donsutherland1

The December 16-17, 2020 snowstorm dumped 10.5" snow on New York City. December cases when 10" or more snow falls have generally seen 20" or more additional snowfall from January to the last snowfall. The historic data suggests that seasonal guidance notwithstanding, the New York City area now has a high probability of seeing 30" or more seasonal snowfall. January-May Snowfall (1869-2020) following December Monthly Snowfall of 10" or More: Number of cases by DJF ENSO: La Niña: 8 Neutral: 13 El Niño: 6 Total: 27 What could result in less snowfall: 1. Collapse of the ongoing La Niña and/or 2. The persistent AO- blocking yields to a sustained AO+ regime

Morning thoughts... Today will be mostly sunny in much of the region. It will remain unseasonably cold. Temperatures will likely top out in the lower and 30s across most of the region. Likely high temperatures around the region include: New York City (Central Park): 31° Newark: 33° Philadelphia: 33° Tomorrow will be variably cloudy with some rain and snow showers, along with moderating temperatures.

IMO, the NWS is an invaluable resource. It is among the most effective and beneficial federal entities and provides proof to skeptics that government can perform at a consistently high level. I appreciate and applaud all that NWS does even with severe resource constraints in recent years.

@bluewave was discussing the areas where 20”-40” snow just fell as having the highest risk of a flood threat, not here.

To date, the Arctic Oscillation (AO) has averaged -1.678 this month. It has been negative on 89% of days. It has been -1.000 or below 78% of the time and -2.000 or below on 39% of the days. A negative AO has been present in more than three-quarters of cases where Philadelphia and New York City saw 6" or greater snowstorms during the December 10-31 period, one of which occurred during December 16-17. Since 2000, December snowfall exceeded 6" or more in 5/7 (71%) of cases in both New York City and Philadelphia when the AO averaged -1.000 or below in December. In Philadelphia, seasonal snowfall exceeded 20" or more in 80% of those December cases when 6" or more snow fell and 30" or more in 60% of those cases. In New York City, seasonal snowfall exceeded 30" or more in all those December cases with 6" or more snowfall and 40" or more in 80% of those cases. The recent snowstorm during what has been a blocky December suggests that seasonal snowfall prospects have increased especially from north of Philadelphia into southern New England. Nevertheless, there remains a chance that the current La Niña could adversely influence the pattern leading to a disappearance of the blocking. Were blocking to disappear, snowfall prospects would diminish. For now, blocking appears likely to continue into the start of January. Temperatures will slowly moderate this weekend. Tomorrow will be fair and still cool. Sunday will be variably cloudy with some rain and snow showers possible. A strong storm could pass to the west of the region after midweek. With the persistence of Arctic blocking, uncertainty about the closing days of December has increased. A possible piece of an Arctic air mass could even push into parts of the region on or just after December 25. Frigid Arctic air is currently centered in northwestern Canada. Earlier today, near record and record cold was reported in parts of the Northwest Territories. At Norman Wells, the temperature fell to -53° (old record: -50°, 1961). At Yellowknife, the temperature fell to a near-record -48°. Historic experience following exceptionally warm November cases suggests that a warmer than normal December remains the base case. Almost 90% of cases with a November mean temperature of 51.5° or above in Central Park (2020 saw the temperature average a record 52.9° in November) went on to record a warmer than normal December and just over three-quarters of such cases saw December register a monthly mean temperature of 40.0° or above. Sustained Arctic blocking will produce a much cooler outcome. The monthly temperature anomaly could wind up only somewhat above normal. The ENSO Region 1+2 anomaly was -0.1°C and the Region 3.4 anomaly was -1.2°C for the week centered around December 9. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.62°C and the ENSO Region 3.4 anomaly has averaged -1.43°C. La Niña conditions will likely prevail at least through the winter. The SOI was +34.10. Today, the preliminary Arctic Oscillation (AO) figure was -2.035. On December 17 the MJO was in Phase 5 at an amplitude of 0.790 (RMM). The December 16-adjusted amplitude was 0.845. Based on the latest guidance, no significant stratospheric warming event is into the final week of December. Since 1950, there have been five cases where a La Niña developed during June-July-August or afterward following an El Niño winter. 4/5 (80%) of those cases saw a predominant EPO+/AO+ winter pattern. The most recent such case was 2016-17. 10/11 (91%) of the La Niña winters that followed an El Niño winter featured a predominantly positive EPO. A predominant EPO+/AO+ pattern is very likely for winter 2020-21. It is likely that the Middle Atlantic and southern New England areas will see a warmer than normal winter with below normal snowfall. The large snowfall that just took place provides a strong contrary signal to the below normal snowfall idea. Should blocking continue, the probability of above normal snowfall would likely increase. Based on sensitivity analysis applied to the latest guidance, there is an implied 54% probability that New York City will have a warmer than normal December. December will likely finish with a mean temperature near 38.0°.

