that a +EPO locks cold in far northern Canada.  So I am surprised to see the warmth up there as well.  Perhaps there it already takes an AGW "offset" into account.

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Thatnks for the response Don.  My understanding is that a +EPO locks cold in far northern Canada.  So I am surprised to see the warmth up there as well.  Perhaps there it already takes an AGW "offset" into account.

The point about AGW is a good possibility. The CFSv2 may well reflect the impact of AGW on the Arctic, as it is a coupled land-ocean-ice-atmosphere model. That aspect of the model may well explain why it sees the Arctic region as much warmer than normal and that warmth may also be playing a role with respect at least to a slice of Canada and Alaska. Unfortunately, unlike with the placement of its height anomalies, one can't really see what impact the coupled nature has.

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No changes in thoughts this morning...

 

1. The AO is now poised to dive strongly negative over the next 7 days according to the GFS ensembles.

2. The EPO is forecast to have a sustained period of negative values for the first time since November 20-30.

3. The PNA remains positive.

4. 500 mb height anomalies have been evolving away from the predominant December pattern.

 

In short, North America is in the midst of a pattern evolution and this transitional state may last 1-2 weeks. If blocking is sustained afterward, sharper and more expansive outbreaks of cold could occur. 

 

Not surprisingly, the CFSv2 adjusted its warm anomalies northward overnight. 

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0Z   DEC 31  EURO   ensembles  are very strong with the signal for east coast winter storm JAN 10-11   as the over pattern and teleconnections  have been screaming  this for   days 

 

 

  followed by  a blast of   TRUE ARCTIC AIR 

 

 

post-9415-0-75247800-1451573723_thumb.pn

 

post-9415-0-34584200-1451573718_thumb.pn

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December will go down in the record books as an historically warm month. Some average monthly temperatures and temperature departures from select cities:

 

December2015_Select_Cities.jpg

 

Back on December 17, the GFS ensembles suddenly indicated that a PNA+ could return. Since then, one has witnessed verification of that forecast and the SOI’s having gone strongly negative. Moreover, the ensembles indicate a sustained period of blocking should develop over the next few days.

 

The upcoming cold shot that will slide eastward in coming days will be fairly sharp. It will serve a reminder that even as December as been lost to an ocean of warmth, winter has not been vanquished. Parts of the Western U.S. already have experienced winter’s fury in the form of a blizzard that paralyzed parts of New Mexico and Texas.

 

That cold shot will very likely yield to moderation with some above normal readings. However, the warmth won’t even begin to approach the levels that had been so common in December. Instead, the pattern will continue to evolve. Near mid-month, give or take a few days, the sustained blocking may well lead to sustained cold.

 

Finally, cities such as Washington and New York, among others, could see increased opportunities for snowfall should the AO-/PNA+ pattern persist. For example, the percentage of days receiving measurable snowfall in New York is 9% above the January average (1950-2015) during such patterns. In Washington, DC, the percentage of such days is nearly 22% above the January average.

 

In sum, the transitional period remains underway. A sustained cold pattern could develop toward mid-month and, opportunities for snowfall could begin to increase ahead of that colder pattern.

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December will go down in the record books as an historically warm month. Some average monthly temperatures and temperature departures from select cities:

 

December2015_Select_Cities.jpg

 

Back on December 17, the GFS ensembles suddenly indicated that a PNA+ could return. Since then, one has witnessed verification of that forecast and the SOI’s having gone strongly negative. Moreover, the ensembles indicate a sustained period of blocking should develop over the next few days.

 

The upcoming cold shot that will slide eastward in coming days will be fairly sharp. It will serve a reminder that even as December as been lost to an ocean of warmth, winter has not been vanquished. Parts of the Western U.S. already have experienced winter’s fury in the form of a blizzard that paralyzed parts of New Mexico and Texas.

 

That cold shot will very likely yield to moderation with some above normal readings. However, the warmth won’t even begin to approach the levels that had been so common in December. Instead, the pattern will continue to evolve. Near mid-month, give or take a few days, the sustained blocking may well lead to sustained cold.

 

Finally, cities such as Washington and New York, among others, could see increased opportunities for snowfall should the AO-/PNA+ pattern persist. For example, the percentage of days receiving measurable snowfall in New York is 9% above the January average (1950-2015) during such patterns. In Washington, DC, the percentage of such days is nearly 22% above the January average.

 

In sum, the transitional period remains underway. A sustained cold pattern could develop toward mid-month and, opportunities for snowfall could begin to increase ahead of that colder pattern.

Don,

 

Not to be too ingratiating, but your opinion is widely respected on this board.  In your own thinking, do you attribute this historically warm December to any particular cause or causes? 

 

I live in eastern NC, with RDU being the nearest major station.  We just had our warmest December ever by a comfortable margin.  Our local weather guy (who I feel is well respected) mentioned that he was having a hard time coming up with a simple explanation as to why.  He is no AGW denier, but he felt that long-term AGW was not satisfying for the huge short-term anomaly.  As for El Nino, he discussed a comparison pool of some of the previous warmest Decembers in RDU history and he noted that not a single one was an El Nino year.

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Don,

 

Not to be too ingratiating, but your opinion is widely respected on this board.  In your own thinking, do you attribute this historically warm December to any particular cause or causes? 