Great point about the flood potential.

Morning thoughts... Today will become partly to mostly sunny in much of the region. It will remain unseasonably cold. Temperatures will likely top out in the lower and middle 30s across most of the region. Likely high temperatures around the region include: New York City (Central Park): 33° Newark: 34° Philadelphia: 36° Temperatures will moderate during the weekend.

In the wake of a snowstorm that brought widespread 10" or above amounts from New York City into New England with several areas of excessive amounts of 30" or more, tomorrow will be fair and unseasonably cold. With the persistence of Arctic blocking, uncertainty about the closing days of December has increased. A possible piece of an Arctic air mass could even push into parts of the region on or just after December 25. Historic experience following exceptionally warm November cases suggests that a warmer than normal December remains the base case. Almost 90% of cases with a November mean temperature of 51.5° or above in Central Park (2020 saw the temperature average a record 52.9° in November) went on to record a warmer than normal December and just over three-quarters of such cases saw December register a monthly mean temperature of 40.0° or above. The probability of December's having a 40.0° or above mean temperature has fallen sharply with the development of strong AO blocking. The ENSO Region 1+2 anomaly was -0.1°C and the Region 3.4 anomaly was -1.2°C for the week centered around December 9. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.62°C and the ENSO Region 3.4 anomaly has averaged -1.43°C. La Niña conditions will likely prevail at least through the winter. The SOI was +34.10. That is the highest figure since March 17, 2018 when the SOI reached +35.90. Today, the preliminary Arctic Oscillation (AO) figure was -2.620. On December 16 the MJO was in Phase 5 at an amplitude of 0.839 (RMM). The December 15-adjusted amplitude was 0.880. Based on the latest guidance, no significant stratospheric warming event is into the final week of December. Since 1950, there have been five cases where a La Niña developed during June-July-August or afterward following an El Niño winter. 4/5 (80%) of those cases saw a predominant EPO+/AO+ winter pattern. The most recent such case was 2016-17. 10/11 (91%) of the La Niña winters that followed an El Niño winter featured a predominantly positive EPO. A predominant EPO+/AO+ pattern is very likely for winter 2020-21. It is likely that the Middle Atlantic and southern New England areas will see a warmer than normal winter with below normal snowfall. The large snowfall that just took place provides a strong contrary signal to the below normal snowfall idea. Should blocking continue, the probability of above normal snowfall would likely increase. Based on sensitivity analysis applied to the latest guidance, there is an implied 55% probability that New York City will have a warmer than normal December. December will likely finish with a mean temperature near 38.1°.

5th earliest. November 26-27, 1898 had 10.0”.

The latest PNS doesn’t contain the near final numbers.

I agree. As long as the AO stays negative, there could be some more chances. As for Central Park, I suspect the final figure will be close to 10”. We’ll probably know soon.

Morning thoughts... Snow will end early this morning as a strengthening storm moves away from the region. It will turn partly cloudy in much of the region during the afternoon, but remain windy. Temperatures will likely top out in the lower and middle 30s across most of the region. Likely high temperatures around the region include: New York City (Central Park): 30° Newark: 32° Philadelphia: 34° In the wake of the snowstorm, temperatures will run below normal for the remainder of the week.

A major snowstorm is now blanketing much of the region. By the time the snow ends tomorrow, much of central Pennsylvania to New England will have picked up a foot or more of snow. Estimated total snowfall amounts include: Allentown: 12"-18" Boston: 8"-16" Bridgeport: 8"-16" Hartford: 12"-18" Islip: 5"-10" New York City: 7"-14" Newark: 7"-14" Philadelphia: 4"-8" Poughkeepsie: 12"-18" Providence: 6"-12" Some areas could see 20" or above snowfall amounts. Since 2010, the three biggest December snowstorms for Boston, New York City, and Philadelphia were: Boston: 18.2", December 25-27, 2010 7.1", December 1-3, 2019 6.4", December 17, 2013 New York City: 20.0", December 25-27, 2010 5.0", December 14-15, 2013 4.6", December 9-10, 2017 Philadelphia: 12.4", December 25-27, 2010 8.6", December 8, 2013 4.1", December 9-10, 2017 During the December 10-31, 1950-2019 period, an AO- was the most important teleconnection for a significant Middle Atlantic (6" or more) snowstorm. Since 1950, the following percentage of 6" or greater snowstorms occurred when the AO was negative: Boston: 50%; New York City: 75%; and, Philadelphia: 88%. There were 8 snowstorms that brought 6" or more snow to at least two of the following cities: Boston, New York City, and Philadelphia. 7/8 (88%) occurred when the AO was negative. All storms from the subset from these 8 storms where 10" or more snow fell in at least one of those cities occurred with a negative AO. The most common synoptic pattern when at least two of the aforementioned cities picked up 6" or more snow (63%) was an AO-/PNA+ combination. The most recent such storm was the December 25-27, 2010 "Boxing Day Blizzard." With the persistence of Arctic blocking, uncertainty about the closing days of December has increased. At present, the guidance has backed off somewhat from the warmth that had been shown previously. Historic experience following exceptionally warm November cases suggests that a warmer than normal December remains the base case. Almost 90% of cases with a November mean temperature of 51.5° or above in Central Park (2020 saw the temperature average a record 52.9° in November) went on to record a warmer than normal December and just over three-quarters of such cases saw December register a monthly mean temperature of 40.0° or above. The probability of December's having a 40.0° or above mean temperature has fallen sharply with the development of strong AO blocking. The ENSO Region 1+2 anomaly was -0.1°C and the Region 3.4 anomaly was -1.2°C for the week centered around December 9. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.62°C and the ENSO Region 3.4 anomaly has averaged -1.43°C. La Niña conditions will likely prevail at least through the winter. The SOI was +28.65. Today, the preliminary Arctic Oscillation (AO) figure was -3.009. That is the lowest AO value since February 1, 2019 when the AO was -3.315. On December 15 the MJO was in Phase 5 at an amplitude of 0.868 (RMM). The December 14-adjusted amplitude was 1.070. Based on the latest guidance, no significant stratospheric warming event is into the final week of December. Since 1950, there have been five cases where a La Niña developed during June-July-August or afterward following an El Niño winter. 4/5 (80%) of those cases saw a predominant EPO+/AO+ winter pattern. The most recent such case was 2016-17. 10/11 (91%) of the La Niña winters that followed an El Niño winter featured a predominantly positive EPO. A predominant EPO+/AO+ pattern is very likely for winter 2020-21. It is likely that the Middle Atlantic and southern New England areas will see a warmer than normal winter with below normal snowfall. Based on sensitivity analysis applied to the latest guidance, there is an implied 56% probability that New York City will have a warmer than normal December. December will likely finish with a mean temperature near 38.4°.