 

I live in eastern NC, with RDU being the nearest major station.  We just had our warmest December ever by a comfortable margin.  Our local weather guy (who I feel is well respected) mentioned that he was having a hard time coming up with a simple explanation as to why.  He is no AGW denier, but he felt that long-term AGW was not satisfying for the huge short-term anomaly.  As for El Nino, he discussed a comparison pool of some of the previous warmest Decembers in RDU history and he noted that not a single one was an El Nino year.

Attribution is difficult.

 

What one has seen is that both the 1877-78 and 1982-83 super El Niño cases resulted in an extreme ridge with somewhat different placement. Natural variability can result in some big extremes. At the same time, the current ENSO and exceptionally warm December occurred within the context of a continued increase in greenhouse gas forcing, so more is involved than just natural variability.

 

If one assumes linear responses to that increased  that increased forcing e.g., the rise in temperatures that has occurred so far, one finds a somewhat increased probability of such events. By itself, such an increased probability would seem to fall short of explaining what happened.

 

The big issue concerns non linear responses. There is at least one paper that proposes a mechanism that may well have contributed to the outcome.

 

http://marine.rutgers.edu/~francis/pres/Francis_Vavrus_2012GL051000_pub.pdf

 

Once attribution and detection become more robust, stronger conclusions may be drawn.  So, at least for now, my view is that there was a combination of factors, including climate change, involved, but I can't go beyond that.

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Attribution is difficult.

 

What one has seen is that both the 1877-78 and 1982-83 super El Niño cases resulted in an extreme ridge with somewhat different placement. Natural variability can result in some big extremes. At the same time, the current ENSO and exceptionally warm December occurred within the context of a continued increase in greenhouse gas forcing, so more is involved than just natural variability.

 

If one assumes linear responses to that increased  that increased forcing e.g., the rise in temperatures that has occurred so far, one finds a somewhat increased probability of such events. By itself, such an increased probability would seem to fall short of explaining what happened.

 

The big issue concerns non linear responses. There is at least one paper that proposes a mechanism that may well have contributed to the outcome.

 

http://marine.rutgers.edu/~francis/pres/Francis_Vavrus_2012GL051000_pub.pdf

 

Once attribution and detection become more robust, stronger conclusions may be drawn.  So, at least for now, my view is that there was a combination of factors, including climate change, involved, but I can't go beyond that.

Hi Don, I heard it's going to be impossible to get a locked in cold pattern because the PV hasn't split?. However, if the central based El Nino is getting in it would be a pattern that could last a long time. I know before that the east based side of the El Nino was showing it's ugly head.

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Attribution is difficult.

 

What one has seen is that both the 1877-78 and 1982-83 super El Niño cases resulted in an extreme ridge with somewhat different placement. Natural variability can result in some big extremes. At the same time, the current ENSO and exceptionally warm December occurred within the context of a continued increase in greenhouse gas forcing, so more is involved than just natural variability.

 

If one assumes linear responses to that increased  that increased forcing e.g., the rise in temperatures that has occurred so far, one finds a somewhat increased probability of such events. By itself, such an increased probability would seem to fall short of explaining what happened.

 

The big issue concerns non linear responses. There is at least one paper that proposes a mechanism that may well have contributed to the outcome.

 

http://marine.rutgers.edu/~francis/pres/Francis_Vavrus_2012GL051000_pub.pdf

 

Once attribution and detection become more robust, stronger conclusions may be drawn.  So, at least for now, my view is that there was a combination of factors, including climate change, involved, but I can't go beyond that. But

So whatever the ultimate attribution, is it correct to say that the immediate mechanism for the much-above-average temperatures in Eastern North America was the presence of an extreme ridge?  In our local weather discussion from the NWS, I saw several mentions of an anomalously strong high pressure in the western Atlantic pumping warm moist air into the east.  Was this the same ridge of which you speak?

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So whatever the ultimate attribution, is it correct to say that the immediate mechanism for the much-above-average temperatures in Eastern North America was the presence of an extreme ridge?  In our local weather discussion from the NWS, I saw several mentions of an anomalously strong high pressure in the western Atlantic pumping warm moist air into the east.  Was this the same ridge of which you speak?

I'm referring to the predominant position of much above average 500 mb height anomalies.

 

Dec2015500mb_Anomalies.jpg
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Tropospheric PV is much easier to break and split. That is independent of any stratospheric response. Part of the PV is on our side of the pole which is good. It will be interesting to see what happens if the Pacific starts to break down a bit after the 20th like models have. That may be related to IO convection firing up.

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Tropospheric PV is much easier to break and split. That is independent of any stratospheric response. Part of the PV is on our side of the pole which is good. It will be interesting to see what happens if the Pacific starts to break down a bit after the 20th like models have. That may be related to IO convection firing up.

The Pacific breaking down is that a good thing for cold and snow lovers or bad?. 

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This morning saw the AO at -1.556 and the PNA at +1.785. The EPO was also negative. This EPO-/AO-/PNA+ combination is the opposite of what prevailed during much of December. It is yet another indication that the predominant December pattern has broken.

 

A downstream response to such a pattern should begin to yield colder outbreaks over a larger part of North America than has been the case so far and eventually the development of a trough in parts of the East. Once that happens (probably near mid-month +/- a few days), the pattern could favor sustained cold. There could also be increased opportunities for snowfall, but we're not yet there.

 

Following the early week cold shot, moderation will take place. Readings will return to above normal values in a large part of North America that just went through December's historic warmth. But that period of moderation will likely be temporary given the current and forecast state of the teleconnections.

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