IMO, 6”-10”. Hopefully, the guidance will still improve a little as the day goes on and things will verify higher in the end.

Morning thoughts... A significant to major Middle Atlantic and New England snowstorm will impact the region later today into early tomorrow. Snow will move into Philadelphia during the afternoon and then New York City area toward or during the evening. Parts of the region will pick up more than a foot of snow. Near blizzard or even blizzard conditions are possible at the height of the storm tomorrow night. Estimated accumulations: Allentown: 12”-18” Boston: 7”-14” Bridgeport: 8”-16” Islip: 5”-10” New York City: 7”-14” Newark: 7”-14” Philadelphia: 4”-8” Poughkeepsie: 12”-18” Providence: 6”-12” Since 1900, there have been only 7 years that saw a snowstorm bring 6” or more snow to Boston, New York City, and Philadelphia in December: 1909, 1912, 1945, 1947, 1960, 2009, and 2010. In some cases, winter proved to be snowier than normal. In one (1912-13), the winter saw below normal snowfall. At New York City, 11.4” snow fell on December 24. The remainder of winter saw just 3.1” snow. In stark contrast, winters 1947-48, 1960-61, 2009-10, and 2010-11 saw much additional snowfall after December. Clouds will increase. Snow will overspread the region during the afternoon or evening. Temperatures will likely top out mainly in the lower and middle 30s across most of the region. Likely high temperatures around the region include: New York City (Central Park): 33° Newark: 35° Philadelphia: 37° In the wake of the snowstorm, temperatures will run below normal for the remainder of the week.

The historic probability of normal to above normal snowfall would increase, especially if December sees 10” or more snow. The last 10 days of December are colder than what had been modeled.

A major storm will develop and move off the northern Delmarva Peninsula and pass south of Long Island Wednesday into Thursday. Parts of the region will pick up a significant snowfall. Estimated snowfall amounts include: Allentown: 12"-18" Boston: 6"-12" Bridgeport: 8"-14" Islip: 4"-8" New York City: 6"-12" Newark: 6"-12" Philadelphia: 4"-8" Poughkeepsie: 12"-18" Providence: 6"-12" Since 2010, the three biggest December snowstorms for Boston, New York City, and Philadelphia were: Boston: 18.2", December 25-27, 2010 7.1", December 1-3, 2019 6.4", December 17, 2013 New York City: 20.0", December 25-27, 2010 5.0", December 14-15, 2013 4.6", December 9-10, 2017 Philadelphia: 12.4", December 25-27, 2010 8.6", December 8, 2013 4.1", December 9-10, 2017 During the December 10-31, 1950-2019 period, an AO- was the most important teleconnection for a significant Middle Atlantic (6" or more) snowstorm. Since 1950, the following percentage of 6" or greater snowstorms occurred when the AO was negative: Boston: 50%; New York City: 75%; and, Philadelphia: 88%. There were 8 snowstorms that brought 6" or more snow to at least two of the following cities: Boston, New York City, and Philadelphia. 7/8 (88%) occurred when the AO was negative. All storms from the subset from these 8 storms where 10" or more snow fell in at least one of those cities occurred with a negative AO. The most common synoptic pattern when at least two of the aforementioned cities picked up 6" or more snow (63%) was an AO-/PNA+ combination. The most recent such storm was the December 25-27, 2010 "Boxing Day Blizzard." With the persistence of Arctic blocking, uncertainty about the closing days of December has increased. At present, the guidance has backed off somewhat from the warmth that had been shown previously. Historic experience following exceptionally warm November cases suggests that a warmer than normal December remains the base case. Almost 90% of cases with a November mean temperature of 51.5° or above in Central Park (2020 saw the temperature average a record 52.9° in November) went on to record a warmer than normal December and just over three-quarters of such cases saw December register a monthly mean temperature of 40.0° or above. The probability of December's having a 40.0° or above mean temperature has fallen sharply with the development of strong AO blocking. The ENSO Region 1+2 anomaly was -0.1°C and the Region 3.4 anomaly was -1.2°C for the week centered around December 9. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.62°C and the ENSO Region 3.4 anomaly has averaged -1.43°C. La Niña conditions will likely prevail at least through the winter. The SOI was +19.88. Today, the preliminary Arctic Oscillation (AO) figure was -2.891. That is the lowest AO value since February 1, 2019 when the AO was -3.315. On December 14 the MJO was in Phase 5 at an amplitude of 1.052 (RMM). The December 13-adjusted amplitude was 1.495. Based on the latest guidance, no significant stratospheric warming event is likely through the third week of December. Since 1950, there have been five cases where a La Niña developed during June-July-August or afterward following an El Niño winter. 4/5 (80%) of those cases saw a predominant EPO+/AO+ winter pattern. The most recent such case was 2016-17. 10/11 (91%) of the La Niña winters that followed an El Niño winter featured a predominantly positive EPO. A predominant EPO+/AO+ pattern is very likely for winter 2020-21. It is likely that the Middle Atlantic and southern New England areas will see a warmer than normal winter with below normal snowfall. Based on sensitivity analysis applied to the latest guidance, there is an implied 64% probability that New York City will have a warmer than normal December. December will likely finish with a mean temperature near 38.7°.

Morning thoughts... Today will partly cloudy and chilly. Temperatures will likely top out in the upper 30s and some lower 40s across most of the region. Likely high temperatures around the region include: New York City (Central Park): 40° Newark: 41° Philadelphia: 41° A significant to major Middle Atlantic and New England snowstorm (6” or more snow) is likely from tomorrow into Thursday. Boston, New York City, Newark, and Philadelphia will likely see 6”-12” with locally higher amounts. North and west of Philadelphia to New York City, an area of 12”-18” snow with some locally higher amounts is likely. Parts of the region will pick up more than a foot of snow. Near blizzard or even blizzard conditions are possible at the height of the storm tomorrow night.

Tomorrow will be the calm before the storm. It will be partly cloudy and chilly. A major storm will develop and move off the northern Delmarva Peninsula and pass south of Long Island Wednesday into Thursday. There remains some uncertainty about the storm's track, which could make a large difference in snowfall amounts along parts of the coastal plain. A general 6"-12" snowfall is likely from Philadelphia to Boston with locally higher amounts. Interior sections away from the coastline have the chance to pick up as much as 18" snow. During the December 10-31, 1950-2019 period, an AO- was the most important teleconnection for a significant Middle Atlantic (6" or more) snowstorm. Since 1950, the following percentage of 6" or greater snowstorms occurred when the AO was negative: Boston: 50%; New York City: 75%; and, Philadelphia: 88%. There were 8 snowstorms that brought 6" or more snow to at least two of the following cities: Boston, New York City, and Philadelphia. 7/8 (88%) occurred when the AO was negative. All storms from the subset from these 8 storms where 10" or more snow fell in at least one of those cities occurred with a negative AO. The most common synoptic pattern when at least two of the aforementioned cities picked up 6" or more snow (63%) was an AO-/PNA+ combination. The most recent such storm was the December 25-27, 2010 "Boxing Day Blizzard." Near record to record cold could develop in northwestern Canada during the middle or latter part of this week. A sustained warmer pattern could develop toward the winter solstice. The closing 10 days of December could wind up generally warmer than normal. The EPS weeklies and subseasonal guidance are in agreement that the last week of December will likely be warmer than normal. Uncertainty has increased, as the AO is likely to remain negative into the closing week of December and perhaps beyond it. Historic experience following exceptionally warm November cases suggests that a warmer than normal December remains the base case. Almost 90% of cases with a November mean temperature of 51.5° or above in Central Park (2020 saw the temperature average a record 52.9° in November) went on to record a warmer than normal December and just over three-quarters of such cases saw December register a monthly mean temperature of 40.0° or above. The probability of December's having a 40.0° or above mean temperature has fallen sharply with the development of strong AO blocking. The ENSO Region 1+2 anomaly was -0.1°C and the Region 3.4 anomaly was -1.2°C for the week centered around December 9. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.62°C and the ENSO Region 3.4 anomaly has averaged -1.43°C. La Niña conditions will likely prevail at least through the winter. The SOI was +21.43. Today, the preliminary Arctic Oscillation (AO) figure was -2.889. That is the lowest AO value since February 1, 2019 when the AO was -3.315. On December 13 the MJO was in Phase 5 at an amplitude of 1.486 (RMM). The December 12-adjusted amplitude was 1.526. Based on the latest guidance, no significant stratospheric warming event is likely through the third week of December. Since 1950, there have been five cases where a La Niña developed during June-July-August or afterward following an El Niño winter. 4/5 (80%) of those cases saw a predominant EPO+/AO+ winter pattern. The most recent such case was 2016-17. 10/11 (91%) of the La Niña winters that followed an El Niño winter featured a predominantly positive EPO. A predominant EPO+/AO+ pattern is very likely for winter 2020-21. It is likely that the Middle Atlantic and southern New England areas will see a warmer than normal winter with below normal snowfall. Based on sensitivity analysis applied to the latest guidance, there is an implied 64% probability that New York City will have a warmer than normal December. December will likely finish with a mean temperature near 39.0°.

Allentown: 6” or more: 98% members; 10” or more: 67% members; 12” or more: 47% members JFK: 6” or more: 78% members; 10” or more: 39% members; 12” or more: 25% members Mount Pocono: 6” or more: 98% members; 10” or more: 78% members; 12” or more: 47% members

Morning thoughts... At 7:25 am, precipitation was moving into the region. Rain had changed to snow in a line running from East Stroudsburg, PA to Poughkeepsie. Today will mainly cloudy and cooler. Some rain that could end as snow, especially north and west of the major cities is likely. Parts of the area could pick up a minor accumulation. Temperatures will likely top out in the lower 40s this morning in most of the region. Likely high temperatures around the region include: New York City (Central Park): 40° Newark: 41° Philadelphia: 42° A significant to major Middle Atlantic and New England snowstorm (6” or more snow) is likely Wednesday into Thursday. The operational GFS, which had been the most suppressed guidance has moved closer in line with the ECMWF and GGEM Models at 6z. A look at the EPS individual members continues to show a significant to perhaps major snowstorm in much of the region. Boston: 6” or more: 84% members; 10” or more: 78% members; 12” or more: 59% members Islip: 6” or more: 90% members; 10” or more: 55% members; 12” or more: 37% members New York City (Central Park): 6” or more: 98% members; 10” or more: 80% members; 12” or more: 43% members Philadelphia: 6” or more: 49% members; 10” or more: 24% members; 12” or more: 14% members Poughkeepsie: 6” or more: 94% members; 10” or more: 78% members; 12” or more: 55% members

Strong Arctic blocking continues. That blocking will likely pave the way for a significant to perhaps major Middle Atlantic and New England snowstorm Wednesday and Thursday. Significant snowfall is likely in such cities as Boston, New York City, Newark, and Philadelphia, as well as north and west of those cities. Before then, tomorrow will be cloudy and noticeably cooler. Rain is likely and parts of the region could see snow with some accumulation. A major storm will then develop and move off the northern Delmarva Peninsula and pass south of Long Island Wednesday into Thursday. There remains some uncertainty about the storm's track, which could make a large difference in snowfall amounts along parts of the coastal plain. The probability of a significant snowfall (6" or more) has increased further for such cities as Philadelphia, New York City, and Boston. Select EPS Ensemble Members: 6" or more: Boston: 90% Islip: 86% New York City: 96% Philadelphia: 67% Poughkeepsie: 94% 10" or more: Boston: 75% Islip: 65% New York City: 73% Philadelphia: 39% Poughkeepsie: 68% During the December 10-31, 1950-2019 period, an AO- was the most important teleconnection for a significant Middle Atlantic (6" or more) snowstorm. Since 1950, the following percentage of 6" or greater snowstorms occurred when the AO was negative: Boston: 50%; New York City: 75%; and, Philadelphia: 88%. There were 8 snowstorms that brought 6" or more snow to at least two of the following cities: Boston, New York City, and Philadelphia. 7/8 (88%) occurred when the AO was negative. All storms from the subset from these 8 storms where 10" or more snow fell in at least one of those cities occurred with a negative AO. The most common synoptic pattern when at least two of the aforementioned cities picked up 6" or more snow (63%) was an AO-/PNA+ combination. The most recent such storm was the December 25-27, 2010 "Boxing Day Blizzard." Near record to record cold could develop in northwestern Canada during the middle or latter part of this week. A sustained warmer pattern could develop toward the winter solstice. The closing 10 days of December could wind up generally warmer than normal. The EPS weeklies and subseasonal guidance are in agreement that the last week of December will likely be warmer than normal. Uncertainty has increased, as the AO is likely to remain negative into the closing week of December and perhaps beyond it. Historic experience following exceptionally warm November cases suggests that a warmer than normal December remains the base case. Almost 90% of cases with a November mean temperature of 51.5° or above in Central Park (2020 saw the temperature average a record 52.9° in November) went on to record a warmer than normal December and just over three-quarters of such cases saw December register a monthly mean temperature of 40.0° or above. The probability of December's having a 40.0° or above mean temperature has fallen sharply with the development of strong AO blocking. The ENSO Region 1+2 anomaly was -0.5°C and the Region 3.4 anomaly was -1.1°C for the week centered around December 2. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.77°C and the ENSO Region 3.4 anomaly has averaged -1.35°C. La Niña conditions will likely prevail at least through the winter. The SOI was +16.97. Today, the preliminary Arctic Oscillation (AO) figure was -2.734. That is the lowest AO value since December 26, 2019 when the AO was -2.773. On December 12 the MJO was in Phase 5 at an amplitude of 1.525 (RMM). The December 11-adjusted amplitude was 1.637. Based on the latest guidance, no significant stratospheric warming event is likely through the third week of December. Since 1950, there have been five cases where a La Niña developed during June-July-August or afterward following an El Niño winter. 4/5 (80%) of those cases saw a predominant EPO+/AO+ winter pattern. The most recent such case was 2016-17. 10/11 (91%) of the La Niña winters that followed an El Niño winter featured a predominantly positive EPO. A predominant EPO+/AO+ pattern is very likely for winter 2020-21. It is likely that the Middle Atlantic and southern New England areas will see a warmer than normal winter with below normal snowfall. Based on sensitivity analysis applied to the latest guidance, there is an implied 63% probability that New York City will have a warmer than normal December. December will likely finish with a mean temperature near 39.0°.

Morning thoughts... Today will be partly to mostly sunny and very mild. Temperatures will likely top out in the upper 50s and lower 60s in most of the region. Likely high temperatures around the region include: New York City (Central Park): 61° Newark: 62° Philadelphia: 63° Colder air will return beginning tomorrow, along with some rain with snow in parts of the region. A significant Middle Atlantic and New England snowstorm (6” or more snow) is likely Wednesday into Thursday. The operational GFS is the most suppressed guidance at present. A look at the EPS individual members shows a farther north extent of the significant snow. Boston: 6” or more: 92% members; 10” or more: 73% members New York City (Central Park): 6” or more: 86% members; 10” or more: 55% members Philadelphia: 6” or more: 35% members; 10” or more: 22% members

The major synoptic story of the opening of December is the return of the negative Arctic Oscillation (AO). Through today, the AO has been negative on 83% of days, including -1.000 or below on 67% of days. The AO has averaged -1.169 this month. During the December 10-31, 1950-2019 period, an AO- was the most important teleconnection for a significant Middle Atlantic (6" or more) snowstorm. Since 1950, the following percentage of 6" or greater snowstorms occurred when the AO was negative: Boston: 50%; New York City: 75%; and, Philadelphia: 88%. There were 8 snowstorms that brought 6" or more snow to at least two of the following cities: Boston, New York City, and Philadelphia. 7/8 (88%) occurred when the AO was negative. All storms from the subset from these 8 storms where 10" or more snow fell in at least one of those cities occurred with a negative AO. The most common synoptic pattern when at least two of the aforementioned cities picked up 6" or more snow (63%) was an AO-/PNA+ combination. The most recent such storm was the December 25-27, 2010 "Boxing Day Blizzard." Colder air will return to the region after the coming weekend. The recent guidance has suggested a fairly impressive cold shot for late next week. The return of the colder air could coincide with some storminess. There is growing model and ensemble support for a significant Middle Atlantic and New England snowstorm next Wednesday into Thursday. There remains some uncertainty about the storm's track, which could make a large difference in snowfall amounts along parts of the coastal plain. Near record to record cold could develop in northwestern Canada during the middle or latter part of next week. A sustained warmer pattern could develop toward the winter solstice. The closing 10 days of December could wind up generally warmer than normal. The EPS weeklies and subseasonal guidance are in agreement that the last week of December will likely be warmer than normal. Uncertainty has increased, as the AO is likely to remain negative into the closing week of December and perhaps beyond it. Historic experience following exceptionally warm November cases suggests that a warmer than normal December remains the base case. Almost 90% of cases with a November mean temperature of 51.5° or above in Central Park (2020 saw the temperature average a record 52.9° in November) went on to record a warmer than normal December and just over three-quarters of such cases saw December register a monthly mean temperature of 40.0° or above. The probability of December's having a 40.0° or above mean temperature has fallen sharply with the development of strong AO blocking. The ENSO Region 1+2 anomaly was -0.5°C and the Region 3.4 anomaly was -1.1°C for the week centered around December 2. For the past six weeks, the ENSO Region 1+2 anomaly has averaged -0.77°C and the ENSO Region 3.4 anomaly has averaged -1.35°C. La Niña conditions will likely prevail at least through the winter. The SOI was +15.88. Today, the preliminary Arctic Oscillation (AO) figure was -2.549. That is the lowest AO value since December 26, 2019 when the AO was -2.773. On December 11 the MJO was in Phase 5 at an amplitude of 1.642 (RMM). The December 10-adjusted amplitude was 1.376. Based on the latest guidance, no significant stratospheric warming event is likely through the third week of December. Since 1950, there have been five cases where a La Niña developed during June-July-August or afterward following an El Niño winter. 4/5 (80%) of those cases saw a predominant EPO+/AO+ winter pattern. The most recent such case was 2016-17. 10/11 (91%) of the La Niña winters that followed an El Niño winter featured a predominantly positive EPO. A predominant EPO+/AO+ pattern is very likely for winter 2020-21. It is likely that the Middle Atlantic and southern New England areas will see a warmer than normal winter with below normal snowfall. Based on sensitivity analysis applied to the latest guidance, there is an implied 58% probability that New York City will have a warmer than normal December. December will likely finish with a mean temperature near 39.0°.

Morning thoughts... Today will be mostly cloudy and mild. A few locations could see a shower. Temperatures will likely top out in the lower and middle 50s in most of the region. Likely high temperatures around the region include: New York City (Central Park): 56° Newark: 56° Philadelphia: 59° Next week could see the return of colder air. There is growing support for a significant Middle Atlantic and New England snowstorm for the middle of next week.